Strange portable genrator voltages

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copper123

Senior Member
Hello,

What would cause a three phase generator that appears to be wired Delta, to give you good reading between phases, A,B BC, CA all at 240 volts, and then get reading of 220 volts between B and Neutral, and C and Neutral? A to Neutral is 120.?
Wild Leg voltage would be expected but not two of the three phases between phase and Neutral.
Generator also has the L0 and ground bounded.



I am a little lost as I have never ran into this before.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
It has no ground reference is my WAG. You are trying to check voltage to ground on an ungrounded sytem. The voltage will float all over the place.


The neutral would have to have a reference. Are you thinking about voltage to ground measurments?

Is it possible there is a separate winding for the 120V power, connected only at point A?

Steve
 

AdrianWint

Senior Member
Location
Midlands, UK
What would cause a three phase generator that appears to be wired Delta...

I think you've answered your own question in the first line. A Delta configuration, by definition, has no neutral star point so can't have a Line to Neutral voltage - only line-to-line voltages make any sense. The L0 & the ground may well be bonded together .... but I suspect that the L0 isnt connected to the winding.

If you are attempting to measure the voltage between Line & 'neutral' with a high resistance meter (like a DVM) you will get a reading due to capacitive coupling, but this voltage is phantom and unable to supply any current.

Can I suggest a small experiment ?..... connect a 240V lamp (or two 120V lamps in series) between a line & this 'neutral' point and see what happens. I suspect that the voltage will fall to zero & the lamp will not glow.
 
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copper123

Senior Member
Thank you for the posts!

As you can see in the picture, the piece of S.O. cord is feeding a small temporary electrical panel. They only carried three wires over and have forgotten the ground. When I put my meter on the two phases feeding the panel, I thought for sure I would have gotten a potential difference of 120 volts between the Neutral bar of the panel and L3 or L2 of the generator lugs. With the bonding jumper installed at the generator you should have a reference. What I discovered was L1 to N0 had a reading of 120, but L2 and L3 to NO (Neutral on the panel) had 220vAC. Two of the three phases to NO should have had a 120 volt reading.

I will try later to upload the picture, as it seems to be having a bit of trouble!
 

kwired

Electron manager
Location
NE Nebraska
Thank you for the posts!

As you can see in the picture, the piece of S.O. cord is feeding a small temporary electrical panel. They only carried three wires over and have forgotten the ground. When I put my meter on the two phases feeding the panel, I thought for sure I would have gotten a potential difference of 120 volts between the Neutral bar of the panel and L3 or L2 of the generator lugs. With the bonding jumper installed at the generator you should have a reference. What I discovered was L1 to N0 had a reading of 120, but L2 and L3 to NO (Neutral on the panel) had 220vAC. Two of the three phases to NO should have had a 120 volt reading.

I will try later to upload the picture, as it seems to be having a bit of trouble!

With bonding jumper installed at generator you have a reference at the generator but if you don't have a grounding path between the generator and the panel or to earth you still essentially an ungrounded source and measurements to ground will be fairly unpredictable.
 

kwired

Electron manager
Location
NE Nebraska
Wouldn't you see that if L1 and N0 are swapped between the panel and generator?
/mike

If L1 and N0 are swapped you will have

L1-L2 = 208 volt
L1-L3 = 120 volt
L2-L3 = 240 volt
L1-N = 120 volt
L2-N = 240 volt
L3-N = 240 volt

Assuming proper grounding and bonding has taken place and it is indeed a 4 wire delta with midpoint of one phase used as N0
 

hurk27

Senior Member
Sounds like the center tap between A and C has opened on the C side to the X0, if so the 240 volts your reading is because it is operating in an open delta. without this connection the generator has no reference to the X0 except through A phase, but I'm surprised your not seeing strange voltages on the other two legs to X0?

Not having an EGC from the generator to the panel has no bearing on the voltage as you are still referencing the X0 to phases if the neutral is bonded at the panel.
 

kwired

Electron manager
Location
NE Nebraska
Sounds like the center tap between A and C has opened on the C side to the X0, if so the 240 volts your reading is because it is operating in an open delta. without this connection the generator has no reference to the X0 except through A phase, but I'm surprised your not seeing strange voltages on the other two legs to X0?

Not having an EGC from the generator to the panel has no bearing on the voltage as you are still referencing the X0 to phases if the neutral is bonded at the panel.

but from what I understand he doesn't have a neutral at the panel either. Just the three phase conductors. If there is no other other path to the generator frame or neutral then the neutral bus in the panel is just floating.
 

hurk27

Senior Member
Yep I missed in post 5 where he said they brought only 3 wire over to the temp panel. if the neutral was not brought to the panel, then in this case yes the neutral is floating at the temp panel and can have the same effect as a lost neutral, if there are loads on the panel this imbalance might be what he is reading,
 

copper123

Senior Member
Thank you Gentleman,
Let me try to describe a bit better:

They have fed a 100 amp, single phase electrical panel via a 8/3 SO cord to a 3 phase generator. The generator and the panel are an estimated 100 feet apart. The SO cord of course has three insulated conductors running in it.
They have fed the panel in this way. Two insulated conductors have been landed on the A and B phase. The third conductor is landed on the ground bar of the panel. The ground bar is solidly bonded to the can via its 8/32 screws.
The panel is feeding a submersion well that is running on 240 volts. The well runs through a well controller. The well controller is fed via a two pole, 30 amp breaker in the panel.
I was asked to just come over and make sure the generator was giving correct readings before they turned it on for the season.
With the generator running, I took readings at the distribution panel. I had 240 volts between the A and B phase of the panel. (Good) I had 120 volts between A phase and the ground. (Good). I had 220 volts between B phase and the ground. (Hmmm). I closed the door on the temp panel, walked the 100 feet to the generator and shut it off. I opened it up and looked for the wiring diagrams. Sure enough, it was wired in a center tapped Delta. The diagram shows the center tap between L2 and L3. They had two phases landed on L1 (black) and L2 (Red) I swapped the red from L2 and landed it on L3. I closed up the equipment, turned on the generator and walked back to the panel. As I thought, the A phase of the panel still had a good reading between phase and ground. (120), but to my dismay, I still had a 220 volt reading from B to ground. This was/is a bit of a puzzle as I have swapped the suspect phase over to the other leg.
Additional background: The generator frame has been grounded via a ground rod, but the small panel was not. (No ground rod was driven)
 

kwired

Electron manager
Location
NE Nebraska
Thank you Gentleman,
Let me try to describe a bit better:

They have fed a 100 amp, single phase electrical panel via a 8/3 SO cord to a 3 phase generator. The generator and the panel are an estimated 100 feet apart. The SO cord of course has three insulated conductors running in it.
They have fed the panel in this way. Two insulated conductors have been landed on the A and B phase. The third conductor is landed on the ground bar of the panel. The ground bar is solidly bonded to the can via its 8/32 screws.
The panel is feeding a submersion well that is running on 240 volts. The well runs through a well controller. The well controller is fed via a two pole, 30 amp breaker in the panel.
I was asked to just come over and make sure the generator was giving correct readings before they turned it on for the season.
With the generator running, I took readings at the distribution panel. I had 240 volts between the A and B phase of the panel. (Good) I had 120 volts between A phase and the ground. (Good). I had 220 volts between B phase and the ground. (Hmmm). I closed the door on the temp panel, walked the 100 feet to the generator and shut it off. I opened it up and looked for the wiring diagrams. Sure enough, it was wired in a center tapped Delta. The diagram shows the center tap between L2 and L3. They had two phases landed on L1 (black) and L2 (Red) I swapped the red from L2 and landed it on L3. I closed up the equipment, turned on the generator and walked back to the panel. As I thought, the A phase of the panel still had a good reading between phase and ground. (120), but to my dismay, I still had a 220 volt reading from B to ground. This was/is a bit of a puzzle as I have swapped the suspect phase over to the other leg.
Additional background: The generator frame has been grounded via a ground rod, but the small panel was not. (No ground rod was driven)


If you do not have any line to neutral loads at the panel you kind of don't have any operational problem here.

You apparently have the wild leg as one of the two phases brought to the panel. When you tried to swap them you somehow mistakenly got the wrong one. Probably still had wild leg and swapped the two 120 volt to ground phases.
What were voltages at generator - you did not mention checking there.

The 240 volt load will operate between any two of the three phases. The only issue is whether or not the circuit breaker is a 240 volt breaker or a 120/240 volt breaker. You can not use the 120/240 rated breaker with the wild leg.
 
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