A client has a critical industrial process that relies on a 100 hp, 460V, three-phase motor. Interruption of power to this motor, even for a brief moment, would cause significant financial loss and safety hazards. Which type of motor starter is specifically designed to resume operation automatically after a power outage, once power has been restored?
- AMagnetic Motor Starter with three-wire control
- BManual Motor Starter
- CReduced Voltage Soft Starter
- DMagnetic Motor Starter with two-wire control
Show answer & explanationAnswer & explanation
Correct answer: D. Magnetic Motor Starter with two-wire control
A magnetic motor starter with two-wire control provides automatic restart upon power restoration. In a two-wire control circuit, the control device (e.g., a pressure switch or float switch) simply makes or breaks the circuit to the starter coil. If power is lost and then restored, the control device will close if its condition is met, re-energizing the starter coil and restarting the motor. This is often used in critical applications where automatic restart is desired, despite potential safety concerns.
Why the other options are wrong
- A. A magnetic motor starter with three-wire control (using start/stop pushbuttons and a holding contact) provides undervoltage protection, meaning it will not automatically restart after a power outage; the 'start' button must be pressed again.
- B. A manual motor starter requires physical interaction to restart after a power outage.
- C. A reduced voltage soft starter primarily manages motor starting current and torque; it doesn't inherently determine automatic restart capability, which is a function of the control circuit design (two-wire vs. three-wire).
Two-Wire vs. Three-Wire Motor Control
Motor control circuits are typically categorized as two-wire or three-wire, referring to the number of control conductors connecting the control device to the starter coil, which determines the motor's restart behavior after a power interruption.
- Two-wire control provides automatic restart upon power restoration (undervoltage release).
- Three-wire control provides undervoltage protection, requiring manual restart after power restoration.
- Three-wire control is generally safer as it prevents unexpected motor restarts.
Memory trick: Two wires: Automatic return. Three wires: Manual turn.