Transformer Voltage

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tkb

tkb

Senior Member
Location
MA
I have a customer that has has a 750kVA dry transformer, 480v primary and a 120/208 secondary.

They are having a low voltage problem in the secondary side.

We measured 470 on the primary and 198 on the secondary.

I there a way to calculate what the secondary voltage should be at a fixed primary voltage.
I know that at 480 volts the sec should be 208. 480 ? 2.31 = 208
At 470 volts is it a linear calculation to get the sec voltage? 470 ? 2.31 = 203

They are considering changing the tap on the transformer to raise the sec voltage.
We don't want a voltage that is too high if the utility voltage raises in the winter and too low in the summer.
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
The relationship is linear, with the exception of the full load impedance. If the transformer impedance is 5.75%, then the closer you get to full load, the secondary voltage will drop an additional 5.75%, which is also relatively linear from no load to full.
 
C

charlietuna

Senior Member
I am assuming these readings were taken without any load ??
Most utilities have a + or - 10 percent regulation, so the 470 volts isn't anything to complain about. But it sounds to me the transformer was never connected right with the voltages you read? I would change the taps to give them the rated voltage at the 470 volt primary reading. I have seen variations in primary service voltage as low as 440 in the daytime and then go up to 525 after business hours, basically caused by system loads. Your customer's loads won't mind a little over-voltage, but many just will not function on under-voltage.
 
zog

zog

Senior Member
Location
Charlotte, NC
tkb said:
I have a customer that has has a 750kVA dry transformer, 480v primary and a 120/208 secondary.

They are having a low voltage problem in the secondary side.

We measured 470 on the primary and 198 on the secondary.

I there a way to calculate what the secondary voltage should be at a fixed primary voltage.
I know that at 480 volts the sec should be 208. 480 ? 2.31 = 208
At 470 volts is it a linear calculation to get the sec voltage? 470 ? 2.31 = 203

They are considering changing the tap on the transformer to raise the sec voltage.
We don't want a voltage that is too high if the utility voltage raises in the winter and too low in the summer.
Click to expand...


You should monitor voltage for a couple weeks with line monitors to see what the variance is before changing your taps. I would not change any taps based on one voltage reading.
 
C

charlietuna

Senior Member
Thats what they make voltage taps for ! Raise it one tap and your secondary should be around 212 volts--then put the monitor back on it for a week to see if you need to raise it again. Some printing equipment is very fussy about undervoltage and we always kept it on the high side to reduce their problems.
 
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