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Torque slip characteristics of induction motor (full guide)

May 20, 2018
Torque slip characteristics of induction motor (full guide)

Torque slip characteristics of induction motor

Torque slip characteristics of induction motor also known as torque-speed characteristics provide us with the 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 vary and in contrast the torque will also vary.

To understand the torque slip characteristics of induction motor we should focus on mathematics relations;

Torque slip characteristics

And from this relation we deduce that curve:

Torque slip characteristics of induction motor

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 at the higher speed ranges.

And we can’t deny that the torque affected by differentiating the rotor resistance and it will be:

Effect of Rotor Resistance on Torque Slip Characteristics

Changing the rotor resistance will affect the slip (s=R2/X2), when we have a constant rotor reactance we would have a maximum torque at the start by adding much resistance in the rotor.

Torque slip characteristics of three phase induction motor:

Torque-slip characteristics of three phase induction motor don’t have big difference than of the main induction motor. We only have three regions or three cases to illustrate and predict variation and they are:

  • Low slip region.
  • Medium slip region.
  • High slip region.

Torque slip characteristics of three phase induction motor

1- 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.

2- 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 outside supplier to operate the motor;

so, we don’t usually use the induction motor as a generator.

3- Breaking mode (high slip region):

In breaking mode or plugging, the polarity of the supply voltage 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 breaking mode because the kinetic energy stored in the revolving load dissipated as heat and also if the stator 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 opposite direction so the produced net torque equals zero so the motor fails to start (the single-phase induction motor isn’t self-starting, it’s another reason to know why it isn’t self-starting).

Torque slip characteristics of single phase induction motor

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.

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