A Reminder for 20D motors & others: RIPPLE CURRENT & brushes

Hi everyone,

In the interest of public information, I wanted to warn people to make some precautions when using the 20D motors near [or past] their limits. Running a DC motor near it’s stall current is often past it’s limits, but dealing with ripple current might improve your ability to edge close to the limits.

[size=150]Short Version[/size]
The motor has low quality brushes relative to the abilities of the rest of the motor (of course, one part of the equation always does have to be the weakest link.)

The effect of this is that if you’re having a low duty cycle PWM signal (for us it was 60%), at a low frequency (say, arduino’s default analogwrite which is roughly 450hz), and a higher input voltage (for us 12v), then you can damage the brushes over time. Even though you might never break the gearbox or overheat the motor, the brushes fail much sooner on this motor.

[size=150]Long Version[/size] [I wrote this as an email and decided to post it on the forums instead, so that more people might benefit from the heads up. I obviously still like Pololu and their products, this is not their fault or inherently a faulty product.]

In the course of our work we came across a few details I thought would reporting about your geared motors. We were busting a few of your 15.5D motors because of excessive torque damaging their gearbox [Edit To be clear, they busted because we were stalling them past their torque limits == bad. Eventually we realized we should upgrade to the 20D 29:1 motor, which was much stronger for our application (did not struggle nearly as much).

We ended up destroying two of those before we discovered why those were dying. I made some simplified calculations on the forum that so long as the gearbox can tolerate it, 3A would not remotely overheat the 20D motor over our 15 second actuation stroke. We also had high quality thrust washers to prevent these motors susceptibility to axial loads [they have bronze radial sleeves/bushings on both ends but not much to help axial loads].

On the second dead motor teardown, I realized the cause of the destruction of these 20D motors was the poor quality brushes they use, combined with our low frequency PWM (the brushes are seriously just a thin strip of metal). We were using 12V Vin at 160/255 duty cycle (~7.5V). Even at stall current (4A)- which we were not drawing- this 7.5V equivalent did not seem likely to immediately overheat the motor. However we were only running 450hz pwm, which likely damaged the brushes by causing a high current ripple. We could even hear this in the healthy-but-pulsating sound of the linear actuator.

[size=200]Solution:[/size]
The solution was to pick up your MCP 5A driver board, enable a current limit of 2.8A in the code [extra precaution, not inherently needed for this problem], and use a 19.6khz pwm signal to drastically remove/eliminate the ripple. The other solution could have been to add choke inductors (these are really regular inductors just called choke), if changing the PWM signal frequency is not possible in your application. So far it’s been holding up and sounding much better. The teardown of both failed motors made us realize that the brushes were overheating and warping and that’s how they died. It’s just that I didn’t notice that until the second motor that failed - because I was incorrectly assuming the windings mysteriously overheated despite my believe they shouldn’t.

UPDATE:

Make sure to make your current limit much less than 3A.

our windings overheated, though the brushes on this one look much less burnt than the other two that died. It’s not actually that our windings persay overheated. The windings have enamel that probably won’t overheat until a high temperature, but they are insulated from the stator by some generic plastic, which smooshes and melted very easily.

This is kind of frustrating. Am going to go buy a quality motor - I am rather disappointed with these.

Hello, Tomek.

I am sorry you have had a frustrating experience with our motors; thank you for the detailed feedback. Could you post some pictures of your dissected motors and your setup? We have not had anyone else complain about these motors being “low quality” or unexpectedly fragile, and we expect the quality to be high relative to comparable motors from other suppliers as we put a lot of effort into evaluating various manufacturers and working with them to improve their gearmotors.

For brushed DC motors, a general rule of thumb is that you should design your system so that the typical load is about 20% or 30% of the stall torque, which corresponds to 0.5 A to 1 A for our 20D motors. Since you are talking about 3 A, is it possible you are using them for something that is way past what one would reasonably expect them to handle? You can often subject such motors to higher loads without breaking them right away, but doing so is rough on them (both mechanically and electrically), and it will shorten their lives.

- Ben

Hi, I’ll try to respond more thoroughly tomorrow. For now I’ll try to double check my post and see if I can weed out any excess negativity from the haste at which I posted during the day. Because I do really like Pololu stuff!

I understand that the motors have limits, and I’m begging for trouble when I push them close to the stall current. That said, I would expect the mechanical limits in an ideal situation to be more closely matched towards the system as a whole, where they might fail from overtorque on gears or overheating. I as a user perhaps have misplaced expectations, because I play with brushless motors a lot, for which of course you can’t destroy the [non existent] brushes. And, it seems, when you start dealing with high power DC motors, the only dc motors I have a bit of experience playing with besides Pololu’s, they could be a different class (because those have brushes that will survive abuse and you’re more likely to overheat the windings)

In my eyes the chief benefits of a DC motor is the potential for delivering high peak torque, but I can see I am biased by how I use things. I also appreciate in email that you let me know the HP version of the 25D motors have carbon brushes [just saying it out loud in case other people have concern.]

The other downside (making it easier for me to bust the motors) is that my design doesn’t allow for a slip clutch the way that using the motor on a wheel (probably the most common use) intrinsically does.

I’m glad to hear that you do like our products! I do not think you were being excessively negative, but I do think you have some minor misconceptions about DC motors, and that is what I was trying to address.

I agree that this would be an ideal situation in some sense, and that is probably what you should strive for if you are custom-designing a motor for a particular application. However, that kind of thing is not really practical for a general-purpose gearmotor. For example, a typical situation for us is to have a single motor that works with several different gearboxes. If the gearbox has a low gear ratio (e.g. 5:1), you will be able to stall it easily with virtually no risk of mechanical damage, and it will mostly just be susceptible to damage from overheating. Conversely, if the gearbox has a high gear ratio (e.g. 500:1), it might tear itself apart long before the load puts the motor at risk of melting.

- Ben

Ah, however, in this case the motor would not overheat and die if you stalled the 5:1 motor. The brushes would die, and this is the problem I find, and it is not incompatible with the motor going over a range of gearboxes. True, some would have the gearbox brake before the motor dies, but those that don’t should die from overheating and not brush damage. The only condition I could see for wanting very weak brushes is when you’re working in an ultra-efficiency circumstance and you want to reduce the minute friction bigger brushes cause (but sooner than that you’d go to thinner laminations on the stator -which prevent eddy current losses.)

I will admit that one of my motors after implementing the high frequency PWM and accidentally jamming the motor backward for a while DID overheat before or in addition to the brushes dieing. This is possibly because the brushes were brand new- but when I took them out they had been charred. The other thing is that these motors use a lower temp plastic guard between the stator and the windings, and not an epoxy veneer that has a much higher melting point. That said, the plastic is not too uncommon in small motors (I found a funny motor manufactor quote online that sold it as the “more environmentally friendly solution” when really they’re both quite small factors in the equation.)

The takeaway for me is that I will still use these motors in high-speed applications and preferably those with some measure of a torque clutch (like wheels!). Where I need low speed torque, however, I’m probably going to run back to the stepper motors as I do think those are higher quality for a better price. The magnets are better, they have bearings instead of bronze sleeves, and they naturally don’t have brushes. Plus your DRV8825 driver board is great!

I think in your email you asked me to send some pictures along. I’ll definitively do that, I keep forgetting to bring a decent macro camera to work. I wish I had a usb microscope ^_^.

In general, the brushes are often the weakest point of the DC motor, especially if the motor has been used for a while. Not only are the brushes subjected to mechanical wear, but they also are subjected to the worst of the electrical wear as they constantly experience arcing as the rotor spins. They should be fine if you are using the motor in its intended operating range (e.g. generally keeping the load from exceeding 20% to 30% of the stall torque while supplying it with close to its rated voltage), but anything beyond that is probably hastening some kind of failure. Please let me know if I am wrong, but I think your position boils down to “why don’t they make the brushes stronger”, and the answer is that they do, on more powerful motors. If you find yourself wanting to have a load that is much higher than 30% of the stall torque for extended periods of time, you really should be using a more powerful motor (with beefier brushes) from the start. Your secondary question might be “why don’t they make the windings weaker so that they are as likely to fail as the brushes”, and I think the answer to that is that there isn’t much to be gained by doing that.

By the way, I suspect that brush failure is more desirable than failure of the windings. The former should effectively cause the motor to stop drawing current while the latter likely leads to some more prolonged runaway process as the windings melt together and the coil resistance drops, and that could be pretty bad for other parts of your system like your motor driver or power supply.

- Ben