Torque-slip Characteristics of 3-Phase Induction Motor
Definition
Torque-slip characteristic is the graph or relationship showing the variation of electromagnetic torque developed by a 3-phase induction motor with respect to slip.
Mathematically, slip is defined as:
where:
- = synchronous speed
- = rotor speed
The torque-slip curve helps identify:
- starting torque,
- maximum or breakdown torque,
- running torque,
- stable and unstable regions of operation.
Main Content
1. Slip and Its Role in Torque Production
Slip is essential for torque generation
- If the rotor were to rotate at synchronous speed, the relative speed between the rotating magnetic field and rotor conductors would be zero, so no emf would be induced in the rotor. Without rotor emf, there would be no rotor current and hence no torque.
Torque increases with slip up to a limit
- As slip increases from zero, rotor emf and rotor current increase, causing torque to rise. However, after a certain point, further increase in slip reduces torque because of rotor reactance effects.
Different operating regions depend on slip
- At very small slip values, the motor operates near synchronous speed with small torque variation. At higher slip values, such as during starting, the motor develops large current and significant torque.
For example, in a motor with synchronous speed of 1500 rpm, if rotor speed is 1440 rpm, then:
This small slip is enough to produce the required running torque.
2. Shape and Features of the Torque-Slip Curve
At standstill or starting condition ()
- The rotor is stationary, so the relative speed is maximum. The induced rotor emf and rotor frequency are maximum, producing the starting torque.
At low slip values near synchronous speed
- Torque is approximately proportional to slip. This region is stable and represents normal motor operation. A small increase in load causes a slight increase in slip, which increases torque to balance the load.
At high slip values
- Torque initially increases and reaches a maximum called breakdown torque or pull-out torque. Beyond this point, torque decreases as slip increases further, making the operation unstable.
The curve generally has these major points:
At
- Torque is zero
At low slip
- Torque varies almost linearly with slip
At maximum torque
- The motor can supply the highest load before stalling
At
- Starting torque is obtained
This characteristic is very useful in selecting motors for different loads such as fans, pumps, conveyors, crushers, and lifts.
3. Mathematical Relationship and Motor Performance
Torque equation of induction motor
- The torque developed by a 3-phase induction motor is given by:
where:
- = rotor emf at standstill
- = rotor resistance
- = rotor reactance at standstill
- = slip
This equation shows that torque depends on slip, rotor resistance, and rotor reactance.
Condition for maximum torque
- Maximum torque occurs when rotor resistance equals slip reactance:
or
This means the slip at which maximum torque occurs depends on rotor resistance.
Effect of rotor resistance
- Increasing rotor resistance shifts the maximum torque point to a higher slip, improving starting torque in slip-ring motors. However, the value of maximum torque remains nearly unchanged.
This is why slip-ring induction motors are preferred for high starting torque applications.
Working / Process
1. When the motor is switched on, the stator creates a rotating magnetic field
- The 3-phase AC supply to the stator produces a rotating magnetic field at synchronous speed.
- The rotor is initially at rest, so slip is 1 and the relative speed between field and rotor is maximum.
2. Rotor emf and current are induced, producing torque
- Because of the relative motion, emf is induced in the rotor conductors.
- This emf causes rotor current to flow, and the interaction between rotor current and stator magnetic field generates torque.
- The motor starts accelerating in the direction of the rotating magnetic field.
3. As rotor speed increases, slip decreases and torque adjusts to load
- When the rotor speed rises, slip reduces.
- The torque changes according to the slip-torque characteristic until it matches the load torque.
- The motor settles at the speed where developed torque equals load torque.
If the load increases, the rotor slows slightly, slip increases, and the motor automatically develops more torque to meet the new load demand. This self-regulating nature is a major advantage of induction motors.
Advantages / Applications
Simple and practical speed-load relationship
- The torque-slip curve clearly shows how the motor responds to changes in load, making analysis and design easier.
Useful for motor selection and starting analysis
- It helps determine whether a motor can provide sufficient starting torque and overload capability for a given application.
Widely applicable in industrial drives
- Induction motors are used in pumps, fans, compressors, conveyors, cranes, elevators, machine tools, and crushers, where torque-slip behavior is important for stable operation.
Summary
- The torque-slip characteristic shows how induction motor torque varies with slip.
- Torque is zero at synchronous speed, rises with slip, reaches maximum torque, and then decreases.
- The curve explains starting torque, running torque, and breakdown torque behavior.
- It is important for understanding motor performance, stability, and load handling.