Slip Ring Induction Motor Resistance

I'm no expert on large motors, so I may not be of help to you, but since no one else has responded so far:

1. Asking correct questions commonly improves the quality of answers. If you are applying power to the Stator AND the Rotor, Then this is NOT an induction motor!

2. 3-phase motors directly connected to fixed-frequency supplies normally run at a single speed. Introducing series resistance does NOT sound like an appropriate method to control the speed of such a motor. These resistors will waste huge amounts of energy!

3. I assume the resistors are in sets of three (9 resistors total), are connected in series with the stator windings, and all three are switched at the same time. Do these resistors have a duty cycle (time on/time off)? I'm suspecting that the resistors were intended only as a method of bringing the motors up to speed gently (soft start), rather than as speed controllers for continuous use.

4. This sounds like a question that should be directed to the manufacturer of the motor and/or the resistor bank.

5. Have you checked the condition of the switch contacts? If the three phases are not closely balanced, the motor efficiency will drop significantly, and the resistors with better contacts will heat more...

The duty cycle of individual resister is vital in designing such resisters. Please visit http://nationalswitchgears.com/ They are pioneer in designing resisters for such applications

You start motor with maximum resistors connected to the rotor's slip rings, and end running up procedure with shorted (Y) rotor. Check control and possibility of miss wiring.

Even i am working to find rotor maximum resistance to get maximum torque n speed of slip ring induction motor.All are suggesting to use VFD starts 220 dollar in US.