Restoring vintage induction motor (DOs and DON'Ts?)

[thexeno]

Hi all,

Fist forgive me if I am in the wrong part of the forum, I just need expert advice on vintage electric motors, so feel free to move this thread.
We are trying to bring up safely an old vintage 3N industrial induction asynchronous motor, squirrel cage - is quite heavy, has a few HP, sadly I don't have the label right now. I have an idea on the steps, but I'd like to get  an opinion. The motor mechanically seems perfect and spins with no friction. It seems to have coils connected in star configuration.

My idea:
1. cleaning the old dirt. open if necessary
2. measure insulation, coil-to-coil, and chassis (with multimeter or even high voltage tester if I can)- If needed open the star connection to measure all individually.
3. replace bad wiring, check the internal wirings that are going to the motor, including PE and chassis
4. get a variable frequency controller (rated for such motor), we have 1N and 3N with differntial protection available to the controller
5. playing around, starting slowly? <- this is the part I am not sure about, on top of wanting to double check the previous steps.

Thanks for any 2 cents given! Let me know if you want to know something more in details, I'll provide the info.

[WattsThat]

Typical motor refurbishment generally includes:

1. Shorted windings require rewinding. If coils look and check ok - then
2. Check winding insulation at 1000vdc test voltage. If no leakage (usually 10G ohm or higher) then surface clean and proceed to bearings
3. Assuming insulation resistance is low, clean typically with steam. Dry throughly by baking at 120-130C for 10-12 hours
4. Varnish dip and bake
5. Recheck insulation resistance. Rewind if insulation resistance still low.
6. Replace bearings if indicated, grease
7. Test

Test is usually direct across line, not with a VFD. Verify locked rotor current at start, confirm no load and full load currents

Important note: Older design motors, prior to AC VFD’s become common, say perhaps as late as year 2000, did not have strengthened insulation systems to withstand the increased voltage stresses created by said VFD’s due to reflected wave. Modern motors have withstand voltages of 1600-1800 voltage which helps prevent shorted end turns which occurs in random wound coil motors.

[thexeno]

Thanks

Typical motor refurbishment generally includes:

1. Shorted windings require rewinding. If coils look and check ok - then
2. Check winding insulation at 1000vdc test voltage. If no leakage (usually 10G ohm or higher) then surface clean and proceed to bearings
3. Assuming insulation resistance is low, clean typically with steam. Dry throughly by baking at 120-130C for 10-12 hours
4. Varnish dip and bake
5. Recheck insulation resistance. Rewind if insulation resistance still low.
6. Replace bearings if indicated, grease
7. Test

Test is usually direct across line, not with a VFD. Verify locked rotor current at start, confirm no load and full load currents

Important note: Older design motors, prior to AC VFD’s become common, say perhaps as late as year 2000, did not have strengthened insulation systems to withstand the increased voltage stresses created by said VFD’s due to reflected wave. Modern motors have withstand voltages of 1600-1800 voltage which helps prevent shorted end turns which occurs in random wound coil motors.

Thanks!! I totally overlooked the high voltage issues of VFD. Also the lack application procedure I was not aware of it - although I hope is not needed, since I don't have equipment to do it...
In the meantime I will then provide myself with an insulation tester then.

[Gyro]

...
2. Check winding insulation at 1000vdc test voltage. If no leakage (usually 10G ohm or higher) then surface clean and proceed to bearings
3. Assuming insulation resistance is low, clean typically with steam. Dry throughly by baking at 120-130C for 10-12 hours
...

A tip from an old Electrical Engineering book I have, from the '30s, which covers big motors and generators/ alternators. You can use carefully controlled low voltage DC current to dry out the windings rather than baking the whole motor. Check the winding temperatures frequently until you're sure they stable and not excessive. Monitor the insulation resistance until it stops increasing. At that point, decide whether it is good enough.