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.

The VFD is not causing this.

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

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.

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.

Joe Gwinn


This effect is probably present in many home shop machines that users have
converted to VFD operation, of which many of them are probably unaware.

Proper earth grounding of a machine is mandatory, regardless of what the
power source is.
There are no perfect insulating materials, only good ones and better ones.
Insulation in motors is rated for maximum voltage and temperatures.

Once the insulation materials are compromised, (arc-over or similar path is
established), in any electrical device, the earth ground path is critical
for safety.

that's why I decided to discontinue.

The motor insulation current leakage (in power line operated, or VFD-driven
motors) is documented for those interested in looking for the info.

Motors that are rated for VFD operation are typically labeled Inverter
Rated
Duty or something similar. The insulation in these motors is specifically
chosen to minimize/reduce leakage currents for the fast rise times of the
VFD output voltages (variable time square waves, in appearance).

A motor that isn't rated for inverter duty could be a dual voltage motor
rated at 240 or 480VAC. If the motor is operated with a VFD output at 240V,
the insulation is generally up to the task of minimizing the current
leakage.
If that motor were operated with a VFD output of 440V or greater, then the
insulation's intended rating will be exceeded.

This is a single voltage (two speed) 220v motor. :-(

i

WB
.........
metalworking projects
www.kwagmire.com/metal_proj.html


"Ignoramus31221" wrote in message
...
I had some bad news last night.

As the drive was running the motor, the motor leaks a lot of current
through its insulation. (like Joe Gwinn story) Not good. Phase
converter does not create this effect, so it is due to voltage spikes
from the drive that this old insulation cannot handle. I do not want
to ignore this issue by grounding, as I feel that it can ruin the
motor.

I may try to locate a line reactor to dampen this, and shorten some
cables, but for now I will go back to the phase converter.

i