12v DC Motor Stalling

[dalhug]

Hi,

I have a copal 12v dc motor that is driven by an allegro A3968slb IC. My load supply is 7.5 volts.

My problem is occassional(1 out of 200) I will find a motor that will spin up initial and slow down then stall. My PWM on the IC is set to about 63Khz. It almost looks like the IC is locked into some kind of feedback loop. I can disconnect the power to the motor and reapply it, but there is no change. If I replace the motor, all problems go away. Also, a motor would typically consume about 30-40ma, 50ma at startup. A stalled motor that is then power with a regular power supply needs about 200ma to start up again. Then it is fine until I put it back into my regular circuit. I changed the PWM resistor to a smaller value to decrease the frequency by a third and that appears to solve the failing motors.

I'm not sure yet if this is a sure fix across the board. Can someone explain what is happening. Could this have something to do with coil inductance?

 

[itsmoked]

MY guess is that you are having inductive kickbacks that are damaging the windings. Have you put an oscilloscope on things?

Other's here will no doubt have more suggestions.

 

[dalhug]

I've attempted scoping the motor and taking current measurements. The signal looks clean for a while then start seeing noise spikes. On a stalled motor I'll see 2 volt spikes with a steady state at 0v. The frequency is at 63khz which is the frequency of my pwm. I'm thinking that this is the pwm trying to start the motor then shuts down.

After I had changed the resistor to lower the PWM frequency I had ran one of the bad motors. It started up where previously it had stalled. Well, after a while this motor will still stall leading me to think that there really is something wrong with these motors. Any ideas on how to determine if they are damaged?