DC motor debate

HI All,
A friend and I were having a debate yesterday. Here are the details:
We're using a fractional HP 12vdc gear motor to run a theatrical special effect. The entire triggering of this effect lasts about 2.5 seconds and occurs once a day, max. Cost, time, and mounting space pressures have led to us using a 24vdc power supply for this motor. We turn the motor on and off using a PWM DC drive, and we never have the motor run even near 50% PWM duty cycle.

If anyone's interested, here's the link to the drive we're using:

http://www.dimensionengineering.com/products/sabertooth2x25

My friend is maintaining that we will eventually damage this motor, as it is seeing double its rated voltage, albeit at a 50% or less duty cycle. I maintain that it can't possibly damage the motor, as 24v is not likely to damage the motor's insulation, the PWM doesn't exceed 50%, and the operation time is only seconds.

Further, isn't a motor inherently an "averaging" device, and will run at the speed as determined by the voltage and the PWM duty cycle? In essence, if we were to run the motor at 50% on 24v, we'd see the same behavior and RPM as if we ran the motor at 12v at 100% duty cycle, correct?

I am willing to concede that at some point, the iron in the motor may saturate and heat generation would increase, but I don't think this is happening here (or if it is, it doesn't matter here).

Which one of us is right? Are we gambling and is the motor likely to die an imminent fiery death, or should we not worry?

PS: replacing the power supply isn't really an option right now, as space considerations allow only for the specific PS we have right now and there are other components that require 24v.

Thanks for helping settle this debate!

-rpoet

rpoet; said:

I maintain that it can't possibly damage the motor, as 24v is not likely to damage the motor's insulation, the PWM doesn't exceed 50%, and the operation time is only seconds.

Click to expand...

+1 here. I have never seen insulation rated at less than 50v.

rpoet; said:

Further, isn't a motor inherently an "averaging" device, and will run at the speed as determined by the voltage and the PWM duty cycle? In essence, if we were to run the motor at 50% on 24v, we'd see the same behavior and RPM as if we ran the motor at 12v at 100% duty cycle, correct?

I am willing to concede that at some point, the iron in the motor may saturate and heat generation would increase, but I don't think this is happening here (or if it is, it doesn't matter here).

Which one of us is right? Are we gambling and is the motor likely to die an imminent fiery death, or should we not worry?

PS: replacing the power supply isn't really an option right now, as space considerations allow only for the specific PS we have right now and there are other components that require 24v.

Thanks for helping settle this debate!

-rpoet

Click to expand...

It's not going to be the voltage doing any harm in your case, it is the heat generated by power. You are limiting the power and the duty cycle is very low (not only the PWM duty cycle, but the motor run/stopped duty cycle). It should be fine. Of course now that you end up winning the debate, some mechanical thing will go awry and the smoke will escape...to be safe, be sure you have time delay fuses just a tad larger than the motor FLA.

What current pulse can the motor take before it starts to de-magnetize?

Would be nice to know some small details such as starting current,
Ramp up of voltage, steady state voltage and current both starting and steady state.
Torque rating of motor and torque demand of load would sure be handy also.

Current is the most important parameter since it is proportonal to torque.

I would venture to say that even driving it off a battery at 24 volt for less than seconds once a day is not going to destroy it soon.
My guess is that at 50% setting on PWM power supply you are OK
Dan Bentler

The universe has a way of laughing at me. Last night during rehearsal, a brownout cooked the power supply running this effect. Replaced with a spare. Lucky all we lost was a 240w 24v power supply. We're looking into protecting at the panel with a 3ph. surge suppressor. At this point the motor is the least of my worries.

Thanks for the help, everyone!


-ropet