Efficiency of wound rotor vs VFD 2

[Barry1961]

I have some preliminary information about a ball mill that is having problems with the input of the drive gearbox failing every couple of years. The last time it failed the input pinion bearing housing had cracked. The assumption is that the motor is starting “hard” and inducing a moment into the input of gear box. A ball mill similar to this one had the same problem which was solved by the use of a fluid coupling between the motor and gearbox.

The motor is a 4000hp WRIM using an old liquid rheostat. I suspect that the rheostat may be overfilled or not functioning correctly. The push now is to add a fluid coupling for a soft start and leave the rheostat in place. The ball mill should be a fairly constant load across its speed range.

Considering the cost involved would it be worthwhile to change to a VFD and put a fixed resistor on the motor? What if the motor is running at half speed?

I know that a VFD is more efficient than a WRIM but I don’t know by how much. I would like to get a feel for how efficient the WRIM is. I would assume that the WRIM is less efficient at lower speeds when driving a constant load. I would also guess that the WRIM motor would have to be oversized some to produce a smooth soft start.

Sorry that I don’t have all the details yet.

Barry1961

 

[jraef]

In the format you have now, the VFD would be far more efficient than the WRIM run by a standard liquid starter system if you are not running at full speed (shorted rotor). At reduced speed, the slip energy coming from the rotor is given up as waste heat. You can get a slip recovery system for the WRIM, and that would in fact make it MORE efficient than the VFD I believe. Running at full speed, the WRIM would be more efficient because of the minor losses in the VFD, 2-5%.

A side concern I have is that the existing system was starting too hard? It sounds to me as though it is not a case of overfilling your liquid starter, but of not ramping it proprly. You should be able to start off with absolute minimum torque using that system, that is it's chief benefit. If someone has just filled the tank to a preset level and is turning on the stator, of course you will get a starting torque surge. I would consider checking into your liquid rheostat system and see if it is functioning and/or being used properly.

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[Barry1961]

Where do you think the break over point would be as far as motor speed?

If they are running at full speed keep the WRIM.

If they are running at 90% speed?

Barry1961

 

[jraef]

Any reduction in speed means waste heat in the WRIM, but I suppose you are correct in thinking there would be a crossover point somewhere as the losses in the VFD become greater than the slip losses. Tough to quantify that however.

This outfit makes both liquid rheostat controllers and slip recovery systems.

http://www.flomatch.com/

In looking at their website at their Wattmiser slip recovery system, they discuss this issue and mention some sort of a graph, but don't show it. I suspect that it is in a paper brochure, and you must email them to get a copy. That may be a good resource for you.

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[aolalde]

Check the liquid rheostat electrolyte density. The resistance could have changed with time and the optimum external secondary resistance departed from the original setup.