On 2008-07-18, Joseph Gwinn wrote:
In article ,
Ignoramus31221 wrote:

On 2008-07-18, Wild_Bill wrote:
You don't quantify the amount of motor insulation leakage current, or
describe how it is that you became aware of it.

I measured it yesterday with an clamp on ammeter, it came out as 9
amps (!). Really a shocking amount.

Nine amps?...!! If that's true, the motor is toast, and should be
replaced. Immediately. When we talk of leakage, we mean a few
milliamps.

Probably a measurement error due to spikes.

No leakage without VFD.

The VFD is not causing this.

Unless the clamp-on is confused by the high-frequency output of the
inverter drive.

That's my feeling.

One way to tell is to hook all three windings of the motor together and
connect them to 110 volts through a light bulb (not through the VFD),
and measure the current to ground. In a perfect motor, the current will
be zero. (The motor will not even try to run, or be in any danger, and
single-phase is what to use.)

Do not use an ordinary ohmmeter for this. The test must be made with
full power voltage, or more. Or a megger, if you have one:
http://en.wikipedia.org/wiki/Megger.

I think that I have a megger.

If the current is in the amps, at least one winding is shorted to the
frame, and the motor must be replaced or rewound.

This is *exactly* why one firmly grounds the frame of a machine tool.

I think that the motor is not leaking 9 amps and it was a measurement
issue. I will try using a analog clampon next time. But there is
leakage of some, unspecified amount.

i