Plansifter and Flour Sifting Machine Electric Motors

A plansifter motor drives, through a belt, the eccentric weight at the centre of a freely suspended sieve cabinet, and this gives the whole box a circular swing. The motor is usually placed inside the swinging body with its shaft vertical and moves with it. In manufacturers' catalogues, flour plansifters with 4 to 8 sections have motors between 3 and 11 kW; control sifters ahead of the flour silo start at 1.1 kW. A plansifter uses much less energy than the roller mills it serves, but it needs high torque at start.

This page explains the working principle of the plansifter, how the vertical belt-driven motor arrangement affects selection, passing through resonance, the sourced power range and the difference from control sifters. For the rest of the mill, see the parent category Flour Mill Electric Motors; ready motors are on Buy Electric Motors Online.

How does a plansifter work?

The plansifter separates the mixed product from each roll passage by particle size into bran, semolina, middlings and flour. The machine consists of two sieve modules and a central drive body between them. The whole structure hangs from the ceiling on glass-fibre or similar flexible canes and does not stand on the floor.

Eccentric weight and circular swing

In the central body a deliberately unbalanced weight turns about a vertical axis. As it turns, the suspended box describes a small circle in the horizontal plane; because the sieves are stacked, the product is sifted on each level by this circular motion and flows down through channels. The speed of the weight equals the number of swings of the box. One maker sheet gives 220 or 240 swings per minute and a swing radius of 40–45 mm for a small plansifter; the two speeds are selected by the pulley fitted on the motor shaft.

Number of sections and sieving area

Plansifters are built with 4, 6 or 8 sections per box and 20–30 sieve frames per section. As the number of sections and sieves rises, the swinging mass grows and motor power moves up one step. In one maker's series, 4-section models come with 4 kW and large 8-section models with 11 kW.

How does the vertical belt-driven arrangement affect selection?

In the classic plansifter the motor is fitted in the central drive body with its shaft parallel to the vertical axis; the pulley on the motor shaft drives the pulley on the shaft end of the eccentric weight through a belt. Power reaches the motor through a flexible cable running alongside one of the suspension canes. This arrangement affects motor selection in four ways.

The motor swings with the body

The motor is attached not to a fixed base but to a box in constant circular motion. The bearings, terminal box and cable entry carry this swing continuously. A tight cable gland, a flexible cable that can move without being pulled, and periodic checks of the terminal connections are therefore important.

Vertical mounting and the DRG range

In vertical operation the rotor weight and the pulley load also load the bearings axially. The motor must be ordered suitable for vertical operation with the shaft down (V1) or up (V3). DRG product pages list the mounting options as B3, B5, B14 and B35; vertical V1 or V3 mounting is not confirmed in the product data. Before ordering a plansifter motor, consult DRG on vertical mounting and confirm the axial load with the machine builder.

The belt ratio sets the motor speed

According to an engineering article, the belt drive gives a high ratio, so a smaller motor can be used and placed where it does not take up valuable space in the body. Example: with a 6-pole motor at 970 rpm and a target of 240 swings per minute, the ratio is 970 / 240 ≈ 4.04; a 120 mm motor pulley needs a 485 mm weight pulley. If a 4-pole motor at 1465 rpm is fitted with the same pulleys, the swing rises to 1465 × 120 / 485 ≈ 362 per minute. Since centrifugal force grows with the square of speed, the force rises about (362 / 240)² ≈ 2.28 times, overloading the suspensions and sieve frames. To keep 240 per minute with a 4-pole motor, the motor pulley must be reduced to about 80 mm (1465 × 80 / 485 ≈ 242). You can repeat the pulley calculation with the pulley belt calculator.

Keep the number of poles

Some catalogue models state a 6-pole motor explicitly; one maker sheet lists a 4 kW, 950 rpm motor for a small plansifter. When replacing the motor, keep the old motor's number of poles and the pulley diameter. If the speed has to change, the pulley diameter is recalculated for the swing rate and the swing range allowed by the machine builder is not exceeded.

Passing through resonance at start and stop

A suspended plansifter has a natural swing frequency below its running swing rate. As the box passes through this range at start, the swing amplitude grows.

Why must starting torque be high?

Engineers of a machine builder write that a plansifter must reach rated speed within a very short time, otherwise it may build up deep resonance. According to the same source, an induction motor delivers up to three times its rated torque until it reaches rated speed, which is why this motor type is chosen for plansifters. The catalogue Ts/Tn of the 6-pole DRG models is 2.0.

Starting method

Star-delta starting reduces starting torque to one third, which may lengthen the time spent in the resonance range. With a soft starter or an inverter the same problem arises if the ramp is not kept short. The starting method and ramp are chosen according to the machine builder's instructions. You can see the differences between methods on the star-delta starting page.

Stopping and steady running

At stop the speed passes the same range and the box swings widely for a while, stressing nearby pipe and flexible sleeve connections. Once at rated speed the box swings freely and uses very little energy; the same article puts its energy use at one twentieth to one thirtieth of what the upstream roller mills draw. The motor therefore runs at low load for long periods, and the real criterion is starting torque rather than power. Some new machines monitor motor load, belt temperature, slip and counter-shaft bearing vibration with sensors.

Click to see current electric motor prices and order nowSEE PRICES →

How many kW is a plansifter motor? Catalogue values

Flour plansifters show motor options of about 3–11 kW depending on the manufacturer's model and number of sections; including control sifters the range is 1.1–11 kW. The values are the motor of one machine; a mill has several plansifters and their total is calculated separately. Compact square sifters on rice and small grain lines go down to 1.5–2.2 kW in catalogues.

Control sifter versus plansifter

The control sifter (rotary sifter or control plansifter) is the machine where flour is sifted one last time before the silo or packing. Its task is not grading but removing foreign matter and oversize particles. There are two types and their motors differ:

The gyratory control sifter works with an eccentric like a small plansifter; one catalogue gives 55 mm eccentricity and a 1.1–1.5 kW motor. In the centrifugal (turbo) control sifter a horizontal rotor throws the flour against a fixed cylindrical screen; the motor sits on the fixed housing, runs horizontally and is rated 5.5–11 kW. An extra sieve box added beside the main plansifter behaves like one of its sections, is not a separate machine, and the motor power grows with the added section.

Which plansifter drives the DRG range can serve

DRG Motor is a manufacturer, supplier and seller of electric motors. The range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron frames; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V. Mounting B3, B5, B14 or B35 is stated in the order note. The 3–11 kW motors of plansifters and the 1.1–11 kW motors of control sifters fall within this range in power and speed; 6-pole (1000 rpm) and 4-pole (1500 rpm) options exist at every step. Horizontal centrifugal control sifter motors are covered by a standard foot- or flange-mounted motor.

Not covered or subject to confirmation: vertical (V1/V3) mounting is not confirmed in the product data, so consult DRG before ordering. Special drive units built into the machine, with drives or sensors, are parts of the machine builder. If the hazardous-area classification requires an Ex motor, the DRG range has no equivalent.

Checklist for choosing a plansifter motor

• Machine plate: motor power, number of poles and swing rate (per minute).
• Diameter of the old motor pulley and of the weight pulley; the ratio must be kept with the new motor.
• Mounting: shaft down or up, flange or foot; suitability for vertical operation.
• Shaft diameter and key: the pulley hub must fit the new shaft.
• Flexible cable and gland: cable suitable for a swinging body, tight gland, cable route without tension.
• Starting: the start must finish quickly; star-delta only with the machine builder's approval.
• Direction at first run: the weight must turn in the direction given in the machine instructions.

DRG motor options for plansifters

All 6-pole models in the table run on a 400 V supply; those of 4 kW and above are wound 400/690 V and the 3 kW model 230/400 V. Frame and shaft dimensions are on the technical drawings page.

DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate).

Motors in this power range

• DRG-90S-4 – 1.1 kW – 1445 rpm – 90S cast iron
• DRG-90L-4 – 1.5 kW – 1445 rpm – 90L cast iron
• DRG-132M2-6 – 5.5 kW – 968 rpm – 132M cast iron
• DRG-160M-6 – 7.5 kW – 970 rpm – 160M cast iron
• DRG-160L-6 – 11 kW – 970 rpm – 160L cast iron

Click to see current prices and order these motors nowSEE PRICES →

Plansifter motor: frequently asked questions

What speed should a plansifter motor have?

What matters is the swing rate of the box, not the motor speed. Catalogues show 6-pole motors (about 950–970 rpm) with belts giving 220–240 swings per minute. Keep the old number of poles when replacing the motor.

Why does a plansifter motor struggle at start?

The box has to pass quickly through its resonance range. A starting method with low torque, a slack belt or a wrong pulley lengthens this passage and the motor current stays high.

Can a plansifter motor be replaced with a horizontal motor?

If the motor works vertically on the swinging body, no; a motor suitable for vertical operation is needed. Have suitability confirmed by DRG and the machine builder before ordering.

Where can I see a flour mill sifter motor price list?

Current prices for plansifter and control sifter codes are shown in the live table on this page.

Send us the plansifter nameplate, pulley diameters and mounting arrangement, and we will settle the right motor and the vertical mounting confirmation together. Phone +90 (542) 666 11 11, e-mail [email protected], contact page, WhatsApp wa.me/905426661111. To order, use the Buy Electric Motors Online page.