Transformer Theory Question

[winnie]

I am looking at a Federal Pacific drive isolation transformer, model FH220CFMD

Its connection diagram is shown as figure 25C here http://www.federalpacific.com/outlinedrawings/wd25.pdf

The transformer is nominally 480V delta to 480/277V wye, 60Hz.

The diagram looks to me like that of a 230/460 dual voltage motor.

My theory question:

Is it plausible that this transformer could be connected as a 240V primary, in analogy to the low voltage connection of a dual voltage motor?

It seems to me that if the two winding halves are symmetric, with the 'taps' provided at the midpoint of the coils rather than at one end, then by jumping from the midpoint to the supply leads you would effectively halve the number of primary turns and double the primary current handling capacity.

Thanks
Jon

 

[VinceS]

Isolation transformers are 1 to 1 with taps on the primary

Isolation transformers are 1 to 1 with taps on the primary

In the figure 25C your input voltage variance can be corrected to 460 by setting the taps. 25C is not specified as a 240 volt primary isolation transformer that would be 25A. As far as plausible to use it should work, ( 1 to 1 is the same 480D in 480Y out, 240D in 240Y out ). Because its specified (listed) for use at 480 not 240 I would consult the MFR for the appropriate derating. I don't quite understand your analogy to a dual voltage motor and transformer taps..

 

[Smart $]

Seems quite plausible to me...

...but I'd require something a bit more substantial than a diagram before connecting it up that way and energizing

 

[cadpoint]

It seems to me that if the two winding halves are symmetric, with the 'taps' provided at the midpoint of the coils rather than at one end, then by jumping from the midpoint to the supply leads you would effectively halve the number of primary turns and double the primary current handling capacity.

Thanks
Jon

P=I?E ? ?

Physics forum
here are two posts from the same thread:

Integral
08.28.05, 05:43

P=IE is directly related to Ohms law, and therefore only holds for purely resistive DC circiuts. In AC circiuts you must consider the reactive impedance. The impedance is dependent upon the frequency as well as the circiut resistance, inductance and capacatance. P=IE will also have a reactive component in AC circiuts, making it different from that of a pure resistive DC circiut.

SGT
08.28.05, 07:17

The instant power in an electric circuit is p(t) = v(t).i(t).
This is allways valid. What happens with reactive circuits and sinusoidal voltages and currents is that the peak value of the power is different of the product of the peak values of voltage and current.
P_{max} = V_{max} . I_{max} . cos(\phi)
Where \phi is the phase angle between voltage and current.

I'd truely have to get back to you on this one, but I always enjoy the presentations and the solutions contribited by the members, here, Too...

 

[jim dungar]

No, it is not the same as a dual voltage motor (or a generator for that matter).

The core of the transformer has been designed for a magnetizing current based on an applied nominal voltage of 480V +/- the tap adjustments, with the two winding halves in series. And, on some transformers the primary windings contain additional turns to compensate for losses in the transformer, this means it is possible (probable?) that the two windings halves are not identical.

 

[winnie]

Vince,

Thanks for the comment. I don't want to operate the transformer as a 240V in 240V out; I want to get full capacity.

Let me try to explain the 'dual voltage motor' thought better.

A common type of motor has a delta winding where each phase winding is actually divided into two parts, and these two parts may be connected in series or parallel, allowing the motor to be used at 230V (parallel) or 460V (series). See 'Delta connected 9 lead:
http://www.patchn.com/index.php?option=com_content&task=view&id=44&Itemid=74

I've redrawn diagram 25 to better express what I am asking about. If I add the jumpers shown as bold freehand lines, will the transformer operate correctly as 240V delta to 480/277V wye ?

I know that I'll have to consult the manufacturer because at the very least there is no guarantee than the two 'coil halves' are equivalent (as they would be in a motor), but I was wondering if there were any other caveats that I need to watch out for.

Thanks

-Jon

Edited to add:

P.S. Looks like other answers came in while I was writing the above up. I think that the key point is Jim's where he says that it is probable that the winding halves are not symmetric. If they are symmetric, I think that the above connection addresses the magnetizing current issue: the volts/turn would be the same, so the magnetizing current per turn would be correct, and you would get 2x the magnetizing current at half the voltage with the same core flux.

 

[brian john]

1 to 1 is the same 480D in 480Y out

A 480 delta to 480/277 wye is not a one to one transformer actually the ratio is 1.732 to 1.

 

[LarryFine]

Is it plausible that this transformer could be connected as a 240V primary, in analogy to the low voltage connection of a dual voltage motor?

It sure looks that way, presuming the numbers indicate accessible individual connections. I'd say that as long as you make sure you use the same taps all around, it should work.

I do completely agree with checking with the manufacturer before doing it that way, though. If the primary is not rated 480/240, the magic smoke could be made to escape . . . once.

 

[winnie]

Well, learn something new....

I spoke with an engineer at Federal Pacific, and he confirmed that the taps are explicitly _not_ right in the center of the winding, and are in fact pretty close to the end. He didn't know the exact number, but 90%-10% is the right idea.

He also explained the reason: when the taps are at the end of the coil, the jumper must be sized to carry the full line current (even though it doesn't normally carry the full line current), but when the taps 'split' a coil, then they need only be sized for the normal coil current. This doesn't quite make sense to me (the current carried is the same in either case), but I guess it has something to do with the standards to which transformers are built.

In any case, while the diagram suggests the possibility, this cannot be used as a 240V primary.

-Jon

 

[LarryFine]

I spoke with an engineer at Federal Pacific, and he confirmed that the taps are explicitly _not_ right in the center of the winding, and are in fact pretty close to the end. He didn't know the exact number, but 90%-10% is the right idea.

That sounds like he's talking about the taps on each winding (or winding half.) The question is whether each half of each secondary, including the taps, is identical to the other.

We already know the taps are near one end of each of their respective windings, and we can calculate the percentages of turns using the percentages of the voltages.

What we want to know is, using the center primary in the OP's linked image for example, whether the winding from H1 to its tap#5 is identical to that from tap #4 to H2.

 

[Smart $]

That sounds like he's talking about the taps on each winding (or winding half.) The question is whether each half of each secondary, including the taps, is identical to the other.

We already know the taps are near one end of each of their respective windings, and we can calculate the percentages of turns using the percentages of the voltages.

What we want to know is, using the center primary in the OP's linked image for example, whether the winding from H1 to its tap#5 is identical to that from tap #4 to H2.

I agree...

 

[winnie]

What we want to know is, using the center primary in the OP's linked image for example, whether the winding from H1 to its tap#5 is identical to that from tap #4 to H2.

When I was talking with the engineer at Federal Pacific, I explicitly asked if the winding halves were equivalent. He said that they were nowhere near equivalent.

He understood what I wanted to do (make a 240V connection), and said that such a connection would not work.

I did manage to find a used transformer on ebay with exactly the primary connection that I'd want, unfortunately it has half my desired capacity.
http://www.industrialsurplus.com/photos ebay/028-042_2.jpg

-Jon