2 winding transformer vs autotransformer impedance

[mshields]

Say you need a 45kVA transformer for 575 to 480V. And lets say a 2 winding transformer has an impedance of 4%. ANd that therefor a full load, the voltage drop would be 4%.

Would there be an impedance and therefore voltage drop advantage to using an autotransformer to accomplish the same thing?

 

[electrofelon]

Say you need a 45kVA transformer for 575 to 480V. And lets say a 2 winding transformer has an impedance of 4%. ANd that therefor a full load, the voltage drop would be 4%.

Would there be an impedance and therefore voltage drop advantage to using an autotransformer to accomplish the same thing?

I dont think its a simple matter of computing VD of a transformer by the impedance. Voltage regulation is more complex than that.

Maybe check out this thread:

Transformer %Z impedance and voltage drop

I need a refresher on what a transformers %Z means in terms of voltage drop across the transformer. For example if a transformer has a 5% impedance does that mean that that the transformer will drop 5% voltage during full load conditions. So in this case a transformer with a 480V rated...

forums.mikeholt.com

 

[winnie]

Agree with electrofelon that voltage drop is not quite as simple as % impedance at full load, mostly because transformer impedance is mostly inductive and thus not 'in line' with the load being served.

But going back to the original question: when you use an autotransformer the bulk of the load is served directly by the supply and only the 'difference' goes through the transformer. So in your case instead of seeing the transformer voltage drop on the full 480V, you would instead see it on the 120V being dropped.

-Jon

 

[mshields]

Ah - Thank you Winnie - I thought it was something like that but obviously wasn't sure. Thanks

Mike

 

[mshields]

So tell me this. If the load is 100% motor load then, the two would equate?

 

[winnie]

So tell me this. If the load is 100% motor load then, the two would equate?

Not quite, but if you have a highly inductive motor load it will be closer.

The thread linked above itself links to a calculator. Worth playing with.

Transformer Calculator – Voltage Disturbance

According to the calculator:
If you have a transformer with 4% impedance and X/R ratio of 7 feeding a unity power factor load (a resistor), and the transformer is loaded to 100%, then the voltage drop at the load will be 0.57%. Instead feeding a load with a power factor of 0.8 the voltage drop will be 2.83%. The load power factor needs to be less than 0.2 to see the full 4% voltage drop.

 

[Sahib]

An autotransformer is more efficient than corresponding two winding transformer and so its voltage drop should be less. No?

 

[Besoeker3]

An autotransformer is more efficient than corresponding two winding transformer and so its voltage drop should be less. No?

And much less impedance especially if the ratios are fairly close as in this case.