# Split Phase Induction Motor – Operation and Characteristics

- Link: https://studyelectrical.com/2014/07/split-phase-induction-motor-working-characteristics.html
- Published: 2014-07-09T06:01:00+00:00
- Author: 

Before going to the operation and characteristics of **split-phase induction motor**
we should first understand [why single phase induction motor is not self-starting](http://www.studyelectrical.com/2014/05/single-phase-induction-motor.html).

To [make a single-phase induction motor self-starting](http://www.studyelectrical.com/2014/05/how-to-make-single-phase-induction.html),
we should somehow produce a **rotating magnetic field**. _This may be achieved by
converting a single-phase supply into two-phase supply through the use of an additional
winding_.

In a split-phase induction motor, the additional winding is known as _auxiliary 
winding_ or _starting winding_. The constructional features of split-phase induction
motors are well explained below.

The stator of a split-phase induction motor is provided with an _auxiliary or starting
winding S _in addition to the _main or running winding_ M to make the single-phase
induction motor self-starting. The starting winding is displaced 90° electrical 
from the main winding and operates only during the brief period when the motor starts
up.

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[⌊Split Phase Induction Motor⌉⌊Split Phase Induction Motor⌉[

 Split Phase Induction Motor

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 The two windings are so designed that the **starting winding S has a high resistance
and relatively small reactance while the main winding M has relatively low resistance
and large reactance** as shown in the schematic connections in the figure.

Consequently, the currents flowing in the two windings have a reasonable phase difference(
25° to 30°) as shown in the phasor diagram in the figure given above. Hence the 
motor behaves like a **two-phase motor**. These two currents produce a revolving
flux and hence make the motor self-starting.

A centrifugal switch is connected in series with the starting winding. Its function
is to automatically disconnect the starting winding from the supply when the motor
has reached **70 to 80 percent** of its full load speed.

The resistance of starting winding may be increased in two ways : 

 1. by connecting a **high resistance in series** with it  or
 2. by choosing **high resistance fine copper wire** for winding purposes.

## Working of Split Phase Induction Motor

The operation of the split-phase induction motor is explained below.

When the two stator windings are energized from a single-phase supply, the main 
winding carries current I_(m) while the starting winding carries current I_(s).

Since main winding is made highly inductive while the starting winding highly resistive,
the currents I_(m) and I_(s) have a reasonable phase angle a (25° to 30°) between
them as shown in Figure.

Consequently, a weak revolving field approximating to that of a 2-phase machine 
is produced which starts the motor. 

The starting torque is given by;

**Ts = kI_(m)I_(s)sinα**
 where k is a constant whose magnitude depends upon the
design of the motor.

 * When the motor reaches about **75% of synchronous speed**, the centrifugal switch
   opens the circuit of the starting winding.
 * The motor then operates as a single-phase induction motor and continues to accelerate
   until it reaches the normal speed. The normal speed of the motor is below the
   synchronous speed and depends upon the load on the motor.

The direction of rotation of such motors can be reversed by reversing the connection
of any one of the stator windings ( not both ). The [rotating magnetic field](https://studyelectrical.com/2014/01/production-of-rotating-magnetic-field.html)
gets reversed and hence the motor rotates in the opposite direction. For this purpose
four leads are brought outside the frame.

## Characteristics of Split Phase Induction Motor

The important characteristics of the split-phase induction motor are given below,

 1. The **starting torque is 150 to 200 percent of the full load torque** with a starting
    current of 6 to 8 times the full load current.
 2. Due to their **low cost**, split-phase induction motors are the most popular single-
    phase motors in the market.
 3. Since the starting winding is made of fine wire, the current density is high and
    the winding heats up quickly. If the starting period exceeds 5 seconds, the winding
    may burn out unless the motor is protected by built-in-thermal relay. This motor
    is, therefore, **suitable where starting periods are not frequent**.
 4. An important characteristic of these motors is that they are essentially constant-
    speed motors. The speed variation is 2-5% from no-load to full load. For this reason,
    they are **usually regarded as constant-speed motors**.
 5. These motors are suitable where a **moderate starting torque** is required and 
    where **starting periods are infrequent**
 6.  * e.g., to drive:_ _
        - _(a) fans _
        - _(b) washing machines _
        - _(c) oil burners _
        - _(d) small machine tools etc._

The power rating of such motors generally lies between 60 W and 250 W.

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