Why is a PMDC motor getting hard, when on load after 10 years of service?

[Hobbyist]

Why is a PMDC motor getting hard, when on load after 10 years of service?

 

[Darkwood]

Your going to need to expand on the term 'getting hard' a bit more, what exactly do you mean, what is happening?

 

[Hobbyist]

Your going to need to expand on the term 'getting hard' a bit more, what exactly do you mean, what is happening?

The concerned PMDC (geared) motor runs a sliding door. The motor shaft rotates the gear assembly, which in turn rotates the spindle and the door panel thus. This is not the problem because when the motor without the 'unwinding lever' assembly is rotated to achieve the above process, everything is ok. However, the moment the unwinding lever is installed on the motor and the motor is made to achieve the above act, it becomes comparatively harder to move the panels.
The unwinding lever is kept to unlock the locking assembly, which holds the door in closed position. The movement of the unwinding lever is done by the motor as it is hanged on the motor shaft.
Please note: We are trying to achieve the free movement of the panels (door operation) without giving supply to the motor, i.e. no electrical supply is involved, only mechanical pulling is tested

 

[Deleted account]

Is this an electrical question?

 

[Hobbyist]

Is this an electrical question?

I want to know from the experience how the rotation of a PMDC motor gets hard after many years of service, which part needs to be looked into.

 

[Darkwood]

You will not achieve this, you will have a high ratio gearbox (provide info if you can) that converts the speed to a very low rpm, in doing so it proportionally increases in available torque and thus will require a very high amount of force to rotate the motor from the gear output side.

Example - If I have a 1500rpm motor and reduce that with a 10-1 gear ratio for a output of 150 rpm, the torque at the output with increase proportionally by 10 fold, thus when you try to drive the motor from the shutters it will take an incredible amount of force to do so, the only way to get around this is to decouple the shaft in some way on the output of the gearbox.

This is why when electric roller shutter doors that are put into chain/manual operation they take a lot of effort to move a small amount, the chain itself is suitably geared to allow you to turn against any gearing or fitted to the input side also the motors sometimes are specifically designed for lower output speeds to start with reducing the need for a high ratio.

 

[Darkwood]

I will add, if you have always normally been able to move the shutters in the past and it is only recently getting hard then it is likely the shutters have been damaged/warped etc or need a service, the runners may want regreasing any running roller wheels that have seized may need replacing etc

 

[Hobbyist]

You will not achieve this, you will have a high ratio gearbox (provide info if you can) that converts the speed to a very low rpm, in doing so it proportionally increases in available torque and thus will require a very high amount of force to rotate the motor from the gear output side.

Example - If I have a 1500rpm motor and reduce that with a 10-1 gear ratio for a output of 150 rpm, the torque at the output with increase proportionally by 10 fold, thus when you try to drive the motor from the shutters it will take an incredible amount of force to do so, the only way to get around this is to decouple the shaft in some way on the output of the gearbox.

This is why when electric roller shutter doors that are put into chain/manual operation they take a lot of effort to move a small amount, the chain itself is suitably geared to allow you to turn against any gearing or fitted to the input side also the motors sometimes are specifically designed for lower output speeds to start with reducing the need for a high ratio.

When i pull the door with a new motor and the old gear assembly (decoupled from the faulty motor), everything is fine. However, when i pull the door with the faulty motor and a new gear assembly (decoupled from a spare new motor), the pull becomes hard. Hence, i can conclude that it is something inside the motor that is making the operation hard.

 

[Darkwood]

You did not present this added info' at the original post, this would have been better to give this info' as it changes how we approach this matter and advise on.

Lets get this straight then and simplified -

New motor and old gearbox is easy to move shutters.
Old motor and new gearbox is hard.

I assume the old motor and old gearbox was hard as well which would lead to a conclusion that the old motor is your problem, can you easily turn the shaft on the old motor, when you do does it turn freely without a grinding feel.. try this with the motor not connected to anything, if it is difficult then it is likely the bearings have failed and after 10yrs it wouldn't surprise me.

I take it the initial problem has been resolved with the new motor so you're just curious at this point and not looking for any solution.

 

[Lucien Nunes]

The bearings may be wearing out. Even if the motor is not stiff in itself, if the drive end carries a gear and the bearing is worn so that it does not mesh accurately, the wrong surfaces on the gear will rub together and cause friction.

Does the motor work correctly on power? An internal short-circuit in the armature, or between commutator segments, can make the motor stiff to turn. A PM motor works equally as a generator and any short will allow current to flow within the armature and create a torque opposing rotation. If the gearbox has a high reduction ratio it might be important that it spins very freely, in which case perhaps even a carbon build-up between the commutator segments will pass enough current to create too much dynamic braking.

 

[Hobbyist]

You did not present this added info' at the original post, this would have been better to give this info' as it changes how we approach this matter and advise on.

Lets get this straight then and simplified -

New motor and old gearbox is easy to move shutters.
Old motor and new gearbox is hard.

I assume the old motor and old gearbox was hard as well which would lead to a conclusion that the old motor is your problem, can you easily turn the shaft on the old motor, when you do does it turn freely without a grinding feel.. try this with the motor not connected to anything, if it is difficult then it is likely the bearings have failed and after 10yrs it wouldn't surprise me.

I take it the initial problem has been resolved with the new motor so you're just curious at this point and not looking for any solution.

I have replaced the bearings and will now check after 2 - 3 days if it works well on the test module i have, in the meantime i wanted to know if any other factors is responsible for it's hardness like weak magnetism etc.

 

[Hobbyist]

I have replaced the bearings and will now check after 2 - 3 days if it works well on the test module i have, in the meantime i wanted to know if any other factors is responsible for it's hardness like weak magnetism etc.

And Thank you Sir for bearing with me as i went a little offtrack.

 

[Darkwood]

See @Lucien Nunes reply above, he's has more working knowledge than me of DC motors and his response may shed some further light on the matter.

 

[Hobbyist]

The bearings may be wearing out. Even if the motor is not stiff in itself, if the drive end carries a gear and the bearing is worn so that it does not mesh accurately, the wrong surfaces on the gear will rub together and cause friction.

Does the motor work correctly on power? An internal short-circuit in the armature, or between commutator segments, can make the motor stiff to turn. A PM motor works equally as a generator and any short will allow current to flow within the armature and create a torque opposing rotation. If the gearbox has a high reduction ratio it might be important that it spins very freely, in which case perhaps even a carbon build-up between the commutator segments will pass enough current to create too much dynamic braking.

Out of 10 faulty motor, 1 donot even work on power, rest does comfortably.

 

[Hobbyist]

The bearings may be wearing out. Even if the motor is not stiff in itself, if the drive end carries a gear and the bearing is worn so that it does not mesh accurately, the wrong surfaces on the gear will rub together and cause friction.

Does the motor work correctly on power? An internal short-circuit in the armature, or between commutator segments, can make the motor stiff to turn. A PM motor works equally as a generator and any short will allow current to flow within the armature and create a torque opposing rotation. If the gearbox has a high reduction ratio it might be important that it spins very freely, in which case perhaps even a carbon build-up between the commutator segments will pass enough current to create too much dynamic braking.

Yes, there is a little carbon build up.

 

[Hobbyist]

Yes, there is a little carbon build up.

The bearings may be wearing out. Even if the motor is not stiff in itself, if the drive end carries a gear and the bearing is worn so that it does not mesh accurately, the wrong surfaces on the gear will rub together and cause friction.

Does the motor work correctly on power? An internal short-circuit in the armature, or between commutator segments, can make the motor stiff to turn. A PM motor works equally as a generator and any short will allow current to flow within the armature and create a torque opposing rotation. If the gearbox has a high reduction ratio it might be important that it spins very freely, in which case perhaps even a carbon build-up between the commutator segments will pass enough current to create too much dynamic braking.

"I have replaced the bearings and will now check after 2 - 3 days if it works well on the test module i have, in the meantime i wanted to know if any other factors is responsible for it's hardness like weak magnetism etc."
Posted the above 2 days ago.
Status now: no change in pulling force even after replacement of bearings
Guide me for any other factors...

 

[Lucien Nunes]

How does the shaft couple to the gearbox - could it be a mechanical alignment issue that is causing a little extra friction even though you have changed the bearings? Can you post a picture of one of the motors so we can understand the mechanical setup?

Do try cleaning the slots between the commutator segments and then polishing the comm, even if only to rule that out. There isn't much else, weak magnetism would cause it to run more freely as the cogging (reluctance) torque will be lower.

You might like to compare the current both on and off load between a new and old motor. That would be quite revealing.

 

[Hobbyist]

How does the shaft couple to the gearbox - could it be a mechanical alignment issue that is causing a little extra friction even though you have changed the bearings? Can you post a picture of one of the motors so we can understand the mechanical setup?

Do try cleaning the slots between the commutator segments and then polishing the comm, even if only to rule that out. There isn't much else, weak magnetism would cause it to run more freely as the cogging (reluctance) torque will be lower.

You might like to compare the current both on and off load between a new and old motor. That would be quite revealing.

 

[Hobbyist]

I have literally pushed the carbon brushes out to rule out it's hindrance (if any). I will check the operation and revert back.

 

[Lucien Nunes]

Is the driving gear of the planetary gearbox attached to the motor armature? Is it plastic or metal? Are you swapping armatures relative to the rest of the gearbox or keeping them as units? Does a new motor armature run freely with an old planetary gearbox?

I am thinking that the gear on the old motor is worn and causing extra friction, or there is wear or play in the gearbox that makes it intolerant of all but a new motor gear. You mentioned replacing the bearings - that's both bearings on the motor shaft. correct?

I would definitely try the current comparison.