I am stumped! Here's the scenario. Two 56" 120V industrial ceilng fans in parallel. One speed control. These two fans are on phase one of a three phase system. Phase two and three are lighting circuits. All small loads. The fan closest to the rheostat drops out of service. Technician checks and the motor has been fried. Checks voltage while second fan is still running. Line to neutral reads 140V on load side of rheostat. Why? When the switch is in the "high" setting or the "low" setting voltage is 120V. When the switch is adjusted between these settings voltage reads 140V. Again why? I can't explain it to my staff cuz I am at a loss.

The speed control works by chopping up the AC waveform, such that it is no longer a nice clean sine wave. Unless you use a "True RMS" multimeter, non-sinusoidal waveforms cause very screwy AC voltage readings in most cases.

Recheck the reading using a good quality TRMS meter, such as a Fluke 87.

The meter used was a Fluke 337. Brand new.

I'll join you in stumpedness, then....:)

Does the working fan change speed properly between the high and low settings?

When you say the motor was "fried", do you mean actual burnt windings/smell, etc., or just an open connection with no visible damage?

Could this be an issue of an open neutral, causing the other loads on phases B and C to affect the voltage on phase A?

Tech's troubleshooting reports no open neutral, and the fan has an open connection with no visible damage. He also reports that the fan housings seem a little on the warm side.

I just wondering is that ceiling fan is shared with common netrual with other lines if that so i will suggest that the fan run it own netural and Ø line as well because the fan speed control have it own ' modifed sine wave ' reduced voltage connection

Merci , Marc

Sounds like just a defective fan motor from here. A shifting neutral would probably have fried the motor violently and also killed the speed control.

As for your voltage readings, take them with a huge grain of salt, so to speak. If the control is electronic, in addition to chopping the wavewform, it would possibly increase the voltage to compensate for lost motor torque.

OTOH, you could have also been reading "inductive kickback" from the other fan's motor.

And as for the housings seeming "a little on the warm side" remember a normally operating modern motor can safely run up to 40 deg. C above ambient, more than hot enough to burn your hands!!

Replace the bad fan and move one.....