Under-Voltage (UV) Coils in Generator Circuit Breakers: Purpose and Common Failures

What is a UV Coil? An Under-Voltage (UV) coil is an essential safety component used in circuit breakers, particularly in generator and power distribution systems. Its primary purpose is to automatically trip (open) the circuit breaker when voltage drops below a certain threshold, protecting the system from operating under unsafe or unstable conditions.

Purpose of a UV Coil

1. Automatic Disconnection at Low Voltage. If the system voltage drops significantly (typically below 70–80% of nominal), the UV coil de-energizes, which causes the breaker to trip. This prevents motors and sensitive equipment from operating with insufficient voltage, avoiding damage.
2. Fail-Safe Mechanism. UV coils are often used as a fail-safe, ensuring that a breaker cannot remain closed (or be closed) unless there is adequate control voltage present — typically coming from the generator itself.
3. Interlocking and Synchronization Systems. In marine and industrial generator systems, UV coils are used in automatic transfer switches (ATS) and synchronization schemes to ensure proper breaker operation based on the system’s logic.

What is the electronic control of motors?

It is necessary to control motors. That is, motors must be started, stopped, and reversed, and, within limits, it must be possible to control their speed.

The controls needed to perform these various functions on DC motors are designed to alter the quantity or the direction; of the current in the field or in the armature of the motor. On DC controls, these functions were shown to consist mainly of resistances, switches, and solenoids.