Chuck Yung
EASA Senior Technical Support Specialist

One of the basic dependable tests for DC fields and synchronous fields is the voltage drop test. This test can be performed using AC or DC voltage, but EASA’s Technical Support Team receives a lot of questions about how to interpret the results. 

Let’s start with the basics of the test. When identical coils are connected in series and we apply a voltage across the set of coils, there should be an equal voltage drop across each field. A common example is four shunt fields in a DC motor. (See Figure 1.) If we apply 120V across the set of four fields, we can expect 30 volts drop across each field. 

Because the AC test is more rigorous, it exaggerates the imbalance resulting from shorted turns. The test behaves like the shorted secondary of a transformer, dramatically affecting the difference in voltages. Offsetting this advantage, the AC test is sensitive to differences in the iron surrounding the coils. For example, a weldment such as a foot or lifting ear can skew the results of an AC drop test. See the circled areas of Figure 1.

Figure 1

A typical 4-pole field frame with four fields and four interpoles. The circled areas indicate features that can influence an AC voltage drop test.

For these reasons, the AC voltage drop is the preferred test. If the AC drop test results are good, no further testing is needed. If the AC drop test is questionable, follow with a DC voltage drop test. Because the AC drop test is more stringent, the acceptance tolerance is +/- 10% of the average. Because the DC drop test is not as probing, the acceptance tolerance is +/- 5% of the average.