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Voltage present on induction motor shaft

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duckpilot

Automotive
Sep 9, 2003
1
I work at an automotive company that builds automatic transmissions for front wheel drive vehicles. We have an end-of-line test that consists of a Reuland Electric 100 HP input drive motor (two pole, 480v) that simulates the engine in a vehicle, and two Reuland 67.5 HP motors (four pole, 480v) to simulate road load (one motor for each driveshaft of the transmission to simulate the two front wheels). All three motors are three-phase induction motors, each one controlled by its own dedicated SWEO flux vector drive. Each motor also has a strain-gage based torque transducer in the driveline. Our plant has eight identical test stands with this setup. We have recently been trying to upgrade the torque transducers due to obsolescence of the existing units, but ran into electrical noise problems on the torque signal of one of the output driveshaft motors. After MUCH diagnosis, we found that the motor in question had a measureable AC voltage (approx. 0.6 volts) between the rotor shaft and the housing of the motor during operation (measured with a DVM). Frequency content of the AC voltage was not determined, but appeared to be essentially random when viewed on an oscilloscope. Testing was performed at 500 RPM, with no load on the motor. The noise on the torque signal is not present when the motor is run at 100 RPM or less, and also decreases significantly when the motor speed exceeds 1200 RPM. The motor in question does not audibly sound any different than any of the other output motors, nor does it show any difference in performance or behavior. I don't know that much about induction motors, but is it possible that we are seeing some sort of leakage current into the rotor from a rotor bar? Is this normal, or does it indicate that we have a loose or otherwise failing rotor bar on that particular motor? The torque transducers we are evaluating as replacements are apparently not sufficiently isolated from such voltages (the old transducers are noise-free on this motor), so if this is something we could possibly expect to see on the other 7 test stands, then these transducers will definitely not work for us. Thanks in advance for any help you can provide.
 
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Hello duckpilot

This is a common phenomena when using IGBT based variable speed drives. It is caused by the very fast switching og the output devices and commonly results in damage to the bearings. If you do a search on bearing damage, I am sure that you will find a lot of information covering the cause and solutions.
One solution, is to put a dv/dt limiting filter between the drive and the motor. There are ather schemes of adding earthing brushes between the motor frame and shaft, special bearings etc. I expect that if you talk to the inverter manufacturers, they will also be able to give advice on this.
Best regards



Mark Empson
 
0.6 volts is not much, but in the wrong spot...
I agree with the above but would also add that a DVM has a very high input impedance, therefore no load. What did work for me in a "noise" situation was to use a loading resistor.
For me about 10Kohms. No more noise.
 
DUCKPILOT- I work in the power generation buisness and shaft voltage is a common effect on our plant generators. What we do is install a brush rigging or copper braids to ride on the saft and connect it to ground to bleed off induced voltage on the shaft.
 
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