Types of Induction Motor
Depending
upon the type of rotor construction used the induction motor are classified as:
1-Squirrel cage three phase induction motor:
The rotor of
the squirrel cage
induction motor is cylindrical in shape and have slots on its
periphery. The slots are not made parallel to each other but are bit skewed as the
skewing prevents magnetic locking of stator and rotor teeth and makes the
working of motor more smooth and quieter. The squirrel cage rotor consists of
aluminum, brass or copper bars (copper bras rotor is shown in the figure
beside). These aluminum, brass or copper bars are called rotor conductors and
are placed in the slots on the periphery of the rotor. The rotor conductors are
permanently shorted by the copper or aluminum rings called the end rings. In
order to provide mechanical strength these rotor conductor are braced to the
end ring and hence form a complete closed circuit resembling like a cage and
hence got its name as "squirrel cage induction motor". The squirrel
cage rotor winding is made symmetrical. As the bars are permanently shorted by
end rings, the rotor resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and
brushes make the construction of Squirrel cage induction
motor very simple and robust and hence widely used
three phase induction motor. These motors have the advantage of adapting any
number of pole pairs. The below diagram shows squirrel cage induction rotor
having aluminum bars short circuit by aluminum end rings.
Advantages
of squirrel cage induction motor.
1.
Its construction is
very simple and rugged.
2.
As there are no
brushes and slip ring, these motors requires less maintenance.
Applications:
Squirrel cage induction motor is used in lathes,
drilling machine, fan, blower printing machines etc
In this type of induction motor the rotor is wound for the same number of poles as that of stator
but it has less number of slots and has less turns per phase of a heavier
conductor.The rotor also carries star or delta winding similar to that of
stator winding. The rotor consists of numbers of slots and rotor winding are
placed inside these slots. The three end terminals are connected together to
form star connection. As its name indicates three phase slip ring induction motor consists of slip
rings connected on same shaft as that of rotor. The three ends of three phase
windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and
hence used for speed control and improving the starting torque of three phase
induction motor. The brushes are used to carry current to and from the
rotor winding. These brushes are further connected to three phase star
connected resistances. At starting, the resistance are connected in rotor circuit and
is gradually cut out as the rotor pick up its speed. When the motor is running
the slip ring are shorted by connecting a metal collar, which connect all slip
ring together and the brushes are also removed. This reduces wear and tear of
the brushes. Due to presence of slip rings and brushes the rotor construction
becomes somewhat complicated therefore it is less used as compare to squirrel
cage induction motor.
Advantages of slip ring induction motor.
It has high starting
torque and low starting current. Possibility of
adding additional resistance to control speed.
Application:
Slip ring induction motor are used where high
starting torque is required i.e in hoists, cranes, elevator etc.
