another slip ring question

[mickeyrench]

Is there a slip ring made just for vfd use? our vfd is about 200' from the motor @ 480 volt about 1 hp not really sure . The slip ring had a hole burn in the track on one of the phases. also it was installed with thhn type wire.
Is there any maintenance needed to the slip ring. I was thinking of the way a commutator on dc motors was cleaned.
Thanks for any help .

 

[Jraef]

Slip rings are used on Wound Rotor motors, as well as Synchronous motors. They are also used on standard Asynchronous induction motors that are attached to rotating machinery where the motor is part of a rotating element and the controls are stationary. Please describe your application ion more detail.

 

[mickeyrench]

It is used to control a motor for a trickling filter. I don't know the exact size or speed of motor. Is there a slip ring rated for vfd ?

 

[Jraef]

It is used to control a motor for a trickling filter. I don't know the exact size or speed of motor. Is there a slip ring rated for vfd ?

Again, it depends on what the slip rings are for. If it is a Wound Rotor or Synchronous motor, you would not put the motor power through the slip rings at all.

The slip rings on a Wound Rotor motor are there to simply add resistance to the rotor circuit. There are some specialized WR motor systems that PULL power from the rotor and put it into a VFD-like system to recover energy from the rotating load, but technically, the PWM of the VFD is not going through the slip rings.

In a Synchronous motor, the slip rings are there to feed DC excitation power to the rotor. The device that is providing that excitation power is often something essentially the same as a DC drive, so maybe you are THINKING it is a VFD, but it is not. That is low power DC being fed, and it is not a problem for the slip rings; they are made for that.

If it is because the motor is attached to something in the trickling filter that is rotating and the motor is fed from a stationary point through the slip rings, then that is a different matter. In general, an application such as this is not a good candidate for powering that motor with a VFD going through slip rings. The output of the VFD is high power pulsed DC in a PWM pattern, very similar to a welder when you start putting it through gaps, such as slip rings and brushes. It tends to deteriorate the connections very fast. You would be better off trying to mount the VFD on the rotating part of the machine along with the motor and putting the Line power TO the VFD through the slip rings.

 

[glene77is]

Jraef,

The OP certainly has a problem project.

Your comment on this is a good reason to read through these threads.
Your insight is not found in textbooks.

 

[Besoeker]

Again, it depends on what the slip rings are for. If it is a Wound Rotor or Synchronous motor, you would not put the motor power through the slip rings at all.

On a wound rotor motor, the sliprings generally* have to handle full totor current continuously. They also have to be able to withstand standstill rotor volts so in a sense, they have to be capable of dealing with rated motor power.
We make variable speed drives for wound rotor machines I just looked at the data on one we did not so long ago. Rated motor power is 2.3MW. Standstill rotor voltage is 1628V and roror current is 815A. I know you can do the arithmetic....:smile:

The slip rings on a Wound Rotor motor are there to simply add resistance to the rotor circuit.

Put otherwise, to allow external starting resistors to be included in the rotor circuit to limit starting current.

There are some specialized WR motor systems that PULL power from the rotor and put it into a VFD-like system to recover energy from the rotating load, but technically, the PWM of the VFD is not going through the slip rings.

I'd have to differ with you there, but that's off topic.

In a Synchronous motor, the slip rings are there to feed DC excitation power to the rotor. The device that is providing that excitation power is often something essentially the same as a DC drive, so maybe you are THINKING it is a VFD, but it is not. That is low power DC being fed, and it is not a problem for the slip rings; they are made for that.

Yes, it's low power, but it's also often fairly low voltage so fairly high current. The last one we did was rated at 540A and 110Vdc for a 5MVA machine. But I take your point about it being essentially the same as a DC drive. In fact, we use the same main control card and most of the same power electronics.