Delta to Wye Problem

[eobryan]

I need to power a x-ray generator from a 480 delta service. The generator needs 277 wye. I happen to have a 3-phase autotransformer with the proper taps.

Can I use the autotransformer with the center tied to safety ground; or, should I buy a delta-to-wye transformer and connect the neutral of the wye to the safety ground of the service?

 

[cranky108]

It depends. 277 is the phase to neutral voltage for a 480 grounded wye source. So what exactly is do you mean 277 wye?

Why do you think an auto transformer will help you?

If the source is truly a 480 delta, and I ask because there should be a ground reference, all you need is a 480 wye to anything delta to generate a ground reference.

 

[eobryan]

I need to reduce the voltage from 480 to 277. I can't get this directly from the delta service. Therefore, I need something to reduce the voltage.

I'm concerned that using the autotransformer will violate some codes because the delta service has no fixed ground. If I use the autotransformer I will directly reference the delta service ground to safety.

Using a delta-to-wye transformer, I will establish a floating ground which I plan to tie to safety.

Perhaps I'm overly concerned, but I don't want to jeopardize the operators with a system ground that jumps. X-ray systems can scram (shuts down abruptly) or form (x-ray tube arcs) which results in massive imbalance problems. The safety ground on my service is beefed up just to handle dumps like this.

 

[cranky108]

From the perspective of someone in the US, there should be a ground reference on the 480. In other countries they do use a floating secondaries.

So I would expect your source, if you are in the US to either have a center ground reference, or a corner ground. That is why I can't jump to a solution like an auto transformer. Besides we usually specify a delta somewhere (at least in higher voltages) for harmonics.

If the 480 V is truly floating, then you should be capcaitivly coupled to a center ground, which means the phase to ground voltage should be about 277 V. And in fact where we supply a 480 V source to a customer, the ground reference is the center point. On our 480 V systems the voltage, phase to ground is 277 V.

 

[davidbeach]

277 wye (line-neutral) = 480 line-line. I think you need to find a competent electrician locally who can assist in the installation.

 

[eobryan]

I understand that the line-line on a wye is the square root of 3 times the line-neutral.

What I have for a source is a fused safety switch box on the wall of a laboratory. There are four wires in that box, the biggest of which is the safety ground. This box is about 30 meters from the electrical closet.

What I need to connect to is a x-ray system that requires five wires (three phases of 277, a neutral, and a safety ground). I should be able to do this safely with a delta-wye transformer of the appropiate size and create a system ground point at the safety ground of the service. A 9 kva delta-wye costs about $1500 USD.

I have a 3-phase autotransformer. The question is can I safely use this autotransformer in this application and save $1500?

 

[DRWeig]

I've worked with services with 480 delta, ungrounded.

If you need the neutral for a single load, can you not get an appropriately sized delta-wye transformer to create a separately-derived source?

I agree with davidbeach, too. 277 wye is not an appropriate expression of what you need --

Alternative: post the wiring diagram for the input power to your x-ray generator so we can see what your needs are.


Best to you,

Goober Dave

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[dpc]

There may be a neutral back at the source of power to this disconnect, but a neutral conductor was not provided, perhaps. A true ungrounded delta 480 V system in the US in a commercial building would be rare. DRWeig's suggestion of a 480 V - 480/277V transformer is a good solution if there is no way to run a 4-wire circuit.

 

[waross]

If you tie the auto-transformer wye point to ground, you will have issue should a ground fault appear somewhere else in your system.
In any event the wye point will not develop a stable neutral. Unbalanced loads will cause issues.
You may want to consider three small dry type transformers to develop a neutral locally for the X-ray machine.
You want transformers with a primary voltage of over 277 Volts. 480 Volt transformers will do fine. The secondary voltage is unimportant.
Connect the primaries in wye and take your neutral point from the wye point. DO NOT GROUND the wye point.
Connect the secondaries in delta and tape up the connections. The delta stabilizes the wye point. It does not support any load. But, before doing that I would check the wiring diagram to see if the neutral may be dispensed with.
To echo DRWeig's suggestion; Post a wiring diagram.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Bill - I'm not sure your solution would pass muster with the inspectors. Biggest electrical problem would be a phase-neutral fault that nothing would clear. You do solve a lot of circulating current problems by floating the neutral with reference to ground, but many codes would have a problem with that floating neutral.

 

[waross]

Hi David;
Under the Canadian code, if grounding a neutral results in a voltage of over 150 Volts to ground, the system need not be grounded. With the wye point left floating you derive a neutral that is stabilized by the delta secondary. A line to ground fault on the system will not affect this as the small bank is not grounded and the system has no neutral. In the event of a line to neutral fault in the equipment the breaker feeding the transformers will clear. The small transformer bank must be capable of withstanding a line to neutral fault until the circuit breaker clears.
But, all that said, if the AHJ says no, it's no.
Did I miss anything??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eobryan]

As requested, I have posted a wiring diagram and I'm glad I did as I find that I need 400 Volts instead of 277 (I mistakenly thought my x-ray system would adapt to anything between 400 and 230). Therefore, my question to all of you has changed since my autotransformer is useless.

Given that I have 480 Volt, delta service in the laboratory, how can I best adapt to the requirements of my x-ray system needing 400 volt, wye configuration?

I work is a massive bureaucracy which means that modifying the service will take too much time and money. Therefore, I prefer to locally do whatever is needed with equipment I purchase under $3000 USD.

Is there a safe way to locally adapt the delta service I have to my x-ray system?

 

[eobryan]

Correction, my autotransformer has an appropiate tap ( 0.48 ratio ). So my original question still stands -- can I safely use the autotransformer?

 

[cranky108]

I still believe you need delta/wye somewhere to stabalize the voltage. Maybe a delta/wye 480/480, and use the auto transformer to step the 480 to 400.

I think 400 V tap is not very common, but maybe reachable in a buck arrangment like the auto transformer.

But if you need to purchase a delta/wye transformer, a buck arrangment may also be possible with that.

 

[waross]

If the machine is 50 Hz, that may be more of a problem than the voltage. Most pumps need more driving torque at faster speeds. If you put 60 Hz to a 50 Hz motor it will run 6/5 faster with no increase in torque. The voltage should also be adjusted up in the ratio of 6/5.
Are there any other pages that you haven't shown us? I see no current, HP or frequency data.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eobryan]

The power requirements are as follows:
3NPE 400/230 V +/- 10%, 50/60 Hz;
8 kVA
16 A time-delay fuses

After considering all of your helpful input, I’ve decided to purchase a 9 kVA 1:1 delta-to-wye transformer which I will attach to the side of my existing floor standing autotransformer. I will also install a fuse block into my autotransformer because my switch box breaker only will go down to 70 Amps. My delta service will power my new transformer and my autotransformer will be paralleled with the output. The neutral I create will be tied to my switch-box safety ground and I will also tie the transformer cases to the same point. This seems to be a cheapest way for me to create a safe source for the x-ray system I need to power.