The rotor core is
cylindrical and slotted on its periphery. The rotor consists of
uninsulated copper or aluminium bars called rotor conductors. The bars
are placed in the slots. These bars are permanently shorted at each end
with the help of conducting copper ring called end ring. The bars are
usually brazed to the end rings to provide good mechanical strength. The
entire structure looks like a cage, forming a closed electrical
circuit. So the rotor is called squirrel cage rotor. The construction is
shown in the Fig. 1.
|Fig. 1 Squirrel cage rotor|
are permanently shorted to each other through end ring, the entire rotor
resistance is very very small. Hence this rotor is also called short
circuited rotor. As rotor itself is short circuited, no external
resistance can have any effect on the rotor resistance. Hence no
external resistance can be introduced in the rotor circuit. So slip ring
and brush assembly is not required for this rotor. Hence the
construction of this rotor is very simple.
Fan blades are generally provided at the ends of the rotor core. This
circulates the air through the machine while operation, providing the
necessary cooling. The air gap between stator and rotor is kept uniform
and as small as possible.
|Fig. 2 Skewing in rotor construction|
- A magnetic hum i.e. noise gets reduced due to skewing hence skewing makes the motor operation quiter.
- It makes the rotor operation smooth.
- The stator and rotor teeth may get magnetically locked. Such a tendency of magnetic locking gets reduced due to skewing.
- It increases the effective transformation ratio between stator and rotor.