ECN Forum Forums General Topics of the Trade General Discussion Area dc motor

what possible checks could be carried out to see if a 1HP dc motor is functioning correctly?
The motor is fitted to a running machine and is tripping the control circuit on over current.
As the brushes where creating a lot of sparks i have cleaned the commutator which seemed to cure the over current problem, until the machine had been used a couple of times then the same problem occured.This did not make sense as i thought the higher ressistance between brushes+commutator connection the lower the current draw?
The over current device is set to 7 amps.
The belts driven by the motor are all set up fine.
ps what am i reading if i use a ac clamp ammeter on a dc source?

With the clamp AC ammeter, you'll read a spike when the DC current switches on, and another when it switches off. In between, any reading you get will be caused by the ripple of the DC current, and will not be accurate in the least. No ripple - no reading.

Radar

How did you 'clean' it? If you used an abrasive of any kind, such as wet & dry paper you may have damaged the copper segments, and/or left metal debris between the lands, or created 'flats' and burrs. The most you should do is use a solvent to gently lift out any carbon dust.
Re-machining a commutator to compensate for wear is a very skilled job, requiring a first class machinist with specialist knowledge and tools to recut the insulation between coppers to the correct width and depth after accurate and concentric turning. That would cost more than a new motor in such a small size. Most likely you need replacement brushes.
( I assume the armature windings are not damaged from your saying the 'cleaning' worked for a few operations ).

Alan

As said before the commutator segments have to be very smooth otherwise excessive sparking will occur. It's likely that some sparks jump across from brush to the steel frame of the mmotor and cause it to trip.

The correct type of brush is also very important, hard / soft ness and copper content in the carbon.

For current measurement the best is a DC analogue ampmeter wired in series. Lot more accurate and fluctuations in load can be better observed than with clamp on dc meters.

Also do an insulation test between armature and shaft, and field windings and frame.

Not sure what voltage this motor is but start at meggering at 250 volts dc first for a 110 volt motor.
If it's a low voltage motor like 24 volts dc then a testlamp and mA meter in series to measure leakage, may have to used.

The lamp is there to control the currentflow in case of a short to earth.

The question of the DC motor the part i am little curious here which type of DC motor you are talking about if series or shunt wound or compound or perment magent motor [ shunt wounded is most common ]

if you saying if get too much sparking you have to check the feild current setting if reading low on feild current it will cause the armture to draw more current than it should be doing .


best thing is double check the feild / armture voltage it should the same as nameplate stated.

for smaller dc motor [ less than 5 hp ] the common voltage will be one of few i will listed here ::

100 v F/ 100 V arm
100 V F/ 200 V arm

80 V F / 80 V arm
160 V F / 160 arm

and i am sure there is about half dozen more listing but i have to get the listing out and print it here

as i will state again please check the name plate and controller very carefully

Merci , Marc