Multiple VFD Motor Circuits in One Conduit

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natfuelbill

Senior Member
Folks -
Any experience or thoughts on installing multiple (>1) motor circuits in one conduit?

Multiple VFDs each running their individual motor, with all the motor circuit conductors installed in a single steel conduit using building wire (not VFD specialized cables).

Any problems or limitations with this?
 
D

DAWGS

Senior Member
Location
Virginia
We do this all the time on conveyor systems, and other systems with small motors and pumps. Never have any kind of problems.
 
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natfuelbill

Senior Member
My application is an existing 480V VFD feeding a 15HP, and I am thinking of adding a 208V VFD feeding a 5HP motor running all wires in a single conduit.
 
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VinceS

Senior Member
Consult the MFR prior to specification

Consult the MFR prior to specification

AB's drives-in001_-en-p.pdf goes on and on and on.... But if you are concerenend about the reflected wave effect on the wires you are installing its a good read. Most MFR's have something like it...

A quote from drives-in001_-en-p.pdf chapter 5 "A single IGBT drive output may have reflected wave transient voltage stresses of up to twice (2 pu or per unit) the DC bus voltage between its own output wires. Multiple drive output wires in a single conduit or wire tray further increase output wire voltage stress between multi-drive output wires that are touching. Drive #1 may have a (+) 2 pu stress while drive #2 may simultaneously have a (-) 2 pu stress"

You can plan around and size wire accordingly, and it may be just fine. For long runs shielded cable is costly but effective in containing interference.

If at all possible I always spec one conduit for each drive's motor/motors. If a customer decided to not desire this installation method, I would ensure it's specified in the contract to avoid liability.
 
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charlietuna

Senior Member
i wouldn't chance it--many manufacturers recommend shielded cable, especially inside equipment ! we always kept the load in separate raceways. and some manufacturers double the warentee when a line reactor is installed before the drive--it does protect the drive !
 
wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
Line reactors should always be specified on drive installations. They are cheap, they help protect against capacitor switching transient overvoltages, and help provide a level of harmonic mitigation.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Line reactors for harmonic attenuation are only cheap for small VFDs where they probably aren't especially useful anyway. The harmonics they attenuate are not typically a major issue with small VFDs.

The capacitor issue is something else, but most of the time the wiring provides enough impedance on its own.

Once in a while line reactors are necessary due to the high level of current available.
 
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wirenut1980

wirenut1980

Senior Member
Location
Plainfield, IN
petersonra said:
Line reactors for harmonic attenuation are only cheap for small VFDs where they probably aren't especially useful anyway. The harmonics they attenuate are not typically a major issue with small VFDs.

The capacitor issue is something else, but most of the time the wiring provides enough impedance on its own.

Once in a while line reactors are necessary due to the high level of current available.
Click to expand...

I suppose "always" is always too strong of a word:grin: . Attached is a voltage and current waveform from a case where a utility capacitor switch was knocking out a chiller with a variable speed drive. I suppose because I work for a utility and visit many customer locations, I see about this more than some...
 
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