Ground Detector Lights Shift Phases

[rovineye]

This is a sipboard installation. 480 V 3 phase ungrounded sub station feeding an elevator with a two speed two winding motor. When the motor is running I get ground indications on ground detector lights at the sub station, and they change the ground indication from phase to phase to phase. Sort of a slow stobe effect so to speak. This happens in both speeds. In fact I have 4 elevators that do this (out of eight). All are new.

IR and PI are normal.

 

[Alex68]

Very intriguing question!

What is the strobe effect frequency? Does it change with the motor speed?
What is common with all motor producing the strobe effect (busbar, location, motor size, cable...)?
What is different from the others?
Is there any other component which can produce this lamp lighting when some motors are running?

Regards
Alex

 

[itsmoked]

You may have excessive leakage via capacitance. As multiple elevators run they beat against each other.

Will this happen when only a single elevator is in operation?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rovineye]

Alex68,
Since the ship isn't right here, I can't see the lights myself (troubleshooting via email). It was described as a second or so between changes. I don't know if it is relative to speed. Good question!

All 8 elevator designs are the same. Different locations on the ship. Things like door operation do not cause this when the hoist motor isn't running, but the door VFD motors are further isolated by a shielded isolation transformer.

itsmoked,
These elvators are not close to each other, fed from multiple sub stations (transformers). It does happen when running alone. The distance from the sub stations to the motor controller is only about 100 feet or less and the distance from the controller to the motor is maybe 25 feet. I am thinking capacitance also, mainly because I don't have another suspect. I do know the duration of the ground indication is not sufficient to pick up my ground fault relay alarm.

There is a brake circuit connected between the UP/DOWN and HS/LS main motor contacts. Schematic is here

http://web.mac.com/sleipness/iWeb/Site/Iligan Pics_files/no1 brake circuit.jpg

. The brake connection at T12 was isolated while running in low speed. The ground issue went away! Voltage to ground was checked at that time on the HS winding, and it was 28 volts to ground. Didn't get a phase to phase reading.

I would expect to get some induced voltage in a two speed motor in the unused winding, but have never really tried to measure it before.

If this was caused by capcitance in the brake circuit, via the single phase connection to the unused winding, why would it change phases?

 

[oldfieldguy]

It sounds like system capacitance to me. It is likely that with your system, your available capacitive coupling to ground is sufficient to allow enough current to illuminate your indicating lamps.

You could try temporarily adding ballasting resistors from each phase to ground, sized to draw a few amps, in parallel with the input to the detector light source voltage. This will give you a resistive wye ground point. If this takes care of the problem, then you have a piece of data as to how your system is working.

old field guy

 

[ScottyUK]

Is the brake circuit a phase controller designed to provide a variable braking effort by injecting an adjustable DC level into the stator?


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Sometimes I only open my mouth to swap feet...

 

[rovineye]

ScottyUK,
No, it is just a simple electric release caliper brake (a brake solenoid which releases the brake).

 

[rovineye]

oldfieldguy

I do recall some old Navy ground detectors that used resistors or some sort of balancing capacitors to get around circuits that had interference from capacitance, such as filters used in the communications suite. I am leaning in this direction.