Brush type servo motor question

Bruce Griffing · Jan 19, 2007

I am repairing an old SWI Proto Trak Plus servo motor. Initially, the motor had a very high resistance (75 ohms). I checked the brushes - they were ok. I then worked the motor manually for many revolutions. Now the resistance is 12 ohms. I am expecting 1 or 2 ohms static resistance. So I conclude that I should disassemble the motor and clean (or turn) the armature. I was just about to take it apart, when it occured to me that some types of dc motors are damaged in disassembly. Will this be a problem? Any suggestions?

macona · Jan 19, 2007

Shouldn't be a problem. Servo motors are intended to be maintained.

Are there any grooves or and other noticable wear on the commutator? If not just chuck it up in a lathe and use something to clean it up. I have been told not to use emery. I have a stone specifically for cleaning slip rings and commutators.

If you do turn it make sure you relieve the mica in the gaps in between the segments. You can often use a thin saw blade for this.

rklopp · Jan 19, 2007

I think you will want to shunt the magnets with a piece of steel, if you pull the armature.

Yankee Metallic · Jan 19, 2007

Dismantling the DC motor shouldn't be a problem. I have a SERVO I have dismantled about 4 times now to replace switches inside that I caused to break when I changed the lever system to fit my application. I haven't had any problems after reassembling it.

bcstractor · Jan 20, 2007

To measure the resistance of such a motor the best way is to lock the rotor and then apply current from a battery. Measure both the current and the voltage across the motor. The resistance is Volt/Current.

This is called the four wire method and measures the resistance under load. Repeat the measurement at different rotor positions. Make sure the current is well below rated and don't apply it for long or the motor will heat up and change resistance.

Chris P

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Brush type servo motor question

Bruce Griffing

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rklopp

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Yankee Metallic

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bcstractor

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