How a PLC Controls Start/Stop of a Motor Using Push Buttons

1. Introduction

Programmable Logic Controllers (PLCs) are fundamental components in modern industrial automation systems. They allow for precise control of machines and processes, including the start and stop of motors. A common setup for controlling a motor is to use a push button interface with the PLC, where the user presses a button to start or stop the motor. This simple yet effective control method is widely used in manufacturing, packaging, and other automated processes.

In this article, we will explain how a PLC controls a motor’s start and stop functions using push buttons, and we will discuss the basic PLC logic required to implement this control.

2. Basic Components

To control a motor using a PLC with push buttons, you typically need the following components:

• PLC: The programmable logic controller that executes the control logic.
• Motor: The load that you wish to control.
• Start Push Button (Normally Open): A button that starts the motor when pressed.
• Stop Push Button (Normally Closed): A button that stops the motor when pressed.
• Contactor/Relay: A device that actually switches the motor on or off.
• HMI (Optional): A human-machine interface to visualize the status or control parameters.

The basic principle is simple: the user presses the Start button to initiate the motor and presses the Stop button to shut the motor down. The PLC processes the signals from the buttons and sends commands to the contactor/relay to control the motor’s operation.

3. PLC Control Logic

The PLC control logic for starting and stopping a motor typically involves a simple ladder diagram, which is one of the most common methods of programming PLCs. Here is an overview of the logic behind the system:

• Start Push Button: When the Start button is pressed, a signal is sent to the PLC input. The PLC logic checks if the motor is in a “stopped” state. If so, it energizes the output to start the motor.
• Stop Push Button: The Stop button, typically wired in series with the motor control, sends a signal to the PLC input. If the button is pressed, the PLC logic checks the motor’s current state. If the motor is running, it de-energizes the output and stops the motor.
• Motor Relay: The PLC output energizes a relay that activates the motor. When the relay is energized, it closes the contact and allows current to flow to the motor, turning it on.

The logic flow can be visualized as:

1. Start Button: When pressed, it triggers a signal to the PLC to start the motor.
2. Stop Button: When pressed, it sends a signal to stop the motor.
3. Relay/Contactor: The PLC outputs a signal to activate the relay, which in turn switches the motor’s power on or off.

4. Ladder Diagram Example

Below is a simple ladder diagram representing the control logic for starting and stopping a motor using push buttons.

|  Start PB  |----] [----(MOTOR ON)----|  Start Motor
|  Stop PB   |----] / [----(MOTOR OFF)---|  Stop Motor

• Start PB (Start Push Button) is normally open and closes when pressed.
• Stop PB (Stop Push Button) is normally closed and opens when pressed.
• MOTOR ON and MOTOR OFF are the relay contacts that control the motor.

The diagram works as follows:

• When the Start PB is pressed, it closes the circuit, allowing current to flow to the relay, which energizes the motor.
• When the Stop PB is pressed, it breaks the circuit and de-energizes the motor.

5. Example PLC Program (Structured Text)

Here is an example of how this logic might be written in Structured Text (ST), a common PLC programming language:

IF StartButton = TRUE THEN
    MotorStatus := TRUE;  // Start motor
ELSIF StopButton = TRUE THEN
    MotorStatus := FALSE; // Stop motor
END_IF;

• StartButton and StopButton represent the inputs from the push buttons.
• MotorStatus represents the state of the motor (True for running, False for stopped).
• The motor will start if the StartButton is pressed and will stop if the StopButton is pressed.

6. Safety Considerations

When controlling motors with PLCs, safety should always be a top priority. Here are some key considerations:

• Emergency Stop: Always include an emergency stop button in the control circuit. This button should cut power to the motor immediately, overriding all other controls.
• Overload Protection: Consider programming the PLC to monitor motor current or temperature to prevent damage from overload conditions.
• Fault Indicators: It’s a good practice to include fault detection and alert systems, which can notify operators if the motor does not start or stop as expected.

7. Conclusion

PLC-based motor control using push buttons is a simple, effective way to start and stop motors in industrial applications. By using the start and stop buttons, PLC logic determines when to energize or de-energize the motor relay, providing reliable control. Implementing safety features like emergency stops, overload protection, and fault indicators further enhances the safety and reliability of the system.

Suggested Diagram:

For your reference, the diagram below can visually represent the circuit and logic involved in controlling the motor with push buttons:

Image Description:

• A simple circuit diagram with Start and Stop push buttons wired to a PLC input.
• A relay or contactor is connected to the PLC output, which in turn controls the motor.
• Labels indicating “Start” and “Stop” button positions, as well as the PLC input and output connections.