Problem with knee mill blowing fuses

We have a knee mill that keeps blowing a fuse on one leg of the 3-phase breaker box. One day it blew the fuse out of the blue, not from overworking or misusing it. Now when we replace the fuse and try to turn the spindle on, it kicks and shudders and tries to spin (like only 2 of the 3 legs are hooked up) but it won't rotate and then blows the fuse.

Anyone have this kind of experience before? Possibly a chip laying on some windings or some such? Any quick and dirty resistance readings I could take to diagnose or rule anything out?

Disconnect the wires from the circuit breaker, then use your ohmmeter:

check from each leg to ground. Turn the Fwd/Rev switch "on" and check again. Should measure "infinite" in both cases.

Check from leg to leg to leg (switch "off"). Should be infinite.

Check from leg to leg to leg (switch "on"). Should be some relatively low resistance, depending on motor Hp and whether it's wired for 480 or 240 volts. Resistance should be the same for all 3 measurements.

If you get different readings on one of the measurements above, remove the cover from the switch and measure the motor leads directly, leg to leg, to leg.

If it's a drum type of switch, usual on a knee mill, my bet is that the switch has a burned contact and the motor is "single-phasing", which will blow fuses/pop circuit breakers.

+1 on the single phasing diagnosis and the inspection of either a cooked or loose contact in a drum switch or mag.

Once a winding shorts in the motor you need a rewind. If the tough insulation is intact, chips and dirt won't affect them unless they cut or penetrate this insulation.

If the controls are ruled out, the motor can be megged which should easily (usually) find any defect in the windings.



Stuart

More than likely, it is a switch problem.
JR

Thanks for the suggestions, guys. Those tips will give me something to inspect tomorrow.

Finally got around to doing some resistance measurements, and everything seems fine as per jdunmyer's post. Measuring from leg to leg to leg with the switch on shows the same low resistance value. The switch isn't bad either - I did the resistance measurements upstream of the drum switch.

I would take the motor to a motor shop and have it load tested. It sounds like one leg is going to ground when you power the mill up.

The motor may read OK with a Ohmmeter but when hit with full voltage under a starting load a winding will short to itself or to ground. A megger 'may' see this problem while the motor is still on the machine but it may not.


Stuart