Saturable Reactors and Magnetic Amplifiers

The Saturable Reactor: A Rugged and Simple Way to Control AC Power

The peculiar transformer symbol seen in the schematic above represents a saturable-core reactor. The top winding serves as the DC control winding, while the bottom winding, known as the "power" winding, carries the controlled AC current. Increasing the DC control current generates a stronger magnetic flux within the reactor core, pushing it closer to saturation. This, in turn, reduces the power winding's inductance, lowering its impedance and allowing more current to flow to the load. Consequently, the DC control current governs the AC current supplied to the load.

Overcoming the Challenges

While the depicted circuit would function, it would not perform optimally. The first concern is the saturable reactor's inherent transformer action. AC current flowing through the power winding induces a voltage in the control winding, potentially disrupting the DC power source. Additionally, saturable reactors tend to regulate AC power in a unidirectional manner. During one half of the AC cycle, the magnetomotive forces (mmfs) from both windings reinforce each other; during the other half, they oppose each other. As a result, the core experiences a higher flux density during one half of the AC cycle than the other, leading to earlier saturation in that half-cycle and preferential conduction of load current in one direction.

Ingenious Solutions

Fortunately, both issues can be resolved with a touch of ingenuity:

Observe the phasing dots on the two reactors: the power windings are "in phase" while the control windings are "out of phase." If both reactors are identical, any voltage induced in the control windings by load current through the power windings will cancel out at the battery terminals, eliminating the first problem mentioned. 

Furthermore, since the DC control current through both reactors produces magnetic fluxes in opposite directions within the reactor cores, one reactor will saturate more during one half-cycle of the AC power while the other reactor will saturate more during the other half, balancing the control action throughout each half-cycle and ensuring symmetrical "throttling" of the AC power. This phasing of control windings can be achieved using two separate reactors as shown, or with a single reactor design employing intelligent winding and core layout.

Magnetic Amplifiers: A Rugged Alternative

Since the amount of power needed to saturate a magnetic core is much lower than the amount of AC power it can control this is a form of amplification. Resulting in so called Magnetic Amplifiers.

This is particularly intriguing as amplification, typically achieved using physically delicate vacuum tubes or electrically sensitive semiconductor devices, can be realized in an exceptionally sturdy device. Magnetic amplifiers do have drawbacks compared to their more fragile counterparts, namely size, weight, nonlinearity, and bandwidth (frequency response), but their inherent simplicity merits a degree of admiration, if not widespread practical application.

Alternative Names

Saturable-core reactors are also known as "saturable-core inductors" or transductors.