Inverse use of a Transformer 2

[ServoWizard]

Hello,

I have a 3 phase "step down" transformer that has multiple HV inputs and two LV outputs. My need is to raise 208/220 up to 230. Mathmatically I can accomplish that by reversing the roles of the HV inputs and LV outputs but I do not possess the knowledge to know whether or not that is possible. I calculated the percentage of reduction by using the HV of 222VAC and the LV of 208VAC:

208 -:- 220 = .9369369

1.0 - .9369369 0630631 + 1.0 = 1.0630631

220 x 1.0630631 = 233.87388VAC

The transformer is rated at 75kvW and my application is 2.2kvW. Is this doable or am I too wet to be working with electricity?

Thanks, Servo

 

[dpc]

The transformer can work in either direction (step-up or step-down) regardless of what the original application was.

As long as the voltage applied is within the nameplate values and the turns ratio gives you the output voltage you need, it will work.

But you will need to make sure the transformer and associated feeders are adequately protected against overloads and short circuits per your local codes. You might need to enlist an electrician or EE to sort that out for you.

 

[electricpete]

You have a very small voltage tranformation ratio. I doubt the hv/lv ratio of your transforer is that small.

What are the voltages of the transformer and taps?

What is a kvw? Did you mean kv or kw or kva?

I agree with dpc any installation needs a thorough review for safety.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[mc5w]

Under U.S. rules you can use buck boost transformers to fudge the small difference in voltage. Under U.S. rules you have a choice between a wye-wye autotransformer or an open delta vee autotransformer.

For this amount of power using a 75 KVA transformer is a waste of time.

Acme Power Distribution has a downloadable application booklet for what you want to do.

Also, some transformers, such as General Electric's Transformore forced air cooled dry transformers cannot be used in reverse because of internal thermal protection that activates an internal molded case switch for the primary. Using the transformer in reverse would put the thermal protection system on the wrong side of the transformer.

 

[ServoWizard]

Hello,

I wish to thank all of you for your input. I was in error when I reference the ratings for the transformer and my application.

The GE model 9T24C401 transformer is rated at 75kVA and my application requires 2.2kVA. The transformer taps are as follows: HV=192--222; 318--526, LV=120 or 208;

Is thermal protection issue of any significance when considering the 75kVA to the 2.2kVA?

Ironically thermal protection is the basis of this request for assistance. My application is a vector duty 3 phase motor that is driven by a variable frequency drive which facilitates operating the motor at 500 to 5400(rated) RPM. Running the 230/460 motor with 208/220 is causing a thermal protection alarm to be set when the motor is operated under a moderate load at 40Hz and under.

The motor thermal protection issue is significant due to it's impact on the machine tool plus the possibility of burning up $2,400.00 motor.

Thanks, Servo

 

[dpc]

Overcurrent and short circuit protection of the transformer is a legal requirement per the National Electrical Code.

I'm not quite sure what this transformer is, based on the taps you've given. I'd suggest doing some tests to verify you will get the output voltage you're expecting.

 

[ServoWizard]

Thanks Guys,

I have not purchased the GE transfomer so I've not lost any money. Due to your replys I believe that I'll search for another solution.

Servo

 

[mc5w]

Acme makes a 208 volt delta primary 277Y480 volts secondary transformer. I believe that SquareD also makes one.

Also, a standard single phase transformer is 277 volts primary, 120x120/240 volts secondary. These might be a better buy because you can use these in another application if you should get a 277Y480 volts service in the future.

I take it that this spindle motor cannot be reconnected for 240 volts or something like that? Also, the cooling fan on the motor mat not be rated for 40 Hertz or less and you would then need to refit this spindle motor with a separately powered cooling fan or blower. This is a likely explanation because at 5,400 RPM a shaft operated fan would use 27 times as much power as at 1,800 RPM.

 

[tinfoil]

Some variable frequency drives give you the ability to adjust the output voltage (the waveforms are synthsized anyways) over a range.

Have you exausted that possibility?

 

[ServoWizard]

Hey Guy's,

Thanks for all of the input. I went into this project with very little knowledge, if any, for what I was wanting to accomplish. I soon learned that 12/24 does not always, if ever, refer to the actual secondary current. The balance of the knowledge came soon after. Beyond that I all can say is "Thankg God for eBay"

servo