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Discussion Starter · #1 ·
I started in this trade in the mid to late 1980's, and even then, DC motors were on their way out in industry in favor of VFD controlled AC motors, servo motors, and steppers. I never really learned the tricky DC motor troubleshooting stuff, for lack of exposure.

My current issue.... two 1/2 hp permanent magnet DC gearmotors, both controlled from one DC drive (both intended to turn the same speed). There is no feedback control. From the drive to the motors is a single set of conductors to the local disconnect, and they split out from there. From the disco to each motor is the exact same distance (about 5 feet, if it matters).

The connected load of each motor is the same. They run "gapping wheels" on each side of a 2-lane conveyor to pace out product that looks a lot like Pepsi-cola in a 16oz plastic bottle. :whistling2: Even with no product passing, one motor will occasionally turn much slower than the other. Checked brushes, amp draw, and gear case oil level. Everything seems normal with the slow motor. Then, out of the blue, the thing will work well for days or weeks, then it will take a notion to run slow for a few hours. What the heck is going on, DC motor guys?
 

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Bilge Rat
motors and controls.........
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Here's one possibility;

The output of a DC controller is DC, but it has tons of ripple. Looking at the current of a DC motor on a scope (fed by a battery, no ripple at the source), it isn't a constant draw. Tons of ripple.

Of course, the ripples are at differing frequencies, but I have to wonder if resonance, especially given 3 sources of ripple, causes one of the motors to effectively see lower voltage than the other.

I haven't seen this in a DC system yet, but I certainly have seen this type of resonance confuse generator regulators.
 

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When you checked the brushes did you take a close look at the commutator, any grooving will cause the brushes to not seat correctly sometimes. Also a good idea to blow the dust out when the brushes are out, make sure the springs on the brushholders are ok.
 

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Bilge Rat
motors and controls.........
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A recording ammeter would point to either a mechanical issue or a motor/power issue.

If the current goes up when it slows down, it'd certainly point to mechanical loading. If it goes down, it's almost certainly not mechanical.
 

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THE "BIG RED MACHINE"
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the armatures may need to be serviced,

Check commentator undercut commentator bars, check to see if armature coils have been burnt, or failure to
armature coils. maybe there's an inspection plate you can open up on the endbell of the motor, examine the commentator see if it looks like its burning or worn out brush grooves in the copper bars. sometimes there adjusting that could be made of the brush carriage, but that should be done after armatures serviced. sometimes the permanent magnets
come loose . does it look like a headache to pull motors our and take them to a motor shop? get a good motor shop that knows DC motors, armature works.

make sure your controls are working correctly then take a look at the motor
 

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I agree with the guys above.. I am leaning towards bad armatures.... I have worked on a few conveyor systems in bottled H2o plants and even upon initial inspection they seemed ok. But once the motor was removed and sent to our motor guy he would always rewind/replace the armature and they would run like new again for a while... (Mind you much like what you are working on these things ran 24/7 so it didn't take long to burn them up)

Ours also had bearings on the output side where the drive was for the rollers that if they were "missed" on a PM would go bad and cause a similar issue....
 

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Discussion Starter · #10 ·
does the factory have any spare units on the "rack" they can interchange with existing unit?
Yeah, matter of fact. I just (understandably) hate throwing parts at something on speculation, even though that is sometimes the most prudent thing to do. Just seems like there should be something testable here. The amp draw stays the same as the other gearmotor when this slowdown event happens, so it doesn't seem unreasonable to rule out mechanical issues w/ the bearings or gear case. I have inspected the com though the brush holes, and sprayed the **** out of it with contact cleaner, and didn't really get much in the way of black goo out. The com looks pretty nice. Just a nice even tarnish, and the bars look fairly proud of the insulators, so they're not really worn down. Thought someone might have some tricky test up their sleeve. I'm pretty sure local maintenance is going to change the little gearmotor since I came up empty. Just bugs me.
 

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Marc. Swap motors and see if the problem follows the motor in question.
Make sure you keep the gear reducer on the motor. If the problem follows the motor, then you need to check both the motor and reducer.
Its entirely possible the gear reducer is your issue.
 

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Discussion Starter · #13 ·
Marc. Swap motors and see if the problem follows the motor in question.
Make sure you keep the gear reducer on the motor. If the problem follows the motor, then you need to check both the motor and reducer.
Its entirely possible the gear reducer is your issue.
I'm sure that's what will be done. A half-horse gearmotor isn't anything I think anyone would ever fix. If the new one cures it, the old one is destined for the trash, I feel sure. Like I say, I just hate guessing (even if it's an educated guess). Everything is testable. I just don't know how to test this one.
 

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I started in this trade in the mid to late 1980's, and even then, DC motors were on their way out in industry in favor of VFD controlled AC motors, servo motors, and steppers. I never really learned the tricky DC motor troubleshooting stuff, for lack of exposure.

My current issue.... two 1/2 hp permanent magnet DC gearmotors, both controlled from one DC drive (both intended to turn the same speed). There is no feedback control. From the drive to the motors is a single set of conductors to the local disconnect, and they split out from there. From the disco to each motor is the exact same distance (about 5 feet, if it matters).

The connected load of each motor is the same. They run "gapping wheels" on each side of a 2-lane conveyor to pace out product that looks a lot like Pepsi-cola in a 16oz plastic bottle. :whistling2: Even with no product passing, one motor will occasionally turn much slower than the other. Checked brushes, amp draw, and gear case oil level. Everything seems normal with the slow motor. Then, out of the blue, the thing will work well for days or weeks, then it will take a notion to run slow for a few hours. What the heck is going on, DC motor guys?
when studying about dc motors my lecture always told me that they are out dated and AC motors are the in thing. so I went through college literally ignoring DC motor theory.

I was wrong,my first job I encountered so many DC motors that I had to go back to books,
that was a lesson learnt.

back to your question if there was a feedback control lets say encoders then we could have faulted them. but since there isn't any I will stick with two theories

1) maybe your gearbox has a problem like bearing or gears themselves

2) the motors just need a replacement that is if the parameters in the drive are ok.

Sent from my HUAWEI Y210-0100 using Tapatalk 2
 

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I'm sure that's what will be done. A half-horse gearmotor isn't anything I think anyone would ever fix. If the new one cures it, the old one is destined for the trash, I feel sure. Like I say, I just hate guessing (even if it's an educated guess). Everything is testable. I just don't know how to test this one.
There is not a whole lot you can do to test a DC motor yourself. Since its permanent magnet, you would need a growler to test the armature.
The only thing you can do as the end user is change brushes and blow out the motor (clean it).
Screwing around with the commutator is not a good idea and usually does more harm than good.
A motor shop would machine the comm and then undercut. Then check for ground. Its all they can do in addition to load testing.

Is this motor 90 or 180 volt? You could put a meter on the motor leads at the motor and see if you get the correct voltage in reference to speed.
At 50% percent full speed, you should have approximately 1/2 full voltage.
I have seen magnets get weak also and you have no way to check that.
You could trim/adjust the drive, but since the other motor runs okay, it sounds like the drive is set up right.
What type of control is it? KB?

Gear boxes sometimes will mess up the electrician as everything checks good, but issues with speed and torque arise and we assume the gear box is good because it turns freely.
Don't let the gear box be the issue and not address it.
If the issue follows the motor and gear box, then you need to check each one to be certain which is at fault.

I'm sorry I missed your "gear motor" comment. Yes, its one single unit and there is no reason to try and take it apart. Is it a true gear motor or a motor and gear box?
If the motor frame is 56C it will come apart. The reducer will also have the motor frame designation on the name plate.
 

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You said they are PMDC motors. Those are typically NOT used in applications like this where you want accurate speed of two motors from one drive. In a PMDC motor, the field strength can vary as the magnets become demagnetized over time and/or temperature/overloading. In other words lots of variables are possible. A drive can compensate for that by varying the armature voltage, but when you have two PMDC motors, the drive cannot compensate for one PM field strength change without affecting the other. Bottom line, bad design likely made to be cheap, not accurate. If the change is not consistent, it's more likely to be related to temperature and/or overloading. But it also could be exacerbated by the fact that when new, there was enough overkill to make that irrelevant but now over time BOTH motors are losing field strength, so any slight change in temperature/overloading becomes more of an issue. Might be time for replacement, and I would do them both.
 

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Discussion Starter · #17 · (Edited)
There is not a whole lot you can do to test a DC motor yourself. Since its permanent magnet, you would need a growler to test the armature.
The only thing you can do as the end user is change brushes and blow out the motor (clean it).
Screwing around with the commutator is not a good idea and usually does more harm than good.
A motor shop would machine the comm and then undercut. Then check for ground. Its all they can do in addition to load testing.

Is this motor 90 or 180 volt? You could put a meter on the motor leads at the motor and see if you get the correct voltage in reference to speed.
At 50% percent full speed, you should have approximately 1/2 full voltage.
I have seen magnets get weak also and you have no way to check that.
You could trim/adjust the drive, but since the other motor runs okay, it sounds like the drive is set up right.
What type of control is it? KB?

Gear boxes sometimes will mess up the electrician as everything checks good, but issues with speed and torque arise and we assume the gear box is good because it turns freely.
Don't let the gear box mess be the issue and not address it.
If the issue follows the motor and gear box, then you need to check each one to be certain which is at fault.

I'm sorry I missed your "gear motor" comment. Yes, its one single unit and there is no reason to try and take it apart.
Yeah, it's a KB drive. Pretty primitive. Has some little wee KB board beside it turning the 4-20ma from the PLC analog output into something useful to the KB drive. Probably 0-10v. Didn't really care much about that end since one drive controls both gearmotors and the one motor works just fine.

Looks like I'm just gonna slap a new gearmotor on, or at least recommend that someone does.
 

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Donuts > Fried Eggs
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...In a PMDC motor, the field strength can vary as the magnets become demagnetized over time and/or temperature/overloading....
I don't have an answer, so I ran this by a couple of guys in our DC motor department this morning and because there's no change in current following the speed change they both said it sounded like a temperature issue affecting the permanent magnet.

Either that or the possibility that there's a loose magnetic pole.
 

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THE "BIG RED MACHINE"
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if one of the two motors is running real hot to the touch, you need to pull it replace it then check the other motor you took out maybe the side where the hot motor comes out of is binding.
 
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