An induction motor is an AC motor whose rotor rotates slightly slower than the stator field.
In the previous article we saw that the stator creates a magnetic field that rotates at a fixed speed (e.g., 3000 rpm or 1500 rpm).
Now we look at the rotor. In other motors, the rotor is complex: it has coils, rings, brushes, or expensive neodymium magnets. In the induction motor, the rotor is… an aluminum cage.
Why Squirrel Cage
If you take the rotor, remove the iron laminations from the core, and leave only the conductor, it literally looks like a hamster or squirrel exercise wheel.
- They are aluminum or copper bars, often skewed to reduce noise and torque oscillations.
- They are joined at the ends by two short-circuit rings.
There are no wires coming out. There are no brushes. It is a solid block, closed upon itself.
The Operating Principle: Induction
How does it move if we don’t connect the rotor to a power source? Remember Faraday and Lorentz from module 5.
The Field Rotates: The stator’s magnetic field passes “grazing” the rotor bars at full speed.
Induction: Because the magnetic field varies with respect to the bars (it cuts them), an electric current is induced in them.
The stator acts as the primary of a transformer and the rotor as the short-circuited secondary.
Lorentz Force: Now we have bars with current immersed in a magnetic field. Result: A Force Appears.
Rotation: The force pushes the bars tangentially, and the rotor starts to chase the magnetic field.
Slip (
Here comes the key physical concept that gives the motor its name.
Suppose the field rotates at 1500 rpm (
If the rotor moves at the same speed as the field, there is no relative motion.
But then:
- The magnetic field “does not cut” the bars (they are parallel).
- If it doesn’t cut the bars → No Induction (Faraday says 0).
- If there is no current → No Force.
- The motor stops pushing.
Conclusion: It is impossible for the rotor to reach synchronous speed. It must always be slightly slower so that the magnetic field keeps “ahead” of it, cutting the bars to generate force.
This speed difference is called Slip.
We can express slip as a per-unit value:
Real Example
Look at the nameplate of an industrial motor with two pole pairs powered at 50 Hz.
- Synchronous Speed (Field): 1500 rpm.
- Nameplate Speed (Shaft): 1440 rpm (approx).
That 60 rpm difference corresponds to a slip of 4 %.
- If the load increases, the rotor slows down slightly, slip increases, and the motor develops more torque until it reaches a new equilibrium. If you exceed its capacity, current and heating increase, and the motor may stall.
Advantages
- Robustness: No brushes on the squirrel cage rotor.
- Cost: Simple construction with iron, aluminum, or copper.
- Maintenance: Usually low, although it requires checking bearings, ventilation, insulation, and connections.
Disadvantages
- Speed Control: With direct line power, the speed remains close to synchronous; to vary it widely, a variable frequency drive is used.
- Starting Current: Can reach several times the rated current, depending on design and starting method.
- Starting Torque: Depends on rotor design and the power supply system.
