Question

Alternating current machines

Answer 1

Alternating current machines generally convert one form of electrical/mechanical energy into another one form of mechanical/electrical energy.

Ac machines have armature winding at stator and field winding connected at rotor. The AC machines are classified into.

• Synchronous motors.
• Asynchronous motors.

Synchronous machine-

Synchronous machines consists of synchronous motors that convert mechanical to electrical energy and synchronous generators convert electrical energy to mechanical energy.

In synchronous machine is a double excited machine the rotor is connected to dc supply and stator is connected to ac supply. The synchronous machine follows,

N_s=120f/p where Ns is speed and f is frequency where p is number of poles . It works on law of interaction.When the conductor is placed in electric field the rotating magnetic field is produced which makes the N-S poles to attract each other known as magnetic locking thus it runs at a constant synchronous speed.

But since poles are interlocked they are unable to run so we need a external force to start the machine by using damper winding or a prime mover once it is started we can remove the external force.Synchronous machine can run at any power factor.

The synchronous machine are classified as single phase and three phase synchronous motor.

Advantage of synchronous motor.

• It has wide lag/lead power factor.
• It has high power rating.
• It has very high efficiency.
• It is cost efficient.

Disadvantages

• It is not self starting.
• Hunting takes place if load is changed.

Induction machine.

Induction machine is also known as asynchronous machine it has speed less than synchronous speed. The stator is supplied by AC voltage thus it produce magnetic flux due to coil and rotor is arranged as short circuited thus current is induced hence known as induction motor.

The induction motors are further classified as.

Single phase induction motor.

• Split phase induction motor-It has low current and moderate torque used in fans and blowers.
• Capacitor start and capacitor run motors-it has caged rotor and two stator windings as main and auxillary winding used in heavy duty machine such as conveyors and pumps.
• Shaded pole induction motor- It is a self starting motor with shaded copper ring used in small devices like hair dryers ,relays etc..

Three phase induction motor.

• Squirrel cage induction motor .
• Slip ring induction motor.

In general single phase induction motor are not self starting unlike 3phase induction motor because they dont have any phase difference that moves towards another phase.

Advantages-

• It is simple to construct.
• It is mechanically strong and robust in nature.
• It has high starting torque and good speed regulation.

Disadvantages.

• Single phase IM are not self starting.
• It has low power factor.
• Speed control is complex.