Answer 6—Start by using the ohmmeter to figure out which leads go to which windings. Then, apply AC voltage to any two leads on one of the windings, using the resistor to limit the current to a reasonable value. Test the remaining leads on that winding (if any) to see if there is any pair that has a higher voltage than the applied voltage. Those will be the outermost connections for that winding. Move the AC power to that pair, and the voltages on the intermediate taps will give you their turns ratios. Test the remaining windings with the voltmeter, finding the pair of leads that has the highest voltage. Record the voltage relative to the applied voltage to get the inter-winding turns ratio and voltages on any taps. To learn about the relative phasing of windings, it is necessary to hook pairs of them in series in both directions until you find the direction in which the voltage adds. Then the windings are in-phase. Note that you don't necessarily know at this point which winding is meant to be the primary. If you have a Variac, you can try carefully increasing the applied voltage to each winding to see whether the transformer saturates magnetically, as indicated by a sharp increase in current draw.

Contributor: Dave Tweed

Published August 2003