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High Leg Delta

edl

Aluminum
Joined
Apr 8, 2008
Location
Southeast USA
Hello - we are in a new space from the 1970s that has "high leg delta" service described here

High-leg delta - Wikipedia

basically A phase is 120, B phase is 208 and C phase is 120

I promptly plugged a 3 phase baldor carbide grinder in and burned it up

I followed the instructions for wiring the 9 motor leads / l1, l2, l3 for "low voltage" 3 phase (208 - 240)

as i think about it, how would a 3 phase motor know that one leg is a "high leg"?

if it can't know, what is the normal practice on these older systems to wire 3 phase? - do you have to step down B phase prior to wiring each machine?? that seems like it could not have been how they designed it back in the day....?

appreciate any insights....
 
I should add that phase to phase it is all 240

so phase to phase:
a-b = 240
bc = 240
ac = 240

i believe this is because the ground is in the middle of the high leg and so effectively creates 2 half legs each at 120

so the 208 is line to neutral
a = 120 line to neutral
b = 208 line to neutral
c = 120 line to neutral

is there a line to neutral load in this motor? - if not, and if it is all phase to phase, should it care that line to neutral on b phase is 208 and not 120?
 
Your motor should have been, 1+7 2+8 3+9 for the lines and 4+5+6 tied and taped, quick put the smoke back in and it should work, Hi Hi...Phil
 
I promptly plugged a 3 phase baldor carbide grinder in and burned it up

I followed the instructions for wiring the 9 motor leads / l1, l2, l3 for "low voltage" 3 phase (208 - 240)

as i think about it, how would a 3 phase motor know that one leg is a "high leg"?
They do not know. Unless they have an internal fault.

Or you create one.

See Phil's guidance.

Not a lot of "mystery" to it.

The MAIN "unwelcome" thing high-leg brings is higher probability of "surprise" to the unwary.

It is not OTHERWISE even all that much greater risk of electrocution, high leg to Earth, than ignorant leg-to-leg 240 VAC split-phase can be if one gets careless. over-confident, STUPID, or maybe just unlucky?

We are not actually MEANT to be grabbing onto live wires in any case, are we?

That's what proper insulating jackets, safe equipment housings, smart termination technology, sane switchgear, and decent enclosures, ducts and conduits are all about.

And safe OPERATING practice and its GOOD habits, even MORE so.
 
My professional machinist friend and mentor who operates his shop from his home has high-leg 3 phase Delta from the power company. They installed it for him free about 35 years ago. He only had to show the power company his need and types of machines he was using at the time. He's been using it ever since for all types of machinery: Two Monarch lathes, two Bridgeports, grinders, and CNCs: MyCenter 1 and Mori Seiki. He's had no problems at all.
 
My professional machinist friend and mentor who operates his shop from his home has high-leg 3 phase Delta from the power company. They installed it for him free about 35 years ago. He only had to show the power company his need and types of machines he was using at the time. He's been using it ever since for all types of machinery: Two Monarch lathes, two Bridgeports, grinders, and CNCs: MyCenter 1 and Mori Seiki. He's had no problems at all.

By contrast, present-day Powerco's - for long years, already - far prefer to install Wye service. But for liability reasons. Not because the loads give a s**t.

Same again my running either of Phase-Perfect or RPC through a Delta-Wye isolation transformer. The Gen set is Wye. "Dialable" 208 to 220+. And I needed to drive extra ground rods for it in any case.

Made plenty of sparks, tripped plenty of breakers, but last actually shocked meself in the 1970's.

Good habits, long hard-wired in ..plus a healthy ration of electrical paranoia.

Even so, I'm going old enough that "dumb ass forgetful" some day possibly biting me at 130-140 VAC is a better deal that at 245-plus VAC.

One EGS / Hevi-Duty 27 KVA worth. Around $600 including inbound freight.
 
high leg delta didn't kill your motor. It was likely ready to go and wye wouldn't have changed that. Any machine running of a rotary phase converter or Phase Perfect @240v is running off a high leg. I've got about 50 machines from the 19220's to new running off 240 delta. I have found that once in a while a machine that has been moved will suddenly have a motor problem. I don't know if it is the shaking and vibration but it happens. Dave
 
Hear tell they make inordinately competent WIVES.. or Tee Vee shows that claim it so...but then again... it was only an old baldwhore of a bump-and-grinder, yah?

ok...i am not that old...but old enough to know if this keeps up mod's unhappy :-)
 
Your motor should have been, 1+7 2+8 3+9 for the lines and 4+5+6 tied and taped, quick put the smoke back in and it should work, Hi Hi...Phil

ok...one more piece of data now ... machine has a forward - off - reverse switch like this:

20200904_165513.jpg20200904_165455.jpg

[don't know why the pictures all load sideways...they are oriented right on my end???]

back of switch is like this:

20200904_165411.jpg


note that the bar in between "B" and "3" is twelve o'clock in real life (front side of switch where "off" is)

you are right that low voltage is 1+7 2+8 3+9 for the lines and 4+5+6 tied and taped, as per this diagram:

20200904_165204.jpg


my question is where do the wires from the switch go? - I am guessing "4" from the switch ties with "4" from motor (and bundles with "5" and "6")?

"2" from the switch goes with "2" + "8" from the motor?

where would "A" from switch go?

where would "B" from switch go?

Thanks!
 
Just to cover the issue, high leg delta is common in rural areas because it requires only one transformer and 4 wires to the building. The three phase is exactly the same as any other three phase except for the location of the common. By putting it in the center of one winding, you get 120 V single phase from the common to either end of that winding, 240 V single phase with a center ground across the full winding, and 240 V three phase from the points of the delta. This gives someone like a farmer who has a few three phase machines and needs standard single phase for his lights, etc. The high leg, sometimes called a wild leg, is high only because of the ground location.

Bill
 
Just to cover the issue, high leg delta is common in rural areas because it requires only one transformer and 4 wires to the building. The three phase is exactly the same as any other three phase except for the location of the common. By putting it in the center of one winding, you get 120 V single phase from the common to either end of that winding, 240 V single phase with a center ground across the full winding, and 240 V three phase from the points of the delta. This gives someone like a farmer who has a few three phase machines and needs standard single phase for his lights, etc. The high leg, sometimes called a wild leg, is high only because of the ground location.

Bill



And it is likely the high leg will let the smoke out of relays, contactors and other control circuts if you put that little guy on the wrong lug.
 
fair enough but I still have the same question: what do I do with 2,4 A&B coming off of the switch?


what should they wire into?

it looks to me like the motor's 3rd phase was left connected to the incoming power. you can use either 2 and 4, or a and b, for the other two phases.

verify that you have electrical continuity between 2 and b, and 1 and a, when in forward or reverse, but not in off.

when you reverse direction the continuity will be between 1 and b, and 2 and a.

its also possible but unlikely that the switch is wired differently, you should be able to determine from the original wiring which wires were input and which go to the motor.

anyhow you may be able to use terminals 3 and 4 to switch the third phase of the motor. while its not kosher to use a 3 phase drum switch as a disconnect, usually a 3 phase reversing switch electrically disconnecs all the phases from the line, but in your case the switch only has 4 wires so that wasn't possible.


often motors are still useable after they let some smoke out.. its steam and dust burning off.. not always the winding insulation.
 
"often motors are still useable after they let some smoke out.. its steam and dust burning off.. not always the winding insulation. "


You must have better motors than I, in 50 years I have never seen that...Phil
 
You must have better motors than I, in 50 years I have never seen that...Phil

Really!!!

Damp wood "flour" or other outdoor humid and insect nested, maybe?

Indoors, the reverse, far more often. Very little warning.

Heat is trapped and so VERY HIGHLY "localized" the weakest part of the winding is already cremated before it has TIME to cook the crud covering it at even a modest distance.

Good health does not "grow back" after that sort of stress. It can only further degrade, faster and worser.
 








 
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