This will take care of the forces acting on the coil to remain acting in the same direction and continue rotation. The direction of rotation of the rotor can also be reversed, by reversing either the field current or armature current depending on the configuration of the field coil and armature coil.
A.C Motors
Electric motors using A.C. supply are called A.C. motors. Classification of a.c motors Alternating current motors are broadly classified into two grouj
I. a. Single-phase a.c motors and
b. Poly-phase a.c motors
II a. Induction motors
b. Synchronous motors.
That is a.c motors can be single phase induction or synchronous motors or polyphase induction or synchronous motors.
Single phase motors are used for low-power requirement and poly phase for higher power requirement.
Induction motors are inexpensive than synchronous motors and are widely used.
Single phase induction motor
This consists of a single phase stator winding with a cage rotor represented schematically in fig. 5.58. The rotors are made up of either copper or aluminium bars that fit into slots in the end
Fig. : 5.58 : Single phase squired cage indication
rings to form a complete electrical circuit. Instead of being concentrated coil, the actual stator winding is distributed in slots in order to give an approximately sinusoidal distributed m.m.f (magneto motive force) in space. When an alternating current is passed through the stator windings, an alternating magnetic field is produced. As a result of electromagnetic induction, emfs are induced in the conductors of the rotor and current flows in the rotor. Such a motor inherently has no starting torque.
Drawbacks of single phase induction motor
Single phase induction motors suffers from several drawbacks. They are,
Low over load capacity
Low efficiency
Low power factor
No self starting
The frequency of the a.c supply determines the speed of the motor. For a two-pole single phase motor supplied with constant frequency supply will alternate the magnetic field with this frequency. This speed of rotation of magnetic field is called synchronous speed. The rotor rotates at slower speed than the rotating stator fields (this is called slip) making the induction impossible. Hence the term asynchronous. Because of this asynchronous motors are sometimes referred to as induction motors. Generally the slip is around 1 to 3 percent.
Three-phase induction motor
Poly-phase induction motor is, by very considerable margin the most widely used a.c motor,
Advantages of poly-phase motor
a. Low cost
b. Simple and extremely rugged construction
c. High efficiency
d. Reasonably good power factor
e. Low maintenance cost
f. Simple starting arrangement.