In most industrial applications we generally find the PLC controlling motor starters. Starters are required for dc motors and induction motors to limit the starting current and protect the armature winging.

A universal motor (ac series motor) is used in this experiment. Although the above motor may be started directly from the rated voltage, it is required to start the motor with a resistor as shown in Figure (b). The starter resistor is to be shorted out after 8 seconds.

Motor Starter Control

Figure a: Ladder program for the motor starter control

Figure b: Electric circuit for the motor starter control

PLC Logic Explanation

The ladder program is shown in Figure (a) is designed for the following sequential task:

1. Start pushbutton is pressed. Reference input SP0 (I:1/0) is closed, the output Coil_C0 (O:3/0) is energized. Contactor C0 closed and the motor starts with the resistance in the circuit.

2. Contact (O:3/0) across pushbutton closes sealing in the output Coil C0.

3. Contact (O:3/0) in the second rung closes and the timer is energized.

4. After 8 seconds TMR is enabled closing the timer contact (T4:0.DN) in the third rung. This will energize output Coil_C1 at (O:3/1). The contactor C1 across the resistor is closed, shorting out the resistor.

5. The stop pushbutton SP1 stops the motor releasing the holding contact across the START contact.

Also Read: Star Delta Motor Starter

Author: Dr. Hadi Saadat

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