PLC motor control program


Motors used in industrial sectors are controlled through PLC, SCADA or other automation controller systems. Here we are looking to the PLC programming of motor control, before that we have to check the hardware approach.

Motor starter control:

The figure below shows the motor starter connection with the plc. The normally open and a normally closed pushbutton might be connected to control a three-phase AC motor.

The motor start coil (M) is wired in series with a momentary start button, normally open, a momentary stop button, normally closed, and normally closed overload relay (OL) contacts.

Pushing the start push button energizes the motor starter (M). This closes the associated contacts M and Ma (auxiliary contact located on the motor starter). When the start button is released, the current continues to flow through the stop button and the Ma contact, and the M coil remains energized.

The motor will run until the normally closed Stop button is pressed, unless the overload relay (OL) contacts open.

PLC Motor Control:

Motor control using PLC is shown below:

A normally open start button is connected to the first input (I0.0), a normally closed stop button is connected to the second input (I0.1), and the normally closed overload relay contacts (part of the starter of the motor) are connected to the third input (I0.2). These inputs are used to control normally open contacts in a ladder logic line programmed in the PLC.

Initially, status bit I0.1 is a logical 1 because the normally closed stop button (NC) is closed. Status bit I0.2 is a logical 1 because the normally closed (NC) overload relay (OL) contacts are closed. However, status bit I0.0 is a logical 0, because the normally open start button has not been pressed.

Program operation:

When the start button is pressed, the CPU receives logic 1 from input I0.0. This causes contact I0.0 to close. The three inputs are now logic 1. The CPU sends a logic 1 to the output Q0.0. The starter motor is energized and the engine starts.

  • The output status bit for Q0.0 is now a 1.

  • On the next scan, when normally open contact Q0.0 is solved, the contact will close and output Q0.0 will stay on even if the Start pushbutton is released.

When the Stop button is pressed, input I0.1 is deactivated, contact I0.1 is opened, output coil Q0.0 is deactivated and the motor is switched off.