What prevents a generator from becoming motorized?

• A. Voltage regulator
• B. Reverse current relay
• C. Overload relay
• D. Start switch

Answer: (B)

The thing being tested is backfeed protection: when a generator could be driven by the electrical system instead of driving it, you want to stop that. A reverse current relay keeps the generator from being motorized by disconnecting it whenever current would flow backward from the bus into the generator.

Here's how it helps: when the engine isn’t turning the generator fast enough, or if another power source on the bus is higher than the generator’s voltage, current would tend to flow from the bus into the generator’s windings. That backflow can spin the generator like a motor and draw power from the system instead of supplying it. The reverse current relay detects this reverse current and opens the circuit to the generator (and often cuts the field), isolating the generator so it can’t be driven as a motor.

Voltage regulators, overload relays, and start switches each perform other protective or control functions, but they don’t specifically prevent the generator from being back-driven and acting as a motor.