Cascade Speed Control of Induction Motor

That kind of speed control is required only where whole system is synchronized e.g in Paper Industry whole machine motors are Synchronized in that case cascade speed control of Induction motors is required

You can't cascade a single anything.

Specifically...

If you don't have two motors on the same shaft where the second motor can derive its supply via slip rings from the first motor's rotor then you cannot cascade their operation. The main motor must be a slip ring induction motor the slave can be either slip ring or squirrel cage.

In it's most basic configuration three speeds (N_a, N_b, N_c) are possible from the same shaft at mains frequency f.

1. Main motor A may be alone on the mains, where N_a = 120f/P_a, where P_a is the number of poles in motor A.

2. Auxiliary or the slave motor B running alone on the mains, where N_b = 120f/P_b, where P_b is the number of poles in motor B.

3. The combination may be in cascade operation. In this operation, the important point is that the phase rotation of the stator fields of the motors "A" and "B" must be in same direction. Thus the synchronous speed of this cascaded motor set is given by Nc = 120f/ (Pa + Pb). _{http://www.brighthubengineering.com/diy-electronics-devices/44182-speed-control-of-slip-ring-motors/}

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