> Why the measured IR for the same machine will varied if testing voltage change?
it won't necessarily change, but it might.
we don't use very low voltages for testing because they might miss nonlinear phenomena like tracking that only show up as increasing current at higher voltages. if you do a dc step voltage test, you will sometimes see this nonlinear exponential-like curve especially when there is contamination.
we don't want to use very high test voltages because that might damage the winding (even a healthy winding).
the standards like ieee 43 take this into account. if you follow the standards for test method, then you have the advantage that your results are comparable to those measured by others, if you don't follow the standards, then you're on your own and subject to being second guessed (did your selection of test voltage allow a problem to be missed or cause winding damage).
I speak mostly about motors. the concern for cable damage (possibly latent) from dc can be even bigger for cables, and things are a bit more complicated and controversial for dc testing of cables
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(2B)+(2B)' ?
