Induction motor that got wet

[Diver300]

I'm trying to fix a log splitter that got left in the rain. The motor is a capacitor-run single phase induction motor, with a rating between about 1 kW and 2 kW. It's about 20 cm diameter over the cooling fins.

The armature had rusted and jammed, but I have freed that off and the motor runs fine, but I still have a problem with earth leakage.

I've got the stator separate from the rest of the motor. There is about 2 mA leakage between the windings and the lamination at 240 V ac. When measured with a multimeter there is something electrochemical going on. In one direction it reads about 3 MOhm, and in the other direction, the reading climbs to about 20 MOhm, and when I immediately read the dc voltage, it's often above 2 V, and decays to zero. The multimeter puts out well over 3 V when measuring high resistances.

Although there is some rust on the laminations, I can't see any damage to the insulation around the winding bundles, but of course it's difficult to inspect everywhere and I can't get to look at most of the insulation without destroying the windings.

Any suggestions as to how I could deal with this without getting it rewound?

 

[MaxHeadRoom78]

You could try baking the stator in the oven for a long period, not too hot ~ 170F ?

 

[rjenkinsgb]

As Max says, cook it thoroughly to ensure it is absolutely dry.

You could then try running some heat-cured epoxy down each stator slot? That can be given plenty of time to help it penetrate any gaps or cracks, then fully heated for a final "flow" in to fine cracks and to set it solid?

example epoxy type

https://www.permabond.com/wp-content/uploads/2023/05/ES558_TDS.pdf

 

[Diver300]

You could try baking the stator in the oven for a long period, not too hot ~ 170F ?

That was one of the things that I thought of trying. I only have to take off the connections box and the cable.

 

[Diver300]

As Max says, cook it thoroughly to ensure it is absolutely dry.

You could then try running some heat-cured epoxy down each stator slot? That can be given plenty of time to help it penetrate any gaps or cracks, then fully heated for a final "flow" in to fine cracks and to set it solid?

example epoxy type

https://www.permabond.com/wp-content/uploads/2023/05/ES558_TDS.pdf

That could work. I would be worried that there could still be a void in the epoxy that contained water / ionic contaminants.

 

[MaxHeadRoom78]

After the heat treatment, the best test is with a megger, if you can get hold of one!

 

[schmitt trigger]

Agree with Max that a Megger or a hypot tester should be used to ensure that the original insulation hasn’t been degraded.

 

[Lightium]

After the heat treatment, the best test is with a megger, if you can get hold of one!

Couldn't you use the micro amp setting on a dmm?

 

[MaxHeadRoom78]

The Megger tests at HV, which is required to test insulation, an ordinary DMM pales in comparison for that task.

 

[rjenkinsgb]

Agree with Max that a Megger or a hypot tester should be used to ensure that the original insulation hasn’t been degraded.

The electrolytic effects with the apparent leakage resistance dependant on polarity & it producing a voltage between the windings and casing imply the insulation has failed at some point?

That's why I think it's worth trying to add further insulation via the epoxy; I'd not try a high voltage test until that has been done.

 

[Lightium]

I should know this, it's just if I don't use information, after awhile it goes away.

 

[Nigel Goodwin]

I should know this, it's just if I don't use information, after awhile it goes away.

Yes, the standard old Megger used 500V DC (at high impedance) from a hand-cranked generator, I used one at work for years - while not dangerous (due to the high impedance source) they are excellent for shocking young apprentices

You could even show them it didn't hurt by letting them shock you first (because holding the clips TIGHTLY reduces the resistance, and pulls the voltage down low) - but they try and hold them loosely, with high resistance, and hence high voltage

Modern electronic ones use an inverter circuit to generate the voltage from a battery, and often have more than one voltage range - and are considerably lower impedance than the old ones, so less suitable for shocking apprentices

 

[MaxHeadRoom78]

, so less suitable for shocking apprentices

We had the old hand crank also, we would connect them to a run capacitor and crank away, whip the leads off and leave it on the bench for unsuspecting apprentices etc. !

 

[Lightium]

I was tinting windows in a church once and had to carefully move lights and tassels with the back of my hand so the lift could be driven between the pews and the windows and boy did I get two inch streamers to my hand.

 

[MaxHeadRoom78]

I was tinting windows in a church once and had to carefully move lights and tassels with the back of my hand so the lift could be driven between the pews and the windows and boy did I get two inch streamers to my hand.

It may have been a sign!

 

[Lightium]

That my body in conductive?

 

[Diver300]

I have baked it at 80 °C for a couple of hours. The leakage current at 240 V has dropped to about 0.9 mA. I'm going to try a couple more hours.

0.9 mA isn't perfect but it's probably good enough. If the insulation does get worse all that will happen is that the RCD will trip.

The best fix would be to get the motor rewound but I don't know if anyone does that any more.

 

[MaxHeadRoom78]

That my body in conductive?

From above !!

 

[MaxHeadRoom78]

The best fix would be to get the motor rewound but I don't know if anyone does that any more.

Most major cities here have at least one motor re-winder/specialist.

 

[rjenkinsgb]

The best fix would be to get the motor rewound but I don't know if anyone does that any more.

It typically costs more than a replacement, with common size motors...

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