measuring 3 phase circuits correctly

[drumsticksplinter]

Hello,

I've just bought a new piece of machinery for my workshop and wired it into my 3 phase power, the same way as I always do. The machine powered up and worked fine until I noticed a fault showing up on 1 of the motor drives. When reading through the fault codes is states over voltage on the main circuit. The drives are single phase 220 - 240vac and I'm in the UK with 240V single phase 415V three phase. I then measured the voltage from the individual phases to neutral and I got some really strange readings.... fluctuating anywhere from 248v to 315v?? I did some investigating and found the cable I was testing with wasn't great, it came with another machine and looks like 2 of the cores had overheated at some point. I've changed the cable now for new, which seems to keep the motor drives happy, but the voltages from the phases to neutral are still all over the place! I have tried 3 different multi meters, all with similar readings.

has anybody got any ideas what I'm missing?

 

[Minder]

Measure between 3ph only, I would not normally expect a neutral supplied, just GND.
What does it read off and on load?
If the machine is rated 415, the lower voltage drives are often fed from a transformer in the enclosure. Is this a CNC machine?
M.

 

[ADRT]

Here in the U.S. We use 480VAC three phase. Normally will see anywhere from 470 - 490 depending on the transformer that is feeding the MCC and where there taps are. Read from each phase to ground is nominally 240VAC. To see suck a high spoke on any phase as you are describing is very odd. We generally will have someone go around and take images with an infrared camera looking for hot spots on the connections to the transformers and inside the breaker panels and motor control buckets. High temratures at these points can indicate loose connections or conductors that have been over stressed. Things like this could cause some very strange readings indeed.

If you do not have access to an infrared, try to find a good laser thermometer. Fluke makes a decent one not sure of the price though. Look for high readings at your termination points. Compare with other circuits and anything that just appears to be a higher then the others should warrant further investigation.

Check conductors and tightness of connections. Best to do this cold, no power, unless you are properly trained and equipped to work it hot.

There has been on many occasions that simply tightening a lug or redoing a connection by cutting the effected conductor back a couple of inches and relanding has solved strange anomalies like you are describing.

 

[drumsticksplinter]

Thanks for the replies!

The system is definitely a 3phase + neutral + earth. The servo drives are mains voltage drives and use 1 phase + neutral in the range of 220 - 240VAC. With no load I measured around 246V from each phase to neutral, once the drives were enabled the voltages I was able to read were very unstable and ranged from 248v to 315v. There is also a 5kw inverter connected in the panel, could this be making the voltages read strange?

The machine is a CNC router yes.

I will re check the connections today and see if I get any different results.

Thanks.

 

[Arouse1973]

Is your multi-meter a true RMS type? Your meter could be trying to resolve a sine wave with harmonic content. You will get strange readings if the meter is just the type that only passes the positive half of the sine-wave and then computes the bottom half as it expect this to be a pure sine wave. It's when it's on load the problem occurs so it must be the load switching causing issue with the meter, because you mentioned the equipment is working correctly now.

Thanks
Adam

 

[signalman72]

Here in the U.S. We use 480VAC three phase. Normally will see anywhere from 470 - 490 depending on the transformer that is feeding the MCC and where there taps are. Read from each phase to ground is nominally 240VAC.
.

You sure that a 480 3P system should read 240 phase to ground? In my experience it has measured about 277.

 

[Minder]

You sure that a 480 3P system should read 240 phase to ground? In my experience it has measured about 277.

He is in the UK where residential distribution is a 240v phase to star neutral, so the 3ph is 415v.
240 x root3.
M.

 

[signalman72]

He is in the UK where residential distribution is a 240v phase to star neutral, so the 3ph is 415v.
240 x root3.
M.

I'll buy that. However, the claim was made that in the US that 480 3ph line to ground should be 240. If you divide 480/1.73, you get ~ 277.

 

[ADRT]

I'll buy that. However, the claim was made that in the US that 480 3ph line to ground should be 240. If you divide 480/1.73, you get ~ 277.

Sorry, should have been a little more specific on this. This depends on how the given transformer has been configured. The systems at the plant I work at are a delta configured system that will give you 240vac to ground. This also creates a wild phase.

 

[Kiwi]

Would using an oscilloscope give a better idea of what is happening to the CNC router power supply?

 

[ADRT]

Is your multi-meter a true RMS type? Your meter could be trying to resolve a sine wave with harmonic content. You will get strange readings if the meter is just the type that only passes the positive half of the sine-wave and then computes the bottom half as it expect this to be a pure sine wave. It's when it's on load the problem occurs so it must be the load switching causing issue with the meter, because you mentioned the equipment is working correctly now.

Thanks
Adam

I actually find this answer to be the most likely even over my own. What I'm talking about will normally cause a higher current reading on one leg, nuisance trips, and more seriously, arc faults.

The other thing to consider here is that overvoltage faults in VFD's are ussually caused by an overvoltage on the DC bus. This can be caused by spinning a large mass at high rates of speed. It can actually overdrive the motor and cause regeneration to the drive. This is ussually dealt with by adding a dynamic brake to the drive in question.

If repairing the cable took care of your problem here, I think your OK.