Single phase Induction Motor

Introduction

• What is single phase induction motors? is an induction motor having a squirrel cage rotor and a single phase stator winding.

Working Principle

Suppose the rotor is at rest and a single phase supply is given to the stator winding. Now the current flowing in the stator winding will produce a m.m.f with in the stator and this m.m.f induces a current in the rotor. Again the induced current inside the rotor will produce a m.m.f with in the rotor itself which is equal in magnitude and opposite in direction with the stator m.m.f. Thus the two m.m.f cancel out each other and as a result there will be no net torque acting on the rotor. There for the rotor will stay at rest. So due to this effect, we have to find another method to start the motor.

Types of Single Phase Induction Motors

Depending on the method used to start the motor : -• 1) Capacitor-start motors• 2) Capacitor-run motors• 3) Capacitor start-and run motors • 4) Shaded-pole motors

1) Capacitor-Start phase induction motor

• A capacitor-Start motor is a spilt phase induction motor with a starting capacitor inserted in series with the start winding creating an LC Circuit which is capable of producing a much greater torque.• An Lc circuit refers to a circuit containing an

inductor w/c connected together they can act as an electrical resonator w/c stores electrical energy.

Working Principle of single phase capacitor-start motor

• In capacitor-start motors the capacitor enables the motor to handle heavy start loads by increasing the strength of the magnetic field created by the windings. The capacitor is individually mounted outside of the motor as a separate unit either on the top or side of the motor with a centrifugal switch located between the capacitor and the start winding. The switch connects the capacitor with the motor at startup and disconnects them when the motor has reached about 75% of its operating speed. And during startup period when the centrifugal switch is closed, capacitor-start motors typically deliver from 250-350% of the full load torque.

Equivalent circuit of capacitor-start motors

Types of single phase capacitor-start motor

Among this the basic types include:-A) Single voltage externally reversible B) Single voltage non reversible

Applications of single phase capacitor-start motors

Capacitor-start from high torque (>175% full load) are used: -

Operation having high starting loads such as: - Elevators

- Compressors & - Refrigerators

• Capacitor-start moderate torque (<175% full load) are used:-

Operation having low starting loads such as:- Fans

- Blowers & - Small pumps.

2) Capacitor run motors Capacitor-run motors are motors having a capacitor connected in series with the start winding in order to increase the running efficiency. Capacitor-run motors use run-capacitors that are designed for continuous duty which are energized the entire time during operation of the motor.

Working principle of single phase capacitor-run motors

• In capacitor-run motors, a run-capacitor is connected to the start winding of the motor and it constantly energizes the start winding while the motor is running. And this creates a 90o phase change between the start winding current and the run winding current making a two phase motor. As a result a rotating magnetic field is created within the motor which causes the rotor to rotate more efficiently.

Advantages and disadvantages of capacitor-run motors

AdvantagesThe capacitor remains in the circuit at all

times thus no centrifugal switch is required.They can be designed to have low vibration

and less noise under full load condition.If properly designed, they are more efficient

than other type of motors.

DisadvantagesSince capacitor start motors have low

starting torque they cannot be used in applications with severe starting conditions.

Application of single phase capacitor-run motors

are mainly used for applications requiring low starting torque and high efficiency such as:- Small compressors,

Pumps & Fans.

3) Capacitor start-and-run motors

Capacitor-start-and-run motors or permanent-split capacitor motors are single phase induction motors having

capacitors connected in the circuit during both the starting and the running period. In this type of motors both the start winding and the run windings are permanently connected to the power source through a capacitor at all times.

Types of single phase capacitor start-and-run motors

Depending on the number of capacitors used: -1. Single value capacitor start-and-run

motors:

2. Two value capacitor start-and-run motors

The two values of capacitance can be obtained using two different methods.

a. By using two capacitors in parallel b. By using a step up transformer

Advantage

• Ability to start heavy loads• Ability to develop 25% overload capacity• Higher efficiency and power factor • Extremely quiet operation

Applications of single phase capacitor start-and-run motors

• Two value capacitor start and run motors are frequently used in applications requiring variable speed such as : -

Air handlers, Blowers and Fanes.

• Single value capacitor start-and-run motors are used in applications requiring low starting torque such as:-

Fans Blowers & Voltage regulators.

4) Shaded pole motors

• A shaded pole motor is a single phase induction motor having one or more short circuited windings acting only on a portion of the magnetic circuit.

• Generally the winding is a closed copper ring embedded in the face of the pole together known as the shaded pole which provides the required rotating field for starting purpose.

Working principles of single phase shaded pole motors

• Now when an alternating current is passed through the field or main winding surrounding the whole pole, the magnetic axis of the pole shifts from the unshaded part to the shaded part. which is analogous with the actual physical movement of the pole. As a result the rotor starts rotating in the direction of this shift from the unshaded part to the shaded part.

Advantages and disadvantages of single phase shaded pole motors

AdvantagesSimple in construction Tough surface Reliable and cheap

DisadvantagesLow starting torque Very little overload capacityLow efficiency (5% for tiny sizes – 35%

for higher ratings)

Applications of single phase shaded-pole motors

• Because of its low starting torque, the shaded pole motor is generally used for

Small fans,Toys, Hairdryers,Ventilators etc.

Conclusion • Generally when we construct single phase

induction motors we must make sure we use appropriate capacitor. If a wrong capacitor is installed on a motor, the motor will not have equally distributed magnetic field and this will cause the rotor to hesitate at those spots with uneven magnetic field distribution and this causes:-Loud disturbing noise

Increase in energy consumption Performance drop Overheat