wiring up slip ring motors 1

[seaweed]

Please advise on the various ways to wire up a 3 phase 6 wire slip ring motor. It is 15 HP 380/415V. The rotor and stator terminals are marked separately. Many thanks

 

[Marke]

I assume that when you refer to a six wire slip ring motor, that you are counting the stator connections and the rotor connections.

To start the machine, you would normally apply full voltage to the stator windings (Using a contactor and overload) with some resistance connected in the rotor circuit. The initial start torque is controlled by the selection of the rotor resistance. As the motor accelerates to full speed, the rotor resistance can be reduced and at full speed, the rotor circuit can be shorted out. Retaining resistors in the rotor circuit during run will reduce the efficiency of the system and also cause the motor to run a little slower.

If you try to start the motor with the rings shorted, there will be a very large start current for a very low torque. In most situations, this would be unacceptable.

The rotor resistance must have a high power rating as it carries a high value of current for the duration of the start. Mark Empson

http://www.lmphotonics.com

 

[seaweed]

Hi. thank you for the advice re slipring motor start and run. Next question is: When you say resistance in the rotors wires, do you mean join them star fashion but with a resistance in each lead to the centre point? If so are we talking about a few ohms or tens of ohms. These resistances could then be shorted out as the motor came up to speed. thanks again

 

[Marke]

The resistors in the rotor circuit can be connected in star or in delta, and the values are dependant on the motor characteristics and the starting characteristics that you require. Usually, slip ring motors are employed because they are able to produce a high starting torque from zero speed to full speed by adjusting the resistor values as the motor accellerates. This my require 3 or four stages of resistance. If you wish to use only one stage of resistance, then I would suggest that you need to talk to your local switchboard builder to do the job for you, but as a rough estimate, assuming a frame voltage of 400 volt (Open circuit rotor voltage) then I would use between 1 and 3 ohms connected in star. Note these resitors need to be able to dissipate in the order of 5KW each for the duration of the start!

Best regards,
Mark. Mark Empson

http://www.lmphotonics.com