Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A client is upgrading an existing motor control system for a 15 hp, 460V, three-phase motor. The existing system uses a mechanical contactor. The client wants to reduce maintenance and increase the lifespan of the control components. Which of the following devices would be most suitable to replace the mechanical contactor for switching the motor's power circuit?
- AControl transformer
- BSolid-state relay (SSR)
- CPushbutton station
- DThermal overload relay
Show answer & explanationAnswer & explanation
Correct answer: B. Solid-state relay (SSR)
Solid-state relays (SSRs) use semiconductor switches (like SCRs or MOSFETs) to switch power to the motor. Unlike mechanical contactors, they have no moving parts, which significantly reduces wear and tear, noise, and maintenance, leading to a longer lifespan, especially in applications with frequent switching.
Why the other options are wrong
- A. A control transformer reduces voltage for the control circuit; it does not switch the motor's power.
- C. A pushbutton station provides operator input for start/stop commands; it does not switch the main motor power.
- D. A thermal overload relay provides overload protection, it does not switch the main motor power circuit.
Solid-State Relay (SSR) for Motor Switching
An electronic switching device that uses semiconductor components (e.g., SCRs, MOSFETs) to switch power to a motor, offering advantages over mechanical contactors in terms of lifespan, noise, and maintenance.
- No moving parts, eliminating mechanical wear.
- Faster switching speeds.
- Longer operational life, especially with frequent switching.
- Generates less electrical noise (arcing).
Memory trick: For a 'silent switch' and 'long life', solid-state is right.