Journeyman Electrician Exam (NEC)Control Devices and MotorsEasy
A client has a critical industrial process requiring a motor control system that must safely stop a 15 hp, 460V, three-phase motor even if the control circuit power is lost. Which type of motor control circuit provides this inherent safety feature?
- ATwo-wire control with a maintained contact switch.
- BFour-wire control with a latching relay.
- CTwo-wire control with a selector switch.
- DThree-wire control with momentary contact pushbuttons.
Show answer & explanationAnswer & explanation
Correct answer: D. Three-wire control with momentary contact pushbuttons.
Three-wire control circuits, using momentary contact pushbuttons and a holding contact, inherently provide low-voltage release. This means if the control circuit power is lost, the motor will stop and will not restart automatically when power is restored.
Why the other options are wrong
- A. Two-wire control with a maintained contact switch will allow the motor to restart automatically upon power restoration, which is not suitable for critical safety applications.
- B. While 'four-wire' control isn't a standard NEC classification, a latching relay would typically maintain its state through power loss, potentially leading to an unsafe automatic restart unless specifically designed for low-voltage release.
- C. Similar to option A, two-wire control will allow automatic restart, lacking the desired safety feature.
Three-Wire Control (Low-Voltage Release)
A motor control circuit design that uses momentary contact pushbuttons and a holding contact to inherently provide low-voltage release, preventing automatic motor restart after a power interruption.
- Requires a 'Start' pushbutton, a 'Stop' pushbutton, and a holding contact.
- Provides an inherent safety feature by requiring manual restart after power loss.
- Commonly used in industrial applications for safety.
Memory trick: Three wires for safety, no surprise restarts!