phase to phase voltage is erratic

[Belnald]

System: 480v, 3 phase control panel. feeding numerous VFDs and 24vdc x-formers.
Issue:
Phase-1 to Phase-2 is steady at 480v with a deviation of 450 to 550
Phase-2 to Phase-3 is erratic at 480v with a deviation of 300 to 475
Phase-3 to Phase-1 is erratic at 480v with a deviation of 50 to 475
Amps is the same on all 3 phases. it varies with load as expected.

The system as a whole runs ok, but this voltage imbalance worries me.
Any ideas what the cause is?

thanks

 

[ActionDave]

If things are running ok why are you checking voltages?

 

[GoldDigger]

And exactly what kind of meter are you using to make the measurement?

I would be very much interested in seeing the three line currents over the same time period. Either at the feed panel where you measured the voltages or on any single large motor that is continuously connected and running.

 

[Belnald]

Started taking the reading to locate the cause of an Overvoltage Fault.
Meter is an Extech 3-phase power analyzer/datalogger. Model# 382100

I am happy to send the Excel file I have if you supply an email address.
total size is 134Mb

 

[Belnald]

Readings were taken at the load side of the main breaker for this panel.
When the system is not running (all motors Idle) amps = near 0, and the phase to phase voltage is steady at 480v. There are no large motors, but 15 <3HP motors on Powerflex525 drives. These motors are conveyors and start/stop independently of each other.

 

[GoldDigger]

Readings were taken at the load side of the main breaker for this panel.
When the system is not running (all motors Idle) amps = near 0, and the phase to phase voltage is steady at 480v. There are no large motors, but 15 <3HP motors on Powerflex525 drives. These motors are conveyors and start/stop independently of each other.

Once you rule out a problem with noise on the pickup leads of the data logger which might be causing false readings, the most straightforward problem would be the intermittent loss of or very high resistance in one of the two phase conductors (specifically the one which is common to both erratic voltage readings.

It is possible that you could lose a line conductor intermittently and the single phased motors already running could produce a back EMF on the open or high resistance lead. This would prevent you from seeing total loss of voltage as the running motors act as rotary three phase converters.
That is one reason why I would like to see current readings, at least on the phase wire common to the two bad readings. If it goes to zero and the motors keep running, you have a partial or complete single phase condition.

 

[Jraef]

So I'm assuming the 480 readings are with the meter set on averaging, and the erratic readings are reading on instantaneous? If so, and all of the loads are VFDs, I'd suspect this is the result of your meter not being able to deal with the harmonics caused by the VFDs. Do you at least have line reactors ahead of them? If not, that's the first thing I'd recommend.

 

[Ingenieur]

what is the period?
time to swing from min to max?
your supply should be 'stiffer' than that

either bad readings or bad connections imo

 

[ptonsparky]

Wild readings tend to make me check the meter and the leads first.

 

[Phil Corso]

Bel​nald...

Where did you insert (connect) 4th wire... neutral or ground?

Regards, Phi Corso

 

[Besoeker]

System: 480v, 3 phase control panel. feeding numerous VFDs and 24vdc x-formers.
Issue:
Phase-1 to Phase-2 is steady at 480v with a deviation of 450 to 550
Phase-2 to Phase-3 is erratic at 480v with a deviation of 300 to 475
Phase-3 to Phase-1 is erratic at 480v with a deviation of 50 to 475
Amps is the same on all 3 phases. it varies with load as expected.

The system as a whole runs ok, but this voltage imbalance worries me.
Any ideas what the cause is?

thanks

Highly unlikely that your supply is doing this.
Try another meter.

 

[ptonsparky]

Define your terms:
Steady.
Erratic
Deviation

How can you get steady with a deviation of almost 100 volts?

 

[kwired]

Once you rule out a problem with noise on the pickup leads of the data logger which might be causing false readings, the most straightforward problem would be the intermittent loss of or very high resistance in one of the two phase conductors (specifically the one which is common to both erratic voltage readings.

It is possible that you could lose a line conductor intermittently and the single phased motors already running could produce a back EMF on the open or high resistance lead. This would prevent you from seeing total loss of voltage as the running motors act as rotary three phase converters.
That is one reason why I would like to see current readings, at least on the phase wire common to the two bad readings. If it goes to zero and the motors keep running, you have a partial or complete single phase condition.

I kind of agree that the partial or complete loss of a "phase conductor" is the direction I would be investigating first. If possible maybe bring in some temporary load and connect it and see how things respond. 10 or 15 kVA three phase space heater would be a great test load, if you still have similar issues you know it isn't harmonics or other solid state controls issues when the test load is simple resistance.

 

[jcassity]

dummy load it like kwired said, then monitor only your dummy loads to see if the problem remains.

my gut tells me ive seen this before where my service source was Wye 3ph 4w yet "some" of my motor loads were 3ph delta! and some of the motor loads were using two phases and a ground only.

is your source panel / service wye?

 

[kwired]

dummy load it like kwired said, then monitor only your dummy loads to see if the problem remains.

my gut tells me ive seen this before where my service source was Wye yet "some" of my loads were delta!

is your source panel / service wye?

That itself shouldn't really matter.

Supply three single phase line to line loads and balance them across all three phases and you essentially have the same thing.

Supply a three phase wye load but with a floating neutral point in that load (like a typical 9 lead motor, or a three phase heater wye connected but with no connection to the supply neutral) and you still sort of have the same thing when it comes to how the source sees the load.

 

[Belnald]

Erratic Voltage Issue Solved.

Erratic Voltage Issue Solved.

I thank all and everyone for their information. with this information I have narrowed down the problem to one of the drives is going bad, and causing the voltage to spike-dip every few seconds.
By monitoring the system while it was idle, the voltage was steady and stable at 480v phase to phase.
By running one motor (VFD) at a time, the voltage stayed right until the defective motor (VFD) was ran. Then the voltage went erratic on two phases. So the culprit was a defective VFD. But not a bad VFD. It was still running the motor and communicating properly.

Thanks everyone for all your help.

 

[GoldDigger]

I thank all and everyone for their information. with this information I have narrowed down the problem to one of the drives is going bad, and causing the voltage to spike-dip every few seconds.
By monitoring the system while it was idle, the voltage was steady and stable at 480v phase to phase.
By running one motor (VFD) at a time, the voltage stayed right until the defective motor (VFD) was ran. Then the voltage went erratic on two phases. So the culprit was a defective VFD. But not a bad VFD. It was still running the motor and communicating properly.

Thanks everyone for all your help.

FWIW the defective drive almost certainly caused current spikes corresponding to the drops in phase voltage. But likely the current measurements did not have the same short time excursion catching that the voltage measurements did.

 

[ptonsparky]

Open question.

What would be causing this? The current draw on L3 must be fairly high even briefly to cause that much voltage swing. I would think that the stiffness of the POCO power supply would be more than adequate for a plant of any size at all.

 

[kwired]

Open question.

What would be causing this? The current draw on L3 must be fairly high even briefly to cause that much voltage swing. I would think that the stiffness of the POCO power supply would be more than adequate for a plant of any size at all.

My thoughts also.

Isolated individual load like an irrigation machine- you might see this sort of thing easier then in a facility that has larger supply. If there were a single large drive in a larger facility doing this it would draw the entire plant down when it does it. If it only effects an individual feeder - he may still have problems on that feeder more so then a drive problem.

 

[ptonsparky]

OP. Was this the last drive you tested or did you go on and check them all? A second time?