Introduction to Induction Motor
One of the most commonly used electric motors in various applications is the Induction Motor. It is also known as an asynchronous motor because it runs at a speed less than its synchronous speed.
From a constructional point of view, the induction motor is the simplest among electrical machines. It operates on the principle of electromagnetic induction—when the stator's rotating magnetic field cuts the stationary rotor, it induces an EMF in the rotor. Induction machines are widely used in industrial, commercial, and residential settings, primarily as three-phase AC motors.
- Simple and rugged construction
- Low cost and minimal maintenance
- High reliability and efficiency
- No need for additional starting motors or synchronization
Principle of Induction Motor
Consider a portion of the 3-phase induction motor as shown in the figure below:
- When the 3-phase stator winding is energized, it creates a rotating magnetic field that rotates at synchronous speed (Ns = 120f/P).
- This rotating field cuts the stationary rotor conductors, inducing EMFs due to relative motion. Since the rotor is short-circuited, current flows through it.
- These current-carrying conductors in the magnetic field experience a mechanical force, resulting in torque, which makes the rotor rotate in the direction of the stator field.
- According to Lenz's Law, the rotor starts rotating in the direction of the stator field to oppose the cause of EMF (i.e., relative motion), thus reducing slip.
Construction of Induction Motor
Stator of Induction Motor
The stator consists of laminated stampings with slots to accommodate three-phase windings spaced 120° apart. The stator is wound for a specific number of poles. There are two common rotor types used: Squirrel Cage and Wound Rotor. No DC excitation is needed, as voltage is induced in the rotor windings via electromagnetic induction.
Rotor of Induction Motor
The rotor is the rotating part, typically a squirrel cage type. It consists of a laminated cylindrical core with axial slots that carry copper or aluminum conductors. These bars are short-circuited by end rings. Historically, the rotor was also referred to as an "anchor" due to its shape and function in early machines.
Types of Induction Motors
1. Single Phase Induction Motor
Single-phase induction motors are not self-starting. Different types based on starting mechanism include: split-phase, shaded pole, and capacitor motors. Capacitor motors are further classified into: capacitor-start, capacitor-run, and permanent capacitor motors.
When supply is applied, the phase difference between currents in main and start windings produces a rotating magnetic field. Once the motor reaches ~70–80% of synchronous speed, a centrifugal switch disconnects the start winding.
Applications of Single Phase Induction Motor
- Pumps
- Compressors
- Small Fans
- Mixers
- Toys
- High-speed Vacuum Cleaners
- Electric Shavers
- Drilling Machines
2. Three Phase Induction Motor
These motors are self-starting and do not require any auxiliary devices like capacitors or switches. The most common types are Squirrel Cage and Slip Ring motors. Squirrel cage motors are favored for their robustness. Slip ring motors are used where high starting torque is needed.
Applications of Three Phase Induction Motor
- Lifts
- Cranes
- Hoists
- Large Exhaust Fans
- Lathe Machines
- Crushers
- Oil Mills
- Textile Industry