Instabilities of push-pull magnetic amplifiers feeding the field of an electric machine

The causes of instability of inductively loaded, center-tap magnetic amplifiers, and the remedies for this instability are well known.1–4 If such a stabilized magnetic amplifier is used to energize the field of an electric machine, such as the shunt field of a d-c generator, the armature voltage of this machine will increase or decrease smoothly with the control voltage of the magnetic amplifier. Since the load current of the magnetic amplifier is unidirectional, the armature voltage of the d-c generator will also be unidirectional. However, in many closed-loop control systems it is necessary to obtain duodirectional out-put of the d-c geneator. In this case the d-c generator will be equipped with two shunt fields; each shunt field is connected to a magnetic amplifier with such polarity that the magnetomotive forces produced by the two shunt fields are subtractive. Then, if the first magnetic amplifier is controlled “full on” and the second magnetic amplifier is controlled “full off,” the armature of the d-c generator develops a voltage of one polarity, and if the the control to the magnetic amplifiers is reversed, the armature voltage of the d-c generator is also reversed. One may expect, furthermore, that if each magnetic amplifier is turned “one half on,” the net magnetomotive force will be zero, and, hence, that the armature voltage of the d-c generator will also be zero; one may also expect that for a gradual increase of control current of one amplifier, and a similar gradual decrease of control current of the other amplifier, the armature voltage of the d-c generator will show a gradual change. The last two expectations, however, are not likely to be fulfilled. Instead, a minute change of the magnetic amplifier control currents will cause a sudden jump of armature voltage from perhaps +50% to −50% (or vice versa) of rated d-c generator armature voltage, and intermediate values of armature voltage will not be obtainable. This instability renders almost useless this otherwise inviting control scheme. It is suspected immediately that magnetic amplifiers are prone to produce instabilities when feeding inductive loads, such as the field of the d-c generator. However, as soon as one of the two magnetic amplifiers is disconnected from its generator field, the other will produce perfect (unidirectional) control of the d-c generator, and vice versa. Hence the instability is of a different nature than suspected.