High Leg Delta Transformer

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LincHawk

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Could someone explain how a Delta high leg works? i was working in a facility the other day installing a new VFD to an existing 3 phase source. When power was turned back on we started testing voltage and 2 phases read voltage and 1 phase didn't read anything, but the motor and VFD work?

is this a Delta system or something else?
 
Depending on the VFD and its settings it can work with two phases present. Most likely you have a grounded Delta but we need to know what the phase to phase voltage is as well. You need to RTFM (...Factory...), because many VFDs do not like either the Grounded Delta or the High Leg.
 
Agree, L-L and L-G voltages. Sounds like a corner grounded delta
 
You really need to double check the manual for the VFD. Some times there are grounding jumpers that need to be moved around to accommodate a delta source.
 
High leg delta has one winding center tapped. This center tap is grounded to produce voltages of 120V, 208V, 120V to ground respectively on the A, B and C phases.
 
LincHawk said:
203v across phases. But phase to ground 0


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All three phases are 0 to ground???
check it A to B, B to C, and A to C.
Then check it A to ground, B to ground, then C to ground.
 
ggunn said:
208V corner grounded with an open (ungrounded) neutral? I am grasping at straws here...
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Yeah, really. Could be that someone created it using single phase 208V transformers they had laying around by connecting them in a delta configuration and grounded one corner (no neutral at all). But the problem with that is, 208V single phase transformers? Those are rare to start with. Makes no sense to have done this on purpose when there are standard off-the-shelf transformers to get you there (208V 3 phase) in a manner consistent with what everyone is going to expect.

maybe it's 240V corner grounded delta with 5% FCBN taps used, and a severe voltage drop on the primary maybe? My straws are grasping at straws...
 
Jraef said:
Yeah, really. Could be that someone created it using single phase 208V transformers they had laying around by connecting them in a delta configuration and grounded one corner (no neutral at all). But the problem with that is, 208V single phase transformers? Those are rare to start with. Makes no sense to have done this on purpose when there are standard off-the-shelf transformers to get you there (208V 3 phase) in a manner consistent with what everyone is going to expect.

maybe it's 240V corner grounded delta with 5% FCBN taps used, and a severe voltage drop on the primary maybe? My straws are grasping at straws...
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or an attempt to make corner ground system out of a 208 wye source and leave the neutral point floating.

If there is 120 volt loads present, maybe something has failed, if no 120 volt loads may be something intentional though unconventional.

Around here any POCO supplied 208 nominal system is likely to read somewhere around 212-216 volts unloaded.
 
ggunn said:
Which is the straw I grasped at in post #11. Maybe so, but why?
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I saw your post and wondered why anyone would do that as I read it...
if they floated the neutral it looks like any single phase loads would be all over the place as far as voltages.
Maybe there aren’t any... but still,...:?
 
Hv&Lv said:
I saw your post and wondered why anyone would do that as I read it...
if they floated the neutral it looks like any single phase loads would be all over the place as far as voltages.
Maybe there aren’t any... but still,...:?
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In some crude attempt to replace a failed corner ground delta maybe?

If they float the neutral you can still have single phase 208 loads, you just don't have any reference on the system that is 120 volts to anything other than that floating wye point. You can't supply 120 volt loads other than to make sure they are absolutely balanced across all three phases - which is what a wye connected motor is. Same goes for 240 3 wire delta (grounded or ungrounded) - there is no 120 volt neutral and only single phase loads you can directly supply would be 240 volt loads.
 
kwired said:
If they float the neutral you can still have single phase 208 loads, you just don't have any reference on the system that is 120 volts to anything other than that floating wye point. You can't supply 120 volt loads other than to make sure they are absolutely balanced across all three phases - which is what a wye connected motor is. Same goes for 240 3 wire delta (grounded or ungrounded) - there is no 120 volt neutral and only single phase loads you can directly supply would be 240 volt loads.
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You could still supply 120v loads if that floating neutral was extended with the line conductor(s), it just wouldn't be a grounded neutral (nor compliant).
 
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