Saturable reactor or magnetic amplifier is a circuit that used to control very large load of AC with very small input DC. The saturable reactor consists of three essential elements : Direct current source, magnetic core with windings, and alternating current source. How it work? The AC load circuit run on to magnetic core and the DC control circuit is also went on to the same core. Ac current flows through winding and Since this current is alternating, the flux set up in the magnetic circuit loop is constantly changing in magnitude and direction. This means the field builds up to a maximum in one direction, collapses, and builds up to a maximum in the opposite direction DC circuit will cause flux which is which is constant in magnitude and constant in direction. This means the field builds up and remains steady state. The AC flux tends to saturate and then desaturate the core because of its cyclical operation. This results in a changing inductive reactance in the load winding.
The DC flux, according to it’s strength, aids or opposes the AC flux in its saturate or desaturate effects in the core. Hence, the DC flux tends to control the AC flux controlling the reactance of the load winding. The use of separate windings on a single core has distinct advantages. Load winding consists of comparatively few turns of heavy wire because of large current requirements of different loads. Control winding Ni consists of many turns of fine wire. Since magnetomotive force depends upon the number of ampere turns, a small current in the control winding produces a magnetomotive force equal to that of the load winding. Usually, DC in the order of milliamperes controls AC in the order of amperes. The following describes the steps in the operation and control of the simple saturable reactor: 1. Zero DC control current in the control loop.
Since only AC current is flowing through the load windings, an extremely high inductive reactance (Xl) is present in the load windings. This is due to the high inductance (L) of the load windings and the action of the varying magnetic field produced. Extremely high inductive reactance in this winding results in a high impedance (Z) which limits the flow of AC current to a low value. This high reactance also causes a large voltage drop across the load windings in series with the load, limiting the current supplied to the load. Since current is limited to a low value to the load, minimum power is transferred to the load since power is a function of current. 2. Increase DC control current in control loop. DC current creates a flux which, when superimposed on the AC flux, collectively saturates the core.
Since the core is near saturation or fully saturated (core unable to hold any more flux lines), the inductive reactance is greatly reduced. This is due to the fact that no additional changing flux can be held by the core. With reduced inductive reactance the impedance of the load windings is greatly reduced.Large AC currents are now permitted to flow through the load. This results in maximum power transfer to the load. Decrease DC control current in control loop. With less DC current flowing there is less total flux in the core and the core desaturates. This results in the core’s ability to support once again the changing flux, creating a high inductive reactance, and resulting in increased impedance in the load winding. Minimum power transfer results since current to the load is greatly reduced.