3 It has been decided to introduce a Programmable Controller into a manufacturing plant to control a three phase induction motor via an autotransformer in order to reduce high starting currents. It is required to include an alarm circuit which is to give an audible and visual alarm when the motor is overloaded. A momentary actuated Reset push button is to be included in the alarm circuit which will turn off the audible alarm but leave the overload lamp on until the motor overload is reset. Once the motor overload is reset the overload lamp is to be de-activated. The conventional relay circuit diagram for this starter is shown below. A single Stop/Start push button control station is required to initiate the following motor control sequence. a. Motor 1 turns on the instant the start push button is operated; b. Motor 2 turns on 10 seconds after Motor 1 starts; c. Motor 3 turns on 15 seconds after Motor 2 starts; d. Motor 3 turns off after running for six seconds; e. Motor 2 turns off 6 seconds after Motor 3 stops; f. Motor 1 turns off 3 seconds after Motor 2 stops. If the Stop push button is operated, or a thermal overload from any motor is tripped, the whole system will shut down. Motor 1 will start the control sequence again next time the Start push button is pressed.
