A brief overview of the working principle of an induction motor is:
- An induction motor has a stator and a rotor. The stator contains wound coils that are supplied with alternating current to produce a rotating magnetic field.
- The rotor is made up of conductive bars shorted by end rings, which act as a squirrel cage. When the stator field rotates, it induces a current in the rotor bars and creates a magnetic field in the rotor.
- The interaction between the rotating stator field and the induced rotor field produces a torque that rotates the rotor. The rotor attempts to follow the stator field in order to minimize the relative motion between them.
- At zero speed, the rotor field is stationary with respect to the rotor. As the rotor picks up speed, a velocity emf is induced in the rotor bars that opposes the generated emf, reducing the rotor current and field strength.
- The rotor reaches synchronous speed when its field isjust strong enough to be pulled along by the stator field. Beyond this point, no further acceleration is possible.
- During starting, the rotor develops a starting torque due to induction and accelerates. Once close to synchronous speed, the induction motor behaves like a transformer with a rotating secondary.
- Slip, which is the difference between synchronous speed and operating speed, must exist for induction to occur and torque to be developed. Common full-load slips range from 2-5%.
Here are some of the main applications of induction motors:
- Industrial drives – Induction motors are extensively used for various industrial applications requiring constant speed drives like fans, pumps, conveyors, compressors, elevators, etc. They are rugged, reliable and inexpensive.
- Electric vehicles – Induction motors are used in electric vehicles for propulsion. They provide high starting torque and are maintenance free.
- Home appliances – Appliances like washing machines, fans, refrigerators, air conditioners, etc. use induction motors due to their low cost and robust operation.
- Commercial applications – Induction motors are used for air conditioning plants, lifts, escalators, rolling mills, machine tools, etc. Their low maintenance requirement makes them suitable for commercial installations.
- Agriculture – Induction motors are used to drive agricultural machinery like electric pumps, processing machinery, barn equipment, etc. Their suitability to work in dusty, humid and hot environments is an advantage.
- Industrial automation – Induction motors are used extensively in industries to drive conveyors, CNC machines, assembly lines, blowers, compressors, pumps, etc. as part of automation.
- Traction applications – Induction motors are used for electric locomotives, trams, hoists, trolley cars, lifts, etc. where high starting torque is required.