Reversing a Transformer 5

[PUMPDESIGNER]

Electrical know little here.
Have 75 KVA transformer, 480 volt primary, 208 volt secondary.

Any reasons why I cannot use this transformer in reverse?
I would like to use it for testing purposes on motors.

Any problems expected?


PUMPDESIGNER

 

[busbar]

If it’s a plain-vanilla ?-Y drytype, for 75kVA you shouldn’t have any major problems. Operate it with the Y-side XO terminal unconnected. The high side will now be an ungrounded-delta arrangement, but for it’s small size shouldn’t be of any concern. Clearly there is no wye-neutral on the high side, so 277V loads are out. [The tap designations are the same, but are reflected as changes on the load/high-side winding.]

Coincidentally there are stock drytype 208?-480Y/277V units intended specifically for stepup service.

 

[PUMPDESIGNER]

Thank you busbar,
What is the difference between intended stup-up service and what I am doing?

I know about the step-up transformers, but this one is in my hands now and will cost me nothing. Willing to junk it but if there are no serious disadvantages I would like to use it.

PUMPDESIGNER

 

[busbar]

480Y/277V 4-wire service is more common and somewhat safer than ungrounded or corner-grounded 3-wire 480V-delta service.

 

[jraef]

Pumpdesigner,
If you are using it to test 460V motors from a 208V service (as I interpret the original post) there would be no reason to have 277Y on the HV side anyway. There is nothing wrong with what you intend to do, it is done all the time.

Quando Omni Flunkus Moritati

 

[djs]

Hello;
All transformers have some small losses. These losses show up as a loss of voltage with respect to the turns ratio. For example a 4:2 transformaer with 480 in will not output exactly 240 under load. It will be slightly less. For this reason most transformers are designed with slightly different turns ratios to correct for this.
In using a step dowm transformer to step up, you will be hit twice by this. Once by the turns ratio, and a second by the losses. If you can stand a slightly lower voltage, then all is good.

 

[jbartos]

Suggestion to the previous posting: The transformers are normally rated on the secondary side. If the transformer impedance is compensated by the transformer turns ratio, then the transformer reversal will provide the nameplate voltage. If not, the transformer reversal can produce higher voltage on the high side. However, everything is dependent on the actual volts available in the outlet. It can be 200V instead of 208V. Then, the whole detail transformer voltage analysis is somewhat irrelevant.

 

[jraef]

And of course, that is one of the reasons why system voltage is typically 480V and motor utilization voltage is typically 460V! Losses are everywhere.


Quando Omni Flunkus Moritati

 

[PUMPDESIGNER]

"Losses are everywhere",

That is why I so eagerly await superconducting motors and transformers.
Not holding my breath though.
But it would be neat to hold a 20 hp motor in the palm of my hand.

PUMPDESIGNER

 

[jraef]

Promises promises...

 

[nbucska]

If you use it only for motor testing for safety you should make sure neither phase can be GND-ed accidentally, ground the motor case and perhaps provide an artificial Y-point to
GND etc...



<nbucska@pcperipherals.com>

 

[PUMPDESIGNER]

nbucska,
We would provide normal case grounding and protection for each incoming phase.
We could also place circuit breaker on secondary outputs if that is helpful.
What do you mean by artificial Y-Point?
Is there some specical concern you have that this transformer may require a little extra protection above normal?

PUMPDESIGNER

 

[nbucska]

If the center of the STAR is not accessible and one line
gets grounded you can have 1.73 times higher voltage
when you touch the case.

If you connect low enough -- equal-- resistances from all three phases to GND, the grounding of one phase would trip
the circuit braeker.

Other solution may be to place the motor under test in a
closed cage and interrupt the input if the cage is opened.


<nbucska@pcperipherals.com>