Boosting Voltage

[Gwhaarmann]

I have a regrigeration compressor that is rated 208/230V This application is in a mobile unit so supply voltage will vary depending upon what is available.. when the supply voltage drops below 208v ( due to long supply lines) the compressor heats up at times and becomes less efficient ( at times even stopping due to internal protection). I would like to install a step-up transformer to correct this situation.
My question is, when the supply voltage IS high enough (208-230) will this affect the transformer?

[likephysics]

Yes. it would.
Step up transformer will not know what the input voltage is. It will step up whatever you feed it.
You should use the step up only if you are sure the voltage will drop below 208v.

[Dmytry]

I would use thicker gauge supply line. How long is it, and what is the power rating of compressor?

And yes that would. Suppose your transformer steps up from 200 to 220 , i.e. by 10% . At 220, it will step up to 242 , furthermore, it is possible for it to saturate if increase in voltage is too big.