Wasteful utility practice?

[peter d]

I've noticed that the various poco's in my area use 120/240 volt transformers when they build a 120/208 service, and only use one 120 volt coil from each transformer (center tap and one outside terminal connection). This leaves the other 120 V coil unused. Isn't it needlessly wasteful of electricity to do it this way? (additional power consumed by the coils that sit there and do nothing.) I realize they do this for inventory reasons, and presumably to save money.

I've noticed very few of the straight 120 V transformers used for the 3-ph services, so it is done that way occasionally.

 

[jim dungar]

I've noticed that the various poco's in my area use 120/240 volt transformers when they build a 120/208 service, and only use one 120 volt coil from each transformer (center tap and one outside terminal connection). This leaves the other 120 V coil unused. Isn't it needlessly wasteful of electricity to do it this way? (additional power consumed by the coils that sit there and do nothing.) I realize they do this for inventory reasons, and presumably to save money.

I've noticed very few of the straight 120 V transformers used for the 3-ph services, so it is done that way occasionally.

Transformer conductor losses only occur when current flows through the conductor. Transformer core losses occur all of the time. Using a larger core than necessary may result in excess core losses.

 

[quogueelectric]

Remember the seinfeld episode when someone said something exposing the post office and all the mail carriers came out to secretly put the hit on that person? And they kept grilling Newman about his link to them? Hope you sleep well at night my friend. Must be routine maintenance when you see 4 linetrucks at 4 am tonight. Maybe call a "friend" to start your truck tommorow.

 

[peter d]

Transformer conductor losses only occur when current flows through the conductor.

I know, but doesn't a transformer that is sitting there unused still consume power due to eddy currents and hysterises losses? So half the transformer (in this case) will never be used and will forever consume a small amount of power to keep the coils humming and warm.

 

[peter d]

Remember the seinfeld episode when someone said something exposing the post office and all the mail carriers came out to secretly put the hit on that person? And they kept grilling Newman about his link to them? Hope you sleep well at night my friend. Must be routine maintenance when you see 4 linetrucks at 4 am tonight. Maybe call a "friend" to start your truck tommorow.

Did the farmer slip something wacky into your feed tonight?

 

[charlie]

. . . This leaves the other 120 V coil unused. Isn't it needlessly wasteful of electricity to do it this way? . . . I've noticed very few of the straight 120 V transformers used for the 3-ph services . . .

The other coil is used. It is connected in parallel with the first one.

Notice that one bushing is not used but both halves of the secondary coils are used.

 

[steve66]

Remember the seinfeld episode when someone said something exposing the post office and all the mail carriers came out to secretly put the hit on that person? And they kept grilling Newman about his link to them? Hope you sleep well at night my friend. Must be routine maintenance when you see 4 linetrucks at 4 am tonight. Maybe call a "friend" to start your truck tommorow.

Don't be silly - that only happens with the phone company

If you have every watched "WKRP", you know what I'm talking about.

 

[gar]

080516-0903 EST

charlie:

I am unfamiliar with pole transformer construction. Is it correct that the transformer is rewired internally for this application by the user, or is it purchased from the manufacturer in this configuration?


peter d:

As a first order approximation the secondary windings used or unused have nothing to do with core losses. Core losses are a function of flux changes.

Suppose a transformer is designed for X KVA, then at full load about half of the I^2*R losses will be in the primary and the other half in the secondary. Some of the I^2*R losses in the primary are from the magnetizing current, but this is moderately small compared to load current losses.

Reflected back to the primary there is an approximately equal distribution of internal impedance between the primary and secondary.

For all practical purposes there are no losses from an unused open secondary. True there would be some very limited eddy current loss from induced currents in the copper, but extremely small.

If one secondary is fully loaded, and the other no-load, then the I^2*R losses compared to a fully loaded transformer are 1/4 (from 1/2 of 1/2 for the secondary) + 1/4 (only half the current in the primary).

Did I make any mistakes in this analysis?

Operated at half load the transformer should have a longer life.

.

 

[charlie]

Is it correct that the transformer is rewired internally for this application by the user?

Yes. The pole mounted transformers have either a removable cover or a hand hole where access can be gained to reconfigure the secondary coils or to change the primary taps. All of the transformers are shipped in with the coils in series and on the nominal taps (we normally ask for 2 - 2?% taps above and below nominal). The larger 1? transformers have four secondary bushings with copper straps between the two center bushings for use at 120/240 volts. If the straps are relocated to connect the two outside bushings (the two on the left and the two on the right), the coils may be paralleled for 120 volt operation. :smile:

 

[jim dungar]

I know, but doesn't a transformer that is sitting there unused still consume power due to eddy currents and hysterises losses? So half the transformer (in this case) will never be used and will forever consume a small amount of power to keep the coils humming and warm.

Which is why my original answer included a statment about core losses.

 

[engy]

If you have every watched "WKRP", you know what I'm talking about.

It's the PHONE COPS!, lets get out of here!

 

[peter d]

If the straps are relocated to connect the two outside bushings (the two on the left and the two on the right), the coils may be paralleled for 120 volt operation. :smile:

The transformers the poco's use here are 3 terminal 120/240 volt type. I have seen 4 terminal transfomers but only for the few remaining delta services we have around here. I'm pretty sure they would be the 240/480 type.

Anyway, thanks for the info. I trying to "edumacate" myself on some of the finer points of transformer operation.