Why my L1-N voltage is different compare to L2-N & L3-N?

Hi all,

I've collected some data from one of the flour mixer machine from our customer last week using a powermonitor 3000 M4. But I do not understand the voltage captured from the system.

L1-N showing 163.2
L2-N showing 327.2
L3-N showing 256.5

I taught all L1,2 & 3 should be running the same voltage? It's a 3 phase 415V system. I've captured for 2 machines, but both showing the same result....please comment.

Please refer to attachment for the graph captured.

My guess is a ground fault.

Mickey said:

My guess is a ground fault.

Click to expand...

Will it cause any harm to the system? The system has been running for a few years. System 3 is fine, all L1,2 & 3 - N are about the same voltage, only system 1 & 2 has different L1-N voltage.

Is there a step down transformer? how is the transformers connected. I once had a project where I had 3 phase 3 wire, 2 step-down transformers 480/120 VAC. I forgot to ground my yellow phase and the voltage becomes:
Red- 120VAC +/-
yellow- about 250VAC
blue- 120VAC +/-

so it depends on the system you want to attain. if it is the right format and connection, then it is probably a gound fault or a short.

regards
Sherine T.

You said you used a power monitor. Then my guess is that you didn't read the voltage, but the power usage. The different values is because of unbalanced load on the three different phases. It certainly cannot be ground fault because it in this kind of system, a ground fault will mean a short circuit.

Did you read voltages between phases or just from each phase to ground?

A ground fault seems to be popular and I will add to that opinion.

I think if it was a short or partial short you would have problems with smoked motors and fried wires already.

Do your systems include a lot of single phase loads? I've read that connecting single phase loads without concern for balancing the system can cause this type of imbalance.

Your graph shows everything ok up to 14:20, then after that you get the unbalanced readings - is that the correct interpretation of the graph ? Can you measure N to earth.

And also measure L1-E, L2-E, L3-E in stead of L1-N, L2-N, L3-N.

Where is the "N" from ?
Is there an external supply L1-L2-L3-PE and you create L-N via a transformer.
Or is there an external supply L1-L2-L3-N-PE or L1-L2-L3-PEN.

voltage is different

voltage is different

ckchew666 said:

Hi all,

I've collected some data from one of the flour mixer machine from our customer last week using a powermonitor 3000 M4. But I do not understand the voltage captured from the system.

L1-N showing 163.2
L2-N showing 327.2
L3-N showing 256.5

I taught all L1,2 & 3 should be running the same voltage? It's a 3 phase 415V system. I've captured for 2 machines, but both showing the same result....please comment.

Please refer to attachment for the graph captured.

Click to expand...

I think your problem is a diferent loads at each line. The only way to compensate this will be a study os the loads and trying to compensate them.

Ckchew,

I think your power is 415 volts, 3-phase, star(wye)connected, with a center grounded Neutral. If it was a ground fault, and you have proper protection in place, the grounded circuit should trip out and leave only the unfaulted equipment running.

On the other hand, if this is actually an ungrounded Delta system, with the N (neutral) floating and undefined, then the readings from phase-to-neutral have no meaning, as the N voltage will jump all over and move around. Verify this by measuring L1-L1, L2-L3, and L3-L1. If they are balanced, then all is okay and there is no N point in the system. If they are not close to the same, then there is a problem.

If you have 3 separate plant Systems, all with the exact same equipment (unlikely), and System 3 is showing balanced voltages, then Systems 1 and 2 have a problem that needs attention! If there are single-phase loads such as electric heaters, then someone may have incorrectly wired those 1-phase loads for Systems 1 and 2 all on phases L2 and L3, making those phases heavily loaded. It appears that the problem only occurs when certain equipment is switched on, so this theory may be worth looking into. Do you know of any 1-phase loads on Systems 1 and 2 that are not on System 3? Also, do you know if Systems 1 & 2 have been modified so that the wiring is different from System 3?

Another possibility is that the potential and current transformers that are necessary for your Power Monitior, are not wired correctly. A small wiring error in connecting the potential transformers can result in a measurement error.

It sounds like this is a "Y" system. Is the center "neutral" grounded?

I doubt that it is unbalanced loads, since you say line to line voltage is almost all equal.

Tom, I must have missed where he said that line-to-line voltages are equal. Maybe it is me that is misunderstanding the description.

I thought that he has a plant that has 3 separate processes or machines ("systems"), and #3 System voltages are okay, but 1 & 2 Systems are unbalanced, as measured by the Power Monitor.

It always amazes me here that people give you TWO whole sentences, describing the problem in one, and asking for help in the other. Don't they realize that we only know the problem as they describe it to us, and our help is only as good as their description of the problem?

I suppose that the people who are good at describing the problems, have already very easily found help before they ever arrive here!

I would have the same question as Tom Jenkins, Is the center "neutral" grounded? If you look at the graph, L1 (red) is going in oposite directions as the other 2 phases.

Gary

Lancie1 said:

Tom, I must have missed where he said that line-to-line voltages are equal.

Click to expand...

I went back and read it again, and you are correct - I misinterpreted what ckchew said. I should have said "check line to line voltages. IF they are the same ..."

Sorry about that.

Back to the Basics

Please go get a meter (or get someone who is qualified with a multimeter) and take these measurements without looking at the power monitor. You can't know if the power monitoring is set up wrong, connected wrong, reading wrong, or if there is truly a problem with the voltage readings until you VERIFY what you are getting. To continue to rely on the power monitor readings without checking them with a good meter is a guessing game at best.