A limit switch, often referred to as a travel switch, is a precision electromechanical device that converts mechanical displacement into an electrical control signal. It serves as the sentinel of motion control systems, ensuring accurate position detection, over-travel protection, and reliable process interlocking. By responding to the physical contact of moving components, it enforces safe, repeatable, and automated equipment behavior.

1. Operating Principle of a Limit Switch

The working process of a limit switch unfolds in several distinct stages, each transforming mechanical motion into electrical action.

1. Trigger Stage

When a moving component—such as a machine tool carriage, conveyor actuator, or sliding platform—reaches a predetermined position, it physically contacts the switch’s actuator head.
The actuator may be a plunger, roller, or lever, depending on the mechanical configuration.

2. Mechanical Transmission

The mechanical force applied to the actuator is transmitted inward through springs, push rods, or linkages.
This causes the movable contact inside the switch to shift from its resting position.

3. Electrical Switching

The altered position of the movable contact changes the state of the internal contacts:

• Normally Closed (NC):
  Initially closed; opens when triggered. Commonly used to cut off control circuits or stop motors.

• Normally Open (NO):
  Initially open; closes when triggered. Used to start another action or send alarm signals.

4. Reset Phase

After the moving component leaves the actuator, the spring mechanism restores the internal contacts to their original state.
The switch is then primed for the next operation.

In essence, the limit switch transforms a mechanical limit into a control signal, ensuring machinery halts, reverses, or triggers an action precisely when required, preventing over-travel, collision, or equipment damage.

2. Wiring Methods Based on Control Requirements

Proper wiring depends on the functional demand—such as stopping a motor, issuing a signal, or controlling direction—and on the contact type selected (NO or NC).
The following example illustrates the most common wiring method: using a limit switch to stop a single-phase motor.

3. Core Components and Wiring Logic

• Power Supply: Single-phase 220V AC

• Limit Switch (SQ): Uses an NC contact, which stays conductive until triggered

• AC Contactor (KM): Controls the motor power circuit

• Motor (M): Receives power through the contactor’s main contacts

• Control Buttons:

• Stop button (SB1): NC

• Start button (SB2): NO

• Auxiliary Contact (KM2): Used for self-locking (maintaining the motor’s run state)

4. Wiring Steps

A. Main Circuit (High-Power Path)

1. L (live) → KM main contact → Motor live terminal

2. N (neutral) → Motor neutral terminal

3. Optional: Add thermal overload relay (FR) to protect against excessive current

B. Control Circuit (Low-Power Control Path)

1. L → Stop button SB1 (NC) → Limit switch SQ (NC) → Start button SB2 (NO)
   → Contactor auxiliary contact KM2 (NO) → Contactor coil (KM) → N

Control Logic

• Under normal conditions, the NC chain (SB1 + SQ) is conductive.

• Pressing the start button energizes the contactor coil, closing the main contacts and starting the motor.

• When the machine reaches its travel limit, the limit switch actuator is triggered, opening the NC contact.

• The control circuit loses power, the contactor drops out, and the motor stops immediately.

This structure provides reliable shutdown at precise physical positions, a necessity for automation systems and safety-critical operations.

5. Important Considerations

• Power must be disconnected before wiring.

• Correctly distinguish NO and NC terminals; they determine control behavior.

• For three-phase motors, only the main power circuit changes; the control wiring logic remains the same.

• For complex systems such as dual-limit mechanisms, reversing circuits, or sensor-feedback loops, additional contacts and logic modules must be integrated.