On Thu, 24 Jan 2008 12:56:52 -0600, bud--
wrote:

M Q wrote:


bud-- wrote:

Mike Hennessey wrote:
...
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of
the spike depends on where in the sine wave the fan is turned off -
random effect. Fans intended to be used on GFCI circuits could be
built with more winding isolation. I haven't seen answer yet that
better fits what happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike
from the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".


If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.


Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.


If a supply wire was near an end of the armature - could be.

Reverse the motor leads in single phase? Doesn't this happen 120
times per second?

Wouldn't an incandescent lamp be a purely resistive load?