Induction Motors

1. Introduction
The induction motor derives its name from the actual fact that ac
voltages square measure induced  within the rotor circuit by the rotating magnetic
field of the mechanical device. In some ways, induction during this motor is
similar to the induction between the first and secondary
windings of a electrical device.
Induction machines square measure the foremost wide used of all electrical
motors. they provide the subsequent enticing features:
straightforward to create and cheaper than corresponding dc
or synchronous motors
and need very little maintenance
cheap asynchronous performance
potency
to many MW
Some disadvantages of induction motors are:
simply controlled as dc motors
massive beginning currents, usually 6-8 x their full load
values
insulant power issue once gently
loaded
                                                                                                                 
2. Construction
A three part induction motor consists basically of 2
parts:
1. Stator
2. Rotor
Stator
Rotor
2.1 Stator
• The mechanical device is that the stationary half is made of high – grade
alloy steel laminations to cut back the eddy current losses.
• The laminations square measure slotted on the inner outer boundary and square measure
insulated from one another.
• These laminations square measure supported in a very mechanical device frame of forged
iron or fictitious plate. The insulated mechanical device
conductors square measure placed in these slots. The mechanical device conductors
are connected to create a three- part winding. The phase
winding could also be either star or delta connected.                                                                                             

               

2.2 Rotor
There square measure two-types of induction motor rotors:
1. Squirrel-cage rotor or just cage rotor (most common)
2. Wound-rotor or slip -ring.
2.2.1 Squirrel-cage
It consists of a cylindrical laminated core with slots nearly
parallel to the shaft axis, or skewed. every slot contains Associate in Nursing
uninsulated bar conductor of metal or copper. At each
end of the rotor, the rotor bar conductors square measure short- circuited
by serious finish rings.

The skewing of cage rotor conductors offers the subsequent
advantages:
1. additional uniform force is made and therefore the noise is reduced
during operation.
                                                                                                         
2. The lockup tendency of the rotor is reduced. During
locking the rotor and mechanical device teeth attract one another thanks to
magnetic action.

2.2.2 Wound-rotor ( slip –ring)
A wound rotor includes a 3 part winding, kind of like the mechanical device
winding. The rotor winding terminals square measure connected to 3 slip
Rings. The slip rings square measure mounted on the shaft with brushes
resting on them. The slips/ brushes permit external resistors to be
connected serial with the winding.
The external resistors square measure chiefly used throughout start-up, under
normal running conditions the windings short circuited
externally.

• Cutaway of a wound‐rotor induction motor
                                                                                                         
3 fundamental principle of operation

When the 3 part mechanical device winding, square measure fed by a 3
phase offer then, a magnetic flux of constant magnitude however
rotating at synchronous speed is about up.
The flux passes through the air gap, sweeps past the rotor
surface then cuts the rotor conductors that, as yet, are
stationary. thanks to the relative speed between the rotating flux
and the stationary conductors, an e.m.f. is induced  within the
conductors. The frequency of the induced  e.m.f. is that the same as
the supply frequency. thanks to closed ways, the rotor currents
circulate. These rotor currents act with the magnetic flux
(produced by the stator) within the air gap and develop a force
which rotates the rotor within the same direction.
The cause that made this within the rotor conductors is
the relative speed between revolving flux of the mechanical device and therefore the
stationary rotor conductors. so to cut back the relative speed,
the rotor starts occupation constant direction as that of the flux
and tries to catch the revolving flux.