Part Winding operation of 3ph motors

[sacem1]

We have several machines which have 20+ HP motors used in machinery which is suposed to need all that power only on very special ocasions, most of the time the input power requiered will be less than half the motor power.

As a result from this we have to start and keep operating high HP motors with low power usage. We have added a second motor usually a 7.5HP conected to the back shaft of the 20HP and we start only the 7.5 and use the 20HP only as a intermediate shaft, when power is needed we simply start the 20HP and let the small 7.5 turn without any power conected, this works but sometimes its not easy to fit a second motor to the back of the main motor due to space limitations.

We start the 20HP motor using only one of the two windings that it has for each current phase and then switch to full operation after the motor has started, but could we just leave the motor running with the starting winding on and the second one with out power indefinitly.

The 20HP with only one winding connected is supposed to develop 45% of its torque and give 9HP at full speed wich should be more than enough to operate the machine and we could really have two motors in one (20HP/9HP) with full windings and partial windings conected.

Has anyone used this or has any experience with it, please share your experience with us.

Thanks

SACEM1

 

[jraef]

sacem1,
If it's any comfort to you, I have done a similar thing with large horizontal shaft impact rock crushers. Essentially, when not crushing they are a giant wind pump, lots of inertia, but little HP required when unloaded. Because our shop power was limited, for testing purposes (no crushing yet) we would run them either with a part winding motor using only the start winding (as you are proposing), or a Y-Delta motor using only the Wye winding. The motors accelerated to full speed in either case, but realize that they had virtually no load on them so the loss of torque was insignificant to us. If you can truly run some parts of your lathe operation at 10HP and only need the 25HP when making deep cuts for instance, it might just work out for you.

I would re-read Marke's post however. It will not be enough to just have separate OL relays on each winding, you must ensure that the current through the first (low power) winding configuration does not exceed the current through it if running in full power mode. Some empirical data collection will be necessary since I doubt even the motor mfr. will have the necessary values. So run the motor at as heavy a load as you can in full 25HP mode, but measure the maximum current draw on only the 1st winding step to be sure. The big risk I see on this is in the operator knowing when the requirements are going to need 10HP or 25HP, or worse yet, 10HP vs 11HP. You may want to also consider adding a shaft power monitor similar to this Emotron shaft power monitor link. Wire it to automatically kick in the remaining winding if you ever exceed a set shaft power requirement.

All that said, waross' last point is still valid. If you are going through the trouble to experiment, you might want to consider this idea and compare your power consumption with the 25HP mode on light usage. You may find that any savings might not be worth all the hassle in the long run.

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