The purpose of these tests is to check for field-coil short-circuited turns, for number of turns/coil, or for short-circuit conductor size. Besides tests at standstill, a test at rated speed is required, as short circuited turns may occur at various speeds.

There are DC and alternating current (AC) voltage tests for the scope. The DC or AC voltage drop across each field coil is measured.

A more than +2% difference between the coil voltage drop indicates possible short-circuits in the respective coils. The method is adequate for salient-pole rotors.

For cylindrical rotors, the DC field-winding resistance is measured and compared with values from previous tests. A smaller resistance indicates that short-circuited turns may be present.

Also, a short-circuited coil with a U-shaped core may be placed to bridge one coil slot. The U-shaped core coil is placed successively on all rotor slots.

The field-winding voltage or the impedance of the winding voltage or the impedance of the exciting coil decreases in case there are some short-circuited turns in the respective field coil.

Alternatively, a Hall flux probe may be moved in the airgap from pole to pole and measures the flux density value and polarity at standstill, with the field coil DC fed at 5 to 10% of rated current value.

If the flux density amplitude is higher or smaller than that for the neighboring poles, some field coil turns are short-circuited (or the airgap is larger) for the corresponding rotor pole. If the flux density does not switch polarity regularly (after each pole), the field coil connections are not correct.

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