Wednesday, 17 July 2013

Single Phase Motors

"Single phase motors are also used to convert Electrical energy to mechanical energy but the difference is that they are operated on single phase supply".

Types of single phase motors:
Single phase motors are generally built in the fractional –horsepower range and are classified into following four basic types:
  1. Single-Phase Induction Motors
      (i) Split-phase type
     (ii) Capacitor type 
     (iii) Shaded-pole type
  1. AC Series Motor or Universal Motor
  2. Repulsion Motors
      (i) Repulsion-start induction-run motor 
      (ii) Repulsion-induction motor
  1. Synchronous Motors
            (i) Reluctance motor 
           (ii) Hysteresis motor

Single phase Induction motors:

Split – Phase Induction Motor

  • Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90o electrical from the main winding and operates for a short time when the motor starts up.
  • When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed is below the synchronous speed and depends upon the load on the motor.

Capacitor-Start Motor

  • The capacitor motor is similar to a split-phase induction motor but with two differences: (i) the starting winding of capacitor start motor have as many turns as the main winding and (ii) a capacitor is connected in series with the starting winding.
  • Capacitors are used to improve
    • The starting-performance
    • Running performance
    • Both starting and running performance of a split-phase induction motor.

Capacitor-Start Capacitor-Run Motor

  • This motor is similar to capacitor-start motor except that starting winding is not opened after starting so that both the windings of the stator remain connected to the supply when running as well as at starting.
Shaded-Pole Motor

  • A shaded-pole motor has salient pole stator similar to the stator of dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in figure F6. A short-circuited turn of copper ring called shading coil is placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-third section of the pole arc is called unshaded portion. The main single phase winding is wound on the entire pole section.  The rotor used is similar to the rotor of any other single phase induction motor.

Single phase Synchronous  Motor:

Synchronous Reluctance Motor:
This motor has variable air-gap which decides the reluctance of the magnetic path. More the air gap, more is the reluctance and vice versa. The reluctance is greatest when the magnetic field of the stator aligns with the cut out portion of the rotor whereas reluctance is minimum when it aligns with the center position of the rotor teeth. Therefore the reluctance of the magnetic circuit is a function of the air-gap.  The salient poles created on the rotor must be equal to the poles on the stator. The rotor salient poles offer low reluctance to the stator flux and therefore, become strongly magnetized.

A.C. Series Motor OR Universal Motor:

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.
§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.


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