Transformer tap change calculation

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isupowerplant

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Location
Ames, Iowa
I need to boost the voltage on the secondary of a 480V:208/120V transformer to account for voltage drop at the secondary panelboard. How do I use the +2.5%/-2.5% tap setting voltages in a calculation to determine the potential secondary voltages, and what is the calculation? Thanks
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
Think of the primary tap change percentage as being a percentage change of the secondary voltage.

Voltage drop is load dependent. If the load is low and you change the primary tap, the secondary voltage will be unusually high.
 
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isupowerplant

Member
Location
Ames, Iowa
OK, so if my existing secondary voltage at the panelboard is 206.9V L-L, and if I change the tap by 2.5%, then my new voltage will be 212.1V L-L ((206.9*0.025)+206.9)? I thought that the existing primary voltage had something to do with the tap settings and secondary voltage too.
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
The tap change essentially changes the ratio of primary to secondary winding turns. You are correct, that if the secondary voltage was 206.9V and you bump up 2.5% on that primary tap change, the secondary will be 212.

If the load reduces (dependant on current) and the secondary voltage rises, your remaining load will see the higher secondary voltage
 
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waedwards87

Member
Location
Bristol, VA, USA
isupowerplant said:
OK, so if my existing secondary voltage at the panelboard is 206.9V L-L, and if I change the tap by 2.5%, then my new voltage will be 212.1V L-L ((206.9*0.025)+206.9)? I thought that the existing primary voltage had something to do with the tap settings and secondary voltage too.
Click to expand...

The primary is tapped to support different incoming voltages but in this instance you are manipulating the turns ratio of the primary to the secondary. In this case you will want to DECREASE the taps by 2.5%.

Actual transformation ratio (Vp/Vs) = 480/206.9 = 2.32 (ratio)

When changing the taps on the primary, you are changing the turns on the primary winding. The secondary turns do not change. So we will divide the primary voltage by the ratio.

480 / 2.32 = 206.9

You are actually going to apply the 1.025 to the turns ratio. You wont be increasing the primary voltage in the real world. In this case, if you multiply the ratio by 1.025 you will DECREASE output voltage. So, we want to step down (or divide).

2.32 / 1.025 = 2.26

And your new output voltage:

480 / 2.26 = 212.4

On small transformers this is a minor, but important detail, especially when your taps are split further apart. Good luck, best wishes!
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
waedwards87 said:
The primary is tapped to support different incoming voltages but in this instance you are manipulating the turns ratio of the primary to the secondary. In this case you will want to DECREASE the taps by 2.5%.

Actual transformation ratio (Vp/Vs) = 480/206.9 = 2.32 (ratio)

When changing the taps on the primary, you are changing the turns on the primary winding. The secondary turns do not change. So we will divide the primary voltage by the ratio.

480 / 2.32 = 206.9

You are actually going to apply the 1.025 to the turns ratio. You wont be increasing the primary voltage in the real world. In this case, if you multiply the ratio by 1.025 you will DECREASE output voltage. So, we want to step down (or divide).

2.32 / 1.025 = 2.26

And your new output voltage:

480 / 2.26 = 212.4

On small transformers this is a minor, but important detail, especially when your taps are split further apart. Good luck, best wishes!
Click to expand...

Very good and straight forward explanation. :thumbsup:
 
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