Slip Ring Induction Motor- VFD Starting

One of the possibilities is that the overload protection device was set incorrectly.

It sounds like it would be better to junk the motor and purchase a replacement rated for inverter duty.

Depending on the motor power, purchase a newone designed for converter supply might be the better solution...

"...We put VFD's on at least 6 slip-ring motors with liquid resistors--"flow matcher" brand....."

Heh, Flomatcher. They are still around believe it or not. I went in there a few years ago to try to talk to them about soft starters for their stator power, I don't think there was any technical person in that building that was under 70 years old. Those guys make a good product, but they are milkling the limits of that technology to the bitter end.

And they continue to insist that VFDs are an abomination!

Jraef,

Good evening to you. Those flomatchers had a lot of mechanical components that all had to work together for the drives to run. However, they are the quietest variable speed drive that I have ever seen and they are infinitely variable in speed.

Mechanical components, let me count the ways--

1. slip rings, brush holders, and brushes (3 sets each),
2. salt solution tank with three electrodes,
3. air compressor and bubbler for measuring liquid level,
4. salt solution in sealed storage tank, pressurized by the air going to the bubbler,
5. radiator and pump to keep the salt solution from boiling, and
6. piping and filters for all the above--had to resist the salt solution corrosion.

Plus the normal electrical components for all of this, including safeties, starters, etc. And don't forget, mandatory regular maintenance (at least 1x per month if not weekly or daily).--John M.

looks like someone with enough knowledge to be dangerous decided to adapt the wrong control in the wrong application. your slip ring is a very high torque motor that requires specific resistances to start and run properly, a VFD simply is the wrong choice for this application, you were offered an excellent link in another response I suggest you read and UNDERSTAND it before throwing away more money and burning up a perfectly good motor due to ignorance

Fredski,

On this one, I have to strongly disagree with your concern. Although you raise a very valid point regarding loads that are of high torque and/or high inertia (at least for soft starters, but not so much with VFD's), the application that is being discussed in this thread is a pumping application which is neither high starting torque nor high inertia. I read many (most) of the references posted in this thread and in them the application of a VFD for booster pumping to constant pressure or lift station pumping is considered a very good VFD conversion from a wound-rotor motor with liquid resistor speed control on the secondary. As I noted in another post, I have done this very application with zero difficulties a number of times.

Thanks--JMM

If your motor is an inverter duty it can withstand the new challenges presented by a vfd, but if it is not, it may fail both electrical and mechanical in nature. Regarding insulation, because of the type of output generated by vfd, temperature rise in the winding may increased which causes great stress upon insulation. For every 10degree celsius rise, the insulation is Reduced to half and if it rises too high, the motor will overload and burned-up in a short time . What have actually burned-up your stator is the positive harmonics which rotates in the direction of the rotor.

Acceptable reply in general, but some confusion bringing the insulation resistance concept to this discussion.. it is not helpful for evaluate the real problem...

The most common problem in applying VFDs to older motors is that whoever considered this concept got cheap at the last minute and decided to ignore the issue of voltage stresses added to the winding insulation. Newer motors now take this into account, but it was never a concern on older ones. It could have been effectively mitigated at the time of the initial installation, but would have cost more money.

For some reason people do this a lot. It's like buying a classic car that needs Premium gas, but you get cheap and put in Regular, then the engine burns up. It's not the fault of the engine or the gas, it's the person at the pump.

If you are having the motor rewound, insist that they use inverter rated magnet wire, that is a minimum first step now. I would also recommend purchase of a sine wave filter on the output of the VFD, or at least a motor terminator to absorb any voltage spikes.

agreed. This might help those that would ike a more detailed explaination explainationhttp://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/motor_tip_sheet14.pdf

We have had problems from 3 phase, slip ring motors recently (stator/rotor winding damage) switching to Inverter duty motors is probably not an option. We have also damaged at leas one drive, possibly a second. Motors are 270kw, 480V, 3phase. Line reactors used @ motor circuit disconnect sw. Any thoughts would be appreciated. Im no way a master on this subject. Thanks.

Many people ignore old threads like this, you should start a fresh one, and in it, describe the damage you speak of in more detail both to the motors and the VFDs, and describe the application. Give us some clues to work with.

Thanks for the reply. I figured on that, and I attempted to start a new thread, chock-full of details, data, info, and such. Either the server was having problems or maybe I was doing something wrong, either way I couldnt post the thread. I will try again later.

Inverter Duty motors - how are they mentioned in name plate ? As I don't know , request for assistance , please .

What is the application and rating of motor and VFD? Slip ring secondary is shorted or what?

I also implemented VFD in slip ring motor for crane application