- 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 a Three-Phase Induction Motor
The torque-slip curve of an induction motor shows how electromagnetic torque changes with rotor speed. For a rotor turning in the same direction as the stator field, slip is the difference between synchronous speed and rotor speed divided by synchronous speed. Its sign and magnitude identify the machine’s operating region.
The complete curve has three operating modes:
Within the motoring mode, the curve is also described by three approximate slip regions:
- Low slip region
- Medium slip region
- High slip region
Motoring Mode
For motoring in the usual direction, slip lies between zero and one. Torque is zero at synchronous speed. Near synchronous speed, torque is approximately proportional to slip, and normal operation is on this low-slip part of the curve.
As slip rises further, torque reaches a maximum or breakdown value and then falls towards the standstill torque at slip one. The linear approximation does not apply across the whole motoring range.
Generating Mode
When a prime mover drives the rotor above synchronous speed in the same direction as the stator field, slip is negative. Electromagnetic torque is then negative under the stated sign convention. The machine receives mechanical power at the shaft and delivers real electrical power through the stator.
The machine still needs magnetising reactive power from the grid, capacitors or a converter. If speed falls below synchronous speed, power flow changes back to motoring. With suitable reactive-power support and control, induction generators are used in practical generating and regenerative-braking systems.
Braking Mode
In plugging, two stator phases are interchanged so the rotating field reverses; the supply voltage polarity is not simply reversed. While the rotor still turns in its original direction, slip is greater than one and the opposing torque can stop it quickly. Both mechanical energy and power drawn from the supply become heat, so current and temperature must be limited. The controller removes the supply near zero speed if reverse rotation is not required.
Regenerative braking is different. If the load drives the rotor above synchronous speed in the same direction, the machine acts as an induction generator and returns real power to the electrical system. The opposing torque tends to bring speed towards synchronous speed, not to zero. Dynamic braking uses another connection and dissipates energy instead of returning it to the supply.
Torque-Slip Characteristics of a Single-Phase Induction Motor
At standstill, the forward and backward rotating fields in a single-phase induction motor produce equal opposing torques, so net starting torque is zero. Unlike a three phase induction motor, it needs a starting winding or another starting method. Once the rotor turns, forward slip falls while backward slip rises, so the two torques are no longer equal.
To start a single phase induction motor, the auxiliary starting arrangement must create a net torque. The motor accelerates in the direction of the stronger starting torque. Clockwise or anticlockwise rotation depends on the winding connections and the chosen reference direction, not on a universal forward direction.
