Can a stepdown Transformer be used as a step-up transformer? How to rate it?

[Snowjob]

I'm reviewing a package where a client wants to take a 15 kVA 600 VAC to 120/240 VAC single phase step down transformer out of service.

The client wants to re use the transformer as a 120/240 to 600 VAC step up transformer.

Is this a common practice? I have never seen this done before but I'm not that experienced as others. Also how do I rate the transformer in checking the calculations?

 

[brian john]

Yes it will work, rating is the same.

You would only run 240 VAC to the transformer forget the neutral and do not ground anything on the 240/120 VAC side.

The output is 600 VAC, would you ground one side f this? I assume you are from Canada?

I know NOTHING about Canadian codes.

 

[Dave6]

+1 to everything above.

I also recomend (maybe required) to mark it in a way that people know it is abnormally wired.

 

[Snowjob]

Its single phase

Its single phase

Its single phase so is neutral wiring still a priority?

 

[GeorgeB]

client wants to take a 600 VAC to 120/240 VAC single phase transformer out of service.

The client wants to re use the transformer as a 120/240 to 600 VAC transformer.

Is this a common practice? I have never seen this done before but I'm not that experienced as others. Also how do I rate the transformer in checking the calculations?

Back when I was younger and an amateur radio operator, we had a friend with the POCO who gave us a pallet of small 2300:230 transformers they used for pole lighting before changing distribution voltage. We ran those backwards for B+ on our homebuilt kW linears and never had any problems. The manufacturer rated them at 1 kVA, I think; we ran 1 kW CW and 2 kW SSB at virtually no temperature rise. (These were oil filled, roughly 1.5 gallon in size, and PROBABLY had PCB ... ) At the same voltages, the transformer does not know the difference in step up and step down.

 

[K2500]

When sizing ocpd, remember inrush current will be higher for reverse fed transformers.

 

[gar]

100217-1258 EST

Snowjob:

You need to find out if the specified voltage ratio is the turns ratio.

Apply 120 V to the 600 volt input. If on the 240 winding you get close to 120* 240 / 600 = 48 V, then the turns ratio equals the voltage ratio. Simultaneously measure both the input voltage and output voltage with accurate meters. If line voltage is not jumping around you can use one meter.

If the no-load output voltage is more like 50.4 V for 120 V input, then the number of secondary turns have been selected to compensate for voltage drop from full load current. I assumed 5% here.

If this transformer with the extra turns on the original secondary is run backwards from 240, then the no load output voltage would be about 5% low, and when operated at full load about 10% low.

You may need to consider the VA rating will be somewhat less when run in the reverse direction. If the transformer will be run at 75% of original rating you should be OK. As an estimate use 15 * 540/600 = 13.5 KVA.

Another view. The original primary current allowed based on the KVA rating is 15,000 / 600 = 25 A. This same maximum current would be allowed in the reversed direction. On that basis the transformer rating would be 540 * 25 = 13.5 KVA.

The normal transformer connection would allow 15,000 /20 = 62.5 A on the secondary. But you can not run it this hard in the reverse direction because of the original primary and which is now the secondary limitation of 25 A. The turns ratio for 5% correction is 600 / (240 * 1.05) = 2.301. Thus the new maximum primary current is 25 * 2.301 = 57.5 A instead of 62.5 A.

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