TRANSFORMER ISSUES

[jaggedben]

Is it possible the transformer has a corner grounded phase, like B, and the oven has a different one like C?

Or perhaps they are supposed to be the same but the circuit is miswired.

 

[LarryFine]

They had to spend over $2,000 to air freight a 440 to 480 volt transformer that we used as a step down transformer.

Did anyone consider using a buck-boost setup?

 

[jim dungar]

Did anyone consider using a buck-boost setup?

That is probably what was supplied. German and Japanese tool manufacturers often used multi-tapped autotransformers for making their equipment accept a wide range of voltages.

Getting the solution from the factory might have been cheaper in the long run, when warranties may be affected.

 

[hillbilly1]

I agree with the others, most likely a bad element in the oven. Years ago I hooked up a range top that was straight 240, used the high leg and another phase, the cooktop didn’t last long, so on the new on I moved it off the high leg. I think those burners didn’t like the higher voltage to ground, and was leaking current to ground. It may be the same style elements if home built, and since it was home built, I bet the used thhn to connect to the elements. So if it was opened up, you will probably see some burnt wires.

 

[ramsy]

Is it possible the transformer has a corner grounded phase, like B, and the oven has a different one like C?

How do you check that?

If disconnect isolates all supply-side phases, wont each appliance leg still ring to ground, since load phases are tied together?

 

[OldBroadcastTech]

My $ 0.02

The original post said ( in part) "The specs are labeled as 3-phase, corner grounded delta (primary)"

Since the original primary is now the secondary, wouldn't this mean that there is an intentional ground on one corner of the delta ?

Just my take on it, I'm probably wrong, but thought it makes sense.

 

[garbo]

Did anyone consider using a buck-boost setup?

So called engineer that was over the electric shop never heard of them. I did suggest purchasing buck boost transformers but was shot down. Same engineer saw nothing wrong In using a romex connector mounted to a 1900 cover being use to support 12/4 cord down from 20' high ceilings to equipment. At least until OSHA wrote it up. Loved when they told me " but that's the way we always done it ".

 

[herding_cats]

You probably have a dead shorted element, popped fuse. Visually check the elements, look for pitted holes or slag. There will be 3 fuses, one for each three phase leg. Put an ohm meter across each element as well.

 

[synchro]

... When the oven is turned on, one phase is pulling up to 120A then the breaker trips in the control panel. ...

If you put a clamp meter around all three of the phase conductors feeding the oven, if a ground fault is causing the trip then it will show up on your meter when the oven is turned on. If there's no ground fault current indicated, then what @herding_cats mentioned about a shorted element is a likely cause. If neither of these indicates that it's causing the problem, then there could be a wiring problem within the oven.

 

[tortuga]

The oven was custom built in house, no. Specs available. 3-phase heating elements, 40 amps each. 240v and fed through a motor control panel.

If it was custom built in house and house voltage is 208 why not order 208 Elements to begin with? Perhaps it was moved?
Anyways why a transformer at all? Cant you just run 240 Elements at 208 and the thermostat will take slightly longer to come to temp? If there is a 24V control transformer you can usually change the taps from 240 to 208.
Why 75kVA? Is the 75kVA transformer for any other loads other than the 25kVA oven?
If I had to spec a transformer for that I would use a SG3A0030BD 208D primary to 240/139 secondary, not a SG3A0075DB.

 

[synchro]

Another alternative would be two single phase autotransformers in an open delta configuration to get 240V L-L from the 208/120V source.

 

[jim dungar]

Another alternative would be two single phase autotransformers in an open delta configuration to get 240V L-L from the 208/120V source.

Not if the requirement is to create a 3 phase corner grounded source as mentioned in the OP.

 

[synchro]

Not if the requirement is to create a 3 phase corner grounded source as mentioned in the OP.

It seems (at least to me) that in the original post, @jeffreylee7469 was using the word "specs" to refer to what was on the label of the transformer itself instead of a design spec for a wiring installation:

"I was asked to look at this transformer. The transformer is being used as a step-up transformer, but I don't see any specs that state the transformer can be used this way. The wiring is 120/208v primary and stepping up to 240v secondary.
The specs are labeled as 3-phase, corner grounded delta (primary) and 3-phase wye (secondary).
The owner had the transformer wired so x1, x2, and x3 (secondary) were the primary and the H1, H2, H3 (primary coils) are the secondary feeding a 3-phase 240v panel."

As indicated in posts 11, 14 and 15, the OP had mistakingly thought that the dashed line (showing the phase shift of H1 relative to X1) was for indicating that the transformer was corner grounded internally. Voltage masurements showed that the delta did appear to be corner grounded somewhere. But in post #11 the OP said: "Nothing in the transformer was manually grounded".

The OP said no specs are available for the custom made oven, and so it's not clear what type of transformer secondary grounding is best for the oven or any other equipment on the same feeder.