What is a saturable reactor?

• A. A transformer with fixed core
• B. A capacitor bank
• C. A control device that uses a small DC current to control large AC current
• D. A device that generates large DC current

Answer: (C)

A saturable reactor is a device that uses a small direct-current bias to control the inductance seen by a large alternating-current circuit. It has a magnetic core and windings; when a DC current is applied, the core is driven toward saturation, which lowers its permeability and reduces inductance. With lower inductance, the AC current can flow more freely. Conversely, without the DC bias (or with a smaller bias), the core remains less saturated, the inductance is higher, and the AC current is more restricted. In short, a small DC current controls a large AC current by changing the core’s saturation state.

This is why it’s described as a device that uses a small DC current to regulate a large AC current. It’s not simply a fixed-core transformer, not a capacitor bank, nor a device that generates large DC current.