single phase induction motors

Introduction :

single phase induction motors is used general lighting purpose in shops,offices houses,school etc single phase ac supply is commonly used.Hence instead of dc motors works on the single phase ac supply are very popularly in use.This type of A.C motors are called single phase induction motors.single phase motors performs a great variety of useful services in the home,the office,the factory,in the business establishments,on the farm,many other places,where electricity is available.

Types of single phase motors :
1.Single phase induction motors :

In practice some arrangement is provided in the single phase induction motors so that the stator flux produced becomes rotating types rather than the alternating type which rotates in one particular direction.So torque produced due to such a rotating magnetic field is unidirectional as there is no oppositely directed torque present.Hence under influence of rotating magnetic field in one direction,the induction motor becomes self starting.It rotates same direction as that of rotating magnetic field.Thus depending upon the methods of producing rotating stator magnetic flux,the single phase induction motor are classified.

A.Split-phase motors :
(i) Resistance-start motor    (ii) Capacitor-start motor   (iii) Permanent-split capacitor motor
(iv) Two-value capacitor motor.
B.Shaded-pole induction motor
C.Reluctance-start induction motor
D.Repulsion-start induction motor
2.Single-phase synchronous motor:
(i) Reluctance motor
(ii) Hysteresis motor
(iii) Sub-synchronous motor
Construction of single phase induction motor :
A Single phase induction motor is similar to 3-phase squirrel cage induction motor.Its rotor is essentially the same that used in 3-phase induction motor.The stator is very similar except for shaded pole motors.There is a uniform air-gap between stator and rotor but no electrical connection between them.
The induction motors never rotates with the synchronous speed but rotates at a speed is slightly less than the synchronous speed.
The rotor construction is of squirrel cage type.This type rotor consists of un-insulated copper or aluminium bars,placed in the slots.The bars are permanently shorted at both ends with help of conducting ring are called end rings.The entire structure looks like cage hence called squirrel cage rotor. 

The air-gap between stator and rotor is kept uniform and as small as possible.The main feature of this rotor is that automatically adjust itself for same number of poles as that of stator winding.

The Stator winding differ in the two following aspects :

• Single phase motors are usually provided with concentric coils.
• These motors normally have two stator winding.s.The stator winding's are main winding and auxiliary winding.These motors operate with both winding's energized,the winding with the heaviest wire is known as the main winding and other called auxiliary winding.
• If motor runs with auxiliary winding open,these winding are usually referred as running and starting winding.
• In most of motors the main winding is placed at the bottom of the slots and the starting winding on top but shifted 90 deg from the running winding. 

working of single phase induction motor :   

When stator winding of a single phase induction motor is connected to single phase A.C supply,a magnetic field is developed,whose axis is always along the axis of stator coils.The magnetic field is produced by the stator coils is pulsating,varying sinusoidally with time.Currents are induced in the rotor conductors by the transformer action,these currents being in such a direction as to oppose the stator mmf wave coincides with that of the stator field the torque angle is therefore zero and no-torque is developed on starting.However ,the rotor is given a push by hand or by other means in the either direction,it will pick up the speed and continue to rotate in the same direction developing operating torque.Thus a single phase induction motor is not self starting motor and requires some special means for starting.  

Disadvantages :

• The main disadvantages of single phase induction motors are 
• Their output is only 50% of the three-phase motor,for a given frame size and temperature rise.
• They have lower power factor.
• Lower efficiency.
• These motors do not have inherent starting torque.
• More expensive than three phase motors of the same output.

Applications :

• Single phase induction motor are in very wide use in industry especially in fractional horse-power field.
• These motors have low starting current and moderate starting torque.These are used for started loads like fans,blowers,grinders,centrifugal pumps,washing machines,oil burners,office equipment etc.