Question 1

If the motor nameplate is unreadable, how do you determine the right new motor for the machine?

Accepted Answer

If the nameplate cannot be read, the new motor is identified by combining the dimensions of the old frame, the speed the machine requires and the demand of the driven load. The order matters: settle the mechanical dimensions first, then the speed, and the power last. Most mistakes are made when estimating the power. What does the frame tell you?Even without a nameplate, the cast frame still carries information. The distance from the underside of the feet to the shaft centre gives the IEC frame number: a motor measuring 90 mm is a 90 frame, one measuring 132 mm is a 132 frame. The shaft diameter and the length of shaft protruding from the motor confirm this estimate; in the DRG range, 132-frame models have a 38×80 mm shaft and 100-frame models a 28×60 mm shaft. If you spot an embossed number on the end shield or the frame, write it down as well.Finding the pole count by measuring speedIf the motor can still run, measure the shaft speed under load with a tachometer. An induction motor tu

Question 2

Can the right product be found from the motor code given by the machine manufacturer?

Accepted Answer

The code supplied by a machine manufacturer is usually that company's own stock or part number; on its own it does not correspond to another brand's product. Once the technical data behind the code (power, speed, frame, mounting arrangement, shaft and voltage) has been obtained, the right product can be matched with confidence. Tell the types of code apartThe code you have can be one of three kinds. A part number only has meaning within that machine maker's spare parts list. A motor supplier's type code, on the other hand, usually contains frame and pole information; in DRG codes, for example, “DRG-112M-4” indicates a 112M frame and 4 poles. The third kind is a general classification tied to the machine model, and it carries the least information.The data sheet to request from the manufacturerOnce you have the code, ask the manufacturer or the seller for the following in writing:Shaft power (kW) and rated speedVoltage, frequency and connection (Δ/Y)Mounting arrangement: foot, flange or

Question 3

Why do different models of the same machine use motors of different power?

Accepted Answer

Models of a machine sharing the same name differ in capacity, working speed, processed material and optional equipment, so their motor power differs too. That is why the figure in a sister model's catalogue is no yardstick for your machine; the data for your own model and serial number should be the basis. The main reasons power changesCapacity: A wider blade, a longer belt or a larger hopper puts more material to work at the same time.Speed: If the shaft turns faster on the higher model, more power is needed for the same torque; power is proportional to torque multiplied by speed.Material: A model planned for softwood and a version processing hardwood or plastic see different loads.Options: If an extra feed unit, brush or fan is connected to the same motor, the total demand grows.Market: Versions prepared for 60 Hz countries may have been given a different motor speed and power.If you use the wrong model's dataIf the motor from the small model is fitted to the large machine, it runs h

Question 4

How do you prepare a motor requirement list for a new production line?

Accepted Answer

The requirement list should be prepared as a table in which every drive point on the line has its own row, and each row records power, speed, mounting arrangement, shaft size, environment and starting conditions. A list like this cuts down the back-and-forth of questions at quotation stage and lowers the risk of receiving the wrong motor. How should the rows be organised?Arrange the list in machine order rather than by motor type: feeding, conveying, processing, packing and so on. If there are several motors with the same specification, use a quantity column, but do not squeeze motors from different locations into a single row; one being outdoors and the other in an enclosed hall can change the selection.ItemDriven machinePower / speedMounting / shaft1Feed belt4 kW / 1500 rpmFoot-mounted, 28×60 mm2Crusher22 kW / 1500 rpmFoot-mounted, 48×110 mm3Dust extraction fan7.5 kW / 3000 rpmFoot-mounted, 38×80 mmThe values in the table are only there to illustrate the layout.Second-layer informati

Question 5

When buying a spare motor, which features must match the existing motor?

Accepted Answer

A spare motor must match the existing motor in power, speed, supply voltage and frequency, type of mounting, shaft diameter and length, and foot or flange fixing dimensions. If these agree, the spare can be placed on the base without extra work when a breakdown happens; brand, colour or frame material come second. What must matchFeatureWhat happens if it differs?Power (kW)If lower, the motor is overloaded; if higher, the frame may be largerSpeed (number of poles)The machine speed changes and product quality suffersVoltage and frequencyIt may not work with the existing control panelMounting arrangement and flangeThe motor cannot be attached to the machineShaft diameter, length, keyThe pulley or coupling will not seatShaft height and foot holesAlignment is lost and the base has to changeWhere some flexibility is acceptableThe position of the terminal box, the frame material and the external appearance may differ, but each has a practical effect. If a cast iron spare is chosen in place of

Question 6

If the machine's operating manual has no motor information, which sources should you check?

Accepted Answer

An operating manual is mostly written for the operator and may contain no motor data. In that case, the machine's rating plate, the wiring diagram, the spare parts list, the purchase documents and correspondence with the manufacturer quoting the serial number are the most reliable sources. Where to look in the documentsWiring diagram: Kept inside the panel door or in a separate file. Each motor is usually labelled “M1, M2” and so on, often with its power written alongside.Spare parts catalogue: In exploded drawings the motor is shown with a part number; the description line often includes kW and speed.Machine rating plate: Shows the total installed power, voltage and frequency. On single-motor machines this value points directly to the motor.Purchase and service records: Old invoices, the maintenance log or the receipt for an earlier rewind may contain the motor values.Sources beyond the paperworkWhen queried with the serial number, the machine manufacturer's technical service can tell

Question 7

Where should you start when looking for a motor for a second-hand machine?

Accepted Answer

When looking for a motor for a second-hand machine, start with the machine itself rather than a motor catalogue: answer three questions first — does the machine turn freely by hand, is a suitable supply available at your site, and is the fitted motor the original one? A motor bought before these points are clear may not fix the real source of the problem. Check the machine before blaming the motorOn a machine the seller describes as having a “burnt-out motor”, the real fault may be a seized bearing, a broken gear or a jammed pump impeller. With the power off, turn the driven side by hand; if it turns heavily or unevenly, mechanical repair comes first. Otherwise the new motor will struggle under the same load.Is the fitted motor the original?Previous users may have fitted whatever motor was to hand after a breakdown. The following signs point to this:Newly drilled holes next to the old ones on the baseSheet packing or blocks added under the motor at a later dateA bush or adaptor piece i

Question 8

Which dimensions can be taken without removing the motor?

Accepted Answer

With the motor still in place, most of the key dimensions — shaft height, shaft diameter and length, the spacing between foot holes, overall length and the outer flange dimensions — can be taken with calipers and a tape measure. During measurement the machine must be isolated and locked off, as you will be working close to rotating parts. Dimension list and methodDimensionIEC letterHow to take itShaft heightHFrom the foot base to the shaft centre; measure from the foot contact surface to the top of the shaft and subtract the shaft radiusShaft diameterDWith calipers, on the part of the shaft not covered by the pulley or couplingShaft lengthEFrom the shaft shoulder to the shaft end; if a hub is fitted, its length helpsFoot hole spacing (across)ABetween hole centres, opposite feetFoot hole spacing (lengthwise)BBetween the two hole centres on the same sideShaft shoulder to front foot holeCFrom the shaft shoulder to the centre of the front foot holeOverall lengthLFrom the shaft end to the r

Question 9

Which photos of the machine should be taken for motor selection?

Accepted Answer

For a sound recommendation, the motor nameplate, the shaft end, the fixing method (feet or flange), the drive element, the direction the terminal box faces and an overall view of the machine should each be photographed separately. Several close, sharp shots rather than one distant picture answer most of the dimension questions up front. Shot-by-shot listNameplate: Square on, without flash glare; if it is hard to read, take a second shot with light coming from the side.Side profile of the motor: Showing the full length from the shaft end to the fan cover.Shaft end: Together with the pulley or coupling hub; place an extended tape measure next to it.Feet or flange: The bolt holes and where they meet the base.Terminal box: Its position with the lid closed and the direction of the cable entry.The whole machine: A wide shot showing where the motor sits on the machine and the space around it.Machine rating plate: Model, serial number and total power.What photos cannot tell youA photo gives no

Question 10

How should the machine's operating environment be described in a motor order?

Accepted Answer

The environment should not be described in an order with vague terms such as “normal” or “harsh conditions”; instead, list the temperature range, altitude, humidity and contact with water, the type of dust, chemical effects, vibration and the air space around the motor one by one. This shows clearly whether a standard motor is adequate. Description templateTemperature: The extreme summer and winter values; is there a furnace, boiler or sun-exposed sheet-metal roof near the motor?Altitude: In metres, if the site is well above sea level.Water and humidity: Is there washdown, does condensation occur, is the motor cleaned with a hose?Dust: Its type, such as flour, sawdust, cement or metal dust, and how much builds up.Chemicals: Ammonia, chlorine, acid vapour or salt-laden air.Vibration: Is it mounted on a vibrating structure such as a screen or crusher?Air space: Is the motor inside a closed cabinet, is the area behind the fan cover open?How far does a standard motor go?DRG motors are buil

Question 11

Can a replacement be sourced locally for the motor on a machine imported from abroad?

Accepted Answer

If the machine uses an IEC-dimensioned motor, finding a replacement is generally possible. Frequency, voltage, the unit of power and the shaft measurement system should be checked first; on machines built for America or some Asian markets, these four points can hold surprises. If you see a 60 Hz nameplateInduction motor speed follows the supply frequency. Built for 60 Hz, a four-pole machine reaches 1800 rpm synchronous; connected to 50 Hz mains it drops into the 1500 rpm class. What matters, therefore, is the speed at which the machine actually runs on your supply. If the machine has so far worked without trouble on a 50 Hz grid, a 50 Hz motor with the same number of poles keeps the current speed. If the machine was set up for the 60 Hz speed, the difference can be offset with the pulley ratio; on pumps and fans, a change in speed directly affects flow and power.HP, voltage and measurement systemPower: One horsepower (HP) is roughly 0.746 kW. A motor marked 10 HP corresponds to approx

Question 12

If the required machine shaft speed is known, how do you choose the motor speed?

Accepted Answer

If the machine shaft speed is close to one of the standard pole speeds (3000, 1500 or 1000 rpm), the motor is chosen from that class and can be connected directly with a coupling. If the speed is far from these values, a transmission (belt and pulley or gearbox) is planned first, and then the number of poles is chosen to keep the transmission ratio reasonable. Options for example speedsMachine shaftPossible motorTransmissionAbout 2900 rpm2-pole (3000)Direct via couplingAbout 1450 rpm4-pole (1500)Direct via couplingAbout 700 rpm4-pole or 6-poleBelt: about 2:1 or 1.4:1100 rpm and below4 or 6-poleGearbox requiredUnder load, induction motors turn a little slower than synchronous; base the target speed calculation on the rated rpm printed on the plate.The number of poles also sets the torqueAt constant power, the torque available at the shaft rises as speed falls. The approximate calculation is: torque (Nm) ≈ power (kW) × 9550 / speed (rpm). An 11 kW motor in the 1500 rpm class gives about

Question 13

On a belt-driven machine, how is the pulley ratio taken into account when choosing motor speed?

Accepted Answer

In a belt drive, machine speed is found with this relationship: nmachine = nmotor × Dmotor pulley / Dmachine pulley. When choosing motor speed, first define the speed the machine needs, then work out the pulley diameters that give this speed with the chosen motor; motor and pulley are decided together. A worked exampleSay a mixer shaft must turn at about 480 rpm and the machine pulley is 300 mm. With a 6-pole motor rated at about 960 rpm, the motor pulley required is 300 × 480 / 960 = 150 mm. If the same job is done with a 4-pole motor rated at about 1440 rpm, the pulley shrinks to 100 mm. Both solutions give the same machine speed; the difference lies in pulley size and the belt's angle of wrap.Which should you choose?The small pulley should not go below the minimum diameter allowed by the belt profile; otherwise the belt fatigues quickly.As the ratio grows, the wrap angle on the small pulley decreases and the risk of slip increases.A higher-speed motor can be smaller and lighter for

Question 14

Before increasing motor power, which machine parts should be checked for strength?

Accepted Answer

A more powerful motor does not by itself make the machine do more work; but during a jam or sudden load it can transmit higher torque to the drive train. So before power is increased, check whether the shaft and key, coupling, belts, bearings, gearbox, machine frame and electrical supply are suited to the new rating. The mechanical chain, link by linkMachine shaft and key: The shaft diameter and key section were chosen for a certain torque. The starting torque of a larger motor can exceed this limit.Coupling: Every coupling has a rated and a maximum torque; the flexible element wears out faster with a stronger motor.Belt and pulley: The number and profile of belts are calculated for the power to be transmitted; as power rises, so does the risk of slipping and snapping.Bearings: Higher belt tension increases the radial load on the bearings.Gearbox: The input power and output torque on its nameplate must not be exceeded; if they are, gear damage follows.Chassis and base: A larger frame i

Question 15

Why does motor weight matter when choosing a motor for a mobile machine?

Accepted Answer

On a mobile machine, motor weight has a direct effect on the machine's resistance to tipping, the load on wheels and chassis, how easily it can be moved by hand or vehicle, and how well it copes with road vibration. For this reason, choosing a lighter frame for the same power and speed matters more than on a fixed bench. Where is the weight felt?Centre of gravity: The motor usually sits at one end of the machine and high up; a heavy frame increases the machine's tendency to tip over.Wheels and axle: On small-wheeled carts, extra load strains the wheel hubs and chassis welds.Trailer and towing: On towed machines, the total weight and the load on the drawbar are limited.Transport and lifting: On machines loaded onto a van or carried by two people, even a few kilograms make a difference.Vibration: A heavy mass shaken on the road applies greater force to the fixing bolts.Choosing the frame materialIn the DRG range, aluminium-frame models cover 0.25–11 kW; for the same power and frame numbe

Question 16

Does the machine's direction of rotation affect the motor order?

Accepted Answer

Standard three-phase induction motors can usually run in either direction; rotation is determined by the phase sequence of the supply and set during installation. So the direction does not normally change the motor model. Even so, on machines where direction is critical, it is important to note it in the order and to check it at the first start. Machines where direction is criticalCentrifugal pumps: A pump running backwards seems to deliver water, but its pressure and flow drop; impellers screwed onto the shaft may work loose.Fans: In reverse the airflow weakens and performance is lost even if the current looks normal.Screw compressors and augers: Reverse rotation can cause mechanical damage within a short time.Backstop arrangements: On inclined conveyors the backstop blocks rotation in the wrong direction and creates strain.How should direction be described?When stating the direction, always give the viewpoint, for example “clockwise when looking at the shaft end”. The arrow on the ma

Question 17

If the machine will stand outdoors, what notes should be added to the motor order?

Accepted Answer

For a machine standing outdoors, writing just “outdoor use” on the order is not enough; the mounting position, exposure to sun and rain, the day-night temperature swing, proximity to the sea, whether the machine will sit idle for long periods and the direction of the cable entry should each be stated. These notes show whether a standard motor is adequate and whether extra protection is needed. Notes to add to the orderMounting position: Horizontal, or vertical with the shaft down? For vertical shaft-down mounting, ask whether a canopy is needed to stop water entering through the fan cover.Overhead protection: Whether there is a shelter or sheet-metal cover above the motor.Sun: Direct summer sun raises frame temperature; include it in the ambient temperature description.Condensation: Cold nights followed by humid mornings can lead to water collecting inside the motor.Salty or polluted air: Corrosion speeds up on the coast or near a fertiliser plant.Idle periods: On a seasonal machine th

Question 18

How can you tell whether a machine starts unloaded or loaded?

Accepted Answer

Whether a machine starts unloaded or loaded can be worked out by looking at whether material remains inside it when it stops, whether there is a device that removes the load before start-up, and how long the run-up takes. This information directly affects the motor's starting torque requirement and the choice of starting method. Watch how it stopsThe clearest clue is how the machine comes to a halt. Does it stop after the feed is cut and it has emptied, or is it left full after a power cut or an emergency stop? If a belt conveyor, for example, stops with material on it, the next start has to set the whole load in motion. Stones left in a crusher's hopper have the same effect.Is there a device that removes the load?A valve on a compressor that releases pressure during start-upA damper on a fan that closes at start-upA discharge valve on a pump that is throttled at start-upA clutch or fluid coupling between machine and motorIf one of these is present and working properly, the machine in

Question 19

What questions should you ask the machine manufacturer before choosing a motor?

Accepted Answer

The questions you put to the machine manufacturer should reveal not only the motor's power but also how it will operate, how it will be attached to the machine and what loads it will face. Asking for the answers in writing, and with a dimension drawing if possible, prevents mismatches from surfacing later. Questions about load and speedWhat power and torque are needed at the machine shaft, and what margin has been added to this value?At what speed must the machine shaft turn; will speed adjustment be required?Does the machine start unloaded or under load; is extra torque needed during starting?How many hours a day does it run, and how many times an hour does it stop and start?Questions about connection and layoutWill the motor be foot-mounted, flange-mounted or both?What are the shaft diameter and length, and what size is the keyway?What are the dimensions of the space set aside for the motor, and which way does the cable enter?For a belt drive, what are the pulley diameters and belt t

Question 20

When describing a motor over the phone, what information must be given in full?

Accepted Answer

To describe a motor fully over the phone, read out the nameplate values together with their units, then explain the fixing method, shaft diameter, shaft height and what the machine does. Giving figures without units is the most frequent cause of misunderstandings during a call. Notes to prepare before the callHeadingWhat to sayPowerStating whether it is kW or HPSpeedThe rpm value on the nameplate, e.g. 1475Voltage and frequencyThe two voltage values on the nameplate and the HzFixing methodFoot-mounted, flange-mounted or bothShaftDiameter and protruding length, in mmShaft heightFrom the foot face resting on the floor to the middle of the shaft, in mmApplicationPump, fan, crusher, conveyor and so onExpressions that get misunderstood“A 380 motor” describes the voltage but does not show the winding type; read both voltage values on the nameplate together.“1400 rpm” points to a 4-pole motor and “2800 rpm” to a 2-pole one; give the number without rounding it.Descriptions such as “large size”

Question 21

How are power and speed determined in motor selection?

Accepted Answer

Motor power (kW) is determined by the load requirement of the driven machine; the motor should be selected close to, but not below, the load. Speed depends on the supply frequency and the number of poles: at 50 Hz, 2 poles correspond to about 3000 rpm, 4 poles to about 1500 rpm, 6 poles to about 1000 rpm and 8 poles to about 750 rpm synchronous speed. How is power calculated? Mechanical power is the product of torque and speed: P (kW) = T (Nm) × n (rpm) / 9550. If the torque the machine requires and its operating speed are known, the necessary power follows from this formula. For pumps and fans, flow, pressure and equipment efficiency are taken into account; for conveyors, load, speed and friction. Losses in transmission components (gearbox, belts) and a reasonable safety margin are added, and the next standard rating is chosen. A heavily oversized motor, however, has lower efficiency and power factor at light load. Speed and slip The actual speed of an induction motor is slightly belo

Question 22

If the new motor's terminal box is on a different side, will there be a problem with installation?

Accepted Answer

A terminal box on a different side does not prevent installation in most cases; the motor's mechanical connection stays the same. However, cable length, the direction the cable entry will face, whether the box lid can be opened, and whether the box clashes with the belt guard, a wall or another part should be checked before ordering. Four points to check on siteCable length: The existing cable was cut to suit the old box; when the box moves to the other side, the cable has to run over or under the motor. If it is too short, rather than adding a joint, renewing the cable should be assessed by qualified electrical personnel.Clashes: The box may touch the belt guard, the machine body or a motor alongside. Compare the box size on the technical drawing with the space available.Lid access: For maintenance, the lid screws must be reachable and the lid must be able to open.Water and dust: If the cable gland faces upwards in the box's new position, there is a risk of water getting in.Can the bo

Question 23

What are the advantages of asynchronous motors?

Accepted Answer

The main advantages of asynchronous (induction) motors are their robust construction, long life, low maintenance requirement and high efficiency. Since they contain no wearing parts such as brushes or commutators, they operate reliably under demanding industrial conditions. A simple, durable design The rotor of an induction motor is usually a so-called squirrel cage: aluminium or copper bars joined at both ends by short-circuit rings. No electrical connection is made to the rotor; current is induced in it by the rotating magnetic field of the stator. As a result, there is no need for brushes, commutators or slip rings that require maintenance. The only moving wear parts are essentially the bearings and the fan. Key advantages Low maintenance: routine work is mostly limited to bearing lubrication, cleaning and checking connections. Direct-on-line operation: connected to a three-phase supply, the motor starts without any additional drive. Easy speed control: with a frequency inverter, sp

Question 24

When the motor's overall length changes, how can you tell whether it will fit the machine?

Accepted Answer

To tell whether a new motor will fit, comparing overall length alone is not enough; check together where the shaft end will sit relative to the machine, whether the foot holes line up with the base, and whether there is enough space behind the fan cover for cooling air. The shaft end is the reference pointThe coupling or pulley is the fixed point between machine and motor. When the motor is longer, the difference usually shows up at the rear, towards the fan cover. So take your measurement backwards from the shaft end: the distance between the shaft end and the obstacle on the machine must be greater than the new motor's overall length.Length varies even with the same frame numberFrameExample DRG modelOverall length (mm)132SDRG-132S-4-GG438132MDRG-132M-4-GG476160MDRG-160M-4-GG608160LDRG-160L-4-GG652The letters S, M and L denote different frame lengths at the same shaft height. This difference also changes the lengthwise distance between the foot holes; hole alignment on the base should

Question 25

Are motors resistant to water and dust?

Accepted Answer

The resistance of electric motors to dust and water is expressed by the IP (Ingress Protection) rating defined in IEC 60529. Our standard industrial motors are rated IP55, which means they are protected against harmful ingress of dust and against water jets from any direction. How to read an IP code The first digit of the IP code indicates protection against solid objects and dust, the second against water: First digit 5: dust-protected; dust is not fully excluded but cannot enter in amounts that would impair operation. First digit 6: dust-tight; no dust enters. Second digit 4: protection against splashing water from any direction. Second digit 5: protection against water jets from any direction. Second digit 6: protection against powerful water jets. Second digit 7: protection against temporary immersion under defined conditions. An IP55 motor can therefore be used safely in dusty workshops and on sites exposed to rain, while IP56 or IP66 is considered for areas washed down heavily or

Question 26

Can the old pulley and coupling be used on the new motor?

Accepted Answer

An old pulley or coupling can be reused if its bore diameter, keyway and hub length suit the new motor's shaft and the part shows no wear, cracking or imbalance. Even when the dimensions match, a worn part will quickly strain the new motor's bearings and belts. Dimensional fitBore diameter: Must equal the shaft diameter. In the DRG range, 132-frame models have a 38 mm shaft and 160-frame models a 42 mm shaft.Keyway: Its width and depth must seat properly with the key supplied with the new motor.Hub length: The protruding shaft length (80 mm on a 132 frame) must support the hub adequately. If most of the hub overhangs the shaft, the load falls on the shaft tip.If the dimensions differ, work such as boring out or fitting a bush depends on the wall thickness of the part and should be assessed by a machinist.Checking the pulley's conditionBelt grooves wear over time; if the belt sits deeper in the groove than normal or a shiny step can be seen on the groove flanks, the pulley is worn out.

Question 27

What should I consider when choosing an electric motor?

Accepted Answer

The main criteria when choosing an electric motor are the application, the required power (kW), speed, energy-efficiency class (IE3/IE4/IE5), mounting type, protection rating (IP) and the conditions of the operating environment. A correctly selected motor delivers energy savings, long life and uninterrupted operation. Electrical and performance criteria Power and torque: assess the machine's continuous load together with its starting torque requirement. Speed: choose 2, 4, 6 or 8 poles, and match with a gearbox or inverter if necessary. Supply: single or three phase, voltage (e.g. 230/400 V, 400/690 V) and frequency (50/60 Hz). Efficiency: for long running hours, IE3 and higher classes reduce operating costs. Duty type: S1 for continuous operation; choose the appropriate duty type for frequent starts and stops. Mechanical and environmental criteria Mounting type: B3 foot-mounted, B5 large flange, B14 small face flange, B35 foot and flange. Frame size and shaft: the IEC frame number, sh

Question 28

If the height of the motor feet changes, which dimensions on the machine should be checked?

Accepted Answer

When the foot height changes, what really changes is the centre height of the motor shaft (dimension H). With a coupling drive, the two shafts must be brought onto the same axis again; with a belt drive, the pulleys must stay in the same plane and the distance between them must be rechecked. In addition, the base, the fixing bolts and the protective guards are reviewed. How big can the difference be?On IEC frames, the shaft height equals the frame number. When the same power falls into a different frame, the difference becomes significant:Existing frameNew frameChange in shaft centre112M (H 112 mm)132S (H 132 mm)Rises by 20 mm132M (H 132 mm)160M (H 160 mm)Rises by 28 mm100L (H 100 mm)90L (H 90 mm)Drops by 10 mmWith a coupling driveIf the motor shaft sits higher than the machine shaft, the coupling runs at an angle, and vibration and bearing load increase. If the motor has dropped, full-surface calibrated shims matching the foot size can be placed underneath; if it has risen, the motor

Question 29

What are the types of electric motors? (AC motor, DC motor, synchronous, asynchronous)

Accepted Answer

Electric motors are divided first by supply type into AC (alternating current) and DC (direct current) motors; AC motors are further classified as synchronous or asynchronous, depending on how the rotor follows the rotating field. The most common type in industry is the three-phase asynchronous motor, thanks to its robust construction and economical operation. AC motors AC motors are durable, long-lasting and low-maintenance, and are widely used in pumps, fans and general industrial applications. There are two main sub-groups: Asynchronous (induction) motors: the rotor turns slightly slower than the stator's rotating field; this difference is called slip. The squirrel-cage rotor has no brushes, so maintenance needs are low. Single-phase and three-phase versions exist. Synchronous motors: the rotor turns at exactly the speed of the rotating field, so speed depends on frequency rather than load. Permanent-magnet synchronous motors and synchronous reluctance motors offer high efficiency,

Question 30

What mounting information should be given when ordering a motor for a vertically operating machine?

Accepted Answer

For a vertical machine, the most critical piece of information in the order is which way the shaft points: down or up. In addition, give the IEC mounting code (IM V1, V3, V5, V6, etc.), the flange type and size, the axial load acting on the shaft and the face by which the motor is suspended from the machine. State the shaft direction and mounting code together "Vertical motor" on its own is not a sufficient description. A flange-suspended arrangement with the shaft pointing down is usually referred to as V1, and a flanged arrangement with the shaft pointing up as V3; foot-mounted vertical arrangements are designated V5 and V6. The code determines the bearing arrangement, the position of the condensate drain holes and, where needed, whether a rain canopy is required. If the machine documentation does not show the code, a side-view photo of the existing motor as installed will also do the job. Flange data On most vertical machines the motor is carried by its flange rather than its feet.

Question 31

What can be done if the foot holes do not match the old base?

Accepted Answer

If the holes do not line up, it is the base that gets adapted, not the motor feet: either new holes or slots are made in the base, or a rigid adapter plate is fitted between the motor and the base. Enlarging the holes in the motor feet, cutting the feet or welding onto the frame is not acceptable; it reduces strength and voids the warranty. First find the cause of the mismatch On IEC frames, the transverse (A) and longitudinal (B) distances between the foot holes and the shaft height (H) are standard values that depend on frame size. The mismatch is usually one of the following: The old motor has a different frame size (e.g. 90L instead of 90S, which changes the longitudinal distance). The old motor was built to a non-IEC dimension system (inch-based or specific to the machine). The base was drilled by hand at some point and the holes have drifted. Writing down the A, B and H values of both motors side by side shows which solution makes sense. You can compare the dimensions using the D

Question 32

How is a pulley adapted to a new motor if the keyway size is different?

Accepted Answer

A keyway difference usually comes from a difference in shaft diameter; if the pulley hub has enough wall thickness, the bore is enlarged to the new shaft diameter and a keyway of the right width for the new shaft is cut. If the hub is thin or the new shaft is smaller than the old one, the solution is a new pulley or a taper bush pulley. Key size depends on shaft diameter On metric shafts, key width is standardised according to the shaft diameter range (DIN 6885 form A parallel key). Values for shaft diameters common on DRG models: Shaft diameter (mm)Key width (mm)Example IEC frame 24890S / 90L 288100L / 112M 3810132S / 132M 4212160M / 160L Always confirm the exact value from the technical drawing of the model concerned. Method by situation New shaft is larger: The pulley is bored out to the new diameter on a lathe, then the keyway is cut by broaching or slotting. The fit between bore and shaft must not be loose. Little hub wall left: Enlarging the bore weakens the pulley; a new pulley

Question 33

Is a coupling connection adequate if the shaft diameter is the same but the shaft is shorter?

Accepted Answer

Partly, it can be; what decides is how many millimetres of the coupling hub sit on the shaft and whether the distance the coupling maker requires between the two shaft ends can be maintained. If the hub grips only part of the shaft, the key load is concentrated on a small area, the hub rocks and the key may be crushed. Why is it shorter? On IEC motors, shaft length (E) is determined by frame size and number of poles. A shaft of the same diameter but shorter may indicate that the two motors belong to different dimension systems or different frame sizes. In that case, compare not only the shaft but also the shaft height and the foot holes. Three dimensions to check for the coupling Engagement length: Where possible, the hub should sit on the shaft over its full length. If the hub face overhangs the end of the shaft, this is acceptable only to a limited extent and only as far as the coupling documentation allows. Key length: The key must work along the whole section inside the hub; using

Question 34

Why should the spigot diameter be measured on a flange-mounted motor?

Accepted Answer

Because in a flange connection it is the spigot, not the bolts, that locates the motor shaft on the machine axis. Even if the bolt holes match exactly, a different spigot diameter means the motor either will not enter its recess or sits in it with play, and the shaft runs off-centre relative to the machine axis. Why don't the bolts centre the motor? Flange holes are made somewhat larger than the bolt; this clearance, which eases assembly, also allows the motor to shift sideways. The spigot and the recess in the machine, by contrast, form a close-tolerance fit. Thanks to this fit, a pump impeller turns without rubbing on its casing, and the seal and bearing at a gearbox input are loaded evenly. Examples of standard flange sizes On IEC flanges with clearance holes (FF type), the flange code refers to the bolt circle diameter, while the spigot diameter is a separate value: Flange codeBolt circle (mm)Spigot diameter (mm)Typical frame FF16516513080 / 90 FF215215180100 / 112 FF265265230132 S

Question 35

How can misalignment between the motor and the machine be prevented in a new installation?

Accepted Answer

The way to prevent misalignment is to follow a set sequence: first the base and foot seating, then rough alignment, then fine alignment with a measuring instrument, and finally a repeat measurement after the bolts are tightened. A connection left "by eye" or set with a straightedge may look fine in the first days, but it noticeably shortens bearing and coupling life. There are two kinds of misalignment Parallel misalignment means the two shaft axes are parallel but offset. Angular misalignment means the axes meet at an angle. In real installations both occur together, in both the vertical and horizontal planes; that is why measurement is done in two planes. Recommended working sequence Clean the base surface; leave no paint blisters, burrs or old rust flakes. Set the motor in place and fit the bolts by hand, then loosen each foot in turn to check for a gap underneath (soft foot). Carry out rough alignment with a straightedge and feeler gauge. Correct in the vertical plane, then in the

Question 36

If there were shims under the old motor, will the new motor need them too?

Accepted Answer

Not necessarily. The old shims were fitted to bring that motor's shaft height level with the machine or to correct a distorted foot; if the new motor's shaft height is different, the same thickness will now create misalignment. The right approach is to note the shims, remove them and set the height of the new motor again based on measurement. Record during removal The number and total thickness of shims under each foot (the four feet may differ) Shim material: wood, rubber, cut sheet metal, stainless steel shims The shaft height from the old motor's nameplate or as you measured it If all four feet carry the same thickness, the purpose was most likely height matching. If a single foot has a different shim, the base is distorted and the new motor may need a similar correction. Predict the height by calculation On IEC motors, the shaft height is the frame number itself: 100 mm on a 100 frame, 132 mm on a 132 frame. The difference between the old and new motor's shaft heights gives the rou

Question 37

How is the belt tensioning allowance kept on a slide rail base with a new motor?

Accepted Answer

The allowance is preserved if the new motor can tension the belt at a point near the middle of the slide rails. If the motor ends up against the far end of the rails at the first installation, there is no room left to take up the belt as it stretches; if it is against the near end, there is no slack available to fit a new belt. What ensures this is choosing the belt length and the motor foot dimensions together. Why do the rails need travel in both directions? Belts bed in and stretch slightly during their first hours of running, so travel is needed in the tensioning direction. The motor must also be able to move towards the machine so that the belt can be fitted without forcing it over the pulley or levering it into the grooves. Belt makers specify an allowance for both directions related to the centre distance; the values vary with the belt section. What to check on the new motor Foot spacing: Does the longitudinal hole spacing of the new motor fit the rail slots? A long-frame model

Question 38

Can the direction of the cable entry on the terminal box be changed?

Accepted Answer

On most IEC motors it is possible: the terminal box sits on a base bolted to the frame and on some designs can be rotated in 90-degree steps; on others there is a gland entry on both sides of the box and the unused side is closed with a blanking plug. Which option applies depends on the model; at DRG it is enough to ask about the model you intend to buy before ordering. Why does it matter? If the cable entry faces a wall, the machine body or a guard, the cable is bent sharply, the gland is strained and the seal is compromised. A cable that does not enter from below can carry water running along it straight into the box. IP55 protection applies only when the box cover, its gasket and the glands are correctly fitted. Do not confuse box orientation with box position Whether the box is on top of the motor or on its side depends on the mounting arrangement and the frame design; this is not changed on site. The point here is only which way the cable opening of the same box faces. If the box

Question 39

What should be specified when ordering a motor to be mounted horizontally on a wall or ceiling?

Accepted Answer

The order should state whether the foot-mounted motor will be fixed to a wall or to a ceiling, on which side the feet will be with the shaft kept horizontal, and the corresponding mounting code (IM B6, B7 or B8). This information matters for condensate drainage, the position of the terminal box and the direction of load on the bearings. What do the mounting codes mean? CodeMounting surfaceShaft IM B3Floor, feet downHorizontal IM B6 / B7Wall; feet on the left or right viewed from the shaft endHorizontal IM B8Ceiling, feet upHorizontal Although a foot-mounted motor can be used in these positions, the frame's condensate holes must end up at the lowest point in the new position; otherwise the water that collects will damage the winding. What to add to the order note The mounting code, or a drawing or photo of the motor as installed Which way the terminal box should face in this position and where the cable will come from What the mounting surface is, given the weight of the motor: concrete

Question 40

How is the motor type determined for a machine that uses both foot and flange mounting?

Accepted Answer

If the machine holds the motor by both its feet and its flange, the type required is foot-and-flange: IM B35 with the large flange with clearance holes, IM B34 with the small flange with tapped holes. Which one to choose is decided by the type and size of the flange on the machine and by whether the feet actually carry load. First understand what carries the load There are two scenarios on machines of this kind. In the first, the motor is fitted by its flange to a pump lantern or a gearbox, and the feet act as supports transferring the weight to the base. In the second, the motor is bolted to the base by its feet, and the flange is used only for a guard or dust cover. In the first case the flange dimensions are critical; in the second, the shaft height and foot holes are. Decision checklist Are the flange holes on the machine clearance holes (bolt tightened with a nut), or are the holes in the motor flange tapped? Clearance holes point to B35, tapped holes to B34. Measure the bolt circ

Question 41

Will the base be adequate when an aluminium-frame motor is replaced with a cast iron one?

Accepted Answer

If the IEC frame size is the same, the fixing dimensions mostly stay the same, so the holes and shaft height line up. The real question is whether the base can carry the extra weight without vibrating. A thick steel or concrete base is usually not a problem; on bases folded from thin sheet or projecting like a cantilever, a check is essential. How are the dimensions compared? In the DRG range, both aluminium and cast iron frame models are available for many power and speed ratings between 0.55 and 11 kW. For example, 3 kW 1500 rpm is frame size 100L in both versions, with a 28×60 mm shaft. Even so, foot thickness, frame length and terminal box position may differ; putting the two side by side on the comparison page is the quickest way. Base checklist Supporting structure: Is the base a short bracket welded to a machine body, or a solid chassis standing on the floor? Flexing: With the motor in place, does the base spring when you press on it by hand? Bolts: If the foot thickness differs

Question 42

Does a flexible coupling tolerate any difference in shaft alignment?

Accepted Answer

No. A flexible coupling is designed to absorb the small errors left after installation and the thermal shifts that occur in operation, not to make up for poor alignment. Each coupling type has separate limits for angular, parallel and axial misalignment, and these limits narrow as speed rises. How flexible is "flexible"? Jaw couplings with an elastomer spider, tyre couplings, pin-and-bush couplings and steel disc couplings each allow a different amount of misalignment. Types with elastomer elements flex comparatively more, while disc couplings are stiffer. The limit values are given in the coupling maker's table; they are maximum values, and for long life it is recommended to keep the alignment well below them. Misalignment ends up in the bearing, not the coupling As the coupling element flexes, it produces a restoring force. This force changes direction with every revolution and is passed on to the motor and machine bearings. The consequences appear over time: Cracking of the elastome

Question 43

What should be done if the key supplied with the new motor does not fit the old pulley?

Accepted Answer

First establish why the key will not seat: is it too wide, too high, or does the pulley bore not fit the shaft at all? Filing the key down, hammering it into the keyway or heating the pulley and forcing it on damages both the shaft and the hub; the right solution is a key of the correct size or a pulley with a suitable keyway cut. Three possible situations Different shaft diameter If the pulley comes from the old motor's shaft and the new shaft is larger, the pulley will not go onto the shaft; the key problem is only the visible side of this. In that case the pulley bore is enlarged and the keyway recut, or the pulley is replaced. Same diameter, different keyway width The old pulley may have been machined with an inch-size or non-standard keyway. If the pulley keyway is narrower, it can be widened; if it is wider, the key runs loose in the keyway and knocks. In that case a stepped key (one side matching the shaft, the other the pulley) or a new pulley can be considered. Key too high If

Question 44

If the belt length can be adjusted, can a motor of a different frame size be used?

Accepted Answer

In most cases, yes; because a belt drive has no rigid axial connection between the motor and the machine shaft, a difference in shaft height is not as critical as in a coupling drive. Even so, being able to adjust the belt length is only one condition; the motor must also be fixable to the base, the pulley must stay in the correct plane and the guard must still close. What changes when the frame size changes? DimensionEffect Shaft height (H)The pulley centre rises or falls; the centre distance and belt angle change. Foot hole spacingNew holes or a plate may be needed in the base or slide rails. Shaft diameter and lengthThe old pulley may not fit; the hub position shifts. Overall lengthThe fan cover may come too close to a wall or guard behind it. Why a different frame? A frame size difference usually arises in one of two ways: a different speed has been chosen at the same power (at the same kW, the frame grows as the number of poles increases), or the power has been increased. If the s

Question 45

Why should the shaft of a new motor be turned by hand before installation?

Accepted Answer

Because it is the easiest way to catch mechanical problems before the motor is connected to the machine. The shaft should turn freely, quietly and smoothly; if there is catching, a rubbing noise, stiffness or anything abnormal in the axial play, the situation should be assessed before the motor is installed. What to look for when turning it by hand Smoothness: The shaft should turn with the same resistance over a full revolution. Stiffening at a particular point may indicate a dented bearing or a distorted frame. Noise: A slight swishing may be considered normal; clicking, grating and metallic rubbing noises are not. Fan: If the rear fan cover has taken a knock, the fan blades rub against it. This sound is easy to hear when the shaft is turned slowly. Runout: Noticeable wobbling of the shaft end during rotation may point to a bent shaft. Why before installation? After coupling or belting up, part of the resistance you feel belongs to the driven equipment, so pinning down the source bec

Question 46

What operating data should be collected before replacing the motor of a hood extraction system?

Accepted Answer

Nameplate data alone is not enough for the new motor; you also need to know how the system works. The main data to collect are the fan type and drive arrangement, motor and fan speeds, the current drawn in operation, the temperature of the extracted air together with its grease and vapour load, operating hours and the condition of the filters in the ductwork. Fan and drive information Is the fan centrifugal (scroll casing), a box fan or a roof fan? Is the motor directly connected to the impeller or driven by belt and pulleys? If belt-driven, the diameters of both pulleys; if direct-driven, the impeller bore size Whether the motor sits in a separate compartment outside the airstream or inside it Operating measurements Before the replacement, if the existing motor can still run, have the current measured on all three phases and compare it with the nameplate current. Taken with the hood covers and filters in their normal positions, this value shows how hard the system is working the motor

Question 47

What should be considered in motor selection if the fan filter clogs frequently?

Accepted Answer

As the filter clogs, system resistance rises and the fan's operating point shifts; how the motor responds to this change depends on the type of impeller. The motor should therefore be chosen not from the old nameplate but from the fan's blade type and the load with the filter both clean and dirty. Frequent clogging is also a system problem in its own right and should be solved first. Fan type is decisive On forward-curved centrifugal fans, power consumption rises markedly with airflow. When the filter clogs and airflow drops, the motor is relieved; the moment the filter is changed, airflow and power jump up. On this type of fan, the motor should be selected for the cleanest filter condition. On backward-curved fans the power curve peaks at a certain point and the tendency to overload is lower. On axial fans, increased resistance can raise the power in some cases. For details, see the forward-curved fan motor page. What to look at when selecting Power limit: The highest shaft power read

Question 48

When a new branch is added to a ventilation duct, will the existing motor still be adequate?

Accepted Answer

A definite answer cannot be given without looking at the fan curve. When the new branch is added as a parallel leg, the fan at the same speed usually delivers more total airflow, but the air reaching each outlet may fall; the motor load may rise or it may stay the same. The right approach is to calculate the airflow and pressure the new branch requires, find the corresponding point on the fan curve and confirm it by measuring the current of the existing motor. What does a new branch do to the system? Adding a parallel leg opens an extra path for the air and can reduce the total resistance; the fan then moves more air, and the power requirement rises, especially on forward-curved fans. Adding an extension to the end of the duct, on the other hand, increases the resistance along that path. Either way, the fan ends up running at a different point on its curve. Information to collect The fan maker's curve, or at least the fan type, impeller diameter and speed The required air volume at the

Question 49

Apart from rain, what conditions should be considered for a fan motor installed on a roof?

Accepted Answer

Several other factors are as important as rain: the roof surface and frame heated by summer sun, wind turning a stopped fan backwards, freezing in winter and condensation caused by day-night temperature swings, build-up of birds' nests and leaves, access for maintenance, and the temperature and contamination of the air the fan draws in. Motor selection is linked to each of these. Heat and sun Motor power is rated for a given ambient temperature. The air around a motor sitting inside an enclosed fan housing in direct sun can become noticeably hotter than the open air. In that case it is important for the motor to get cooling air and for the housing to be ventilated; if necessary, a margin is left when choosing the power. Class F insulation provides thermal endurance but is no substitute for cooling. Wind and reverse rotation A stationary roof fan can turn freely due to wind or pressure differences inside the building. When the motor is then switched on, it has to start while the impelle

Question 50

Is choosing a standard fan motor sufficient for a motor used in a paint booth?

Accepted Answer

In most cases, no. Solvent vapour is produced in a paint booth, and the booth interior and extract duct may be classified as an explosive atmosphere. A motor located in such a zone, or in contact with that air, must be certified for explosive atmospheres; a standard IP55 fan motor does not meet this requirement. The zone classification is what decides Which motor may be used is determined by the hazardous area classification and the explosion protection document commissioned by the operator. In this study, the booth interior, the duct and fan casing, and the place where the motor stands are each assessed separately. The motor type is chosen according to this document and the opinion of a specialist in the field; it is not decided by the guesswork of the seller or user. DRG's standard range DRG Motor's range consists of IP55, Class F insulated, self-ventilated three-phase asynchronous IEC motors. Motors certified for explosive atmospheres (Ex) are not part of this range. If the paint bo

Question 51

Should the motor power be recalculated when the fan impeller is changed?

Accepted Answer

If the new impeller is a spare of the same size and type as the old one, no calculation is usually needed. If its diameter, width, number or angle of blades, blade type or weight has changed, the motor power must definitely be reassessed; changes that look small can make a big difference to the power requirement. Why is the effect of diameter so large? For geometrically similar impellers turning at the same speed, the power absorbed rises roughly with the fifth power of the diameter. In other words, even a relatively small increase in diameter raises the motor load noticeably. This is an approximate relationship and the actual value depends on the fan curve and system resistance, but it shows why the "a few centimetres won't matter" approach is wrong. What to check besides power Starting time A heavier or larger-diameter impeller accelerates more slowly because of its inertia. The longer the start, the longer the motor draws high current and heats up. With heavy impellers, it should be

Question 52

To change the air volume of a belt-driven fan, should the motor or the pulley be changed?

Accepted Answer

Air volume changes with fan speed, and on a belt-driven fan the main factor setting the fan speed is the pulley ratio. The first step is therefore a pulley change; the motor is replaced only if the power needed at the new fan speed exceeds the existing motor's power, or if the speed ratio cannot reasonably be achieved with pulleys. Relationship between speed, airflow and power Fan speed is found from n_fan = n_motor × D_motor pulley / D_fan pulley. According to the approximate fan laws, airflow is directly proportional to speed, pressure varies with the square of speed and power with the cube of speed. An example calculation: Fan speed changeAirflow (approx.)Power required (approx.) 10% increase10% increaseabout 33% increase 10% decrease10% decreaseabout 27% decrease These values assume that the system stays the same; the actual result depends on the impeller type and duct resistance. When a pulley change is enough Reducing airflow: It is enough to make the motor pulley smaller or the

Question 53

Why does the airflow drop while the fan motor current is normal?

Accepted Answer

A normal-looking current may show that the motor is running healthily, but it does not prove that the airflow is sufficient. The problem is usually on the fan side: belt slip, a dirty impeller, a clogged filter, a damper in the wrong position, a leak in the ductwork or an impeller turning the wrong way. Moreover, on some fan types the current also drops when airflow falls; the value taken as "normal" may in fact be lower than it used to be. Interpreting the current correctly On forward-curved centrifugal fans, the motor load decreases as airflow falls. So on a fan with a clogged filter, the current may be below the nameplate value and be perceived as "normal". The real comparison should be made with a reference value measured when the system was healthy. If there is no reference, recording the current with the system in clean condition for future use is a good habit. Order of checks Direction of rotation: A centrifugal fan running backwards still delivers air, but noticeably less. Comp

Question 54

Why does a reverse-rotating fan seem to run but deliver too little air?

Accepted Answer

When a fan impeller turns the wrong way, the motor runs with a normal sound and often with a reasonable current, so the fault goes unnoticed at first glance. However, because the blade profile meets the air at the wrong angle, the pressure and airflow produced by the impeller drop noticeably. The problem lies not in the motor itself but in the direction in which it is made to turn. Reverse rotation affects radial and axial fans differently In scroll-housing radial fans, air still enters through the inlet and leaves through the outlet even when the impeller runs backwards; since the flow direction does not change, the user assumes the system is fine because "air is coming out". In reality, backward-curved blades are forced to work like forward-curved ones, the scroll tries to collect the air the wrong way round, and the resulting airflow stays far below the design value. In axial fans, by contrast, the direction of airflow reverses: an extractor that should draw air out starts blowing a

Question 55

How should hot air be taken into account when selecting a motor for a boiler or oven fan?

Accepted Answer

Because hot air is lighter than cold air, a fan running at the same speed draws less power in hot operation; the real load appears at the first start-up, while the oven is still cold. The motor should therefore be sized not for the hot operating point but for cold start and intermediate conditions. The second issue is the transfer of gas temperature to the motor through the shaft and the frame. When density changes, fan power changes too The power drawn by a fan is roughly proportional to the density of the gas it handles. The power figure given by the fan supplier usually refers to gas at operating temperature. When the boiler stops and cools down, the same impeller moves much denser air and the shaft power rises noticeably. If there is no closed-damper start, gradual heating or speed control, the motor rating should be checked against the cold-air condition. Asking for two separate power values (hot and cold) on the fan data sheet is the safest approach. How heat reaches the motor Co

Question 56

Is it safe to replace an extractor fan motor with a higher-speed model?

Accepted Answer

In most cases it is not safe. An extractor impeller is designed and balanced for a specific speed; as speed rises, the power the impeller demands increases roughly with the cube of speed. Fitting a 3000 rpm motor in place of a 1500 rpm one means, for the same impeller, roughly eight times the power demand and a risk of exceeding the mechanical limits of the impeller. Why does the power demand grow so quickly? According to the fan laws, airflow is roughly proportional to speed, pressure to the square of speed and shaft power to the cube of speed (with deviations depending on impeller type and duct curve). Even small increases in speed are noticeable: a 10% speed-up made with a pulley raises the power by about one third. If a higher-speed motor of the same kW rating is fitted, the motor is overloaded, the protection trips or the winding overheats and its service life is shortened. Risks beyond the motor Impeller strength: Centrifugal force grows as blade tip speed rises; impellers made o

Question 57

Why is the motor slide base important when replacing the motor of an air handling unit fan?

Accepted Answer

On a belt-driven air handling unit fan, the slide base is the part that determines both the position of the motor and the belt tension. If the new motor's foot holes do not fit the slide base, if the shaft height is different or if there is no tensioning margin left on the base, the motor will not run properly even if its kW rating and speed are correct. The size and condition of the slide base should therefore be checked before the motor. What problems does the slide base prevent? The slide base allows the motor to be moved along the belt direction so that tension can be adjusted, and lets the motor be brought closer for a belt change without being removed. A loose belt causes slipping, heating and loss of airflow; an over-tightened belt places excessive radial load on the motor's drive-end bearing and on the fan bearings. Because the AHU section is cramped, when the slide base is inadequate service staff often make do by fixing the motor at an angle; this upsets pulley alignment and

Question 58

How does impeller wear in a dust collection fan change the motor load?

Accepted Answer

Because a worn impeller transfers less energy to the air, airflow and pressure drop, and the motor current usually falls too. The real danger lies not in the load but in the imbalance and vibration created by uneven wear. In addition, when the worn impeller is replaced with a new one, the fan demands full load again; this is when it becomes clear whether the motor rating is sufficient. Abrasion and build-up have different effects Hard particles such as sand, metal dust or glass fibre wear down the blade tips and the front shroud; as the blades get shorter, the pressure produced by the impeller falls. With moist, oily or sticky dusts the opposite happens: the layer building up on the blades increases the mass and inertia of the impeller and lengthens the run-up time. When this layer breaks away piece by piece, the impeller suddenly goes out of balance. The effect of vibration on the motor If the impeller is mounted directly on the motor shaft, the imbalance force is transmitted to the m

Question 59

Why do humidity and crop spraying matter when selecting a motor for a greenhouse ventilation fan?

Accepted Answer

For a greenhouse fan, the motor is selected according to the power and speed the fan requires, the humid, chemical-laden air of the greenhouse and the electrical supply. The total load of the group of fans switching on at the same time, not just a single fan, must also be taken into account. Motor power should be taken from the fan data sheet, and protection requirements from the way the greenhouse is operated. Why is greenhouse air so demanding? Relative humidity inside a greenhouse climbs very high in the morning, and temperature rises quickly during the day. Fogging and pad cooling systems carry water droplets around the motor; after spraying, the air may contain sulphur, copper or other crop-protection residues. An enclosed, IP55-protected motor offers protection against dust and water jets; however, chemical residue and constant condensation gradually take their toll on the paint, bolts and terminal box seals. Where possible, it is an advantage to place the motor away from the fog

Question 60

What should be investigated before replacing the motor of an enclosed car park fan?

Accepted Answer

The first thing to investigate is whether the fan only provides day-to-day ventilation or also has a smoke extraction role in the event of fire. On fans that are part of the smoke mode, the motor belongs to a fan–motor unit documented to run for a set time at high temperature, and replacing it with a standard motor may invalidate the system's approval. Once this has been established, information on the speed arrangement, direction and mounting is gathered. Verify the fan's duty from the documentation If the fan nameplate shows a temperature–time class, the project drawings mention smoke exhaust, or the fire scenario assigns a task to this fan, the motor must be replaced by the installation's authorised contractor with the approval of the fan manufacturer. DRG's standard three-phase motors can be considered for normal ventilation duties; they are not recommended for a fan expected to operate in smoke mode. Speed, direction and control arrangement Two-speed: Some car park fans have low-

Question 61

How does the slow run-up of a heavy-impeller fan affect motor selection?

Accepted Answer

With a heavy-impeller fan, the issue is not the running power but the time the impeller takes to reach full speed. During this time the motor runs with a starting current several times its nameplate current, and heat builds up quickly in the winding; the longer the start, the more likely the protection is to trip or the insulation to age unnecessarily. Selection must therefore take into account the fan's moment of inertia and the starting method to be used. Three factors that determine run-up Run-up time is directly proportional to the impeller's moment of inertia (J) and inversely proportional to the surplus torque the motor provides above the fan torque during starting. Because fan torque rises with the square of speed, the surplus torque is large at the start of the run-up and narrows as full speed approaches. If motor torque is reduced, this margin shrinks and the time lengthens. Total inertia of the impeller and pulleys (requested from the fan manufacturer in kg·m²) In a belt driv

Question 62

How do you choose between a 1500 rpm and a 3000 rpm motor for the same fan?

Accepted Answer

If the impeller is mounted directly on the motor shaft, there is no choice: the motor speed must match the fan's design speed. On belt-driven fans, either speed can be used, and the decision rests on pulley ratio, noise, bearing load and motor torque. In most belt-driven fan applications a 1500 rpm motor gives a more balanced result, but this is not a rule. Direct-coupled fan On a direct-coupled fan, the impeller's airflow, pressure and power values are given for a single speed. Driving an impeller designed for 1500 rpm with a 3000 rpm motor increases the power roughly eightfold and strains the mechanical limits of the impeller; the reverse means the fan will not deliver the required air. The right question here is not "which speed is better" but "which speed was the fan built for". Comparison for a belt-driven fan Criterion1500 rpm (4-pole)3000 rpm (2-pole) Shaft torque (same kW)About twice as highLower Pulley ratio (slow impeller)Small ratio, large motor pulleyLarge ratio, small moto

Question 63

Is fitting a smaller motor the right way to reduce fan airflow?

Accepted Answer

No, it is not. Airflow is determined not by the motor's kW rating but by impeller speed and system resistance. At the same speed, a lower-power motor tries to drive the fan at the same rate and the fan demands the same power; the result is not less air but an overloaded, overheating motor. Why does a smaller motor not work? An induction motor runs slightly below the speed set by the supply frequency and the number of poles. If you fit a 2.2 kW, 1500 rpm motor instead of a 4 kW one, the impeller still turns at around 1450 rpm, so the fan still tries to move the same air and draw the same shaft power. Under load the motor's slip increases, its current rises above the rated value, and either the protection trips or the winding suffers long-term damage. Starting becomes harder too. The right ways to reduce airflow Changing pulleys: On a belt-driven fan, the fan pulley is made larger or the motor pulley smaller; the impeller slows down. Changing the pole number: On a direct-coupled fan, a l

Question 64

How should humidity and water spray be assessed when selecting a cooling tower fan motor?

Accepted Answer

What stresses the motor in a cooling tower is not external water such as rain, but the saturated humid air and entrained water droplets carried by the fan itself. The assessment starts with where the motor sits: inside the airflow or outside the casing. Condensation, mounting orientation and the effect of water treatment chemicals are then considered. The motor's position is decisive In induced-draft towers the fan sits at the top of the tower and the discharged air is almost saturated. If the motor is within this airflow, it operates in a permanently wet environment. In many designs the motor is placed outside the tower wall and the fan is driven through a long shaft; the motor is then exposed only to outdoor conditions. In forced-draft towers the fan at the bottom pushes dry air in, so the motor works in easier conditions but may still be exposed to splashing water. What does IP55 cover, and what does it not? IP55 protection is defined against dust and low-pressure water jets from an

Replacing a Failed Motor