Parasitic oscillations have been experienced in a voltage and speed control circuits using a rotating amplifier of simple rototrol type, whose control winding is energized by a high gain direct current electronic amplifier. The oscillation has persisted even when the rotor of the rotating amplifier has stopped.
The reverse reaction which have been found in the unit response oscillogram of the open loop transfer function is analyzed, and the cause of the parasitic oscillation is clarified to be attributable to the parasitic coupling of electromagnetic nature between the control and the series compensating winding of the rotating amplifiers, which operates on positive feedback in the control circuit. The analytical expression of the open loop transfer function reveals that the control circuit is a non-minimum phase shift one, whose zero exists in the right half plane.
One of the most efficient and logical method of suppressing the oscillation is presented, that is, to transfer all zeros of the transfer function to the left half plane by neutralizing the effect of the parasitic coupling.
Four examples are given with criteria which may be useful for system design, that is, the first is one using the third winding magnetically coupled to both control and series compensating windings, the second using differentiating transformer, the third using parallel additions and the last using parallel additions with a slight variation to the third.
Control circuits using rotating amplifiers different from the simple rototrol type are also considered to be nearly identical with respect to the parasitic oscillation and the method of suppressing it.
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