3 Phase WYE/Delta transformer

[AccUbonD]

Ran across one of these recently and it has me stumped

Wye primary 208Vac
Delta secondary 220Vac

3 legs on secondary one goes to ground/neutral

How is this 3 phase with 0v read to ground on the one leg?

Thanks in advance

 

[JohnK]

Just curious, you reading 220 between all 3 legs? Problem is when you get 2 legs grounded.

 

[AccUbonD]

This is a European transformer made back in 1984.

If your read between any 3 legs on secondary you read the 220V however if you read each to ground you get the potential on 2 of the legs. The third which is ground by schematic drawing but has a neutral jumper reads of course 0v. I just can't understand how that is 3 phase.

 

[Miguel Cervantes]

Found this on Google. Don't have a clue what it means but I hope it helps. Not much on electrical stuff myself.

http://www.electriciantalk.com/f9/120-240-volt-3-phase-delta-high-leg-5166/

 

[Jim Ammons]

Maybe this will help!

Delta-wye transformer
From Wikipedia, the free encyclopedia
Not to be confused with Y-Δ transform.
schematic

A delta-wye transformer is a type of three-phase electric power transformer design that employs delta-connected windings on its primary and wye/star connected windings on its secondary. A neutral wire can be provided on wye output side. It can be a single three-phase transformer, or built from three independent single-phase units. An equivalent term is delta-star transformer.
One possible delta-wye transformer configuration.[1]

Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three-phase distribution systems.

An example would be a distribution transformer with a delta primary, running on three 11 kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 415 V, with the domestic voltage of 240 available between each phase and the earthed (grounded) neutral point.

The delta winding allows third-harmonic currents to circulate within the transformer, and prevents third-harmonic currents from flowing in the supply line. [2] Delta connected windings are not common for higher transmission voltages (138 kV and above) owing to the higher cost of insulation compared with a wye connection. [3]

Delta-wye transformers introduce a 30, 150, 270 or 330 degree phase shift.[4] Thus they cannot be paralleled with wye-wye (or delta-delta) transformers. However, they can be paralleled with identical configurations and some different configurations of other delta-wye (or wye-delta with some attention) transformers.

 

[Artfuldodger]

I'm not an electrician either but I did read this on the internet;

Occasionally, (especially in older facilities or in oil well and irrigation applications), you may encounter a three-phase delta service with one phase (leg) connected to ground, called a "corner grounded delta" or "grounded leg service".

http://www.ccontrolsys.com/w/Corner-Grounded_Delta_Circuits

I've ran across older building that has a high leg or bastar_ed leg before. This was a way of getting 208volts and 120volts of the same transformer.

Interesting on the corner grounded delta though. I've never heard of that. I too wonder how you'd get 3-phase, must research.

"Used to reduce wiring costs by using a service cable with only two insulated conductors rather then the three insulated conductors used in a convention three phase service entrance."

Link has a diagram;
http://ctlsys.com/electrical_service_types_and_voltages/

 

[Artfuldodger]

Here is a discussion showing a main disconnect fore one of these transformers;

http://www.electriciantalk.com/f30/480v-3-phase-corner-grounded-delta-120585/

 

[Bubba Watson]

Look at the drawing from MC, the ground symbol is center tapped on a winding. If you use the ground as your reference point for your voltmeter you will get 120/120/208. Now if you check line voltage with your voltmeter ( remember on a delta connected transformer line voltage and phase voltage are the same) you will read 240 three phase . You see how we have three phase power with a center tapped grounded winding that we are using for other loads.
Now take and redraw it but move the ground to the end of a winding ( this can only be done once on a delta connected transformer ) we have not changed your three phase connection. But if we reference our voltmeter to ground we will read 240 volts. But we no longer have 120 /120/208. I assume this is a dry transformer in a plant somewhere, where they had a 240 three phase motor they wanted to use, good thing about corner grounded delta connections you don't have to pull another conductor. I hope this helped

 

[JohnK]

Leg to leg 220, it's 3 phase. Ground one leg it's still 3 phase but that leg reads 0 to ground, legs still read 220 leg to leg, still 3 phase. Maybe I don't understand the question.

 

[AccUbonD]

Post 6 linked with diagram labeled "Three Phase Two Wire Corner-Grounded Delta" is what I was dealing with. I was hoping someone could explain to me slowly how one leg is neutral but is 3 legs out of phase with one another or how is tricks a 3PH motor to run. I just can't wrap my head around that knowing one leg reads 0v to ground even though you do read 220v across all legs. I thank all you guys for your input.

 

[Milkman]

The three phases are each creating a magnetic field that spins the rotor in the motor.

Neither of them are using ground in doing this, just creating their opposing magnetic powers.

Trying to utilize the odd leg for single phase loads would be an issue.

 

[Artfuldodger]

What keeps all of the voltage on the ground leg from going to ground? It look like there wouldn't be any amperage on the grounded leg to create the magnetic field for the off phase to spin the motor?
Like that grounded leg would be more of neutral than another leg with the equal amps of the other non-grounded two legs.

I know it works but can't picture it in my head;

Voltages are as follows:
A-B: 240 VAC,
A-C: 240 VAC,
B-C: 240 VAC,
A-G: 240 VAC,
B-G: 240 VAC,
C-G: 0 VAC.
("G" = Ground / Equipment Grounding Conductor / Grounded Conductor).

 

[Artfuldodger]

Her is another thread on one showing the disconnect which requires only two fuses. The grounded leg of course isn't grounded.

http://www.electriciantalk.com/f30/480v-3-phase-corner-grounded-delta-120585/

 

[JohnK]

Post 6 linked with diagram labeled "Three Phase Two Wire Corner-Grounded Delta" is what I was dealing with. I was hoping someone could explain to me slowly how one leg is neutral but is 3 legs out of phase with one another or how is tricks a 3PH motor to run. I just can't wrap my head around that knowing one leg reads 0v to ground even though you do read 220v across all legs. I thank all you guys for your input.

At ain't neutral, it's grounded by intent or accident one 3 phase leg has run to ground. If no other leg is grounded it doesn't trip out. If it was a neutral the other legs would read 120v to the neutral and it would not be 3 phase.