When testing de-energized synchronous or DC motor field poles with the PdMA MCE® remember that different sized field coils with different numbers of turns have very different inductance values. Although the majority of DC field poles exhibit higher inductance values and are best tested at lower frequencies there are occasions where field pole inductance falls below the normal inductance range. In these cases, you can simply increase the test frequency and test the field poles again. If the inductance value of the field poles were historically in the high inductance range, but are trending lower or have suddenly dropped into a lower range requiring a change in test frequency, you may be experiencing a short in your field pole windings. Shorted field pole windings can create a loss or reduction in speed control for a DC motor and difficulty synchronizing for a synchronous motor. In this case performing individual field pole inductance and resistance measurements as well as an AC Drop test may be required to isolate the shorted field pole. Another key indicator of a shorted field pole on a synchronous motor is a change in the In-Rush/Start-Up performance of the synchronous motor. Faulty field poles will reduce the lock-in or synchronizing strength of the field poles when DC is applied resulting in extra work from the damper or amortisseure windings as well as a large transient current on the stator windings. As always the In-Rush/Start-Up test is most effective when compared to a normal baseline In-Rush/Start-Up test taken when the motor was healthy.
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