Testing stepper motors

[monkeypuzzle]

I have a denford easimill 3 that has blown 2 stepper drives in succesion, i have traced the four wires for the motor on the X axis from the drive card right through to the motor and at all times i can read the resistance of the relevant pairs of coils (about 1.8 Ohm), this figure compares exactly with the other motors.
I suspect the stepper motor even though the coils test ok.
My first question being, is there a way of testing the motor apart from using the drive card.
My second question is, how easy is it to repair the drive cards myself, as i believe the first thing to go on them is the output transistors.
The stepper drive cards are Parker Digiplan CD20 drives, i would be gratefull for any assistance from your wealth of knowledge.

Steve

[Mac T. Knife]

If you haven' read about my fiasco, I finally found a coil shorted to the motor can.

[vladdy]

a coil intermittently shorted to the housing would be one possible explanation for that.. While the coils themselves may read correctly, you might want to check resistance between the individual leads themselves and the shield. I assume you have a seperate shield [or ground] from the controller board to the motor itself??

[H500]

What type of ohmeter are you using to do the measurements? Most meters cannot measure such low resistances because of lead resistance. Try shorting your probes together and see what reading you get. The 1.8 ohm might just be the resistance of your leads.

A quick an dirty test is to connect one coil at a time to a power supply through a power resistor to limit the current to about 1 amps. If the rotor locks up solidly each time, then the motor is probably Ok.

[monkeypuzzle]

Thank you for your comments re coils shorting, i have now tested for this with my wind-up Megger and that was an all clear.

I am using a digital multimeter with a lead resistance of 0.2 Ohms, my tests were more for comparing a known working motor against a suspect one, as mentioned they compare exactly.

Thanks for the sugestion re: testing, i have tested the motor with a bench power supply with a current limiter and both coils locked up solid.

I cannot now find anything wrong with wiring or the motor, so i have either an intermitant fault or in physically checking every connection i may have cured it.

I will let you know as soon as i can find another CD20 drive to test it with.

Thanks for you help. Steve

[Khalid]

Can you tell us what went wrong?