[atunguyd]

Hi people I need help.

I have a mill that has what looks like a KBIC-240 speed controller in it. Lately the mill motor has been playing up and getting really hot. Also when I try to run anything over 70% speed there is serious arcing coming from the motor's brushes the arcing lights up the roof of my workshop.

I took the motor into a motor specialist who told me that the armature was ok but it needed new bushes, needed the commutator to be skimmed etc. Long story short I finally got the motor back but the arcing is still there so I am feeling somewhat burnt at the moment.

I opened up the speed controller today and I cant seem to see anything wrong with it, I removed and tested the SCR's and they seem fine.

One thing that bothers me is that when I put the output of the controller on my oscilloscope I see that when I approach the 70% and above power figure the waveform has as much negative voltage on it as there is positive which concerns me. At lowest setting I have the normal wedge shaped waveform peaking at about 190V but the bottom of the wave drops to about -19V. At full power the positive wedge is still about 190V and wider, but there is a bottom almost square wave that is about -170V. I have been doing these tests with a 60W lightbulb connected to the output of the KBIC so this negative voltage is not back EMF from the motor.

There seems to be a few experienced people on this board when it comes to the KBIC controllers so I was hoping to bounce this off them. For starters is this waveform normal? If not where should I look on the board to fix it up.

I am currently living in South Africa so getting another controller is costly and will take time.

Thanks in advance

Anthony

[Al_The_Man]

Certainly sounds like you have an issue with the controller, I have never 'scoped the output, but it almost sounds like you are getting an AC signal on the motor at that point, but if one of the SCR's were mis-firing it usually would cause a short circuit and blow either a diode or an SCR?
Is there a Baldor rep near you? They sell the same controller, relabeled .
Al.

[atunguyd]

Nope no Baldor Rep but there is these guys although they are on the other side of the country.
If it comes to the worst I will get a replacement controller I am just hoping someone here can confirm that the waveform I am seeing is incorrect first. I don't want to go and buy another controller if this one if working.

I will recheck this morning that I am not doing anything stupid - I am running the controller off the mains and my scope off an isolation transformer.

[atunguyd]

Ok small stupidity on my side - I had my scope set to AC coupling - when I changed it to DC I now get my waveform all above the zero line. What is somewhat concerning is that I am getting a peak voltage of 312V which may go a long way to explain the arcing and overheating of the motor. Can anyone confirm if this 312V is normal (my mains is 240V AC).

If it is not I am not sure which component is causing this higher value

[Al_The_Man]

The peak rectified 240vac will give you ~340vDC.
Minus the drop across the armature resistor etc, 312v peak is around what I would expect.
Other causes of sparking and high current are magnetic field loss or Earth leakage, sometimes caused by carbon tracking at the brush holder.
Al.

[atunguyd]

Thanks, so then the controller is probably perfect and my problem lies with the motor? I will take it back to the guys that "serviced" it and make sure that they tested it correctly under load and with the correct voltage applied

[Al_The_Man]

It is a job to tell definitively without further testing, but I would get them to test the motor on something approaching a purer DC if possible and also give it a torque test.
As well as insulation test to frame.
Al.

[wizard]

Personally I'd look at the motor a bit more. The problem is you are describing some rather intensive arcing, unless lighting up the shop is due to working in the dark. I've found KBIC controllers to be rather rugged, they can fail of course, but I haven't seen a controller caused failure like you are describing. In fact I've seen machines go through dozens of motors before a KBIC controller will have failed.

There are some comming problems that crop up with brushed motors related to those carbon brushes. One already mentioned is arcing to ground over the brush holders. Carbon can also embed itself into armature windings basically ruining the armature requiring replacement. In some cases I've see carbon so built up in the motor that it basically shorts electrical connections to ground.

As to your motor repairer I have to wonder if he knows what he is doing, I'd try another shop if at all possible. However it might not be worth your effort if this is a smallish standard frame motor. Sometime it is cheaper just to buy and install a new motor. Obviously the size of the motor impacts replacement decisions but a new motor would quickly remove one variable.

Another consideration is motor load. Are your spindle bearings OK? Heavy load can lead to fast armature and brush wear. This is where an ammeter and a log book come in handy if you can keep track of historical load and no load current draws. Your motor could simply have failed from normal use but without understanding the spindle condition you can't be sure.