Understanding Autotransformers..........

Status
Not open for further replies.
D

dionysius

Senior Member
Location
WA
The source is 277/480V 3 phase. The loads are small fans (120V) and HVAC indoor units (aka air coils) (230 V).

Would a step down autotransformer (single or 3 phase) be a natural choice for this application since it might be lower cost (less Cu)???

Would any of the following limitations be worthwhile worrying about???? When might isolation be required???

Limitations of an autotransformer relative to a normal trafo are:

(1) It does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either.

(2) A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output.

(3) Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output).


 
With the limited experience that I have, I would lean towards a step down transformer vs buck&boost. If it was a single machine that needed a different voltage then a buck&boost.
 
dionysius said:
The source is 277/480V 3 phase. The loads are small fans (120V) and HVAC indoor units (aka air coils) (230 V).

Would a step down autotransformer (single or 3 phase) be a natural choice for this application since it might be lower cost (less Cu)???

Would any of the following limitations be worthwhile worrying about???? When might isolation be required???

Limitations of an autotransformer relative to a normal trafo are:

(1) It does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either.

(2) A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output.

(3) Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output).


Click to expand...
How many units are you supplying? Are you looking to install one auto transformer at each piece of equipment or something common to all of the units?

Isolation transformers is probably the best solution, autotransformers are typically only used when you need a slight change in voltage, you are changing it enough you will have enough copper and steel in the autotransformer that you may as well go with an isolation transformer.

Disconnecting means for 120/240 volt equipment you are supplying probably costs more if you supply it from the 277/480 system because you will need 480 or 600 volt gear instead of 120 or 240 volt gear. A $2 snap switch is an acceptable local disconnect for the 120 volt blower units, bump that up to 600 volt rated safety switch and you may be spending $150 on it.
 
Your 230V devices are probably intended to have two hot legs (I assume they are 1ph) that are each 120V to ground. An autotransformer from a 277V phase would result in a European-type 240V output where one end is grounded and the other is 240V to ground. Your equipment may not like this.
 
kwired said:
Isolation transformers is probably the best solution, autotransformers are typically only used when you need a slight change in voltage, you are changing it enough you will have enough copper and steel in the autotransformer that you may as well go with an isolation transformer.
Click to expand...
I agree. But would go for one common unit.
 
dionysius said:
The source is 277/480V 3 phase. The loads are small fans (120V) and HVAC indoor units (aka air coils) (230 V).

Would a step down autotransformer (single or 3 phase) be a natural choice for this application since it might be lower cost (less Cu)???

Would any of the following limitations be worthwhile worrying about???? When might isolation be required???

Limitations of an autotransformer relative to a normal trafo are:

(1) It does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either.

(2) A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output.

(3) Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output).


Click to expand...

Auto transformers aren't a very cost effective way to go with such a large voltage change. They are more economical to use when adjusting for smaller voltage change such as 6, 12, 16, and 32v buck /boost applications. As the ratio increases the kva increases to a point where using a common isolation transformer is not that more expensive in addition to being standard.
Remember that autotransformers only need to be sized with enough kva to make the voltage adjustment and not the entire kva of the load as with an isolation type transformer.
 
how is it that any building does not naturally have some form of utilization voltage that provides 120v? you would run the loads you talked about from 208/120 service.
 
Status
Not open for further replies.
Top