Self-Saturated Magnetic Amplifiers

In Feedback in Magnetic Amplifiers it was seen that the use of feedback windings greatly increases the gain of a magnetic amplifier. Several circuits have been devised to provide the feedback by means of the load circuit and thus eliminate the extra feedback winding. Such circuits are termed self-saturating. A "building-block" or elementary self-saturating component is the half-wave circuit of Fig. 214, from which several magnetic amplifiers may be formed.

Fig. 214. (a) Half-wave self-saturated magnetic amplifier circuit and (b) load and rectifier voltage wave shapes.

Impedance Z in the control circuit prevents short-circuiting the reactor. It may be the control winding of another reactor in a practical amplifier. Rectifier RX prevents current flow into the load in one direction, so that the core tends to remain in a continually saturated condition. This condition is modified by negative control winding NI/in., which opposes the load winding NI/in. and permits the core to become unsaturated during the portion of the cycle when there is no load current flowing. The greater the control NI/in., the less the average output current. Transfer characteristics are similar to those of Fig. 211. Ideally the circuit has 100 per cent feedback.

Assuming the core to be saturated at all times with zero control current, current flows into the load throughout the whole positive half-cycle and is zero for the whole negative half-cycle. With a given value of negative control current, reactor inductance is high at the start of the positive half-cycle and load current does not build up appreciably until an angle Θ₁ is reached when the core saturates. Then it climbs rapidly and causes most of the supply voltage to appear across the load as shown by the curve marked e_L in Fig. 214(b) for the remainder of the positive half-cycle. As negative control current increases, so does angle Θ₁. In the limit Θ₁ = 180°; that is, with large negative control current, virtually no load current flows. The similarity of load voltage wave shape to thyratron action is at once evident. It has led to the use of the same terminology. Angle Θ₁ is often called the firing angle of a magnetic amplifier. Load voltage is reduced as Θ₁ increases, approximately as in Fig. 190. There are some important differences, too:

(a)   Reactor inductance is never infinite, and magnetizing current is therefore not zero. This means that during the interval Θ₁ a small current flows into the load. The change in reactor inductance at the firing instant is not instantaneous; the time required for the inductance to change limits the sharpness of load current rise.

(b)   Even with tight coupling between control and load windings, the saturated reactor inductance is measurable. This saturated inductance causes the load current to rise with finite slope.

(c)   After load voltage reaches its peak and starts to drop along with the alternating supply voltage e, core flux continues at saturation density. An instant a is reached when the load voltage exceeds the supply voltage. Beyond a, the reactor inductance increases and magnetizing current decreases, but at a rate slower than the supply voltage because of eddy currents in the core.

(d)   After supply voltage e in Fig. 214(b) reaches zero, the reactor continues to absorb the voltage until the core flux is reset to a value dependent on the control current, that is, until angle Θ₂ is reached. Then part of the negative supply voltage rises suddenly across rectifier RX as shown by the wave form of e_R.

During the interval 0-Θ₁ the reactor inductance is high and virtually all the supply voltage appears across it. The voltage time integral ∫ e dt represented by the reactor flux increase during this interval is equal to ∫ e dt during π-Θ₂. That is, the energy stored in the core before the firing instant is given up during the negative half-cycle of supply voltage.

Self-saturated magnetic amplifiers have transfer curves similar to that of Fig. 212(a). A small amount of additional positive feedback makes them bistable. Negative feedback makes the transfer curve more linear but reduces the gain. Ordinates and abscissas may be current, ampere-turns, or oersteds, as for simple magnetic amplifiers.