Plastic Washing Line Squeezer Electric Motor: Speed and Starting Load

On the squeezer of a plastic washing line, the motor drives a slow-turning screw through a gearbox, and its hardest moment is the start, when it tries to turn a screw that is full of wet, compacted material. Two questions therefore come first in the selection: with which motor speed and gearbox ratio is the screw speed reached, and can the motor deliver enough torque at start to turn a full screw? Power only makes sense once these two questions are answered.

A squeezer and a centrifugal dryer are not the same load

Two different machines are used on a washing line to dry the material separated from the water, and from the motor's point of view they have nothing in common. Before ordering, it must be clear which one is involved.

• Screw squeezer: compresses thin washed material such as film and bags inside a conical barrel with a screw and forces the water out under pressure. The screw turns slowly and the torque is very high; the motor is usually 4 or 6-pole and connected through a gearbox.
• Centrifugal (friction) dryer: the rotor turns at high speed and throws water against a screen by centrifugal effect. The load comes mostly from mass inertia; the motor is usually belt-driven and the speed is high.

This article focuses on the first group, the screw squeezer. With a centrifugal dryer, accelerating a heavy rotor at start-up is the problem; with a squeezer, the problem is a stopped screw locked in by material.

The speed chain: motor, gearbox and screw

The squeezer screw turns at a few tens of revolutions per minute. This low speed is reached by reducing the motor speed through a gearbox, which increases torque by the same ratio. The basic rule in a motor replacement is that if the gearbox ratio does not change, the pole count of the motor should not change either.

For example, with a hypothetical 1:20 gearbox, a 4-pole motor running at 1,485 rpm turns the screw at about 74 rpm. If a 6-pole motor running at 985 rpm is fitted to the same gearbox, the screw drops to about 49 rpm. When the screw slows down, capacity falls, the material stays longer in the barrel and the degree of compression changes. A change in pole count is therefore not just a motor decision but one that also changes the process.

Even if the pole count stays the same, if the power is to be increased, the permitted input power and output torque of the gearbox must be checked. When the motor is larger, the torque transmitted to the gearbox at the moment of a jam is also larger; the weak link then becomes the gears or the coupling rather than the motor.

Why is the starting load so high?

During normal operation there is a continuously flowing, relatively uniform material inside the screw. When the line stops unexpectedly, the wet film remains compacted in the screw; the water drains, the material cools and sticks to the barrel. At the next start the motor has to break this plug.

• Power cut or emergency stop: the screw stops full; the start is at its heaviest.
• Long standstill: material left in the barrel becomes denser, and resistance to the first turn increases in the cooled zones.
• Feed imbalance: if the belt or feed screw sends too much material, the screw is overloaded even while running and the motor current jumps.

The torque an induction motor can deliver at start is a certain multiple of its rated torque, and this ratio is given in the motor catalogue. If the full screw exceeds this value, the motor cannot turn, the current stays high and the protection trips. In this case the right solution is usually not a larger motor, but an operating procedure that empties the screw before starting and a suitable starting method.

Choosing the starting method according to the starting load

On a squeezer, the starting method directly determines whether the start succeeds. The table below shows general tendencies; the final decision is made with the machine builder's data and the actual starting conditions on site.

If a frequency inverter is to be used, the general statement on our website applies: our three-phase series are suitable for speed control with a frequency inverter; for long periods of full-torque operation at low speed an external fan may be needed, and if the cable between drive and motor is long a dv/dt or sine filter is considered. Details are on the inverter FAQ page.

If you have the power and speed of your existing motor at hand, review suitable HEM Motor models and current prices.

Current and heating in continuous operation

In most plants the squeezer runs without interruption throughout the shift. Motor current fluctuates with the feed rate; as the operator increases the feed to raise capacity, the motor approaches its rated current. Operation that keeps the current permanently above the rated value shortens winding life.

A practical approach is to adjust the speed of the feed belt according to the main motor current. This way the screw is never overfilled and the motor stays below its rated current. For a wet environment, IP55 protection is suitable against water splashes; however, the motor must not be exposed directly to a washing water jet or be placed under a drain line. Ambient conditions should be stated with the order.

Example models

Examples from the power range used with gearboxes on squeezers are listed below. All are 4-pole, cast iron frame, IP55 protection and insulation class F, supplied at 400/690 V (Δ/Y), 50 Hz.

• 225M-4: 45 kW, 1,485 rpm, IEC 225M, rated torque 289.4 Nm.
• 250M-4: 55 kW, 1,485 rpm, IEC 250M, rated torque 353.7 Nm.
• 280S-4: 75 kW, 1,486 rpm, IEC 280S, rated torque 482 Nm, service factor 1.15. This reserve is a safety margin; it is not used to undersize the motor.

Which power is suitable is determined by the existing motor and gearbox data on the machine. Current prices are on the product pages; for other applications in the plastics sector, see the plastic and rubber processing motors page.

Conclusion and what we need for a quotation

On a squeezer, motor selection is built on keeping the gearbox ratio and managing a start with a full screw. Changing the pole count changes the process; with starting problems, the starting method and emptying procedure are examined first. Please send us:

• A nameplate photo of the existing motor (power, speed, voltage, frame code)
• The gearbox nameplate or ratio and the input connection type
• The starting method in use and the problem experienced at start
• Mounting arrangement and the environment the motor is in

You can see suitable models and current prices on the buy electric motors page. Send the nameplate photo, speed and mounting details of your existing motor through the nameplate submission page and request a quotation for a suitable model. Our contact page is open for other questions.

Frequently Asked Questions

Does changing a squeezer motor from 4-pole to 6-pole gain torque?

Torque at the motor shaft increases, but the screw slows down by about one third and capacity falls. A pole change is not recommended without changing the gearbox ratio.

What should be done if the motor does not turn when starting with a full screw?

Check the starting method first; star-delta and soft starters reduce starting torque. In most cases a procedure for emptying the screw after a stop is more effective than a larger motor.

Can a larger motor damage the gearbox?

It can. At the moment of a jam a more powerful motor transmits more torque to the gearbox; the gearbox input power and output torque limits must be checked.

Are a centrifugal dryer motor and a squeezer motor selected the same way?

No. A centrifugal dryer turns a high-inertia rotor at high speed; a squeezer is a screw that needs high torque at low speed. Speed, connection and starting conditions are different.