Motor Torque

Torque is the turning moment exerted by the motor and is usually expressed in Newton metres. It is simply a force of 'so many' Newtons exerted at 'so many' metres radius.

The available torque from a motor varies throughout the starting sequence and certain points on a speed-torque curve are specifically classified.

These are the starting torque (also known as the locked rotor torque), pull out torque (or breakdown torque), pull up torque and the full load torque. Generally the first three are expressed as a percentage of the full load torque.

  • The starting torque is the instantaneous torque developed at the moment of starting.
  • The pull up torque is the minimum torque exerted by the motor during the accelerating period.
  • The pull out torque is the maximum torque exerted by the motor during the accelerating period.

(If a reduced current starter is used these torque figures are reduced.)

The full load torque of a motor in Newton metres can be calculated from the fundamental formula: Full load torque (Nm) = Rated kW x 9550

The motor must be chosen with the necessary torque to drive the load at the required speed. This is fairly obvious but the motor must also deliver sufficient torque to bring the load up to full speed, so it is essential to establish the starting requirements of the driven load. If heavy overloads of short duration will be encountered, maximum available torque (pullout torque) must also be taken into consideration. Thus it is sometimes the pull out torque during the starting period, but much more frequently the starting torque from standstill, which first influences the type of motor or starter selected.

It is possible by incorrect selection for reduced current starting, and particularly for drives which require an increasing torque with speed, for the motor to reach a stable speed well below full speed. Then, at changeover to full current, the peak current can be well in excess of the initial reduced starting current.

Broadly speaking, the majority of industrial drives are covered by the following five torque groups.

1. Constant torque of approximately full load torque - a common requirement for such items as positive displacement pumps, conveyors etc.

2. Constant torque of 20% to 40% full load torque - line shaft drives, for example, where maximum duty is well in excess of normal operating load.

3. Initial torque is above full load torque - on drives with heavy starting duty, such as compressors without unloaders.

4. Torque varies as the square of the speed and reaches full load torque at full speed -- for such items as high speed exhaust fans which require little initial torque.

5. Torque varies as the square of the speed and reaches 40-50% full load torque at full speed - such as a centrifugal pump with variable head needing reserve capacity to handle peak loads.

The required torque values for most machines should be available from the manufacturer. If not, then an approximation based on previous experience must be made. If this is the case make a selection that is well oversize.

As the full load torque of a motor is directly related to the full load current, it can be approximately assessed by taking the current readings when driving the motor at the required speed. The following table lists some common average values of starting torque as a percentage of full load torque.