Buck 240V delta 3 phase to 208?

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HipHopHead

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Hey guys. Is there any way to buck a 240v delta(high leg) 4 wire down to a 3phase 208 4 wire? I heard somewhere that the high leg may not make it possible. Customer has an old manufacturing machine and has been burning out boards, Manufacturer told him that it should work fine within 10% but hes at around 243v which is not within that threshold. can this be bucked? or am i looking at a standard transformer? Thanks guys
 
Sometimes the 208V L-G voltage in a high leg may cause failures in MOV protection devices and/or other parts in the electronics. Was the manufacturer told that this was a high leg and not just 240V L-L ?
 
synchro said:
Sometimes the 208V L-G voltage in a high leg may cause failures in MOV protection devices and/or other parts in the electronics. Was the manufacturer told that this was a high leg and not just 240V L-L ?
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Good question and i doubt it. Heading over there in about an hour to dig a little deeper. What Ive been told is that its a 3 phase machine rated at 208V and he has a 3 phase 240V service. My assumption is that its a high leg but Ill know more in a bit. Thanks for the help
 
HipHopHead said:
Hey guys. Is there any way to buck a 240v delta(high leg) 4 wire down to a 3phase 208 4 wire?
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No. You can not use autotransformer, like a buck-boost, to go from a delta to a wye.

If you realy need 208Y/120 4 wire you will need a different transformer.

If you use a buck transformer on a 240/120V delta it will result in a 208/104V delta which I don't think your equipment will like.
If you can ignore the load side neutral, and voltages to ground, then you can use the arrangements others have suggested.
 
Hi guys. Took a look yesterday. It’s a high leg delta 240. It’s an old Cnc machine that requires a neutral. Load current 21.5kva. Getting a price for a 30kva delta/wye transformer. Thanks for the responses. This is truly an amazing resource.
 
HipHopHead said:
Hi guys. Took a look yesterday. It’s a high leg delta 240. It’s an old Cnc machine that requires a neutral. Load current 21.5kva. Getting a price for a 30kva delta/wye transformer. Thanks for the responses. This is truly an amazing resource.
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Yep, you need a "real" transformer to get wye from a delta.
 
jim dungar said:
No. You can not use autotransformer, like a buck-boost, to go from a delta to a wye.

If you realy need 208Y/120 4 wire you will need a different transformer.

If you use a buck transformer on a 240/120V delta it will result in a 208/104V delta which I don't think your equipment will like.
If you can ignore the load side neutral, and voltages to ground, then you can use the arrangements others have suggested.
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Depends on what you want to do. Isn't a "grounding transformer" on an ungrounded three wire delta just that?

No you can't use autotransformer on 240 volt high leg and get 208/120 out of it.

You can reduce volts for line to line loads, keeping in mind the high leg is still going to be 208 to ground and could impact any surge protection devices if they are connected to this. You can still run 120 volt loads directly off the incoming supply on two of the three phases, which can be handy if there is limited 120 volt needs. If you need to supply significant number of 120 volt loads and want them balanced across the three phase supply, you really need a SDS with wye secondary.
 
kwired said:
Depends on what you want to do. Isn't a "grounding transformer" on an ungrounded three wire delta just that?
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You typically use either a zig-zag arrangement or a wye-delta to create a neutral/grounding point

Yes, a zig-zag is a type of autotransformer but it is not like what would be used to buck-boost.
 
jim dungar said:
You typically use either a zig-zag arrangement or a wye-delta to create a neutral/grounding point

Yes, a zig-zag is a type of autotransformer but it is not like what would be used to buck-boost.
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I agree what is marketed as "buck-boost" is pretty limited. How it gets connected is what makes it become an autotransformer.
 
HipHopHead said:
Hi guys. Took a look yesterday. It’s a high leg delta 240. It’s an old Cnc machine that requires a neutral.
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One thing to check into in this regard;
SOMETIMES, the CNC machine needs the neutral of a 208V 3 phase 4 wire feed, because they are tapping off from phase to neutral for 120V control power going to the electronics or other controls inside. When it was DESIGNED for a 208V Wye system, they could have tapped off of ANY phase to neutral and it would be fine. But when it was hooked up to the 240V 4 wire system, you can only tap from A or C to neutral to get 120V, but there may be something inside tapped off of the B phase and that is what is getting damaged. If so, all you really need to do is either move that connection to one of the other phases, or just use a 240-120Vcontrol power transformer to power everything. I've found this issue more than once in similar situations.

Motors on the CNC machine are usually driven by servo amps or VFDs and do not care whether it is 208V or 240V, the sensitive part are really just the controls and the power supplies for them.
 
210325-1556 EDT

Most CNC machines I have contact with are delta input with various taps to get the desired DC bus voltage. In addition to the delta input there is an EGC that has negligible current flow. Somewhere else 120 V is derived.

If I have a wye source of 240 V line to line, and its neutral as my source, then I can add a buck transformer of 22 V to each phase, and produce either a delta or wye source with the original neutral as the neutral point.

From the bucked point to the end of its phase I have (240-22) = 218 V, but from a bucked point to another bucked point I have about 208 V. Neutral is still in the center of the new output, 208 V, as well as the original 240 V. The phase angles of the new 208 V delta, differ somewhat from the original delta.

Check my math.

.
 
Jraef said:
One thing to check into in this regard;
SOMETIMES, the CNC machine needs the neutral of a 208V 3 phase 4 wire feed, because they are tapping off from phase to neutral for 120V control power going to the electronics or other controls inside. When it was DESIGNED for a 208V Wye system, they could have tapped off of ANY phase to neutral and it would be fine. But when it was hooked up to the 240V 4 wire system, you can only tap from A or C to neutral to get 120V, but there may be something inside tapped off of the B phase and that is what is getting damaged. If so, all you really need to do is either move that connection to one of the other phases, or just use a 240-120Vcontrol power transformer to power everything. I've found this issue more than once in similar situations.

Motors on the CNC machine are usually driven by servo amps or VFDs and do not care whether it is 208V or 240V, the sensitive part are really just the controls and the power supplies for them.
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Had that issue with a German tanning bed, it was so new to the states, they sent an engineer with it. The service was delta with a high leg, engineer said I didn’t need a neutral, but did need buck/boost to take it down to 208, but something didn’t look right at the power terminal block. Got to tracing it back, found the factory using the ground as a neutral for the controls! Located the hot to the controls, and made sure the high leg wasn’t on it! The tanning bed got you so hot, it had air conditioning ducted to it. It was supposed to be a speed tanner, I think you laid in it 15 or so minutes, you came out red, then the next day you were tan! The lamps looked like those miniature metal halide bulbs. You were probably getting cooked with ultraviolet radiation! LOL!
 
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