Asynchronous tests for D and Qaxis subtransient time constants

The paper draws attention to difficulties in measuring the subtransient time constants, which are relatively fast in machines with rated outputs of the order of tens to hundreds of kilowatts. The frequency-response approaches, consisting of asynchronous tests with field open- and also short-circuited, are proposed as the most suitable test methods, and corrections for resistance effects are developed. A new experimental technique is used to separate the D and Q axis quantities in these asynchronous tests. The Park transformation is performed by physical means, using two 3-phase shaft-driven tachometers and Hall-effect (or quarter-square) multipliers. This results in generated voltages proportional to the hypothetical D and Q axis voltages and currents in the machine. From an ultraviolet-recorder output, one can then obtain the D and Q axis operational admittances or impedances. A curious anomaly in the measured operational impedances in the vicinity of 50% slip is explored, and is shown to be due to crosscoupling between the D and Qaxes through the stator resistance. In small machines, this effect is experimentally evident over a much wider range of slips than might be expected. It is inferred that this effect should also be present in even the largest machines provided that the stator resistance is not identically zero, although it would be evident over a much smaller range of slip. The effect depends on rotor asymmetry, either electric or magnetic, and is related to the Görges phenomenon.