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Torque Slip Characteristics

July 12, 2020

Torque slip characteristics of induction motor are also known as torque-speed characteristics. They provide us with information about the variation of torque with the slip. And the variation of the slip obtained with the variation of speed. When the speed varies, the slip varies and in contrast, the torque will also vary.

To understand the torque slip characteristics of induction motor we should focus on the mathematics relations. And from this relation, we deduce that curve. This curve provides us with information about the operation of an induction motor.

At the synchronous speed slip (S)=0.

So, the motor will produce zero torque. In normal operating conditions (between pullout speed and synchronous speed) we have nearly a linear relationship between the torque and the slip. Also, the maximum torque induced ranges from a higher speed.

And we can’t deny that the torque is affected by differentiating the rotor resistance. Changing the rotor resistance will affect the slip (s=R2/X2). When we have a constant rotor reactance, we would have maximum torque at the start by adding much resistance in the rotor.

Torque slip characteristics of the three-phase induction motor

Torque-slip characteristics of three-phase induction motor do not have a big difference from the main induction motor. We only have three regions or three cases to illustrate and predict variation and they are:

  • Generating mode(Low slip region)
  • Motoring mode (Medium slip region)
  • Breaking mode (High slip region)

Generating mode (low slip region)

In this mode, the motor runs in speed above the synchronous speed and it’s driven by a prime mover.

We supply the stator with a three-phase supply that supplies electrical energy.

In this case, we have a negative slip and torque so the motor receives mechanical energy to deliver electrical energy.

In generation mode, we require reactive power from an outside supplier to operate the motor; so, we don’t usually use the induction motor as a generator.

Motoring mode (medium slip region)

In this mode, we supply the stator with voltage and the motor rotates below the synchronous speed.

The slip varies from zero at no load to 1 at the standstill; so, the torque varies from zero to the full load torque.

That means the torque is directly proportional to the slip; The more slip we have the more torque will be produced and vice versa.

Breaking mode (high slip region)

In braking mode or plugging, the polarity of the supply voltage is changed so the motor rotates in the reverse direction and as a natural result, the motor will stop.

We only use this method when we want to stop the motor within a very short period of time.

And we try to disconnect the stator from the supply before the motor enters this braking mode. Because the kinetic energy stored in the revolving load dissipated as heat.

Also if the stator is still connected, the motor receives power from which also dissipated as heat. So in this case, the motor would develop enormous heat energy.

Torque slip characteristics of single-phase induction motor

In the single-phase induction motor we have forward and backward fields. In unity slip, the forward and the backward field produce equal torque but in the opposite direction. So the produced net torque equals zero so the motor fails to start.

The single-phase induction motor isn’t self-starting. To start the motor or to have a starting torque we can increase the forward speed of the motor. Hence the forward slip and the reverse torque will decrease and the forward torque will increase which makes the motor start.

That means there would be a difference of torque between the forward and the backward field to start the single-phase motor.

When we have a larger forward field torque than the backward field torque, the motor rotates in forward (anti-clockwise) direction. When we have a larger backward field torque than the forward field torque the motor rotates in backward (clockwise) direction.