MODULE: ELECTRICAL ENGINEERING TECHNOLOGY III/ELECTRICAL MACHINES & CONTROL
UNIT 3: SINGLE PHASE A.C MOTOR
INTRODUCTION
Single phase induction motor is an AC motor where electrical energy is converted to mechanical energy to perform
some physical task. This induction motor requires only one power phase for their proper operation. They are
commonly used in low power applications, in domestic and industrial use. Simple construction, cheap cost, better
reliability, eases to repair and better maintenance are some of its marketable advantages.
Construction of Single-Phase Induction Motor
The main components of the Single-Phase induction motor are stator and rotor. Stator is known to be the
stationary part. Usually, the single-phase alternating supply is given to the stator winding. Rotor is the rotating part
of the motor. Rotor is connected to the mechanical load with the help of a shaft. A squirrel cage rotor is used here.
It has a laminated iron core with many slots. Rotor slots are closed or semi-closed type. The rotor windings are
symmetrical and at the same type it is short circuited. An air gap is there between the rotor and the stator. The
most practical applications of this motor are in refrigerators, clocks, drills, pumps, washing machines etc. The stator
winding in the 1Ø induction motor has two parts: Main Winding and Auxiliary Winding. Usually, the Auxiliary
winding is perpendicular to the main winding. In 1Ø induction motor the winding with more turns is known as main
winding. While the other wire is called an auxiliary winding.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 1
Engineering Trainer – KMTC - Kisumu Campus
, MODULE: ELECTRICAL ENGINEERING TECHNOLOGY III/ELECTRICAL MACHINES & CONTROL
UNIT 3: SINGLE PHASE A.C MOTOR
WORKING PRINCIPLE OF SINGLE-PHASE INDUCTION MOTOR
When the stator of a single-phase motor is fed with single phase supply, it produces alternating flux in the stator
winding. The alternating current flowing through stator winding causes induced current in the rotor bars (of
the squirrel cage rotor ) according to Faraday's law of electromagnetic induction. This induced current in the rotor
will also produce alternating flux. Even after both alternating fluxes are set up, the motor fails to start (the reason is
explained below). However, if the rotor is given an initial start by external force in either direction, then motor
accelerates to its final speed and keeps running with its rated speed. This behavior of a single-phase motor can be
explained by double-field revolving theory.
Double-field revolving theory
The double-field revolving theory states that, any alternating quantity (here, alternating flux) can be resolved into
two components having magnitude half of the maximum magnitude of the alternating quantity, and both these
components rotating in opposite direction.
Following figures will help you understanding the double field revolving theory.
Why single-phase induction motor is not self-starting?
The stator of a single-phase induction motor is wound with single phase winding. When the stator is fed with a
single-phase supply, it produces alternating flux (which alternates along one space axis only). Alternating flux acting
on a squirrel cage rotor cannot produce rotation, only revolving flux can. That is why a single-phase induction
motor is not self-starting.
How to make single phase induction motor self-starting?
As explained above, single phase induction motor is not self-starting. To make it self-starting, it can be
temporarily converted into a two-phase motor while starting. This can be achieved by introducing an
additional 'starting winding' also called as auxiliary winding.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 2
Engineering Trainer – KMTC - Kisumu Campus
UNIT 3: SINGLE PHASE A.C MOTOR
INTRODUCTION
Single phase induction motor is an AC motor where electrical energy is converted to mechanical energy to perform
some physical task. This induction motor requires only one power phase for their proper operation. They are
commonly used in low power applications, in domestic and industrial use. Simple construction, cheap cost, better
reliability, eases to repair and better maintenance are some of its marketable advantages.
Construction of Single-Phase Induction Motor
The main components of the Single-Phase induction motor are stator and rotor. Stator is known to be the
stationary part. Usually, the single-phase alternating supply is given to the stator winding. Rotor is the rotating part
of the motor. Rotor is connected to the mechanical load with the help of a shaft. A squirrel cage rotor is used here.
It has a laminated iron core with many slots. Rotor slots are closed or semi-closed type. The rotor windings are
symmetrical and at the same type it is short circuited. An air gap is there between the rotor and the stator. The
most practical applications of this motor are in refrigerators, clocks, drills, pumps, washing machines etc. The stator
winding in the 1Ø induction motor has two parts: Main Winding and Auxiliary Winding. Usually, the Auxiliary
winding is perpendicular to the main winding. In 1Ø induction motor the winding with more turns is known as main
winding. While the other wire is called an auxiliary winding.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 1
Engineering Trainer – KMTC - Kisumu Campus
, MODULE: ELECTRICAL ENGINEERING TECHNOLOGY III/ELECTRICAL MACHINES & CONTROL
UNIT 3: SINGLE PHASE A.C MOTOR
WORKING PRINCIPLE OF SINGLE-PHASE INDUCTION MOTOR
When the stator of a single-phase motor is fed with single phase supply, it produces alternating flux in the stator
winding. The alternating current flowing through stator winding causes induced current in the rotor bars (of
the squirrel cage rotor ) according to Faraday's law of electromagnetic induction. This induced current in the rotor
will also produce alternating flux. Even after both alternating fluxes are set up, the motor fails to start (the reason is
explained below). However, if the rotor is given an initial start by external force in either direction, then motor
accelerates to its final speed and keeps running with its rated speed. This behavior of a single-phase motor can be
explained by double-field revolving theory.
Double-field revolving theory
The double-field revolving theory states that, any alternating quantity (here, alternating flux) can be resolved into
two components having magnitude half of the maximum magnitude of the alternating quantity, and both these
components rotating in opposite direction.
Following figures will help you understanding the double field revolving theory.
Why single-phase induction motor is not self-starting?
The stator of a single-phase induction motor is wound with single phase winding. When the stator is fed with a
single-phase supply, it produces alternating flux (which alternates along one space axis only). Alternating flux acting
on a squirrel cage rotor cannot produce rotation, only revolving flux can. That is why a single-phase induction
motor is not self-starting.
How to make single phase induction motor self-starting?
As explained above, single phase induction motor is not self-starting. To make it self-starting, it can be
temporarily converted into a two-phase motor while starting. This can be achieved by introducing an
additional 'starting winding' also called as auxiliary winding.
Prepared by Omondi Ferdinand – 0735 766 013/0712 747 442 2
Engineering Trainer – KMTC - Kisumu Campus
