Field Pole Voltage Drop Testing: Finding Shorted Turns in Rotors

What it is

The field pole voltage drop test, often called the AC pole drop test, is a comparison test on the field winding of a rotating machine. A low-voltage alternating current is passed through the whole field circuit, and a voltmeter is used to measure the voltage across each individual pole, one after another. The test is applied to salient-pole synchronous motors and generators, whose rotor carries a series of wound poles, and to direct-current motors and generators, whose stationary field poles are connected in series. It is a simple, quick check that needs only a variable low-voltage alternating source, a current meter and a good voltmeter. The point is not the absolute reading on any one pole but how the poles compare with each other. Poles that are electrically identical in design should show nearly the same voltage when the same current flows through them, so a pole that reads differently from the rest points to a change in that pole's winding or in the connections on either side of it.

Why it is done

Field winding faults are hard to see and develop quietly. Turn-to-turn insulation on a rotor pole can fail from heat, centrifugal stress, contamination or vibration, leaving a few turns shorted together. The direct-current resistance of the field changes very little when a few turns are lost, so an ordinary resistance reading may pass the machine while a real fault is developing. Shorted turns reduce the magnetic strength of the pole, unbalance the machine's field, raise vibration, increase heating in the remaining turns and, left alone, can progress to a ground fault and a rotor failure that takes the machine out of service for a long and costly repair. Loose or high-resistance pole-to-pole connections also heat up under load and can open. Running the test during acceptance confirms that the rotor was not damaged in transport and gives a baseline, and running it at maintenance outages catches changes early enough to plan a repair.

The principle

At power frequency the impedance of a coil is dominated by its inductance, and inductance depends strongly on the number of turns and on how well they link the pole's magnetic circuit. With the same alternating current flowing through every pole in series, the voltage across each pole is proportional to that pole's impedance. A healthy pole of a given design therefore drops about the same voltage as its siblings. When some turns on one pole are shorted, the shorted turns behave like a secondary winding of a transformer, carrying a circulating current that opposes the flux. The effective inductance of the pole falls, so the voltage across it falls too, and the pole stands out as low compared with the others. A high-resistance connection between poles has a different signature, showing a raised drop across the connection itself or a disturbed total. A direct-current measurement cannot make this comparison because inductance plays no part at steady state, which is why the alternating test finds what a resistance reading misses.

How it is performed

The machine is de-energized, isolated from every source, locked out and grounded under the owner's procedures, and the field circuit is disconnected from the exciter, the rectifier or the slip rings' supply so that no electronic equipment sees the test voltage. On a synchronous machine the test can be done on the rotor with the poles accessible, or through the slip rings, with the rotor at standstill. A low-voltage alternating source from a variable transformer is connected to the field winding, with a meter to monitor the current and a separate voltmeter to read each pole. The voltage and current are brought to a level the standard or manufacturer allows, kept low enough that the winding is not stressed, and held steady while the voltmeter probes are placed across each pole in sequence, taking care that the probes contact the pole terminals and not paint or corrosion. On a direct-current machine the field poles are measured in the same way, with the brushes lifted or the armature circuit isolated. The same source, the same current and the same probe points are used on repeat tests, and where the rotor can turn, readings are repeated at more than one rotor position, because the result may vary slightly with the position of the pole relative to the stator. The winding temperature is noted. After the test the source is reduced to zero and switched off before the leads are removed.

What is recorded

The record shows the source current used, the voltage measured across each pole, and the order in which the poles were measured, together with the rotor position where that matters. It also notes the winding temperature, the identity and calibration status of the instruments, and the as-found condition of the field, such as clean and dry or visibly contaminated, as well as any lead that was lifted to isolate the circuit. The pole-to-pole comparison is only meaningful when later readers can see that the same conditions applied, so the way the test was set up is documented along with the readings themselves.

How results are judged

No single reading decides the outcome, and this page does not state numeric limits. The primary criterion is comparison among the poles of the same winding: poles of the same design should read closely alike, and the one that departs from the group is the one to investigate. A low pole usually suggests shorted turns, while an unusually high drop can indicate a poor connection or a problem in the measurement setup. The second criterion is comparison with the manufacturer's data, and the third is comparison with earlier results on the same machine, where drift between tests is as informative as the absolute difference between poles. Rotor position effects are considered, and repeat readings should reproduce. The scope of this measurement and the conditions that call for it are covered in ANSI/NETA ATS-2025 for acceptance testing and in ANSI/NETA MTS-2023 for maintenance testing, where it is listed as an optional test on the machines it applies to. For the values and the governing wording, consult your own licensed copy of the standards.

Variations

At acceptance the test establishes that the field winding is sound after manufacture and shipping, and the result becomes the baseline for the machine's service life. In maintenance it is repeated at outages and compared with that baseline, and it is especially useful after a period of overload, a fault or a rewind. On salient-pole synchronous machines the poles rotate with the rotor, so access is through the slip rings or the exposed pole connections, and rotor position may matter. On direct-current machines the field poles are stationary and easier to reach, and the test may also cover interpoles and compensating windings according to the manufacturer's guidance. Cylindrical-rotor machines have distributed windings, so this pole-by-pole approach does not apply to them, and other methods such as recurrent surge comparison or impedance checks are used. The test works best alongside winding resistance, insulation resistance and, where available, surge comparison, because each finds a different kind of fault.