A transformer rated for certain input voltage at 60 Hz, will operate with the same efficiency ( and primary magnetising current) if the voltage and frequency are scaled together.
Eg. 120 volts @ 60 Hz.
The size (VA) of the core has no effect on this characteristic.
As usual, Phil is sort of right, but misses two points of some practical importance, which are that the inductance of a transformer is more or less proprtional to its size, and that in order to get an adequate inductance out of a small core you have to put on lots of turns of very fine wire, where the constant thickness of the insulating enamel occupies a disproportionate amount of the winding window.
These two effects conspire to make small transformers disproportionately lossy at low frequencies - check out the difference in regulation between a 6VA and a 12VA mains transformer.
For higher VA ratings, the (constant) thickness of the enamel layer on the thicker wire you end up using ceases to eat up any significant proportion of your winding window, and size becomes progressively less important.
Right, Phil. That has absolutely nothing to do with power transformers. You want to minimize both the iron, and the copper losses to keep the cost down. BTW, how well do your microphone transformers work at 120 VAC?
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Michael A. Terrell
Central Florida
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