Motor capacitance to ground check

You need to add some more details to your question.

Motor's single phase power in kW or hp? Or

Winding capacitance
or C in filters in VFD,

or...

I would be surprised if the capacitance to ground be important, I have worked years ago on big AC & DC motors, we were more interested in the value of the insulation to ground and 1 Megohm was considered to be the low end of safe......less than that and we had to fix it!!

I will be very interested to see what other CR4 people will write in answer to your question.....

I agree

Andy,

I agree with 1 Megaohm, as I use the same concept. Thankyou for mentioning it.

....less than that and we had to fix it!!

That's easier said than done.

Out of interest, please give a couple of examples from something relatively easy (like drying out a damp motor) to something complex (like a complete rewind).

In your experience what was your best and worst case - what did you have to do 'to fix it['

On DC Motors, it was usually a problem of dirt (carbon dust), remove armature & field windings, clean and re-impregnate the field windings.

Armatures, if defective were replaced and the old one sent back to the manufacturer if cleaning and skimming of the commutator were simply not enough, (an earth to the main shaft for example), but this was rarely the case, it was usually field windings that had earthing problems.....

Usually though it was just a clean and replacement of worn down brushes, new bearings.

On AC motors it was usually just a clean and/or bearing replacement, I remember very few earth problems as the field windings were very well embedded in some green colored stuff, god knows what it was!!

AC motors are in my experience generally much more reliable and require far less "hands on".

Thanks Andy.

Just as I thought; a lifetime's work summed up in a few words.

Thanks

Cliff.

Here's a bit of my experience.

Once worked for a cement manufacturing firm on Job Training. We had a lagre induction motor which overheated beyond ambient temperature of 40 °C as we checked with thermal gun, can't remember the actual temperature. The motor tripped several times during previous night shift. Besides that the motor was in operation for quite a number of years. We (2 electricians and I) executed insulation test, found that the motor insulation went below 1 megaohm.

We hired a contractor who rewinded the motor with good insulation. After which we tested, all was O.K. Motor operational temperature was at ambiemt 40-45°C, as with good insulation.

Our induction motor was back on the production line.

General rule of thumb: not less than 1 Mohm for any resistance to ground.

It would appear that most of us do not see the need for a capacitance check!

Maybe you misunderstood the need for a high voltage "resistance" check, that is a resistance measurement made with what we called a "Megger" which used a small hand cranked generator to produce a high (could be painfull!) ac voltage, to calculate the resistance of all windings/fields to ground....(I am sure that the equivalent unit today uses batteries and a voltage stepup system!)

Just measuring with a multimeter is not good enough as the voltage used is too low to break down some "illegal" paths to ground!

In the UK, I believe the "Megger" range of instruments were produced by Avo of Dover......

The equivalent unit today is the Insulation Ohmega Tester.

In the context of a capacitance-loaded 1/4 wave vertical antenna, as much as possible.

...DUUUH!!!