Utility High Leg, Open Delta Phase Converter, and a Motor That Can't Use Either

C

C4ctus99

Member
Location
Jacksonville Fl
Occupation
Electrical Contractor
Good Evening, been running into an increasingly problematic situation with a customer.

Service from Utility is 120V/240V 3P with B phase in panel reading 210V phase to ground. They recently got a new upcut saw and after consulting with the manufacturer I was informed that I cannot use a service with a high leg for that motor. The customer did not want to pay the extra money for a new service (would have saved a lot of headache) and in my ignorant state I decided to try using a rotary phase converter to take the two good legs and generate a third phase. Today I learned that the converter is wired open delta and the manufactured phase is 240V to ground. I am now looking at potentially getting an isolation transformer to rectify the high leg issue at point of use.

With all that being said, motor wiring and transformers are not my strong suit (through an unfortunate blunder of the staff I did not get to take that course during my time in the apprenticeship) and some help and direction would be appreciated.

My question is, would there be a way to rewire the idler (motor) portion of the phase converter to eliminate the high leg issue? This would save me a hassle and some cash trying to return the converter. Also, I have a very basic understanding of the different types of transformers and there functions but if there are any threads y'all could point me to explaining some of the difference between open delta, closed delta, grounded, unground, wye, and open wye I would be much obliged.

Thanks,
Jonathan W
 
3 phase motor I assume, hence the phase converter?

I’m not a motor expert by any means, but I’ve never heard of a 3 phase motor not being able to utilize a high leg service.

Is there a VFD involved?

Only place a high leg really comes into play is on phase to neutral loads. ( Without getting into transformer loading and imbalance on open delta).
 
Yessir, 3 phase motor. I was thinking the converter would give me 120V to ground and 240V phase to phase but was woefully incorrect.

No VFD unless it is internal to the machine, but I do not think so.

Yes, that is my understanding however the manufacturer is adamant that you cant have a high leg.
 
C4ctus99 said:
My question is, would there be a way to rewire the idler (motor) portion of the phase converter to eliminate the high leg issue?
Click to expand...
No. This is how phase converters work.

Purchase (2) single phase transformers and wire them in an open delta to open delta configuration. Using standard isolation style transformers will give you a separatly derived 240V 3-phase 3 wire output.
 
C4ctus99 said:
I was informed that I cannot use a service with a high leg for that motor.
Click to expand...
I agree with 7 Delta. The motor won't care if the service is a high leg or not. If there are any single phase controls in the machine that would be something to be careful about but you would just need to wire them so that it's not on the high leg.
 
jim dungar said:
Purchase (2) single phase transformers and wire them in an open delta to open delta configuration. Using standard isolation style transformers will give you a separatly derived 240V 3-phase 3 wire output.
Click to expand...
This won't provide a balanced L-N 3 phase system.

If the OP needs a balanced 3 phase supply, then you need a wye secondary (or equivalent).

In a nutshell: a delta transformer has 3 coils for 3 phases, connected in a triangle ( or the Greek letter delta) The wye has 3 coils for 3 phases, connected like the spokes of a wheel (or the letter 'Y'). An open delta leaves off one coil of the triangle but leaves the 3 endpoints so you still get 3 phases.

You can pick any transformer secondary terminal as the one to ground, to create your voltage references.

Only the wye secondary has a neutral terminal that is equal voltage to all 3 hot terminals. So if you can't have a 'high leg', then you must have a wye secondary.

In principle you could build a rotary phase converter that generated a wye output, but no one makes these. You'd get lower capacity out of the same idler motor, and most motor loads don't care about the high leg.
 
C4ctus99 said:
Good Evening, been running into an increasingly problematic situation with a customer.

Service from Utility is 120V/240V 3P with B phase in panel reading 210V phase to ground. They recently got a new upcut saw and after consulting with the manufacturer I was informed that I cannot use a service with a high leg for that motor. The customer did not want to pay the extra money for a new service (would have saved a lot of headache)
Click to expand...
I think you may be looking for a 240V 'drive isolation transformer', I am not sure if what kind of delta matters its more the wye vs delta.
 
I agree, ditch the roto phase. I'm not ready to jump into buying a transformer yet. It's not clear why the high leg service won't work other than "The manufacturer said so". I'm not one to ignore manufacturer instructions outright, but they can be as wrong as they are right. More detail is needed.
 
jim dungar said:
Not listed as one of the requirements in the OP.
Changing the specifications usually means changing the recommendation.
Click to expand...

True. I inferred that the manufacturer required a balanced supply from the statement that a high leg was prohibited.

But the actual requirement from the manufacturer is not clear. As others have noted, the motor itself should be perfectly happy with a high leg supply.

C4ctus99 said:
Service from Utility is 120V/240V 3P with B phase in panel reading 210V phase to ground. They recently got a new upcut saw and after consulting with the manufacturer I was informed that I cannot use a service with a high leg for that motor.
Click to expand...
 
Welcome to the forum.

A rotary converter puts out a high-leg delta, so it gains you nothing.

The simple fix is a 240D to 240Y transformer.

We'd need to know why the manufacturer says no to comment further.
 
C4ctus99 said:
My question is, would there be a way to rewire the idler (motor) portion of the phase converter to eliminate the high leg issue? This would save me a hassle and some cash trying to return the converter. Also, I have a very basic understanding of the different types of transformers and there functions but if there are any threads y'all could point me to explaining some of the difference between open delta, closed delta, grounded, unground, wye, and open wye I would be much obliged.

Thanks,
Jonathan W
Click to expand...
Go To this page and look at Scott Thompson's transformer drawings.
 
ActionDave said:
I agree with 7 Delta. The motor won't care if the service is a high leg or not. If there are any single phase controls in the machine that would be something to be careful about but you would just need to wire them so that it's not on the high leg.
Click to expand...
That was my understanding and common consensus amongst everyone I have talked too. I verified the control circuit is single phase so that would not be an issue. The problem is the manufacturer specifically states not to have a high leg. If it wasn't an $85,000 saw I might would be tempted to let it ride. Voltage to ground on the high leg is 210V and that is out of spec for the machine.
 
winnie said:
This won't provide a balanced L-N 3 phase system.

If the OP needs a balanced 3 phase supply, then you need a wye secondary (or equivalent).
Click to expand...
Not entirely certain that it requires a fully balanced system, but certainly it needs 240V phase to phase and 120V phase to ground max.
winnie said:
You can pick any transformer secondary terminal as the one to ground, to create your voltage references.

Only the wye secondary has a neutral terminal that is equal voltage to all 3 hot terminals. So if you can't have a 'high leg', then you must have a wye secondary.
Click to expand...
This definitely helps fill some of the knowledge gap. Is there no delta configuration in 3 phase that gives 120V to ground on all three legs?
 
jim dungar said:
Not listed as one of the requirements in the OP.
Changing the specifications usually means changing the recommendation.
Click to expand...
ActionDave said:
It's not clear why the high leg service won't work other than "The manufacturer said so". I'm not one to ignore manufacturer instructions outright, but they can be as wrong as they are right. More detail is needed.
Click to expand...
Unfortunately the tech support does not know the answer either other than "the specs say not to use a high leg." The machine costs $85,000 so I have zero interest in experimenting to find out why

LarryFine said:
Welcome to the forum.

A rotary converter puts out a high-leg delta, so it gains you nothing.

The simple fix is a 240D to 240Y transformer.

We'd need to know why the manufacturer says no to comment further.
Click to expand...
Thanks for the welcome. Seems as though Delta-Wye will be the best solution as I cannot seem to convince them to replace there service

roger said:
Go To this page and look at Scott Thompson's transformer drawings.
Click to expand...
Thank you sir, I will take a look
 
C4ctus99 said:
Not entirely certain that it requires a fully balanced system, but certainly it needs 240V phase to phase and 120V phase to ground max.
Click to expand...
That is physically impossible.

In a balanced system you either get 208V phase to phase and 120V phase to ground, or you get 240V phase to phase and 139V phase to ground.

C4ctus99 said:
This definitely helps fill some of the knowledge gap. Is there no delta configuration in 3 phase that gives 120V to ground on all three legs?
Click to expand...

Nope. Your choices are:

240V phase to phase, _undefined_ phase to ground (could be anything from 0 to > 240V and will be different on all phases). This is an 'ungrounded' sustem.
240V phase to phase, 240V phase to ground on two phases, 0V on one phase This is a 'corner grounded' system.
240V phase to phase, 120V phase to ground on two phases, 208V (ish) phase to ground on the 'high leg'. This is a high leg delta. It can be open or closed delta.
240V phase to phase, 139V phase to ground on all three phases. This is a _delta_ system with some sort of grounding transformer to make it more like a wye system.
 
C4ctus99 said:
Not entirely certain that it requires a fully balanced system, but certainly it needs 240V phase to phase and 120V phase to ground max.
Click to expand...
No, that's incorrect. A high-leg delta has 240v L-L, 120v L1-N and L3-N, and 208v L2-N.

C4ctus99 said:
Is there no delta configuration in 3 phase that gives 120V to ground on all three legs?
Click to expand...
Nope, only a 208v L-L wye has 120v L-N on all three lines
 
C4ctus99 said:
Voltage to ground on the high leg is 210V and that is out of spec for the machine.
Click to expand...
If this is true, your only choice to use one the previously mentioned wye transformers 240Y/139V (look for a drive isolation transformer) or a 208Y/120V ( a standard transformer but it won't provide 240V).

You are up against the laws of physics versus a clueless manufacturer's contact. My guess is that you are not dealing with actual engineering support. The engineering department probably has some suggested method rather than a simple don't do it. The saw is much more than a simply motor, it likely contains several filters for multiple drives and electronics.
 
You must log in or register to reply here.
Top