Electrical Machines · Single-Phase Motors

Single-Phase Motors: Basics & Build

Dr. Mithun Mondal BITS Pilani, Hyderabad Campus Electrical Machines

Demonstrative Video

SECTION 01

Contents

  • Introduction to Single-Phase Motors, Construction and Qualitative Examination of \(1-\phi\) Induction Motors

  • Principle of Operation - Double Revolving Field Theory

  • Torque Produced in \(1-\phi\) Induction Motors

  • Types of \(1-\phi\) Induction Motors

  • Equivalent Circuit of \(1-\phi\) Induction Motors

SECTION 02

Single-Phase Motors

  • Operate from a single-phase supply

  • Most commonly used electric motors in domestic and industrial applications

  • Small size motors of fractional kW ratings

  • Domestic applications - Fans, hair dryers, washing machines, vacuum cleaners, mixers, refrigerators, food-processors etc.

  • Industries - compressors, pumps, small power tools, office machinery, blowers, small farming equipments etc.

  • Motor manufacturing industry developed different types of motors designs to meet the user needs.

SECTION 03

Classification of Single-Phase Motors

  • Classified based on their construction and methods of starting

  • Broadly classified into 3 main types:

    • Induction motors

    • Synchronous motors

    • Commutator motors

    SPIM 1
    Spim 1
SECTION 04

Single-Phase Induction Motors

  • Similar to three-phase IM, except

    • stator is provided with single-phase winding

    • auxiliary or starting winding for starting purpose

  • Distributed stator winding and a squirrel-cage rotor

  • Single phase supply \(\Rightarrow\) stator winding \(\Rightarrow\) alternating flux \(\Rightarrow\) alternates along one space axis only

  • Not synchronously rotating flux as developed by two or three phase stator winding supply

  • Alternating flux cannot rotate a stationary squirrel-cage rotor that is only possible with rotating flux

  • Single-phase IM are not self-starting

  • If initially the rotor is rotated by hand or using a small motor, in either direction, torque will produce and accelerate to its final speed

  • Possible only when the applied torque is not too high

SECTION 05

Construction and Qualitative Examination

  • Laminated iron core with two windings arranged perpendicularly

  • Two windings : main and auxiliary

  • Stator winding is distributed in slots for a sinusoidal mmf space distribution.

    SPIM 2
    SPIM 3

    Spim 2
    Spim 3

Single-phase winding generates equal forward and backward rotating mmf waves

  • Single-phase winding generates equal forward and backward rotating mmf waves.

  • Due to symmetry, the motor exhibits no starting torque at standstill, producing equal torque in both directions.

  • An auxiliary winding resolves this, creating a net torque in the intended direction for motor initiation and sustained operation.
    SPIM 4
    Spim 4

  • “single-phase” motors are truly two-phase machines

  • squirrel cage rotor, which has a laminated iron core with slots

  • Squirrel cage rotor:

    SPIM 5
    Spim 5

  • Aluminium bars are molded on the slots and short-circuited at both ends with a ring.