I just bought an old excello mill that has slo-syn stepper motors. rated at 1.3V 20 amps. Typically you can measure ohms on the coil to determine the center tap however measuring these I get .1 and eventually 0.00 ohms.
I made a stepper driver and I am able to drive the motor but only at 100 steps per second maximum. Of course the tiny wires on the breadboard don't allow much current so that is not surprising.
Looking at the scope I used 2 12 volt motorcycle batteries in parallel and a 1 ohm power resistor, can't believe the voltage on the center taps goes to about 1 volt, these things must take a huge power supply.
Question, should I see at least some resistance when measuring these coils? I connected the motor with some diodes and a capacitor and a hand drill produces about 20 amps using a 2 ohm resistor the scope shows 9 to 12 volts sine wave while spinning (voltage over resistor) so the windings don't seem to be shorted. Other steppers (new ones) I see resistance when measuring but these 25 pound 1100 oz beasts show little or no resistance.
Also when I did find the center (I think) there is no difference in current when I connect ground to both of the windings on that center tap, i would think the current should be double when both coils on one phase are connected, funny thing is I get the same current connecting any of the three on either phase.
I guess I will make a driver with some beefy wires and give the 20 amps a go but I still would like to understand why I see 0 ohms and the stepper can generate so much power as a generator and does step at 100 steps per second full steps with some amount of torque, A proper driver may make these work but they may just become generators for a windmill or I may want to sell them on ebay but I wan't to know if they actually are any good first.
I have 4 of them.