208V-480V transformer on a 120/240V 3-phase system

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guitarman21632

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Federalsburg, MD USA
I have a customer with a 3-phase 120/240V system. They have a transformer that is 208V-480V that they want to use for an HVAC motor testing station. Because of the 240V I am getting extremely higher voltages on the 480V side; approximately 80V higher. There are a total of 7 taps on the transformer and currently the taps are set at the 3rd tap. Am I correct in that if I change the taps to a lower voltage I should be able to dial the 480V down to a lower voltage? In other words bring the 560V I have down closer to 480V?
 
A lot of dry transformers have 5 taps to get within ?5%. If you've got 7 taps, then it's probably +5% to -10% and tap 3 is your nominal 480V tap.

You need to confirm that this is the case by checking the nameplate before doing anything, but if that's true, then bucking down to the -10% tap will get you 498V (and possibly lower once loaded). This would be within the acceptable operating range of your 480V motors.
 
Are the taps on the 208 or 480 volt side of transformer? (is is a step up or a step down)

The source side should be the side with taps, and if I understand correctly you are transforming 240 to 480 but are using a 208 to 480 transformer.
 
big john said:
Works out the same either way.

208V * 90% * 2.307 = 431V
208V * 2.307 = 479V * 90% = 431V
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Voltage may work out the same, but if you don't have the taps on the supply side, you would be overexciting coils designed for 208 volts with 240 volts. Common actual voltage of a 240 volts system in this neck of the woods is often 246-250 volts so that raises excitation even more.
 
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