Journeyman Electrician Exam (NEC)Equipment and DevicesEasy
A technician is troubleshooting a motor control circuit where a limit switch is used to detect the position of a conveyor belt. What is the primary function of this limit switch in the control circuit?
- ATo provide overcurrent protection for the motor.
- BTo change the speed of the conveyor belt motor.
- CTo isolate power to the motor for maintenance.
- DTo sense a physical condition and initiate a control action.
Show answer & explanationAnswer & explanation
Correct answer: D. To sense a physical condition and initiate a control action.
A limit switch is a type of pilot device or control device. Its primary function is to sense the presence, absence, or position of an object (a physical condition) and convert that mechanical motion into an electrical signal, which then initiates a control action in the motor control circuit.
Why the other options are wrong
- A. Overcurrent protection is provided by circuit breakers or fuses, not a limit switch.
- B. Changing motor speed is done by motor drives or variable frequency drives, not a limit switch.
- C. Power isolation is typically done by a disconnect switch, not a limit switch.
Pilot Device Function
A control device (such as a push button, limit switch, or pressure switch) that senses a specific condition or input and provides a signal to initiate or stop an action in a control circuit, without directly carrying the main power circuit current.
- Operates in the control circuit, not the power circuit.
- Senses various conditions: position, pressure, temperature, presence, etc.
- Provides input for automated control sequences.
Memory trick: Pilot devices don't power, they just command with their sensing power!