# VFD-Line Side Readings



## AaronJ (Oct 28, 2009)

Hi everyone, have a quick question. We were working on a VFD that is 277v.

So on the line side of the VFD when off, we get the 277v reading. Once we kick it on, the line side starts reading crazy numbers. The VFD was running at 22%, but the voltage is jumping all around from anywhere of 30v to 330v on the line side. 

Thanks in advance!


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## micromind (Aug 11, 2007)

Is the system 277/480Y or is it some sort of ungrounded 480 ∆ or grounded B?


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## just the cowboy (Sep 4, 2013)

*277 ??*

Are you sure it is a 277v drive, never seen one. If it is a 480 volt drive stop reading from line to ground or neutral, read phase to phase.

If wye feed check and see if drive is good for a wye feed, sometime a jumper has to be removed


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## AaronJ (Oct 28, 2009)

micromind said:


> Is the system 277/480Y or is it some sort of ungrounded 480 ∆ or grounded B?


This is a 277/480


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## AaronJ (Oct 28, 2009)

just the cowboy said:


> Are you sure it is a 277v drive, never seen one. If it is a 480 volt drive stop reading from line to ground or neutral, read phase to phase.
> 
> If wye feed check and see if drive is good for a wye feed, sometime a jumper has to be removed


This is a 277/480 system. We are getting the same readings phase to phase and to the ground.


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## just the cowboy (Sep 4, 2013)

*Something is wrong big time*



AaronJ said:


> This is a 277/480 system. We are getting the same readings phase to phase and to the ground.


On a 277/480 volt system you can't get the same readings phase to phase AND phase to ground, it don't work that way. So you are saying you are getting 277 volts phase to phase with drive OFF? Sounds like blown fuse somewhere. You SHOULD read 480 volts phase to phase from A-B, B-C, A-C. If you have 277 you are reading a backfeed from a grounded or something connected to a phase conductor.


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## frenchelectrican (Mar 15, 2007)

AaronJ said:


> Hi everyone, have a quick question. We were working on a VFD that is 277v.
> 
> So on the line side of the VFD when off, we get the 277v reading. Once we kick it on, the line side starts reading crazy numbers. The VFD was running at 22%, but the voltage is jumping all around from anywhere of 30v to 330v on the line side.
> 
> Thanks in advance!


Did you check the fuse ? 

Did you check the connection at the VSD or other locations? 

Did you check the voltage at the breaker or main disconnection location ?

I am leaning toward to blown fuse or bad breaker or bad connection somewhere on that circuit on line side of VSD unit. that is one of three possiblity that can cause the issue with it.

and by the way many VSD will have some kind warning on display when you loose phase conductor incomming side unless it is disabled on single phase mode.


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## paulengr (Oct 8, 2017)

I’m a drives field engineer.

First, do not take readings on a drive without a true RMS meter. Many Fluke meters are useless on drives work. Non-RMS meters use some simple electrical tricks to approximate RMS values as long as the input is a sine wave. A VFD isn’t so you can’t use them.

Almost ALL 480 VFDs run phase-to-phase. Phase to ground is meaningless except many of them have surge arresters called MOVs. If you exceed the drive’s input voltage on the transistors it “self commutators” which is a nice way of saying short circuits and fails almost instantly. The best protection is with phase-to-ground MOVs but phase-to-phase can be used too. Drives may have none, both (often with a jumper), or one or the other. You need to know and configure it correctly.

Finally the input Power can be solidly grounded, ungrounded, or resistance grounded. If it’s solidly grounded it should stay pretty close to 277 to neutral or ground all the time. If you measure to an isolated ground all bets are off because that’s kind of meaningless (measuring against something you don’t control). The ground lug that MUST be connected at least to the motor with a separate ground wire on the drive is your reference point. Don’t trust a screw that might be painted/disconnected in the cabinet.

If the power system is resistance grounded and you have a fault somewhere, voltages will be close to 480/480/0 (anywhere not just at the drive) so it’s time to go find and fix it. It may not be the drive at all and almost always isn’t.

If you have an ungrounded system, line to ground readings are erratic everywhere. You can’t use them. It is possible to be ungrounded even accidentally and go for a long time without knowing it. The drive will work but for instance the surge arresters can be blown out and the switching going on in the drive will produce wild outputs.


Sent from my iPhone using Tapatalk


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