Torques in Electrical Induction Motors

Torques used to describe and classify electrical motors

Torque is the turning force through a radius with the units - Nm - in the SI-system and - lb ft - in the imperial system.

The torque developed by an asynchronous induction motor varies when the motor accelerates from full stop (or zero speed) to maximum operating speed.

electric motor current torque curves

Locked Rotor or Starting Torque

The Locked Rotor Torque or Starting Torque is the torque the electrical motor develop when its starts at rest or zero speed.

A high Starting Torque is more important for application or machines hard to start - as positive displacement pumps, cranes etc. A lower Starting Torque can be accepted for centrifugal fans or pumps where the start load is low or close to zero.

Pull-up Torque

The Pull-up Torque is the minimum torque developed by the electrical motor when it runs from zero to full-load speed (before it reaches the break-down torque point)

When the motor starts and begins to accelerate the torque in general decrease until it reach a low point at a certain speed - the pull-up torque - before the torque increases until it reach the highest torque at a higher speed - the break-down torque - point.

The pull-up torque may be critical for applications that needs power to go through some temporary barriers achieving the working conditions.

Break-down Torque

The Break-down Torque is the highest torque available before the torque decreases when the machine continues to accelerate to the working conditions.

Full-load (Rated) Torque or Braking Torque

The Full-load Torque is the torque required to produce the rated power of the electrical motor at full-load speed.

In imperial units the Full-load Torque can be expressed as

T =  5252 Php / nr         (1)


T = full-load torque (lb ft)

Php = rated horsepower

nr = rated rotational speed (rev/min, rpm)

In metric units the rated torque can be expressed as

T = 9550 PkW / nr             (2)


T = rated torque (Nm)

PkW = rated power (kW)

nr = rated rotational speed (rpm)

Example - Electrical Motor and Braking Torque

The torque of a 60 hp motor rotating at 1725 rpm can be expressed as:

Tfl = (60 hp) 5,252 / (1725 rpm)

    = 182.7 lb ft

NEMA Design

NEMA (National Electrical Manufacturers Association) have classified electrical motors in four different designs where torques and starting-load inertia are important criteria.

IEC/NEMA Standard Torques (percent of full load torque)
Power (hp) 2 Pole 4 Pole
Locked Rotor Torque Pull Up Torque Break Down Torque Locked Rotor Torque Pull Up Torque Break Down Torque
3 170/160 110/110 200/230 180/215 120/150 200/250
5 160/150 110/105 200/215 170/185 120/130 200/225
7.5 150/140 100/100 200/200 160/175 110/120 200/215
10 150/135 100/100 200/200 160/165 110/115 200/200
15 - 20 140/130 100/100 200/200 150/150 110/105 200/200

Accelerating Torque

Accelerating Torque = Available Motor Torque - Load Torque

Reduced Voltage Soft Starters

Reduced Voltage Soft Starters are used to limit the start current reducing the Locked Rotor Torque or Starting Torque and are common in applications which is hard to start or must be handled with care - like positive displacement pumps, cranes, elevators and similar.

Related Topics

  • Electrical - Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors

Related Documents

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