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Unread 06-15-2003, 10:59 PM
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PWM and burning out motors

This is for a project not related to FIRST (and possibly related to the team listed in my signature), but I was hoping that there would be someone out there in the FIRST community who might be able to help out with a problem my team is having.

We are using very expensive motors that we purchase from overseas, and they are rated at 6 volts. However, in past years, we have driven the same motors (with different gearheads) at up to 13 volts. We have never (not once) burnt out a motor in previous years at these voltages. In previous years, we used PWM at 4000 hZ and ~13 V peak to control the speed of the motors. This year, we are also using PWM, but at 400 hZ and ~15V peak.

We are burning out motors (we've burnt out about 10 so far) and have tried everything we can think of to keep them from burning out. They burn out (we have verified the brushes get fried or broken off) seemingly at random -- not necessarily due to excessive use -- and the motors often are not even that warm when we touch them after they malfunction. We have changed our control algorithms to reduce the maximum duty cycle to less than 40%, and even that does not stop them from burning out.

I think it is the PWM frequency that is too low, and as a result the 15V spikes are being applied too long and the very tips of the brushes are heating up VERY fast during this time, causing failure at the very tips. But I can not find any books or web sites that treat PWM any differently than an average voltage.

So my question to you engineers is this: Have you seen anything like this, and do you think raising the PWM frequency back to 4000 hZ would solve this? Our electrical engineers are hesitant to switch to 4000 hZ because they say performance drops as the PWM frequency increases... But I think a small performance drop is a small price to pay if the motors stop failing. The lead time is 6-8 weeks on new motors, and with the competition 2 weeks away, we can not afford to burn any more out!

Thanks very much!
- Patrick
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Unread 06-15-2003, 11:22 PM
sanddrag sanddrag is offline
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By a system of elimination I have concluded the following:

Your motors worked at 4000Hz. Your motors burnt up at 400Hz. Problem - too low of an operating frequency. Solution - move back to the higher frequency.

Now I don't know why the frequency should matter but apparently it does.

As for the performance, it seems to me that it would be better as the frequency increases with a smoother throttle response.

PS. What's so special about these motors that they are so expensive and from overseas?
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Unread 06-16-2003, 01:45 AM
Kevin A Kevin A is offline
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What if you put a capacitor in parallel with the motor? Smooth out the spikes. Maybe throw an ammeter in series with it to catch voltage spikes.

HTH
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Unread 06-16-2003, 08:10 AM
Dave Flowerday Dave Flowerday is offline
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Quote:
Originally posted by sanddrag
By a system of elimination I have concluded the following:

Your motors worked at 4000Hz. Your motors burnt up at 400Hz. Problem - too low of an operating frequency. Solution - move back to the higher frequency.
Except that they changed both the frequency AND the voltage. 2 variables changed, so it could be either that's leading to the failures.

However, I would also suspect that the higher frequency is contributing more to the problem than the voltage increase. Unfortunately, it sounds like you were just getting lucky with the motors when using the old parameters, if changing them by such a small amount has such bad consequences. If it were me and I didn't have any more spares left, I wouldn't take any chances and would revert to values that worked before. Ideally that would mean changing both frequency and voltage back to known good values.

Quote:
Our electrical engineers are hesitant to switch to 4000 hZ because they say performance drops as the PWM frequency increases
Sounds to me like performance is non-existant with the current settings.
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Unread 06-16-2003, 04:40 PM
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Quote:
We are burning out motors (we've burnt out about 10 so far) and have tried everything we can think of to keep them from burning out. They burn out (we have verified the brushes get fried or broken off) seemingly at random -- not necessarily due to excessive use -- and the motors often are not even that warm when we touch them after they malfunction. We have changed our control algorithms to reduce the maximum duty cycle to less than 40%, and even that does not stop them from burning out.
It could be that the brushes on this motor are crap. Some motor's brushes are very fragile. The drill motors are one of these motors that come in mind. One thing which may do it is to support the brushes. Btw what motors are you using?? I may be able to hook you up with something that may be a good alternative.

Quote:
Our electrical engineers are hesitant to switch to 4000 hZ because they say performance drops as the PWM frequency increases
According to my book PWM frequency is an finicky thing to answer. Higher frequency is less likely to cause a mechanical vibration in the motor. Lower frequencies may cause your motors to vibrate or whine.

Here is an equation for determining PWM frequency:
2(pi)(f)(L) is much greater than R
where f is switching frequency
L is armature inductance
R is armature resistance
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Last edited by Adam Y. : 06-16-2003 at 04:49 PM.
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Unread 06-18-2003, 12:36 AM
Random Dude Random Dude is offline
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Well the main problem I see is that you are applying 2.5 times the rated voltage of the motor. Even if you use PWM to vary the speed of the motor the excessive power is still being applied to the motors.

I'm willing to bet that if you increase the frequency to 4000Hz you will still have the same exact problem, even if you limit the duty cycle to < 40%.

My reasoning:

At both 400Hz and 4000Hz with 40% duty cycle the .4 seconds of every 1 second (obvious but bear with me)

The reason the motors keep burning out is overheating due to the excessive power passing through them.

Now all that changing to 4000Hz would due is to apply the same power for more, shorter lengths, with pauses in between. At 4000Hz those "pauses" are ~150uS (.00015 S). That period is most likely too short to effectively radiate any large amount of the power just applied (Exact numbers would require the spec sheets of the motors)

However, even if the motors did happen to run at 4000Hz <40% duty cycle, you would definitely begin to run into problems again once you increase the duty cycle range toward 100%.



Summary:(I.E. the short version)

Decrease the voltage back to 13V.

Even that is technically pushing the limits of the motors but you have experience that they work with that voltage, so...
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Unread 06-18-2003, 09:42 AM
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Quote:
Well the main problem I see is that you are applying 2.5 times the rated voltage of the motor. Even if you use PWM to vary the speed of the motor the excessive power is still being applied to the motors.
Hmm... that could be the problem or it couldn't be. It is a common practice to overvolt a motor. If you are running these things near stall then in all likely hood you will destroy them but if you are running them near no load then there shouldn't be a problem.

By the way what are the specs of the motors you are using?
That would make everyone's life easier.
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Unread 06-19-2003, 02:30 PM
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Patrick,
Where PWM frequency comes into play in this application is that there is a distinct correlation with the amount of time that current is being delivered by the brush and how often the brush is moving to a different commutator segment. If the PWM frequency is such that the controller goes through two or more transitions per segment then duty cycle is close to average power delivered. If the PWM frequency is such that less than one transition occurs per commutator segment then the duty cycle and average power differ greatly. I think this is what is happening to your motors. With the higher(than rated) applied voltage your are sending very high currents to each segment that are enough to get the motor turning but because the the next commutator in line (or the next two or three) may not receive any current at all, all power delivered to the motor occurs over a very few windings. You can experiment to verify this. Measure the electrical power input (volts times amps) and measure the motor mechanical power out and I am sure you will see a big difference.
The lowering of PWM frequency and the raising of the voltage both contributed to the motor failing.
Now you did not mention what the commutator looks like in the failed motors, but I am willing to bet it is all dark and burned, and probably has lot's of pitting as well. Let us know what you find.
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