A.C. SINGLE PHASE INDUCTION MOTORS

The motor which works on a.c. single phase supply is known as single phase a.c. motor. It has only one set of winding on the stator. The single phase motors are not self starting because self starting rotational torque in a stationary rotor is produced by the rotating magnetic field of the stator. The stator field produced in single phase motor is only alternating but is not rotating as in the case of two or three phase stator winding. The pulsating field flux is incapable of producing a rotational torque in a stationary rotor. That is why the single phase motors are not self starting.

To make it self started an auxiliary (starting) winding is provided in the stator frame connected parallel to the main motor winding to form a second phase with some phase difference which produces a field with its poles between the main poles. The effect of these two fields in the production of a rotating magnetic field which makes the motor self started.

Main Difference between single phase & three phase motor:

i)                    Three phase induction motor is a self starting motor whereas single phase induction motor has no starting torque.

ii)                  For the same output and speed a single phase induction motor has lower efficiency and power factor than a three phase induction motor.

iii)                For the same output and speed a single phase induction motor is heavier than a three phase induction motor.

iv)                A single phase induction motor requires always a starting device whereas a three phase induction motor can start without any starting device.

Types of AC single phase motors:

A.       Induction type	1. Split phase	Resistance start type	Capacitor type
			a.        Capacitor start
			b.       Capacitor start-Capacitor run
			c.        Capacitor run
	2.Commutator  type	AC series motor or Universal motor	Repulsion motor
			a.        Plain Repulsion motor
			b.       Repulsion-Induction motor
			c.        Repulsion start Induction run motor.
	3.Synchronous type	Reluctance motor	Hysteresis motor.

 Split phase induction motor: It is a single phase induction motor. It has two windings on the stator, one is starting winding and the second is main or running winding. The two windings area placed 90^{0 ,} electrical apart and are connected in parallel across the supply. The rotor itself is of the squirrel cage type. Due to phase difference in the currents of the two windings a rotating magnetic field is set up thereby creating a rotational torque on the rotor.

Two windings area provided in the stator of split phase motor to split up one phase into two phases in order to produce a rotating magnetic field.

Starting winding is the high resistance winding in a single phase motor. It has less turns than the main winding to reduce the inductive reactance and to allow more current. The starting and running windings can be identified with the help of a test lamp connected in series with the windings. When a low voltage supply is applied to the winding-lamp series combination the high resistance starting winding will give dim light and the running winding will give comparatively bright light. Split phase motor have Slow starting torque.

Centrifugal switch is fitted on the shaft on the rotor and electrically connected in series with the starting winding in order to disconnect this winding automatically when the motor attains 90% of its full load speed.

If the starting winding of single phase motor does not cut off during running at full load speed, the motor will draw excessive current giving a humming noise and the starting winding may be burnt out due to excessive heating. Double layer chain or concentric type windings are used in split phase motor. By interchanging the connections externally to either the main or the starting winding on the stator, the direction of rotation of split phase motor can be reversed.

The split phase motors are called resistance start motor because it has a high resistance starting winding. It is also called a duel voltage motor because the two stator windings are so arranged that the motor may work on either 110 volts or 220 volts.

Uses of split phase motor: These are used for constant speed operation even under varying load condition where moderate torque is desirable e.g. Fans, Blowers, Centrifugal pumps, Refrigerators etc.

 Capacitor start motor: It is an AC single phase motor having two windings as in split phase motor. One heavy duty electrolyte capacitor is connected in series with the starting winding in order to produce the phase difference between the currents in the two windings and thereby producing starting torque. As soon as the motor reaches 90% of its full load speed the capacitor and starting windings are cut out of the circuit and the pulsating field is enough to maintain the speed of the motor.

The capacitor is used in series with the auxiliary starting winding to give a phase displacement in between starting and running winding thereby improving starting torque.

If the capacitor motor will not start due to punctured,  because the phase difference between the magnetic field of starting and main motor winding will not exist. As result rotating magnetic field will not be produced to make the motor self started.

The capacitor start motor has a high starting torque.

If the Capacitance of a capacitor start motor is reduced, the starting torque will be reduced. Squirrel cage rotor is used for this capacitor start motor & it is usually a duel voltage motor.

The direction of rotation of capacitor start motor can be reversed by reversing either the running or starting winding leads to the supply lines.

Uses: Capacitor start motors are suitable for grinders, drilling machines, blowers, refrigerators and air conditioners where good starting torque and constant speed under varying loads are required.

 Permanent Capacitor motor: It is an AC single phase motor having two windings as in a split phase motor. But it has no centrifugal switch. The capacitor is permanently connected in series with the starting winding even on running condition. Squirrel cage rotor is used in permanent capacitor motor & is a low torque motor. The direction of rotation of permanent capacitor motor can be reversed by reversing the connection of the starting winding to the line.

Uses: These are used only for ceiling and table fans where high starting torque is not required. The starting winding is not cut off in the ceiling fans because it has same turns of same wire as of running windings.

Capacitor start capacitor run motor: It is similar to capacitor start motor. But the exception is that two capacitors are used in this type of motor. One of large capacity is connected with the starting winding during starting only. When the motor is speeded up the starting capacitor is disconnected by means of centrifugal switch and the motor left running with a smaller capacitor in the circuit.

It has squirrel cage rotor. The direction of rotation of a capacitor start capacitor run motor can be reversed by reversing the connection of either starting winding or running winding.

Advantages: Comparatively starting torque is good, operation is quick, power factor is high and the motor is free from radio interference. Capacitor run motor operates at high power factor.

 Shaded pole motor: It is an AC single phase induction motor in which a portion of each pole face of the stator core is encircled by a copper ring. Current induced in the ring by the stator flux causes the magnetic field over it lagging behind that of the remaining portion of the pole face which is not encircled. The phase difference between the fields produces a rotating magnetic field and the motor is self starting on light loads.

The motor is named Shaded pole motor because the shading rings are provided on each pole face the motor signifies its name as shaded pole motor. Squirrel cage rotor is used in shaded pole motor.

Meaning of shaded coil and shaded pole: A short circuited copper ring or a coil of low resistance placed on a portion of the pole face in a shaded pole motor is called the shaded coil and the portion of the pole face encircled by shaded coil is called the shaded pole. 1/3^rd to ½ part of the full pole face is shaded. Shaded pole motor have no high starting torque. It rotates towards the direction approaching the shading coil. The direction of rotation can not be reversed electrically but it is possible mechanically only by changing the position of the shading coil on the other portion of the pole face. The speed of the single phase capacitor motor and shaded pole motor can be varied by varying the supply voltage with the help of regulator connected in series with the supply line.

Uses: This motor is used for small fans.

AC single phase commutator motor: The single phase AC motors, in which the rotors are wound and the winding leads are brought out and connected to the commutators mounted on the same shaft, are known as AC commutator motors.

Universal motor: It is a single phase fractional horse power motor ranging from 1/100 HP to ½ HP in which the field winding and the armature are connected in series just as in a DC series motor. This motor can be operated on both AC and DC line at high speeds. 300% to 500% starting torque can be obtained with starting current 2 to 3 times the full load current. Since it is used on both AC and DC supply so it is called as uni8versal motor.

 Can a DC series motor be operated as a single phase series motor satisfactorily If not, why?

A DC series motor can be operated as a single phase series motor with the application of a single phase AC supply but not satisfactorily because with the application of AC supply eddy current loss will be excessive due to alternating field flux, excessive sparking on the commutator will be resulted, reactance drop in the field winding will be abnormally high and the power factor will be very low.

The compensating winding is used in universal motor to improve the commutation. The direction of rotation of universal motor can be reversed by reversing the direction of current either in the field winding or in the armature winding by interchanging their leads. By varying the supply voltage with the help of a regulator connected in series with the supply line, the speed of universal motor can be varied. The universal motor can achieve about 5000 r.p.m. speed.

Uses: These motors are widely used in domestic and commercial appliances like Hair driers, Portable drills, blowers, sewing machines, vacuum cleaners, kitchen appliances etc.

Repulsion motor: The simple repulsion motor consists of a field system and an armature with a commutator just like a series motor. The distributed type field winding wound on the laminated stator slots is fed from the single phase AC mains. The armature winding is of drum type like DC motor and is connected to the commutator. The brushes on the commutator are permanently short circuited and are not electrically connected to the main circuit at all. These motors are operated by the reaction between a magnetic flux created by current in the stator winding and induced current in the rotor conductors. The induced rotor current produces the same polarity on the rotor as the nearest stator pole due to which the torque is produced by the repulsion between these like poles in the direction of shifted brush from the maximum current position.

 Repulsion start induction motor: It is a repulsion motor in which a centrifugally operated device is provided on the rotor. When the motor reaches a certain critical speed during starting this device then moves the contacts which short circuits all its commutator segments. Thus the motor starts from rest with a high torque as a repulsion motor and then runs as an induction motor above the critical speed with fairly constant speed on varying load.

 Repulsion Induction Motor: It is a repulsion motor in which an additional squirrel cage winding is provided underneath the insulated winding on the common slots of rotor. The stator winding is similar to that of an induction motor. Inner cage winding has no effect during starting owing to high reactance and the motor starts as a repulsion motor. As the motor speeds up the effect of the squirrel cage winding increases and the torque is produced by the combined effect of two windings, so that the motor has fairly constant speed on varying load.

The direction of rotation of repulsion start induction run motor can be reversed by shifting the position of the carbon brushes round the commutator in reverse direction. When commutator segments are short circuited,  repulsion start induction run single phase motor runs as an induction motor.

Uses of repulsion motors:  These motors are used where sturdy motor with large starting torque and adjustable but constant speed is required such as for refrigerators, compressor pumps, coil winders and other devices particularly those which have a high inertia and prolonged starting period.

Slots of the stator of motor are somewhat bended at some angle to minimize the leakage reactance between the stator and rotor and also to avoid humming sound and magnetic locking.

The starting winding of single phase motors is kept on the upper portion of the slots to produce good starting torque.

The starting and running windings are wound on the stator at 90⁰ electrical degrees apart.

The capacity of a capacitor fitted in the ceiling or table fans is 2 to 3.15 microfarads.

Reluctance motor: When the rotor of a split phase motor is designed with salient poles it will start as an induction motor, attain synchronous speed and continue to run at a constant synchronous speed. It is in fact a self starting synchronous motor without DC excitation having variable magnetic reluctance in the air gap and hence called reluctance motor.

Applications of single phase reluctance motor:  Since reluctance motors are generally made in the fractional horse power sizes and run at constant speed there are particularly suitable to signaling devices, recording instruments, regulating and control equipments, many kinds of timers and phonographs.

Hysteresis motor: The hysteresis motor is nothing but a fractional horse power synchronous motor in which the mechanical torque is produced by the effect of hysteresis. The hysteresis effect is magnified by making the rotor with a group of specially hardened steel rings instead of the usual thin silicon steel laminations. The rotor has no teeth or winding of any sort which makes the motor extremely quiet in operation without any mechanical and magnetic vibrations. The motor is made in very small sizes only about 1/100 HP or less.

 Applications of hysteresis motors: The hysteresis motors are particularly useful for electric clocks, small timing devices and record player motors for its noiseless characteristics and easy synchronizing capacity of heavy inertia loads.

 Types of single phase motor is used for the following:

Appliances	Type of single phase motor
Refrigerator	Split phase or Capacitor start motor
Compressor	Repulsion start induction motor
Lathe	Capacitor start motor
Room cooler	Capacitor run motor
Mixing machine	Universal motor
Electric drill machine	Universal motor
Hooter	Universal motor
Wood working machinery	Repulsion motor
Sweing machine	Universal motor
Blower	Universal motor
Vacuum cleaner	Universal motor
Small fan	Shaded pole motor
Grinder	Capacitor start motor
Centrifugal pump	Split phase motor
Ceiling fan	Split phase motor or permanent capacitor motor.
Table fan	Split phase motor or permanent capacitor motor.

SQUIRREL CAGE MOTOR

                      Advantage                    Disadvantage

1.      Heavy duty                              Low starting torque

2.      No rotor winding                     Starting current high 

3.      On load, PF is strong               on no load, pf is low

4.      Self start                                  On no load – slip decreases, speed decreases On load, slip

                                                increases as stator & rotor speed depends upon the difference.