Duct-Type Centrifugal Fan Motor: Selection for Circular and Rectangular Ducts

The motor of a duct fan is handled in one of two ways, depending on the fan's size: most small duct fans for circular and rectangular ducts have a motor built into the impeller, so when it fails you replace the fan, not the motor; large industrial belt-driven or direct-drive in-line centrifugal fans use a standard IEC-frame three-phase induction motor, chosen from the fan's shaft power, speed and mounting. For example, a belt-driven duct fan moving 5,400 m³/h at 600 Pa static pressure needs about 1.5 kW of shaft power if the fan efficiency is assumed to be 0.60; with belt losses and a margin, a 2.2 kW 4-pole motor is enough.

This article covers centrifugal fans installed in series inside a duct: which fans let you buy the motor separately and which do not, what information you need to collect to choose a motor, and which models in the DRG range fit the job. You can see suitable motors and current prices among the 120 models on the Buy Electric Motors Online page. Overview: Fan Motors.

"Duct type" is a housing name: the impeller inside is centrifugal

Fan terms describe three different things. Axial, centrifugal and mixed flow describe the operating principle; wall, duct and roof type describe the housing and installation; words such as smoke extraction or dust collection describe the purpose. "Duct type" belongs to the housing/installation group. So a search for "duct fan" may lead to fans with axial, mixed-flow or centrifugal impellers. This article is about those with a centrifugal (radial) impeller.

In a duct-type centrifugal fan, air enters the impeller axially, is thrown outward between the blades, and the housing turns it back onto the duct axis. The fan can therefore be bolted into the middle of a duct with two flanges, without a separate scroll housing or a 90-degree turn. Two housings are common on the market:

• Circular in-line fan: fits directly into circular ducts in the 100–315 mm class; used in homes, offices, toilet extracts and small kitchen lines.
• Rectangular (square) duct fan: a box housing built to the rectangular duct size; used in shops, restaurants, workshops and warehouses without an air handling unit. Large sizes have service doors and belt drives.

Difference from a duct-type axial fan: pressure

In a duct-type axial fan, air passes straight through the impeller; it gives high flow for its diameter, but its flow drops quickly as duct resistance rises. A centrifugal impeller produces clearly higher static pressure at the same diameter and speed. If the duct is long, or the line includes a filter, heating coil, silencer or many bends, a duct-type centrifugal fan is chosen. Higher pressure at the same flow means higher shaft power. So the motor rating of an old axial fan is no guide for the centrifugal duct fan that replaces it.

Where is the motor in this fan: integrated, direct drive or belt

Small duct fans with integrated motors

Most circular in-line fans and small rectangular duct fans come with an external rotor motor: the rotating part of the motor sits in the impeller hub, and impeller and motor are balanced together at the factory. In one manufacturer's catalogue, the entire circular duct fan series uses single-phase, 230 V, IP44 external rotor motors, with maximum input power between 73 and 352 W [2]. The same design is common in rectangular duct fans [3]. In these fans the motor is not a separate part with a shaft end and flange; a standard IEC-frame motor cannot be fitted. When the motor fails, the right solution is to buy the same fan, or a new fan giving the same flow and pressure. DRG does not sell external rotor or integrated EC fan motors.

Large direct-drive in-line fans

In medium-sized in-line fans, the impeller is mounted directly on the shaft of a standard motor. The impeller speed is the motor speed. In one manufacturer's catalogue, the direct-drive square in-line series offers two motor speeds (950 and 1425 rpm) [1]. In this arrangement the motor is often inside or right next to the airstream. The same catalogue gives a maximum air temperature of 54 °C for the direct-drive series and 82 °C for the belt-driven series, where the motor sits outside [1].

Belt-driven industrial duct fans

In large square duct fans, the motor sits outside the casing on an adjustable motor base attached to it, and drives the fan shaft through belts and pulleys. Because it stays out of the airstream, the motor is cooled by its own fan and is not affected by dirty or hot air. The fan speed is set by the pulley ratio, so the motor speed can be chosen independently of the fan speed. This is the arrangement where DRG motors fit most directly.

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Data needed to select the motor, with a worked example

Selecting the fan, the fan curve, and the impeller and housing design is the job of the fan manufacturer or the design engineer. The motor supplier's job is to provide a motor that matches the shaft power, speed, mounting and ambient conditions resulting from that work. Before asking for a motor, collect the following:

• Flow (m³/h) and static pressure (Pa) from the fan label, or the operating point from the fan catalogue.
• Fan shaft speed and drive type: direct or belt; if belt, the diameters of both pulleys.
• The old motor's nameplate: kW, rpm, IEC frame, voltage, mounting type.
• Shaft diameter and length, pulley hub, key size.
• Temperature and content of the air handled (oil mist, moisture, dust).
• How the fan hangs in the duct: motor on top, at the side or underneath.
• Starting: direct-on-line, star-delta or inverter.

Shaft power is found with P_shaft = Q·Δp / η_fan (Q in m³/s, Δp in Pa, P in W). Example: a shop extract line must move 5,400 m³/h (1.5 m³/s) at 600 Pa static pressure, including the filter and silencer. If the fan efficiency is assumed to be 0.60, shaft power is 1.5 × 600 / 0.60 ≈ 1,500 W. Taking belt drive efficiency as 0.95, the power required from the motor is about 1.58 kW; with a 15% margin it rises to 1.82 kW, and the next standard rating is 2.2 kW. If the fan is to run at 1,150 rpm, a 1450 rpm 4-pole motor gives a pulley ratio of about 1.26. If the fan speed is later raised to 1,250 rpm to gain flow, shaft power rises with the cube of speed to about 1.93 kW; the power required from the motor becomes 2.03 kW and the 2.2 kW motor's margin is almost gone. If you plan to raise the speed, choose the motor one size up. You can repeat this calculation with your own values in the Pump & Fan Motor Power tool and check the pulley ratio with the pulley and belt calculator.

Power range, speed and pole selection

No fixed kW can be assigned to a duct-type centrifugal fan: power depends on impeller diameter, flow, static pressure, blade type, speed, duct and filter resistance, air temperature and density, and belt losses. As a sourced example, one manufacturer's catalogue lists motor options from 0.18 kW to 7.5 kW for a belt-driven square in-line fan series [1]; in small circular fans of the same kind, motor input power does not exceed 352 W [2].

On speed, what is specific to duct fans is this: belt-driven fans mostly use a 4-pole motor (1500 rpm class), because the fan speed is set by the pulleys anyway, and a 4-pole motor is quieter at the same power and comfortable in terms of belt speed. A 2-pole motor is considered only if a very high fan speed is needed with a small pulley. In a direct-drive fan, the pole count is set by the fan manufacturer; the impeller is designed for that speed. Replacing a 4-pole motor with a 2-pole one theoretically raises shaft power about eightfold and puts both the motor and the impeller at risk. If speed adjustment is needed, use an inverter; our affinity laws article explains how fan power falls as speed drops.

Mounting and motor connection

Hanging the fan in the duct and the motor's IEC mounting type are separate matters. The duct fan may be hung from the ceiling on rods, set on a floor base or fixed to a wall bracket; the motor is attached to the fan's own base. In belt-driven duct fans, the motor is usually foot-mounted (B3) and bolted to the adjustable base. One manufacturer's catalogue states that the motor can be on top of, under or beside the fan and must be left accessible for service [1]. The exact mounting type is taken from the fan manufacturer's drawing.

• Terminal box orientation: if the fan hangs from the ceiling, the box should face down or towards the service door; the cable should enter from below.
• Belt tension: an over-tight belt puts radial load on the motor bearing; fit the pulley close to the shaft shoulder.
• Voltage: in the DRG range, models up to 3 kW are 230/400 V and 4 kW and above are 400/690 V; star-delta starting requires a 400/690 V motor. See the star-delta starting page for details.
• Direction of rotation: a centrifugal impeller still moves some air when running backwards; a direction error shows up as low flow. Check against the arrow on the casing before commissioning. The connection must be made by a qualified electrician.

If the duct fan is to be used for smoke extraction in a fire, the fan and motor set must hold the relevant smoke extraction certification (EN 12101-3); DRG's standard motors are not sold with this certification. The selection in this article applies to fans in normal ventilation mode.

Matching with DRG motors

DRG Motor is a manufacturer, supplier and seller of electric motors. Our range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 and B35 (with an order note). Among duct-type centrifugal fans, the match is belt-driven and direct-drive fans with IEC motors. The price and specification table for the 4-pole ratings most often used in belt-driven duct fans appears right after the power section; for the 2.2 kW requirement in the example, the DRG-100L1-4 is suitable.

If there is oil mist or moisture and the motor is close to the airstream, you may prefer the cast iron frame. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate). For motor selection on radial fans with a scroll housing, see our centrifugal scroll fan motor article.

DRG motors for inline centrifugal fans

• DRG-801-4 – 0.55 kW – 1400 rpm – 80 aluminium
• DRG-90S-4 – 1.1 kW – 1445 rpm – 90S aluminium
• DRG-90L-4 – 1.5 kW – 1445 rpm – 90L aluminium
• DRG-100L1-4 – 2.2 kW – 1450 rpm – 100L aluminium
• DRG-100L2-4 – 3 kW – 1450 rpm – 100L aluminium
• DRG-112M-4 – 4 kW – 1450 rpm – 112M aluminium
• DRG-132S-4 – 5.5 kW – 1460 rpm – 132S cast iron
• DRG-132M-4 – 7.5 kW – 1460 rpm – 132M cast iron

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Inline centrifugal fan motor: FAQ

My duct fan motor burned out; can I replace just the motor?

Not if it is an external rotor motor built into the impeller; then the fan must be replaced. If it is a standard foot- or flange-mounted motor with a pulley or impeller hub on its shaft, a three-phase motor with the same kW, speed and frame as its nameplate can be fitted.

Should I choose a duct-type centrifugal fan or a duct-type axial fan?

If the duct is short and has little resistance, an axial fan may be enough. With filters, coils, silencers and long runs, a centrifugal duct fan provides higher pressure. The design engineer makes the choice from the fan curve.

What motor speed should I buy for a belt-driven duct fan?

Mostly a 4-pole, 1500 rpm class motor. For example, the DRG-100L1-4 runs at 1450 rpm; the fan speed is set by the pulley ratio.

How many amps does a 2.2 kW duct fan motor draw?

The 4-pole DRG-100L1-4 draws 4.52 A at 400 V and 4.76 A at 380 V. Set the overload relay to the nameplate current.

Send us the fan label, the old motor's nameplate and the pulley sizes, and we will confirm the right DRG model together. Phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and current prices, visit Buy Electric Motors Online.

Sources

1. Greenheck – Models SQ-M / BSQ-M Centrifugal Square Inline Duct Fans (catalogue) – belt-drive motor options 0.18–7.5 kW; direct drive 950/1425 rpm; maximum air temperature 54 °C / 82 °C; motor position.
2. Kruger – TSK II Series In-Line Centrifugal Duct Fan (catalogue) – single-phase 230 V IP44 external rotor motor, maximum input power 73–352 W.
3. Systemair – KE/KT Rectangular Duct Fans – AC external rotor motors in rectangular duct fans.