Torque and Inverter Compatibility When Replacing a Plastic Extruder Electric Motor

When replacing the motor of a plastic extruder, the first value to look at is not power but the torque required by the screw shaft. An extruder screw is a constant-torque load that requires roughly the same torque even when speed falls; the new motor must therefore be able to deliver continuously the torque needed at the lowest and highest screw speeds the machine runs at, and be able to cool within this range together with the inverter. When replacing an old DC motor with a three-phase induction motor and a frequency inverter, these two conditions are more important than matching the power rating.

What does an extruder screw require from the motor?

In an extruder, the screw conveys granules through a heated barrel, compresses them and pushes the melt into the die. The friction on the screw surface and the resistance of the melt create a torque requirement that is largely independent of speed. Roughly speaking: when the screw turns at half speed, the required torque stays about the same while the power consumed halves. This is completely different from pumps and fans, whose load falls quickly with speed.

The main factors that change the torque requirement are:

• Material and melt viscosity: Different polymers on the same machine require different torque. If the barrel temperature is kept low, the melt thickens and torque increases.
• Die and screen resistance: A screen pack that is starting to clog raises back pressure and therefore torque.
• Cold start: If the screw is turned before the barrel has warmed up sufficiently, torque rises sharply. This is a process error; upsizing the motor is not the solution.

Torque comparison before replacement

The rated torque at the motor shaft is calculated from power and speed: torque (Nm) ≈ 9,550 × power (kW) / speed (rpm). Find this value from the old motor's nameplate and compare it with the rated torque of the new motor. Two motors with the same power but different speeds have different torque; in an extruder, torque is what must be compared.

Switching from a DC motor to an AC motor

The nameplate of DC motors, often found on older extruders, usually shows a "base speed"; the motor runs at constant torque up to this speed. The base speed of the DC motor may differ from the rated speed of a 4-pole induction motor. In this case there are two routes:

• If the same shaft speed is targeted: The new motor's rated torque must not be lower than the old motor's torque at base speed. The speed at the gearbox input is maintained.
• If the speed is different: The screw speed is maintained by adapting the pulley ratio or the gearbox input; in this case the torque required at the shaft is also recalculated with the ratio.

DC motors are often built with special flange and shaft dimensions. Since the connection of an IEC-frame induction motor may differ, adaptation of the base, coupling or pulley may be required. For dimensions see the 2D technical drawings by frame; mounting arrangements are offered according to the frame and the flange size is confirmed with the order.

Inverter compatibility: speed range and cooling

Because the screw speed of an extruder is adjusted to the process, the induction motor almost always runs on a frequency inverter. HEM Motor's three-phase series are suitable for speed control with a frequency inverter; for long periods at low speed with full torque an external fan may be required, and if the cable between the inverter and the motor is long, a dv/dt or sine filter is considered. These conditions must be stated with the order. For general information see the question on inverter operation.

• Below 50 Hz: The motor can deliver its rated torque, but cooling decreases because the shaft-mounted fan also slows down. If the extruder runs for long periods at low screw speed and high torque, this is why the motor heats up; an external (independent) fan meets this need.
• Above 50 Hz: Because voltage stays constant, the motor enters the field-weakening range; power remains roughly constant while usable torque falls. With a constant-torque extruder load, raising the frequency to increase screw speed can lead to insufficient torque.
• Control precision: Speed stability and response to load changes depend on the type and setting of the inverter. If line precision requirements are high, select the inverter together with the inverter supplier; the standard motor does not determine this performance on its own.

When the motor is first run on the inverter, the nameplate values (voltage, current, speed, frequency) must be entered correctly into the inverter. For commissioning steps, see our first start-up check article.

Gearbox and coupling side

An extruder gearbox has a thrust bearing that carries the axial force generated backwards by the screw. This force is not transmitted to the motor, but the gearbox's input speed and torque capacity must be compatible with the motor. If the motor is connected to the gearbox by belt and pulley, the pulley ratio determines the screw speed; if it is connected directly with a coupling, shaft height and alignment are checked. The new motor's torque must not exceed the gearbox's input torque limit; if power is to be increased, the gearbox data are examined separately.

Example motors for extruder drives

The 4-pole cast iron models below are examples of the power and torque classes seen on medium-sized extruder main drives. The exact selection is made from the old motor's nameplate torque and the line's speed range. You can review suitable HEM Motor models and current prices on the buy electric motors page.

All models have a cast iron frame, IP55, class F insulation and a 400/690 V (Δ/Y), 50 Hz supply. For torque values at other powers see the torque table, and for sector options the plastic and rubber processing motors page.

Hypothetical example: AC conversion of a DC-driven extruder

This example is hypothetical. Suppose a profile extruder has a 30 kW DC motor with a base speed of 1,500 rpm. The torque calculated from the nameplate is about 191 Nm. Since the 200L-4 model has a rated torque of 194.2 Nm and a rated speed of 1,475 rpm, a torque match is achieved and the speed at the gearbox input is practically maintained. If the line mostly runs between about fifty and sixty percent of rated speed, the motor stays at full torque but low fan speed in this range; therefore the need for an external fan is stated with the order. In addition, if the old DC motor's flange is not of IEC size, an adapter flange or base modification is planned.

Conclusion and what is needed for the order

In an extruder motor replacement, the decisive factors are the continuous torque required by the screw, the speed range used by the line, cooling at low speed and inverter-motor compatibility. The power rating is the result of these values, not the starting point. For the order, send the following:

• A photo of the old motor's nameplate (including base speed and armature values if DC)
• The line's typical and lowest screw speed, and the material processed
• The inverter to be used and the inverter-motor cable length
• Connection type: belt, coupling or flange; gearbox nameplate details

Suitable models and current prices are on our online sales page. Send the nameplate photo, speed and mounting details of your existing motor via the nameplate submission page and request a quote for a suitable model. For detailed questions you can write to us via the contact page.

Frequently Asked Questions

When selecting an extruder motor, should you look at power or torque?

Torque. The screw is a constant-torque load; the new motor's rated torque must not be lower than the old motor's torque and the torque the line requires.

Can an induction motor be fitted instead of a DC extruder motor?

Yes, with suitable torque, a frequency inverter and adaptation of the mechanical connection. If there is long operation at low speed, the need for an external fan should be stated with the order.

Can I raise the frequency above 50 Hz to increase screw speed?

Above 50 Hz usable torque falls. With a constant-torque extruder load this can lead to insufficient torque; if more screw speed is needed, the pulley or gearbox ratio should be considered.

Why does the extruder motor heat up at low speed?

While the motor runs at full torque, cooling decreases because the shaft-mounted fan slows down. On lines that run at low speed for long periods, an external fan solves this problem.