hello! it's been a while since I have posted here.

I have a PWM output driving a MOSFET IRFB7545PBF Product Datasheet (mouser.com) that is driving a small hobby style 12v DC motor (0.14 amp draw). The motor has a snubbing diode across the + and - of the motor.
The motor cycles 4 seconds ON and 5 minutes OFF at 55% power.
There are 3 of these motors in the circuit. All 3 side by side with 4" 20awg wire leads connected to the circuit board.

Issue: After a random amount of time, could be a few days to a few weeks to a few months, 1 or 2 of the motors will refuse to run. I haven't seen all 3 stop at the same time.

My confusion: All I have to do is just "nudge" only 1 of the motors and all is good again. This has happened enough times with different systems to make me curious as to what's going on - and possibly over think it.

My first thought is I need to find a better-quality motor, these are fairly cheap motors and possibly the commutators are losing contact, or maybe the internal connections are sketchy. All external connections are good, soldered and heat shrink so no corrosion at the motor contacts. Furthermore, this issue has cropped up in the past 3 or 4 moths after switching to this motor about 2 years ago and I don't believe I had this issue prior to that. So the motor itself very well could be the issue.

Or, is there some type of static charge being built up that I am discharging by touching the motor. Should I be using a small CAP across the +/- of the motors? Is there even a way to test for this? Should I be grounding the motor casing?
The issue is so random and a lot of time in between and I can't "make it happen". There doesn't seem to be an EMI issue as everything else works as it should, so maybe it is just a crappy motor.

I don't know much about DC motors so thought I would get some expert thoughts on the issue as I am most likely over thinking the problem.

Thanks for listening to my random thoughts.

 

No one here is a mind reader, AFAIK !
Rather than guess, apply a logical method, one way or another. Test the suspects one at a time.
The best way is to apply a method outlined , advice such as this https://blog.isa.org/autoquiz-logical-analysis-troubleshooting-method-industrial-process

 

4 seconds ON and 5 minutes OFF

Not sure if you've mistyped. If your statement is correct then I'd suspect a heating issue. Things will heat up quicker than they will cool off.

Rather than guess, apply a logical method, one way or another. Test the suspects one at a time.

I agree.

Is the 4S on 5M off legit? Or a typo?

 

I suspect that the sleeve bearings on the motors have dried out after two years of trouble free operation, causing them to stick on startup. What are you using for a power supply? When the system fails, I believe that the voltage across the motors is dropping because one motor is drawing a heavy stall current. That could cause the other motors to fail to start. You can verify that by measuring the supply voltage during the fault condition.
The solution would be to change the power supply to one that can deliver enough voltage stability during startup and service the bearings by cleaning and lubricating them.

 

No one here is a mind reader, AFAIK !
Rather than guess, apply a logical method, one way or another. Test the suspects one at a time.
The best way is to apply a method outlined , advice such as this https://blog.isa.org/autoquiz-logical-analysis-troubleshooting-method-industrial-process

That what I would like to do - but like I said, the issue may take days, weeks, or months to occur. And when it does, it only takes a bit of movement, and the issue goes away. So it's not like I can do a whole lot of testing. But yes, I'm trying to take a logical approach, don't expect anyone to be a mind reader.

Is the 4S on 5M off legit? Or a typo?

Yes, the motor runs for 4 seconds and off for 5 minutes. The motors aren't getting hot in 4 seconds. I already checked everything for temperatures, all is good.

I suspect that the sleeve bearings on the motors have dried out after two years of trouble free operation, causing them to stick on startup.

Maybe I didn't explain - these are hobby type motors. 24mm x 30mm with a 2mm shaft. light duty stuff, not industrial. the only bearing they have is a brass sleeve bearing and the motor does not come apart. But maybe a shot of dry lube wouldn't hurt. The systems I am having issues with are only about 8-month-old or less, it was 2 years ago that I switched to the motors I am using now, and I don't recall having this issue prior to that. Thats why I think maybe bad run of motors.

What are you using for a power supply? When the system fails, I believe that the voltage across the motors is dropping because one motor is drawing a heavy stall current. That could cause the other motors to fail to start. You can verify that by measuring the supply voltage during the fault condition.

I will try to measure that - if and when the stall happens again. But the supply is from a battery bank of 8 AGM type batteries that are rated at 255amp/hours. They may be wired in series/Parallel for 24v (center tapped for 12volt operations) or wired in parallel for 12v. Charge rate is Bulk 14.8 and float 13.8 - I don't think power is an issues. By the way, the motors that stalled were Motor 1 and motor 3, motor 2 (in the middle) didn't stall. Another clue I figured out today - Motor 1 and motor 3 were under a light load while motor 2 wasn't. That may play a part in the cause.

I stated I was only running them at 55% power using PWM, I'm thinking I might "jolt them" at full power for maybe half a second - then back off to 55%. If there is a small load at start, then maybe the 55% isn't enough at times. I have a faulty unit on my test bench now and I am trying to establish an approximate timeline as to when this occurs so any changes, I make I can hopefully verify if it helped or not.

Thanks for everyone's reply - I like to think out loud and bounce things off people - It helps me sort things out.

 

That what I would like to do - but like I said, the issue may take days, weeks, or months to occur. And when it does, it only takes a bit of movement, and the issue goes away. So it's not like I can do a whole lot of testing. But yes, I'm trying to take a logical approach, don't expect anyone to be a mind reader.



Yes, the motor runs for 4 seconds and off for 5 minutes. The motors aren't getting hot in 4 seconds. I already checked everything for temperatures, all is good.



Maybe I didn't explain - these are hobby type motors. 24mm x 30mm with a 2mm shaft. light duty stuff, not industrial. the only bearing they have is a brass sleeve bearing and the motor does not come apart. But maybe a shot of dry lube wouldn't hurt. The systems I am having issues with are only about 8-month-old or less, it was 2 years ago that I switched to the motors I am using now, and I don't recall having this issue prior to that. Thats why I think maybe bad run of motors.

Yes, you mentioned that these are small, low current hobby motors. That is why I suspect the bearings. The better ones have phosphor-bronze sleeve bearings but the cheaper ones are brass and will dry out in time adding considerable load to the shaft. A good squirt of alcohol will clean away the dried oil. Then a very small amount of thin oil usually solves the problem for a while, but don't get any on the commutator or brushes.

 

Most "Hobby" type Motors have VERY CHEAP Brushes that can
very easily develop a bad connection to the Commutator.
I would venture to guess that the only reason that they ever lasted as
long as they did is because You are running them at ~55%-PWM.

The question is, are the Motors rated for ~6-Volts,
but You are running them at maybe ~12-Volts with ~55%PWM.

Even ~6-Volts might be very optimistic for these Motors.

The usual cheap-Hobby-Motors are NOT designed for long-term reliability.

Premium Bearing-materials, and proper, replaceable, Carbon-Brushes are required.

3-Phase Drone-Motors would be a good place to consider,
they usually have very-good-Ball-Bearings, no Brushes, and
depending on the Controller used, can have tremendous "Start-Up-Torque" for their size,
and can very easily be precisely Speed-Controlled.
The Motors aren't all that expensive,
but You will need to build a 3-Phase-Controller to suit your needs.
.
.
.

 

My first guess is commutator oxidation. The short run, long delay sounds a lot like an air freshener application device. If a quick shot of contact cleaner spray fixes it for a few months then that was the problem. All it takes is one bad spot. You may also have a connection issue or a friction point in whatever the associated mechanism is.

 

Your power source is more than adequate to drive those motors. I assume that you are using power MOS FETs to switch the power to them. Are you sure that you are driving the gates with enough voltage to maintain full switching voltage across the motors when they are stalled? You can check that by measuring the voltage across a motor while you manually stall it.

 

My first guess is commutator oxidation.

Agreed. But there is another possibility:

the issue may take days, weeks, or months to occur. And when it does, it only takes a bit of movement, and the issue goes away. So it's not like I can do a whole lot of testing.

Could be a bad solder joint. Not much likely, but a possibility. Still, as MisterBill2 said, dirty or oxidized commutator may be at issue. Have you tried spraying a little contact cleaner on the com's?

 

Agreed. But there is another possibility:

Could be a bad solder joint. Not much likely, but a possibility. Still, as MisterBill2 said, dirty or oxidized commutator may be at issue. Have you tried spraying a little contact cleaner on the com's?

A lot of small hobby motors have carbon brushes. If yours do, don't use contact cleaner. It will impregnate the carbon and cause the brushes to wear away quickly.

 

Most "Hobby" type Motors have VERY CHEAP ...

You got that right - all the motors I looked at in this size/type all look like they are manufactured the same. Haven't really found one that looks "high quality". The ones I am using now are rated "for use in medical equipment" so I was hoping they were a bit higher quality than ones you would get from Radio Shack. (did I just show my age?)

I have considered the 3 phase motors but space constraints for the drivers may make use of those difficult - but not impossible.

My first guess is commutator oxidation.

This is something I have considered as well and a good thing to look at - some systems are installed in a damp environment. I will see if I can find out if this one on my test bench was installed in a dry or damp location. These are installed in RVs, so some get installed in the interior and some get installed in the bays.

Are you sure that you are driving the gates with enough voltage to maintain full switching voltage across the motors when they are stalled? You can check that by measuring the voltage across a motor while you manually stall it.

Good question - I will test that and check the specs of the MOSFET again. Something I didn't consider and had forgotten about. The MOSFETs I was using became obsolete and I had to choose a different MOSFET about a year ago. I simply went with the "Recommended replacement" from my supplier so didn't consider that or look at it that closely. Thanks.

Could be a bad solder joint.

One of the first things I checked, solder is good. Haven't tried any cleaners or lube as of yet. I want this system on my bench to fail again so I have a chance to check some things.

Thanks, lots of good info here.

 

Ashot of contact cleaner toward the motors commutator may be the fix. Cheap and easy and usually safe.

 

A lot of small hobby motors have carbon brushes. If yours do, don't use contact cleaner. It will impregnate the carbon and cause the brushes to wear away quickly.

I did not know that. Good information. Incidentally I've never tried cleaning brushes. As for cleaning the commutator I've always used a very fine sand paper, 600 grit or higher.

 

For cleaning comm's and seating brushes, traditionally a Com stone is used, while the motor is running, BTW
https://www.amazon.ca/Brush-Seater-Commutator-Cleaning-Stone/dp/B07P6YGWBX

 

One condition that will cause a small motor to not start until it is rotated a bit is a poor contact between a brush and the commutator. Then the light motion and the contact is good.
Is it possible to put an analog ohm meter across a failed motor and then gently and slowly rotate it, watching for a jump in the resistance.

 

...a poor contact between a brush and the commutator....

Almost certainly the problem in my opinion, especially given the extremely light duty cycle and pwm drive as well.

 

Thus a shot of contact cleaner should reduce the problem quite a lot. Cheap and fairly easy, depending, of course.
The question of how critical is the function of these motors has not been stated.
If the function is vital, then it may be better to replace them with better motors.
"Faulhauber" is a supplier of a better line of small DC motors.

 

How to stop a dc motor when it is drawing more current

 

How to stop a dc motor when it is drawing more current

Cutting power maybe?