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Torques in Electrical Induction Motors

There are some common torques used to describe and classify electrical motors

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 in applications as 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 P_hp / n_r (1)

where

T = full-load torque (lb ft)

P_hp = rated horsepower

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

In metric units the rated torque can be expressed as

T = 9550 P_kW / n_r (2)

where

T = rated torque (Nm)

P_kW = rated power (kW)

n_r = 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:

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

= 182.7 lb ft

NEMA Design

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

IEC/NEMA Standard Torques (lb ft)
Power (hp)	2 Pole			4 Pole
Power (hp)	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 starting current and reducing the Locked Rotor Torque or Starting Torque and are common in applications which is hard to start or must be handled with care - as positive displacement pumps, cranes, elevators and similar.