INDUSTRIAL AUTOMATION | Draw diagram of DOL starter power & control circuit for 3-phase Induction Motor for forward-stop-reverse operation. Explain its working.

Forward-Stop-Reverse type DOL starter for 3-phase Induction Motor: 

The power and control circuit of DOL starter with Forward-Stop-Reverse control are shown in figures (a) and (b) above respectively. In power circuit, two contactors (F and R) are used to provide electric supply with opposite phase sequence to motor. When contactor F is ON, the three-phase supply with phase sequence L1-L2-L3 is provided to motor and it runs in Forward direction. However, when contactor R is ON, the three-phase supply with reversed phase sequence L1-L3-L is provided to motor and it runs in Reverse direction.

In control circuit, when push-button ‘For’ is pressed, the contactor coil ‘F’ get energized through ‘Stop’ push-button, pressed ‘For’ push-button and R2 NC contact. Therefore, the contactor ‘F’ get closed and in power circuit, the three phase supply with phase sequence L1-L2-L is provided to motor and it runs in Forward direction. The operation of contactor ‘F1’ causes closing of NO contact ‘F2’ and opening of NC contact ‘F3’. The contact F being connected in parallel with push-button ‘For’, it holds ON the contactor ‘F’ after releasing push-button ‘For’. Now even if somebody presses push-button ‘Rev’, the contactor ‘R’ cannot be energized as the NC contact F2 is open. So if we wish to reverse the direction of rotation, we need to press ‘Stop’ push-button first to stop the motor. Pressing of ‘Stop’ push-button causes interruption of current of forward contactor coil ‘F’. Therefore, the contactor ‘F’ gets de energized and NC contact F regains its original closed state.

Then only the direction of rotation can be reversed by pressing push-button ‘Rev’. It causes the current to flow through contactor coil ‘R’, energizing contactor ‘R’ and closing its NO contacts. In power circuit, the contactor R get closed providing three-phase supply with reversed phase sequence L2 to motor and it runs in Reverse direction.

Thus during transition from Forward to Reverse or vice-versa, we need to Stop the motor, hence this control is referred as ‘Forward-Stop-Reverse’ control.