Static Accuracy Performance of the Selsyn Generator Control-Transformer System

The design of an accurate selsyn generator or control transformer may be accomplished by considering equations 47 and 48 or 55 respectively. The slot openings only increase or decrease certain errors given by these formulas. If according to these formulas a certain harmonic error is small or zero, the slot openings also cause a small or zero additional error. Exception of this rule has to be made only in the case that the errors caused by two different harmonic fields cancel each other. All errors of order (6n-3) can be avoided by making the rotor or stator winding insensitive to even harmonics. Therefore the winding also has to be symmetric to a line vertical to its axis. This is only possible if the punching contains an even number of slots. Curve a in Figure 10 shows a control transformer error curve. It corresponds to a control transformer with a 2-slot round rotor and a 15-slot stator winding, as shown in Figure 4. The 6th-harmonic error is caused by a 7th-harmonic flux and should reach, according to equation 55, an approximate maximum of 0.0013 radian, or 4.5 minutes. The actual 6th-harmonic error is 3.7 minutes. Curve ``b´´ shows the error curve of a generator with a 15-slot stator winding and a salient pole rotor of 90-degree pole arc. The 6th-harmonic error has been eliminated by using a stator as before but coils of different number of turns. Thus it was possible to obtain a 7th harmonic reduction factor equal to zero.