- Single Phase Induction Motor Definition: A single-phase induction motor is an electrical motor that converts single-phase electrical energy into mechanical energy using magnetic interactions.
- Construction: The construction features two main parts—stator and rotor—with the stator receiving AC power and the rotor designed to rotate and drive mechanical loads.
- Working Principle: These motors use alternating magnetic fields produced in the stator to induce current in the rotor, creating the torque necessary for rotation.
- Self-Starting Challenge: Unlike three-phase motors, single-phase induction motors are not self-starting because the opposing magnetic forces at startup cancel out, producing no torque.
- Improving Startability: To become self-starting, these motors employ techniques like split-phase and capacitor methods to create an initial rotating magnetic field.
What is a Single Phase Induction Motor?
A single phase induction motor is an asynchronous AC motor fed from a single-phase supply. Homes and shops use single-phase more than a three phase system because the wiring is cheaper for modest loads. Industry still uses three-phase for larger shaft power.
The motor itself is simple to make and cheap to service, so it drives vacuum cleaners, fans, washing machines, centrifugal pumps and blowers.
Single phase AC motors are classed as:
- Single phase induction motors or asynchronous motors.
- Single phase synchronous motors.
- Commutator motors.
This page covers construction and the working principle of single phase induction motor.
Construction of Single Phase Induction Motor
Like any other electrical motor an asynchronous motor has two main parts, the rotor and the stator.
Stator:
The stator is the stationary part of the induction motor. A single phase AC supply is given to the stator of a single phase induction motor.
Rotor:
The rotor is the rotating part. It drives the mechanical load through the shaft. In a single-phase induction motor the rotor is almost always a squirrel cage rotor type.
The construction of a single-phase induction motor is close to a squirrel-cage three-phase induction motor. The stator has two windings (main and auxiliary) rather than the three-phase winding of a three phase induction motor.
Stator of Single Phase Induction Motor
The stator of the single-phase induction motor has laminated stampings on its periphery to cut eddy-current loss. Slots in those stampings hold the stator or main winding. The stampings are silicon steel to cut hysteresis loss. When a single-phase AC supply is applied to the stator winding, a magnetic field is produced. The motor then runs slightly below synchronous speed Ns. Synchronous speed Ns is given by 
Where,
f = supply voltage frequency,
P = No. of poles of the motor.
The stator of a single-phase induction motor is like that of a three-phase induction motor, with two differences in the winding.
- Firstly, single-phase induction motors mostly use concentric coils. Concentric coils make it easy to set the number of turns per coil. The mmf distribution is almost sinusoidal.
- In every type except the shaded pole motor, an asynchronous motor has two stator windings: the main winding and the auxiliary winding. Those windings sit at right angles, in space quadrature.
Rotor of Single Phase Induction Motor
The rotor of a single-phase induction motor matches the squirrel-cage rotor of a three-phase induction motor: cylindrical, with slots around the periphery. The slots are skewed rather than parallel to the shaft. Skew stops magnetic locking of stator and rotor teeth and makes the working of induction motor smoother and quieter (i.e. less noisy).
The squirrel cage rotor uses aluminum, brass or copper bars. Those bars are the rotor conductors and sit in the rotor slots. Copper or aluminum rings, the end rings, permanently short the rotor conductors.
The bars are braced to the end rings for strength, so the circuit looks like a cage and gives the motor its name. Because the rings short the bars, rotor resistance is low and extra resistance cannot be added. With no slip rings or brushes, the construction of single phase induction motor stays simple and robust.
Working Principle of Single Phase Induction Motor
NOTE: Any electrical motor, AC or DC, needs two fluxes. Torque comes from the interaction of those fluxes.
When a single-phase AC supply is applied to the stator winding of a single phase induction motor, alternating current flows in the stator or main winding. That current produces an alternating flux called main flux. The main flux also links the rotor conductors and cuts them.
According to the Faraday’s law of electromagnetic induction, emf is induced in the rotor. The rotor circuit is closed, so current flows in the rotor. That rotor current produces rotor flux. The machine is an induction motor because that flux comes from induction. Main flux and rotor flux together produce the torque that turns the rotor.
Why Single Phase Induction Motor is not Self Starting?
Double-field revolving theory splits any alternating quantity into two components. Each has half the peak magnitude of the original, and the two rotate in opposite directions. A flux φ can be split as
The two components rotate opposite ways: if one φm/2 is clockwise, the other φm / 2 is anticlockwise.
When a single-phase AC supply is applied to the stator winding of a single phase induction motor, it produces a flux of magnitude φm. Double-field revolving theory splits that alternating flux φm into two components of magnitude φm/2. Each component rotates at synchronous speed Ns, in opposite directions.
Call the two components the forward flux φf and the backward flux φb. Their resultant at any instant is the instantaneous stator flux at that instant.
At standstill the forward and backward fluxes cancel. Net torque on the rotor is zero, so single phase induction motors are not self-starting.
Methods for Making Single Phase Induction as Self Starting Motor
The stator flux is alternating, and at standstill its two components cancel, so there is no net torque. If the stator flux is made rotating in one direction instead, the induction motor becomes self-starting.
A rotating magnetic field needs two alternating fluxes with a phase angle between them. Their interaction produces a resultant flux that rotates in space in one direction.
Once the motor is running, the extra flux can be removed. The motor keeps running on the main flux alone. The usual ways of making an asynchronous motor self-starting give four main types of single phase induction motor namely,
- Split phase induction motor,
- Capacitor start inductor motor,
- Capacitor start capacitor run induction motor,
- Shaded pole induction motor.
- Permanent split capacitor motor or single value capacitor motor.
Comparison between Single Phase and Three Phase Induction Motors
- Single phase induction motors are simpler, more reliable and cheaper at small ratings than three phase induction motors.
- The electrical power factor of single phase induction motors is lower than that of three phase induction motors.
- For the same frame size, a single-phase induction motor typically delivers about half the output of a three-phase induction motor.
- Starting torque is also low for asynchronous motors / single phase induction motors.
- Single phase induction motors are less efficient than three phase induction motors of similar rating.
Single phase induction motors are simple and cheap at small ratings. Common sizes run from a fraction of a kilowatt up to a few kilowatts; 1 kW is not a hard limit.
