Rubber Mill Slip Ring Motor Experience ?

Hi

We are looking at a 100Hp 12 pole 490 rpm Slip Ring motor on a rubber mill. The existing system is a resistance reversing starter from about 1930.

We are pricing to replace the complete panel with a new starter panel using a vapormatic (electrolytic slip ring starter) for the slip rings instead of a resistance bank. This we have no problem with.

The customer has also asked for a price on using a VSD on the slip ring motor (with slip rings shorted out), has anyone got experience of this ? Any problems to forsee if we use a VSD ?

The VSD would have brake resistors fitted for the quick stopping and reversing.

edit: Just to add the stator has just been rewound so there is no problem with old 1930's insualtion!

I don't see any reason why a properly selected VFD wouldn't work on this application. You would have to size it by peak overload amps, not hp or continuous running amps.

The slip rings would normally be shorted together or sometimes just a little resistance left in when operating on an inverter.

Operationally, you would get much better speed control. The slip ring motor (actually a wound rotor motor) is not really a speed regulating device but rather a torque regulator. No doubt, you have found that, even with lots of resistance in the rotor circuit, at light loads or no loads, the mill will run up to full speed. This will not happen when running on a VFD.

A word of caution. Picking the right size VFD to handle peak currents is a job best left to a good drive-motor application engineer. Conventional sizing techniques will NOT get you a successful result!

I have seen many wound rotor motors with the slip rings shorted and run on a VFD. Make sure the VFD carrier frequency is set to its lowest value, usually 2k or 2.5k. You best method of shorting the rotor AKA slip rings, is in the motor junction box.

You also DO NOT need to over size the drive. If you have a 20 hp motor, use a 20 hp drive. If you get a vector control, even open loop control, you will be supprised and pleased at the low speed torque and accurate speed control.

Leadfoot said:

You best method of shorting the rotor AKA slip rings, is in the motor junction box.

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I don't agree with this.

Surely this means that you would have to continue with brush/brushgear maintenance.

Wouldn't it be better to short the rings on the rotor and completely remove the brushgear?

krk said:

I don't agree with this.

Surely this means that you would have to continue with brush/brushgear maintenance.

Wouldn't it be better to short the rings on the rotor and completely remove the brushgear?

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That is not always easy to accomplish. Yes keeping the slip rings and brushes does potentially keep maintenance at a higher level. TO short the slip rings together, you have to remove then from the rotor and that means pulling the motor end bell off, removing the slip ring assembly and then shorting the rotor leads. The connection you make must be capable of handling the rotational forces from being part of the rotor. That might prove to be a greater problem that continuing to use the slip rings and brushes.

In my view, shorting the slip rings on the rotor is the only right way to do it.

As for oversizing the drive, Leadfoot, I simply do not agree with your judgement on that. First, a rubber mill is a batch process with short-term overloads going all over the place, sometimes even approaching the breakdown torque of the motor. You have to size the drive to cover that current requirement or stalling will occur. Second, this is a twelve pole motor so current, especially magnetizing current, is considerably higher than a four pole motor of the same hp. You have to cover that higher current requirement with the drive or, again, stalling or nuisance drive overcurrent tripping will occur.

This drive has to be sized as I described above or it will not adequately cover the application conditions.

This oversizing has nothing to do with the old practice of always oversizing AC drives because they couldn't make torque.

I know nothing about slip ring motors, but I have worked on rubber mills quite a bit. I can testify to what DickDV says about shock loads, and I can't emphasize enough the SAFETY aspect of this application.

My mills were contactor driven and would slam the contactor and motor in reverse momentarily to stop the mill when the e-stop was activated. The mill must stop within a certain distance or someone will eventually be seriously injured or killed. The mill must stop within a certain distance or someone will eventually be seriously injured or killed.

What ever you do, the drive cannot fault. The mill must stop hard and fast no matter what.

JM2cents

If you size the drive so its amp capacity is capable of delivering the FLA of the motor, including low base speed motors, that to me is sizing the drive to the motor.

I am not argueing to NOT size the drive for the job. I am stating my belief that a properly sized drive, NOT oversized just in case, will handle the job.

If the drive spends a LONG time running at the high torque levels, then a larger size is definately appropriate for additional cooling and some redundency.

A year ago we changed a 60 inch rubber mill over to a drive. It had 150 Hp at 720 rpm, we where a little concerned about the starting torque, but have had no trouble. It has worked like a dream. In sizing the drive we went by motor current and the drive says 250 hp instead of 150 hp. We used to break knockouts on this mill several times a year and haven`t lost a one since going to a drive.We have also changed several old slip ring motors on metal rollers to drives and on these applications you need the torque Dick was talking about. One of them was a 75 hp slip ring and we just shorted the rings the others where smaller. None of the operators have complained about losing power, so they must still have enough starting torque. I wish we had drives on all our mills!

Okie, that's a good point about stopping. That would mandate full capacity snubber braking on the drive as well.

Leadfoot, if you size the drive to motor FLA, then you have completely ignored the shortterm overload issues. On many applications, that would not be a problem but where there are heavy shortterm overloads, it is simply a mistake.

If, for example, you use the motor FLA and size the drive using the normal duty or variable torque rating, you will get only 10% motor overload capacity for 1 minute in 10 minutes. That's clearly not enough in this application.

If , on the other hand, you use the motor FLA and size the drive using the heavy duty or constant torque rating, the drive will be bigger and the shortterm overload will be 50% for 1 minute in 10 minutes. Still not enough in this case.

A NEMA Design B motor will normally develop around 220% torque at breakdown. At that torque the current will have risen to about 240% of FLA. If you expect to get breakdown torque out of that motor, you had better have selected a drive that can provide the 240% current, however many amps that is, for as long as the application requires that the torque be there. Note that there is no regard here for motor hp. You must size the drive for the peak current required.

That's about as simple as I can make it. I hope the logic is clear.

The Stator is 150Amps at 400Vac

If we go down this route then the rotor will be shorted out at the windings.

I will be visiting site to take a look at the old panel on wednesday, will keep you informed.

Dick - Question for you on Invertor braking

On the old system the reversing starter plugged the reverse contactor if the bar in front of the mill is hit. Apparently the motor stops dead. Will the Invertor do as good a job ?

Again, if the inverter and snubber is sized correctly, then I would expect the braking torque to be around 180% of nameplate. That plus the friction losses should give you essentially torque limit stopping which is all the motor can deliver even when plugged. More torque than that and I would have to be concerned about damaging the rest of the power train.

One other thing, 504bloke, check to see if the existing motor is belt coupled or chain/sprocket coupled to the roll mill and let us know.

DickDV,

I am used to mostly CT drives and the 150% overload ratings. I also see what you were trying to get me to understand now. When you say snubber, what do you mean DB or RC type network?

504 Bloke,

I have seen a few VFD's and instant stopping. Lots of DB were involved and reversing was not an issue either.

DickDV said:

Second, this is a twelve pole motor so current, especially magnetizing current, is considerably higher than a four pole motor of the same hp.

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Are you saying the nameplate current changes with the number of poles? That is given the same nameplate voltage and HP rating, the nameplate current changes when going from 4 poles to 8 poles to 12 poles?

Are you saying the nameplate current changes with the number of poles? That is given the same nameplate voltage and HP rating, the nameplate current changes when going from 4 poles to 8 poles to 12 poles?

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Yes, both power factor and efficiency fall when number of poles rises. Going from a 4 pole to a 12 pole will result in a current increase of approx. 15% to 20%.