Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A motor control circuit is designed to operate a 5 hp, 460V, three-phase motor. The control circuit uses a 120V transformer to supply power to the control devices. What type of control scheme typically provides inherent undervoltage protection for the motor?
- AJogging control
- BTwo-wire control
- CPlugging control
- DThree-wire control
Show answer & explanationAnswer & explanation
Correct answer: D. Three-wire control
Three-wire control (also known as three-wire maintained or low-voltage release) uses a momentary start button and a normally open holding contact on the motor starter to maintain the circuit. If power is lost or voltage drops significantly, the holding contact will open, and the motor will not restart automatically when power is restored, thus providing undervoltage protection.
Why the other options are wrong
- A. Jogging control is a momentary operation for positioning, not primarily for undervoltage protection.
- B. Two-wire control (low-voltage protection) automatically restarts the motor upon voltage restoration, which can be hazardous.
- C. Plugging control is a method of braking a motor by reversing its phase sequence, unrelated to undervoltage protection.
Three-Wire Control (Low-Voltage Release)
A motor control scheme using momentary pushbuttons and a holding contact to prevent automatic restart of the motor after a power failure or significant voltage drop, thus providing undervoltage protection.
- Uses momentary Start/Stop buttons.
- Relies on a holding contact (auxiliary contact) for maintained operation.
- Provides 'low-voltage release' (undervoltage protection).
- Safer for many industrial applications.
Memory trick: Three wires are for safety's sake, preventing the motor's unexpected wake.