Magnetic amplifier

A transducer or magnetic amplifier is an electrical engineering component to control alternating currents by small direct currents. It consists in the simplest case of a (air- gap-free ) magnetic circuit whose material is characterized by a highly non-linear, low-hysteresis magnetization curve. The magnetization takes place through two coils, one of which leads the alternating current to be controlled. The second coil carries a current with a large DC component which acts as a control current.

There are two operating modes are possible. In the region of small alternating currents (small modulation ), the magnetic characteristic at the operating be regarded as linear. The DC control current is used in this case to shift the operating point. In this operating mode, the AC side a transducer coil with an electrically variable inductance dar. The alternating current is sinusoidal. The second mode (large modulation ) with large alternating currents used for power control. The alternating current can be different in this case strongly from the sinusoidal shape.

Small modulation (small signal operation)

The circuit shown on the right is a typical example. Effective on the AC side of the choke inductance (L = u1/di1 ) is changed by changing the bias of the associated non-linear magnetic core. To this end, the second coil is traversed by a variable in size dc control current i2. In the range of small control currents results in the greatest inductance decreases, the more the control circuit shifts the operating point of the magnetic circuit towards saturation. By changing the inductance changes in the AC circuit the impedance of the choke. In this way, the power in the effective resistance R is varied. Without control current, the power is small, for maximum control current seeks the power in the resistor against uw / R. Due to the requirement of the linearity of the power gain of such a transductor P ( control ) / P ( control) as a rule under 1

Large-signal operation

In order to control larger voltages and powers, the large-signal operation must be provided. To explain the operation of the circuit in this case is the simplest circuit used and the boundary conditions are greatly simplified. So are the ac components of i2 that occur due to the transformed voltages and generate losses, reduced by a very large inductance vernachläsigbare values ​​and thus i2 almost a direct stream. To simplify is to apply in Figure 2 further illustrated the abstract magnetization characteristic. Also to be present in addition to the marked load resistor no parasitic resistive components.

To reach the intermittent operation immediately, one begins by viewing the highest saturation and the highest current. The characteristic point 1 is the load current i1 is equal to zero. The magnetization is only by i2. At point 2, the core comes out of saturation and the current i1 is negative and almost as large as - i2. Because the onset of desaturation occurs voltage on the windings. It results to u1 = uw - R · i1. Because of the applied voltage, the current drops slightly further into the negative. It reaches its most negative value at point 3, where because of the zero crossing of the voltage, the direction of change of flux u1 reverses. By the positive voltage of the core is magnetized to saturation again (point 2). Because of the saturation of the full voltage generator is switched to the load. Hence the current flowing yields i1 = uw / R. In this case, point 1 will overflow and will be recovered in the next current zero crossing. The process is repeated.

Push-pull circuit

If you switch two arrangements shown in figure 1 on the AC side in parallel and allows the control current flowing through the two secondary windings in a different direction so the control flow acts out of phase in the Teiltransduktoren. The voltages induced in the DC link voltages compensated. For small modulation of the currents and voltages remain almost sinusoidal. Due to the parallel connection doubles the possible load current and for the compensation of the induced AC voltage, the losses in the DC circuit reduce significantly.

For the large-signal operation a renewed consideration of the processes to be employed, since the superposition theorem does not apply because of the exploited ( magnetic ) nonlinearities. This is assumed in the circuit shown in Figure 4 and the associated Bepfeilung. For the images of point 4 of the magnetization curve was i2 = box in Figure 5 as the effect of the control current ⅔ IS and the turns ratio equal to 1.

The origin of the magnetization characteristic of the load current is equal to zero and the currents ia and ib just need to be as large as the DC control currents, but counter these. With the same winding direction and the Bepfeilung by Figure 4 is thus ia = ib = - i2 and i2. For the total current i1 = ia ib = 0 at point 4 is ia = 0 and ib = 2 i2 = i1. With the onset of saturation of a coil, the AC voltage is zero and the current I1 is bounded by the load to the applied AC voltage to the two windings. The winding currents are ia = i1 IS - i2 and i2 ib = -IS. If the load current (resistive ) at the zero crossing of the voltage to zero, the core question comes out of saturation. During the negative voltage half-cycle, the nuclei are rückmagnetisiert. During this time, the load current is zero and the currents ia and ib go back to the values ​​at the origin of the magnetization curve. Then, through their negative branch in the same way. The main time profiles are shown in Figure 6.

With respect to the loss of the push-pull circuit is significantly less expensive than circuit shown in Figure 1, because the offset induced in the DC power circuit and AC voltages because, as is apparent from Figures 2 and 5, a relatively small control current is needed.

You can use an additional advantage even when anti-parallel switches the power windings and the control current to flow in the same direction through the coils. In this case, the control windings can be combined into one. The construction of such an arrangement is shown schematically in Figure 7. The iron body has three legs, in this case. This results in the simplified theory, no significant material savings, as the control winding twice the iron cross section shall enclose in this case. Taking into account the occurring in practice, the iron and copper losses, the saving of material becomes larger.

The current and voltage waveforms and the control function of the push-pull circuit correspond to those of an AC power controller (Power Electronics ) with phase angle control. Because of this, the efficiency is better and also easier and less expensive, it has the transducer, replaced as a control device for AC currents.

Other circuits

For three push-pull circuits, a transducer for three-phase loads can be built. There were also three-phase circuits, which could be controlled by appropriate returns with DC voltage. Like all transducers are also they have been displaced by the semiconductor technology.

A special feature is the Pungs choke, which was used to amplitude modulation. It was built according to Figure 7, but controlled in contrast to treated push-pull circuit, not 50 Hz applications, but radio frequencies. Your load was not even approximately a resistor; she worked on the tuned antenna resonant circuit of the transmitter. The shown waveforms of the push-pull circuit can not therefore be transmitted to this application. According to the literature this transductor goes back to Leo Pungs.