What is the formula for determing the step-up or down ratio for transformers? Remember studying this in tradeschool but thats been a long while ago. Thanks, Russell,

I am not sure how to answer your question as the way it is worded does not lend itself to the answer I thing you are looking for.

The stepup or stepdown ratio on a transformer is determined by the number of turns in both the secondary and primary coils of the transformer. If the number of turns in both are the same, then the voltages are equal. If the number is different then the voltage is determined by:

(Ns/Np)Vp=Vs

where:
Ns = turns in secondary
Np = turns in primary
Vp = voltage primary
Vs = voltage secondary

This is a good approximation at 60 Hz.

If taps are provided, they are just removing or adding turns to the side that they are provided on. For example: If the secondary had 100 turns a 2% tap of 102 turns would decrease the secondary voltage by about 2%, whereas a 2% tap of 98 turns would increase the voltage by about 2%. The opposite would happen if the taps are on the primary.

Thanks for your answer.Sorry my wording was not correct.

The tap explanation confused me.If the secondary windings are increased by 2% then the voltage is increased by a like amount.Did I not understand the example?
Chris

Adding 2 turns to the secondary winding will increase the output voltage, assuming that everything on the primary side stays the same. The secondary voltage would be reduced if extra turns were added to the primary winding (and the primary supply voltage stayed the same).

Bear with me guys. So if you have multiple taps, you are utilizing more or less of the primary or secondary windings depending on the taps position?

The purpose of the primary taps is to match the turns to the primary voltage as close as possible.
The installer would connect to the tap that is closest to the actual primary voltage.

The transformer will only put out the rated secondary voltage if the volts-per-turn in the primary is correct.



Ed

So if I connect the 600 volt primary supply to say, the 630v tap then the secondary voltage would be less than 240/120?

Yes, if you connected the 600 volt primary feeder to the 630 tap, the 600 volts would be divided between too many turns, thus reducing the volts-per-turn.

This would induce less than rated volts-per-turn into the secondary, causing the secondary voltage to be too low.

Ed

Thanks Ed, I'm learning!