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slip ring motor


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#1 Sparkles

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Posted 05 December 2005 - 03:07 AM

I understand the operation of the motor and the addition of resistance to the rotor circuit to vary the torque and speed as necessary. My question is that with only three leads coming from the rotor, how is this resistance wired into the circuit. Is there somewhere I could obtain schematics for this type of motor and starter setup?
I guess what I mean is that are the three leads A-B-C or are they A-center tap and C. thereby introducing the resistance in the circuit between A and C phase?
The unit I'm working on has two starters for speed control and a resistor bank above the control cabinet. No starters energized low speed, one energized med speed, two starters high speed.

2nd question is how much load is actually on the brushes on the slip rings. I amprobed it at med speed and showed only one amp. The unit is a 20 hp. motor driving a huge ancient fan for air movement in the building.

thanks in advance for your help.
;p;;p;

#2 Sparkles

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Posted 11 December 2005 - 04:17 AM

Did I forget to say PLEASE HELP?

#3 marke

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Posted 11 December 2005 - 05:38 PM

Hello Sparkles

Welcome to the forum.
I have been slow replying to posts as I have been overseas, sorry about that!!

The rotor resistors can be wired as a star circuit, or as a delta circuit depending on the resistive values used. It is a typical three phase load.

When you talk about starters, I assume that you are referring to contactors in the rotor circuit. The total circuit is generally know as the starter.
I assume that there is a number of resistor stages, probably star connected. as each contactor closes, more resistance is shorted out reducing the amount of resistance in the rotor circuit, with the "high speed" contactor shorting the total rotor circuit.

Typically, the rotor current is similar to the stator current, depending on the voltage ratio between the stator and rotor. If the rotor voltage is equal to the stator voltage, the currents will be similar. (The voltages are quoted on the motor nameplate.) The frequency of the rotor current can be very low and this will read incorrectly on many clamp meters.

Best regards,

#4 Sparkles

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Posted 11 December 2005 - 09:02 PM

Thanks for your reply. Would a true RMS clamp on meter give me the true amperage reading?

You're right when you said contactor instead of starter. Actually it has a starter w/overloads for the stator winding and 2 contactors for the rotor windings.No contactors energized=low speed/starting-one contactor=med speed-two contactors= high speed. You're correct with two contactors energized all the resistance is out of the circuit.

#5 marke

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Posted 11 December 2005 - 09:12 PM

Hello Sparkles

It depends on the frequency bandwidth of the meter. If the meter can measure DC, it should be OK. If it is AC only, it probably will give errors.

Best regards,




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