Trying to get 120/240v with no high leg

[gar]

210622-2115 EDT

drcampbell:

I post #28 you said --- " It's better described as single-phase, split phase, since there's no phase angle between the two hot legs."

When you use the neutral point, transformer secondary center tap, as your reference, then there are two hot voltages that differ in phase by 180 degrees. Therefore, two phases.

This is a very useful means to create a full wave rectified output using two diodes, and that can be referenced to a chassis by the transformer center tap. This further allows several different DC voltages to be created from the same secondary with different taps on the secondary coils, all using the single center tap connection to the chassis. If bridge rectifiers were use, then separate isolated secondary windings would be required. Further voltage doubler rectifiers could be added all referenced to that single secondary center tap.

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[jaggedben]

210622-2115 EDT

drcampbell:

I post #28 you said --- " It's better described as single-phase, split phase, since there's no phase angle between the two hot legs."

When you use the neutral point, transformer secondary center tap, as your reference, then there are two hot voltages that differ in phase by 180 degrees. Therefore, two phases.

...

Oh god, not this again.

 

[LarryFine]

Oh god, not this again.

I SO thought the same thing, but decided to let it ride.

 

[zbang]

If the three phase load is substantial enough now they might have closed the delta that once was open.

If it ever was open; high-leg implies, but doesn't guarantee, an open delta, that's up to the PoCo.

 

[Flicker Index]

It's possible with uncommon transformers. 415Y/240 with center tapped transformer that is grounded on the end in the knot, but you'll end up with 0-120-240 rather than 120-0-120.

 

[Besoeker3]

They're also used for low-power rectifier systems. Ford alternators, in particular.

I did quite a lot of those for anodising plants. Up to 60,000A in some cases. Not exactly low power.....

 

[Ivan Zapata]

Hello all,

just like Zalkon mentioned, I saw a 120/240V 3 phase system with no high leg.

Last month I installed an air compressor at a local shop and it used a 240V 3 phase induction motor, so I installed a 3 pole breaker in the panel, wired it up and it worked as it should. Once I was done, I went back to the panel to check something else and I checked voltage between each phase and neutral and all of them read 120, even B phase.

I didnt ask the shop owner how had he installed that service since I was already on my way out. From the city's distribution lines, 3 wires came down to his transformer and then from the secondary side all wires were routed into a small room (which I wasnt given access to since there was no need for me to be in there) and then the conduit led to the main panel.

I know there is no way B phase could show 120 instead of 208 but I think there must be some additional equipment installed so it gives out 120 in B phase.

I might go back there to do another job so I'll make sure I inspect everything from the transformer to the panel to find out how is that possible.

 

[ggunn]

I did my apprenticeship in Oakland Ca. Worked there for 11 years. It was all new construction. Every 120/240v panel was just that, no 208v phase to phase or high leg. I moved to Wa. and all I find is 120/208v or 120/240v with a high leg. No one here has ever seen such a thing.

That's because 240/120V three phase wye without a high leg does not exist.

 

[jim dungar]

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