PLC Interlocking Explained for Beginners: Motor Interlock and Practical Examples

Easy Electra September 07, 2026

PLC Interlocking Explained for Beginners: Motor Interlock and Practical Examples



Introduction

PLC interlocking is an important concept used in industrial automation to prevent two or more operations from running when they should not operate at the same time.

Interlocking is commonly used for motors, pumps, valves, conveyors, machines, and automatic processes.

For example, a motor may be required to run in either the Forward direction or the Reverse direction, but both directions must not be activated at the same time.

A PLC interlock can prevent this condition by checking whether one operation is already active before allowing another operation to start.

In this article, we will learn what PLC interlocking is, why it is used, and how to create simple motor interlocking logic.

What Is PLC Interlocking?

PLC interlocking is a control method that prevents an unwanted operation from occurring when another condition is active.

In simple terms:

Condition A ON → Condition B Cannot Turn ON

For example:

Motor Forward ON → Motor Reverse OFF

This prevents conflicting commands from being activated at the same time.

Interlocking can be created using PLC logic, electrical wiring, or a combination of both.

Why Is Interlocking Used?

Interlocking is used to improve machine control and prevent conflicting operations.

Common applications include:

  • Forward and reverse motor control
  • Two motor control systems
  • Pump changeover systems
  • Conveyor systems
  • Valve control
  • Automatic machine sequences
  • Heating and cooling systems
  • Process control
  • Equipment changeover

For example, if two pumps perform different functions and only one should operate at a time, PLC interlocking can prevent both pumps from being commanded ON simultaneously.

Electrical Interlocking

Electrical interlocking uses physical electrical contacts to prevent conflicting devices from operating together.

A common example is a Forward and Reverse motor starter.

The Forward contactor can use a Normally Closed auxiliary contact from the Reverse contactor.

Similarly, the Reverse contactor can use a Normally Closed auxiliary contact from the Forward contactor.

This creates a physical interlock between the two contactors.

Electrical interlocking can provide an additional layer of protection in control systems.

PLC Interlocking

PLC interlocking uses programming logic to control whether an operation is allowed to start.

For example, before activating the Reverse motor output, the PLC checks whether the Forward motor output is already ON.

If the Forward output is ON, the Reverse command is blocked.

The basic logic is:

Reverse Command + Forward OFF → Reverse ON

Reverse Command + Forward ON → Reverse OFF

This logic prevents the PLC from activating both commands simultaneously.

Motor Interlocking Example

Consider a motor that can rotate in two directions.

The system has:

  • Forward push button
  • Reverse push button
  • Forward contactor
  • Reverse contactor
  • PLC digital inputs
  • PLC digital outputs
  • Motor

The operator can press either Forward or Reverse.

However, the Forward and Reverse contactors must not be activated at the same time.

The PLC program therefore checks the opposite command before activating an output.

Basic Ladder Logic

A simplified Forward control can be represented conceptually as:

Forward Button + Reverse OFF → Forward Output ON

A simplified Reverse control can be represented as:

Reverse Button + Forward OFF → Reverse Output ON

The opposite output condition acts as an interlock.

This means the Forward output cannot turn ON if Reverse is already active, and Reverse cannot turn ON if Forward is already active.

The exact Ladder Logic instructions and addresses depend on the PLC platform being used.

Forward and Reverse Motor Interlocking

Forward and Reverse motor control is one of the most common examples used to understand PLC interlocking.

Suppose the Forward output is Q0.0 and the Reverse output is Q0.1.

The Forward command can be allowed only when Q0.1 is OFF.

The Reverse command can be allowed only when Q0.0 is OFF.

The concept is:

Forward Button → Reverse Output OFF → Forward Output

Reverse Button → Forward Output OFF → Reverse Output

This creates mutual interlocking between the two motor directions.

Two Motor Interlocking

Interlocking is not limited to Forward and Reverse motor control.

It can also be used when two separate motors must not operate at the same time.

For example, Motor 1 may be used for one production process while Motor 2 is used for another process.

If both motors running together could cause a process problem, the PLC can use interlocking logic.

The basic concept is:

Motor 1 ON → Motor 2 Blocked

Motor 2 ON → Motor 1 Blocked

This ensures that only the permitted motor operates.

Sensor-Based Interlocking

PLC interlocking can also use sensors.

For example, a machine may have a sensor that confirms whether a safety door is closed before allowing a motor to start.

The PLC checks the sensor condition before activating the motor output.

The basic concept is:

Start Command + Door Closed → Motor Allowed

Start Command + Door Open → Motor Blocked

The exact safety implementation depends on the machine risk assessment and applicable safety system requirements. A standard PLC input should not automatically be treated as a safety-rated function.

Practical Industrial Example

Consider a conveyor system with two conveyors.

Conveyor 2 should operate only when Conveyor 1 is running.

If Conveyor 1 stops, Conveyor 2 should also be prevented from starting.

The PLC can use the status of Conveyor 1 as an interlock condition for Conveyor 2.

The sequence can be:

Start Conveyor 1 → Conveyor 1 Running → Conveyor 2 Allowed

If Conveyor 1 stops:

Conveyor 1 OFF → Conveyor 2 Blocked

This type of interlocking can help maintain the correct sequence of a production process.

Common Beginner Mistakes

Beginners can make several mistakes when creating PLC interlocking logic.

One common mistake is forgetting to include the opposite output as an interlock condition.

Another mistake is allowing both outputs to become active because of incorrect Ladder Logic.

It is also important to understand the difference between a normal control interlock and a safety function.

For safety-critical applications, appropriate safety-rated hardware and control architecture should be used according to the machine design and applicable standards.

How to Troubleshoot Interlocking

If an interlocked output is not turning ON, check the following:

  • Check the input command.
  • Check the opposite output status.
  • Check the interlock condition.
  • Check the PLC input status.
  • Check the PLC output status.
  • Check the Ladder Logic conditions.
  • Check for active faults or alarms.
  • Check the electrical wiring.
  • Check the contactor and field device.

Online monitoring in the PLC programming software can help identify which condition is preventing the output from becoming active.

Important Safety Considerations

PLC interlocking is useful for machine control, but ordinary PLC logic should not automatically be considered a safety system.

Safety functions such as emergency stops, guard monitoring, and other hazardous-motion protection may require dedicated safety relays, safety PLCs, safety I/O, or other safety-rated components depending on the application.

Always follow the machine manufacturer's documentation, electrical drawings, risk assessment, and applicable safety requirements.

How to Practice PLC Interlocking

The best way to understand interlocking is to practice simple control programs.

Start with these exercises:

  • Forward and Reverse motor interlocking
  • Two motor mutual interlocking
  • Two pump changeover control
  • Conveyor sequence interlocking
  • Sensor-based machine interlocking
  • Motor and valve interlocking
  • Timer and interlock combination
  • Alarm and fault interlocking

After understanding these examples, you can combine interlocking with timers, counters, sensors, HMI commands, and automatic sequences.

Conclusion

PLC interlocking is an important concept in industrial automation.

It prevents conflicting operations and helps ensure that machines operate according to the required control sequence.

Forward and Reverse motor control is one of the best examples for beginners because it clearly demonstrates how one output can prevent another output from operating.

Understanding interlocking will help you build more reliable PLC programs and troubleshoot industrial machines more effectively.

In the next article, we can learn about PLC memory bits, latching, Set and Reset instructions, and how they are used in industrial automation.

Meta Description

Learn PLC interlocking for beginners with practical motor-control examples. Understand Forward and Reverse interlocking, two-motor control, sensor interlocks, Ladder Logic, troubleshooting, and safety considerations.

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