3 phase high leg problem

[cpinetree]

I have a guy on a job - 3phase 240/120v high leg.
They are burning up 3 phase motors.

Here are the readings on the incoming lines, before the main. These have also been checked with a load (Compressor, no difference)

A to ground - 122V
B to ground - 121v
C to ground - 206v (little low)

A to B - 243v
A to C - 242v
B to C - 230v <-- this seems to be the problem

How / why can there be a 12/13v differential when the other legs seem correct.

What to check next?

 

[Frank DuVal]

What overload protection is the 3 phase motor using?

Could be an open delta three phase service, so with a missing transformer, voltage might not be balanced. 5% imbalance does not sound motor destroying to me. What are the current readings on the subject motors?

 

[Jraef]

A 5% current imbalance would not be bad, but a 5% VOLTAGE imbalance is horrible and the likely cause of the motor failure, especially if you have basic inexpensive overload relays that don’t protect for that.

As to why the voltage is unbalanced like that, the likely cause is too much 120V load on that transformer. Those delta high leg transformer setups are supposed to be used where the MAJORITY of loads are 3 phase and in many cases (depending on the transformer configuration) no more than 5% of the loads are 120V single phase. The problem is, the original owner was told this when the service was put in, but the building changes hands over the years and subsequent users don’t get that message, so they keep adding 120V circuits without a permit (which would have ostensibly caught that in the review process). Now, after that fact, your options may be limited to changing over to a 208Y120 system and balancing the loads, or adding a buck-boost transformer to your low leg. The problem with that is that it doesn’t solve the issue of having too much 120V and you may still be overheating that transformer.

 

[cpinetree]

Thanks we'll check the overloads at the motor.

Definitely way too many 120v loads at the office / shop area where the problem is. The only 3 phase load is the compressor.

The rest of the facility coming off the same pole (additional services) utilizes the 3 phase high leg more efficiently.

 

[don_resqcapt19]

A 5% current imbalance would not be bad, but a 5% VOLTAGE imbalance is horrible and the likely cause of the motor failure, especially if you have basic inexpensive overload relays that don’t protect for that.

As to why the voltage is unbalanced like that, the likely cause is too much 120V load on that transformer. Those delta high leg transformer setups are supposed to be used where the MAJORITY of loads are 3 phase and in many cases (depending on the transformer configuration) no more than 5% of the loads are 120V single phase. The problem is, the original owner was told this when the service was put in, but the building changes hands over the years and subsequent users don’t get that message, so they keep adding 120V circuits without a permit (which would have ostensibly caught that in the review process). Now, after that fact, your options may be limited to changing over to a 208Y120 system and balancing the loads, or adding a buck-boost transformer to your low leg. The problem with that is that it doesn’t solve the issue of having too much 120V and you may still be overheating that transformer.

The 5% limit only applies to a single three phase transformer that supplies the wild leg system. I have never seen such a transformer, but know they exist.
The utility will use two or three transformers for this type of service with a larger one supplying the 120/240 volt single phase loads. Around here that type of service is typically used where the 3 phase load is very small as compared to the single phase load. Most often they will install this as an open delta. The local utility calls the two transformers the "lighter" and the "kicker", with the lighter supplying the single phase loads and the kicker adding the third phase for the 3 phase loads. Of course that doesn't mean that the single phase loads have not been expanded to the point where they caused the voltage imbalance, but I would expect two legs with a lower voltage if that was the case.

 

[gar]

180816-1729 EDT

Something is very wrong.

The sum of the voltages around a delta has to equal zero. The numbers provided do not add up correctly for close to 120 degrees between phases.

Drawing the phasors for the given line to line voltages I get about 202.4 V for C to neutral. This is somewhat lower than the 206 measured to ground. Is ground neutral or earth?

It would be useful to actually measure phase difference between vector AB and AC.

It is not clear what the load conditions were when the measurements were made. I would like to know the voltages when the transformers are totally unloaded. The C to ground should be made to neutral and not ground. Are there two or three pole transformers? How do the voltages change with no load on A-N-B and a maximum load is applied to AC and separately to BC..

 

[ptonsparky]

Gar,
I would suspect that if the AC is the only three phase load, he has no neutral at that starter. He most likely has a ground, EG, and is measuring to it, not earth. Very few of my voltage readings at motor controls/starter would be done to a neutral, almost always to an Equipment ‘ground’, not earth.

 

[kwired]

Gar,
I would suspect that if the AC is the only three phase load, he has no neutral at that starter. He most likely has a ground, EG, and is measuring to it, not earth. Very few of my voltage readings at motor controls/starter would be done to a neutral, almost always to an Equipment ‘ground’, not earth.

And the motor won't care what the voltage to neutral/ground is anyway, it only connects to A,B and C. But as mentioned in OP 230 B-C is a problem when other two readings are 242 and 243, especially on motor that is producing near rated output.

 

[jaggedben]

Correct me if I'm wrong, but if it's a high-leg system with a bonded neutral, it would make less sense for the high-leg to EGC voltage to be 'a little low'. The neutral might see some voltage drop due to load, the EGC shouldn't.

 

[gar]

180816-2220

If you have a wild leg delta that is balanced with 243 V line to line, then the wild leg to neutral would read 210 V, not 207.

If the line to line voltages are not equal, then the wild leg to neutral will not be 210 (based on 243 line to line).

Whether the neutral is used or not there is still a useful voltage measurement using it to see what is happening in the circuit.

As I previously stated the vector sum of the voltages around the delta must equal zero. This means the vector sum is a closed triangle. If line to line voltages are equal, then it is an equilateral triangle, anything else and it is not.

The question here is where is the cause of the unbalance. The measured voltages at this time do not correlate with a normal circuit. The 243 and 242 vectors are not related by 120 degrees based on the 230 V measurement.

It is important to have totally unloaded values, and then loaded values and currents.

.

 

[ActionDave]

I have a guy on a job - 3phase 240/120v high leg.
They are burning up 3 phase motors........

How / why can there be a 12/13v differential when the other legs seem correct.

What to check next?

Try double rotating the phases at the starter so you don't change rotation on the motor (I hope this makes sense) and see if the problem stays on the same phase or follows the wire.

 

[Little Bill]

Try double rotating the phases at the starter so you don't change rotation on the motor (I hope this makes sense) and see if the problem stays on the same phase or follows the wire.

I'm not seeing how you can swap any two wires and not affect the rotation. I see what you are trying to accomplish but can't see how you would do it.:?

Let's say the rotation goes A-B-C and you double rotated, what would the rotation be when finished?

 

[wwhitney]

Swapping one pair of wires reverses rotation. Do it again (with a different pair), and you are back to the original rotation.

Cheers, Wayne

 

[Little Bill]

Swapping one pair of wires reverses rotation. Do it again (with a different pair), and you are back to the original rotation.

Cheers, Wayne

I was (in my mind) trying to keep the high leg in the same position but realized that Dave said at the starter and it doesn't matter where it is on a 3Ø load/machine. That's unless a neutral is used in the equipment.

 

[gar]

180817-0246 EDT

If you have two phases from a balanced 3 phase source, meaning the same voltages and 120 degree phase spacing, and two identical transformers from two of these phases with 240 V output connected as an open delta, then the output open phase must read 240 V.

.

 

[GoldDigger]

180817-0246 EDT

If you have two phases from a balanced 3 phase source, meaning the same voltages and 120 degree phase spacing, and two identical transformers from two of these phases with 240 V output connected as an open delta, then the output open phase must read 240 V.

.

And if the 240/120 pot (A-C) is at a 120 degree angle to the stinger pot (typically B-C) based on the primary phasing, but the secondary voltage on the stinger pot is lower for any reason, then the triangle can close but with the A-B (open) leg at an angle other than 120 degrees. This is one of the known problems with an open delta source.

 

[gar]

180817-0328 EDT

GoldDigger:

But his measured AB is 243 and AC is 242. That makes his results strange. From memory his values make the C node about 11 V shifted from being vertical above N (neutral).

.

 

[cpinetree]

Gar,
I would suspect that if the AC is the only three phase load, he has no neutral at that starter. He most likely has a ground, EG, and is measuring to it, not earth. Very few of my voltage readings at motor controls/starter would be done to a neutral, almost always to an Equipment ‘ground’, not earth.

Correct no neutral at motor

 

[ptonsparky]

Try double rotating the phases at the starter so you don't change rotation on the motor (I hope this makes sense) and see if the problem stays on the same phase or follows the wire.

Remind me.

That would be for a current imbalance, correct?
If it follows the wire, the problem is with the motor.
If it stays the same, the problem is at the source.

Sometimes rotating it does correct or lessen the current imbalance and that has more to do with the motor windings.

OP. What are the amp readings of the motor(s) while in operation?

 

[kwired]

Remind me.

That would be for a current imbalance, correct?
If it follows the wire, the problem is with the motor.
If it stays the same, the problem is at the source.

Sometimes rotating it does correct or lessen the current imbalance and that has more to do with the motor windings.

OP. What are the amp readings of the motor(s) while in operation?

Other way around isn't it?

If problem follows the wire the problem is with the source.
If it stays the same problem is with the motor.