- Torque Slip Characteristics Definition: The torque slip characteristics of an induction motor describe how its torque changes with slip.
- Slip: Slip is the difference between synchronous speed and actual rotor speed, divided by synchronous speed.
- Motoring Mode: In motoring mode, the motor runs below synchronous speed with torque proportional to slip.
- Generating Mode: In generating mode, the motor runs above synchronous speed, needing external reactive power to generate electricity.
- Braking Mode: Braking mode stops the motor quickly by reversing its direction, dissipating kinetic energy as heat.
Torque Slip Characteristics of Three Phase Induction Motor
The torque slip curve of an induction motor shows how torque changes with slip. Slip is defined as the difference between synchronous speed and actual rotor speed, divided by synchronous speed. When speed changes, slip and the corresponding torque also change.
The curve can be described in three modes of operation-
The torque-slip characteristic curve can be divided roughly into three regions:
- Low slip region
- Medium slip region
- High slip region
Motoring Mode
In this mode, power is supplied to the stator, and the motor rotates below synchronous speed. The motor torque ranges from zero to full load torque as the slip changes. Slip ranges from zero at no load to one at standstill. The curve shows that torque is directly proportional to slip.
That is, more is the slip, more will be the torque produced and vice-versa. The linear relationship simplifies the calculation of motor parameter to great extent.
Generating Mode
In this mode of operation induction motor runs above the synchronous speed and it should be driven by a prime mover. The stator winding is connected to a three phase supply in which it supplies electrical energy. Actually, in this case, the torque and slip both are negative so the motor receives mechanical energy and delivers electrical energy. Induction motor is not much used as generator because it requires reactive power for its operation.
That is, reactive power should be supplied from outside and if it runs below the synchronous speed by any means, it consumes electrical energy rather than giving it at the output. So, as far as possible, induction generators are generally avoided.
Braking Mode
In braking mode, the motor’s supply voltage polarity is reversed, causing it to rotate in the opposite direction and stop. This method, called plugging, is used to quickly stop the motor. The kinetic energy in the load is dissipated as heat, along with the power still received from the stator. Thus, the stator is disconnected before the motor enters braking mode to avoid excessive heat.
If load which the motor drives accelerates the motor in the same direction as the motor is rotating, the speed of the motor may increase more than synchronous speed. In this case, it acts as an induction generator which supplies electrical energy to the mains which tends to slow down the motor to its synchronous speed, in this case the motor stops. This type of breaking principle is called dynamic or regenerative breaking.
Torque Slip Characteristics of Single Phase Induction Motor
At a slip of one, forward and backward fields in a single-phase induction motor create equal but opposite torques, resulting in zero net torque, so the motor fails to start. Unlike three phase induction motor, these motors are not self-starting and need an external method to provide starting torque. Increasing the forward speed decreases forward slip, increasing forward torque and decreasing reverse torque, thus starting the motor.
To start a single phase induction motor, a difference in torque between the forward and backward fields is needed. If the forward field torque is greater, the motor rotates in the forward (anti-clockwise) direction. If the backward field torque is greater, the motor rotates in the backward (clockwise) direction.
