mulitple conductors feeding exhaust fans thru frequency drives causing back feeds

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littledog

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We have 1-conduit with 6#10 and ground wire for 2 exhaust fans
these are wired thru 2 separate frequency drive units and then to the fans
when one fan is taken off line there is an 18volt reading from each phase to ground on the disconnected feeder
What can cause this situation
Help please
 
littledog said:
We have 1-conduit with 6#10 and ground wire for 2 exhaust fans
these are wired thru 2 separate frequency drive units and then to the fans
when one fan is taken off line there is an 18volt reading from each phase to ground on the disconnected feeder
What can cause this situation
Help please
Click to expand...

Its not unusual to get some kind of voltage reading if you are using a high impedance meter like a digital meter.
 
Every drive I have wired was a 3 pipe system.
1 pipe for power in, could have more than one 60hz feeder in it.
1pipe for power out per drive as the freq. out isn't 60hz and will induce on to any other wires in the same pipe.
1pipe for control
 
sameguy said:
Every drive I have wired was a 3 pipe system.
1 pipe for power in, could have more than one 60hz feeder in it.
1pipe for power out per drive as the freq. out isn't 60hz and will induce on to any other wires in the same pipe.
1pipe for control
Click to expand...


We have installed separate conduit systems for all control wiring
The feed from the panel goes directly to the FD (2) and then out the the exhaust fans with a single conduit with the 6 #10 and the ground
Can i be experiancing harmonic distortion causing this 24V reading?
What do you think would clear this us just short of running a separate conduit to each motor of which no one will pay for as its 100' run in a 20' ceiling
help is appreciated
littledog
 
I would think that imposing a different wave form than the drive is trying to put out would cause the motor to run at a different speed. Then the BMS/EMS would tell the drive to change to get the right speed and cause the drive to "hunt" for lack of better words.

Not a drive pro. but the drives i have done did demand 3 pipe systems.
 
I've heard, but not tested or installed myself, that if you tightly braid the wire for each circuit before you pull through the pipe there is way less interferance. Like a cat5. Can anyone confirm this?
 
Bob's point is a good one. You need to use a LowZ meter to check this voltage. Three-way travellers often show up to 60V on a high impedance meter (10MOhm input resistance) when in reality there is no voltage present.
 
littledog said:
We have 1-conduit with 6#10 and ground wire for 2 exhaust fans
these are wired thru 2 separate frequency drive units and then to the fans
when one fan is taken off line there is an 18volt reading from each phase to ground on the disconnected feeder
What can cause this situation
Help please
Click to expand...

The installation is NOT a good idea, the two drive load side conductors should be wired in a SEPARATE RGS or EMT conduit or shielded cable. Running the two feeds together actually can damage the drives themselves and/or interfere with each others operation.
 
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