No load losses - normal vs reverse fed

[iwire]

We have wired a number of 75 kva 480 primary using 100 amp breakers and I can't remember any of them tripping on energizing even though I was told on some forum that it would not work.

 

[electrofelon]

So we kind of got off topic with the inrush issue. Recall that the OP was about no load losses. The reason I ask relates to the step up step down setup mentioned in my previous post. Originally I had 2 old GE 10 KVA open units - old - 50's-60's maybe? They showed a no load loss of about 300 watts each or 3% ouch! I dont have a way to measure the power on the 2400 side so all I can do is measure the step up by itself, then energize the step down and note the addition. The first transformer isnt really under no load then I suppose but I imagine its close. In this case, both the normal fed and reverse fed seemed to have the same losses.

I upgraded recently to an olsun 2400 to 240 15 KVA dry type used in reverse for the step up, and a 15 KVA cooper pad mount for the step down. The Olsun measures 130 watts. The padmount shows 70 watts which is quite impressively low - .46%. I know .5% is the factor typically given, but I have seen some literature on a similar small padmount units and the manufacturer stated the no load losses at .7% so I figure they just run a little higher than larger units. So that is what got me thinking about this and I was questioning whether the higher losses in the dry type are because it is reverse fed, or just because its not as good a design for no load losses.

 

[Besoeker]

It's called a compensated winding.

OK.
How is this compensation achieved?
Be precise and concise if you can.

 

[Besoeker]

Ding! Everyone to their corners.

Transformer impedance causes full load voltage to be lower than no load voltage.

The only question is whether a transformer is designed to produce its nominal voltage at full load or at zero load.

I suspect you know the answer to that. Look at the nameplate voltages.

But, either way, it affects the turns ratio.

 

[electrofelon]

OK.
How is this compensation achieved?
Be precise and concise if you can.

I admit to knowing nothing beyond conjecture, but why wouldnt it just be a few more/less turns? Also, why not just leave the ratio proportionate to nominal voltages and let the user adjust the taps if they so desire?

 

[Besoeker]

I admit to knowing nothing beyond conjecture, but why wouldnt it just be a few more/less turns? Also, why not just leave the ratio proportionate to nominal voltages and let the user adjust the taps if they so desire?

You could. That changes the turns ratio. That's how tap changers work.

 

[electrofelon]

You could. That changes the turns ratio. That's how tap changers work.

I concur.

 

[topgone]

So we kind of got off topic with the inrush issue. Recall that the OP was about no load losses. The reason I ask relates to the step up step down setup mentioned in my previous post. Originally I had 2 old GE 10 KVA open units - old - 50's-60's maybe? They showed a no load loss of about 300 watts each or 3% ouch! I dont have a way to measure the power on the 2400 side so all I can do is measure the step up by itself, then energize the step down and note the addition. The first transformer isnt really under no load then I suppose but I imagine its close. In this case, both the normal fed and reverse fed seemed to have the same losses.

I upgraded recently to an olsun 2400 to 240 15 KVA dry type used in reverse for the step up, and a 15 KVA cooper pad mount for the step down. The Olsun measures 130 watts. The padmount shows 70 watts which is quite impressively low - .46%. I know .5% is the factor typically given, but I have seen some literature on a similar small padmount units and the manufacturer stated the no load losses at .7% so I figure they just run a little higher than larger units. So that is what got me thinking about this and I was questioning whether the higher losses in the dry type are because it is reverse fed, or just because its not as good a design for no load losses.

I guess it's not too late chiming in.
If you have two, identical single-phase units, why not try the Sumpner back to back transformer test?

 

[electrofelon]

I guess it's not too late chiming in.
If you have two, identical single-phase units, why not try the Sumpner back to back transformer test?
View attachment 16470

interesting, thank you. Ill have to digest it a bit and get back to you.......

 

[big john]

How is this compensation achieved?

I've never tested a compensated winding transformer, so I don't have empirical proof, but my understanding is that it's accomplished by adding winding turns so that the VD through the loaded transformer is closer to nameplate nominal than it would be without the compensation.

That's why I mentioned that the power transformers I test all have ratios that very precisely match the nameplate voltages: They aren't being compensated with added turns.

 

[Besoeker]

I've never tested a compensated winding transformer, so I don't have empirical proof, but my understanding is that it's accomplished by adding winding turns so that the VD through the loaded transformer is closer to nameplate nominal than it would be without the compensation.

That's why I mentioned that the power transformers I test all have ratios that very precisely match the nameplate voltages: They aren't being compensated with added turns.

But the output voltage will very some depending on loading.

 

[iwire]

But the output voltage will very some depending on loading.

Just as it will with compensated windings. The only difference is a compensated winding is going to have a higher, unloaded voltage than the label indicates.

You can't escape circuit impedance with extra windings in a transformer, all will have some voltage drop due to loading.

 

[Besoeker]

Just as it will with compensated windings. The only difference is a compensated winding is going to have a higher, unloaded voltage than the label indicates.

You can't escape circuit impedance with extra windings in a transformer, all will have some voltage drop due to loading.

Which is why you need to use no load voltages to get turns ratio.

 

[GoldDigger]

Which is why you need to use no load voltages to get turns ratio.

Right. And for US (NEMA) transformers you will find that it matches the nominal voltage ratio.

Sent from my XT1585 using Tapatalk

 

[iwire]

Which is why you need to use no load voltages to get turns ratio.

Sure, but that is not what we have been talking about.

My understanding of this thread is this.

It seems you feel all transformers have compensation, my position has been 'not the ones I work on'

 

[Besoeker]

Right. And for US (NEMA) transformers you will find that it matches the nominal voltage ratio.

So the on-load voltage ratio will be different to the turns ratio since the turns ratio matches the no load voltage ratio.

 

[Besoeker]

Sure, but that is not what we have been talking about.

My understanding of this thread is this.

It seems you feel all transformers have compensation, my position has been 'not the ones I work on'

Nor the ones I have worked on.

 

[GoldDigger]

So the on-load voltage ratio will be different to the turns ratio since the turns ratio matches the no load voltage ratio.

Absolutely.
It seems to me that your position is that since you feel that compensated windings are a good design idea, it must be true that all manufacturers build that way.
The opposing position states that the world just does not work that way.

Sent from my XT1585 using Tapatalk

 

[Besoeker]

Absolutely.
It seems to me that your position is that since you feel that compensated windings are a good design idea, it must be true that all manufacturers build that way.

I think no such thing and I have no idea how you came to that conclusion.

 

[iwire]

Nor the ones I have worked on.

:slaphead:

Can you explain how we ended up down this rabbit hole?

There's another thing to think about. Say, for simplicity, the transformer is designed to step down from 240V primary to 120V secondary the turns ratio is likely to lower than 2:1.
This because the 120V would be designed to be the on-load voltage so it would be higher off-load. If you feed it with 120V you will most likely get less than 240V out.

It has to be to get the right on-load voltage. It isn't a fudge.

That's kinda the point - to get the correct on-load voltage. It isn't a fudge.
Turns ratio test is done at no-load so the voltage is higher than the rated on load voltage. This means that turns ratio for my 240/120V tranny is not exactly 2:1.

From the link you posted:
"Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and backfeeding will result in lower than expected output voltage."
The very point I've been making!
The comment about lower than expected voltage doesn't just apply to control transformers. It is just basic transformer calcs. The no-load voltage is higher than the on-load voltage. Thus, the turns ratio cannot be the same as the designed operational voltage ratio. It really is that simple.

It may not change what you do and you don't think it affects you. That's fine. Maybe not a big difference, one you can ignore for many practical purposes. But it is there.


Yes. As was I, so I'm not sure what point you are making.

Maybe just say you made a mistake?