Measuring motor short

I am working on an application where I need to make sure the case of a dc motor has not become shorted to the armature windings. Normally the case is isolated, but on the rear occation, the case becomes connected to the power supply through the windings. This is normally caused by the insulators on the windings being damaged which causes a short to the case.
The motor is normally powered with voltage between 12 and 24 volts. The short in the windings can occur at any point in the windings. So normally the case is floating but once shorted the case can see voltages anywhere between 0 and 24.
Does anyone have any ideas how i might be able to determine that the case is no longer isolated while the motors is powered and running?

jonfarrugia said:

I am working on an application where I need to make sure the case of a dc motor has not become shorted to the armature windings. Normally the case is isolated, but on the rear occation, the case becomes connected to the power supply through the windings. This is normally caused by the insulators on the windings being damaged which causes a short to the case.
The motor is normally powered with voltage between 12 and 24 volts. The short in the windings can occur at any point in the windings. So normally the case is floating but once shorted the case can see voltages anywhere between 0 and 24.
Does anyone have any ideas how i might be able to determine that the case is no longer isolated while the motors is powered and running?

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Isn't the Case of the Motor Grounded, or does it need to be floating for some reason? Do you just want an Alarm, or do you want to have this go to a PLC so it can take some action?

Stu....

sthompson said:

Isn't the Case of the Motor Grounded, or does it need to be floating for some reason? Do you just want an Alarm, or do you want to have this go to a PLC so it can take some action?

Stu....

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The case is normally floating/isolated. I need to detect using a plc input when the case is ceases to be isolated.

jonfarrugia said:

The case is normally floating/isolated. I need to detect using a plc input when the case is ceases to be isolated.

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Since you can't depend on the Voltage you will see, a standard 24VDC Input will probably not work. How about an Analog Input set for Voltage? You will have to check the Specifications carefully to make sure it will not be damaged if it receives a full 24VDC. If it could be damaged, you will have to take steps to limit the Voltage or protect the Analog Input in some other way. Then you can set up your PLC Program to respond if the Input Voltage exceeds something like 5 VDC. This assumes that other equipment around it, or the operation of the motor itself doesn't induce stray voltages into the case. In that case, you might need a drain Resistor to ground to pull it down.

I am just musing here, so some experimentation may be required. Maybe someone will pop in with a better idea, or has had to deal with a similar problem.

Stu....

sthompson said:

Since you can't depend on the Voltage you will see, a standard 24VDC Input will probably not work. How about an Analog Input set for Voltage? You will have to check the Specifications carefully to make sure it will not be damaged if it receives a full 24VDC. If it could be damaged, you will have to take steps to limit the Voltage or protect the Analog Input in some other way. Then you can set up your PLC Program to respond if the Input Voltage exceeds something like 5 VDC. This assumes that other equipment around it, or the operation of the motor itself doesn't induce stray voltages into the case. In that case, you might need a drain Resistor to ground to pull it down.

I am just musing here, so some experimentation may be required. Maybe someone will pop in with a better idea, or has had to deal with a similar problem.

Stu....

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The only problem with your solution is that the case could be shorted to the 0 vdc of the motor supply. Depending on where the windings get shorted the case potential could range from 0 vdc to 24vdc. I need to make sure it remains floating.

jonfarrugia said:

The only problem with your solution is that the case could be shorted to the 0 vdc of the motor supply. Depending on where the windings get shorted the case potential could range from 0 vdc to 24vdc. I need to make sure it remains floating.

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Well, I am not sure how you are going to detect 0 VDC. There are a lot of very smart people on this list, so hopefully they can make a suggestion. Good luck.

Stu....

As example in Navy submarine batteries had a ground detector. A resistor was tied to ground on one side and on other another resistor went to pos and one to neg. A switch was used to select resistors.

Another way of doing it was a resistor tied to ground in the center of the battery 126 cell so between 63 and 64. Current flow was read.

YOu can look up ground detectors on web.

Bender makes one that may be applicable to you. Can supply model No tomorrow.

Why are you asking? Do you get that many grounds? Do you clean your motors - brushes do wear and carbon dust does conduct. Are you using these in a dusty or wet environment? Are you overloading thus overheating motors and breaking down insulation? Last but not least why 24 VDC?

What about a simple megger test? Even with a ground detector that is what we relied on - except for battery of course.
Dan Bentler

I know this is a radical thought but, why not fix the motor so you don't have to worry about it?

Or even, ground the case of the motor and protect the power supply with fuses. When a fuse is blown, the motor has a short to the case.

My guess is he is trying to detect and correct the short on his terms BEFORE it shuts him down. I'm guessing this isn't an issue for him until he gets more than one armature short to case. The blown fuses would shut him down just as surely as a blown drive, albeit less expensively.

Keith

If your problems are due to carbon buildup as someone mentioned above, you may be able to use a product like this to set your threshold, and catch before windings are truly grounded:

http://207.56.194.41/ground-fault-relay-grounded-dc/

Many times, if not too far gone, a winding can be cleaned, re-dipped and baked and be good as new at a fraction of the cost. Most shops consider this reconditioning along with turning the armature, changing the brushes, and bearings.

If you just want to see a dead short there's plenty of ways to do that including better fusing or circuit breakers with GF detection. The breakers usually have an aux. for sending a signal to a PLC or alarm.

DickDV said:

I know this is a radical thought but, why not fix the motor so you don't have to worry about it?

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Granted we have no idea of what kind of DC motor he has and we have no idea of the application or the demands he is placing on motor. But he did not tell us these "insignificant details".

If he cannot get 10 years service out of a motor
1. correctly sized to the load
2 of proper design for the environment
3. WITH a PM system to check brush wear etc

then there is something wrong. If he does the 3 items most of his problems should go away. Putting a ground detector on it seems like placing band aids on gangrene to me.

Dan Bentler