Delayed turn-on and turn-off control circuit

A delayed turn-on and turn-off control circuit for an electric motor comprising:

• an electrical power source having a positive terminal and a ground terminal;
• a motor relay having a normally open contactor connected in series between the electric motor and one of the terminals of the power source, said contactor being movable between an opened position at which the electric motor is disconnected from the power source and a closed position at which the electric motor is connected to the power source, and an electrical actuator means connected to the contactor for moving the contactor to the closed position in response to said actuator means being energized and to said opened position in response to said actuator means being de-energized;
• means for providing an electrical current at a predetermined value;
• a motor start switch connected to the current providing means and being movable between a normally open position and a closed position;
• a power silicon controlled rectifier connected in series between the electrical actuator means and the ground terminal of the power source for energizing the electrical actuator means in response to receiving a first electrical signal and for de-energizing the electrical actuator means in response to receiving a second electrical signal, said power silicon control rectifier having a control gate for receiving the first electrical signal;
• a turn-on delay means connected to the motor start switch and to the control gate of the power silicon controlled rectifier for delivering said first electrical signal to the control gate only after the running of a first predetermined period of time after the closing of the motor start switch;
• a turn-off delay means connected to the motor start switch and to the power silicon controlled rectifier for delivering said second signal to the power silicon controlled rectifier only after the running of a second predetermined period of time after the opening of the motor start switch, said turn-off delay means including a commutating capacitor, means to charge the commutating capacitor to the voltage of the power source only when the power silicon controlled recitifer is conductive, a commutating silicon controlled rectifier connected between the commutating capacitor and the grounded side of the power silicon controlled rectifier, means for gating on the commutating silicon controlled rectifier after said second predetermined period of time following the opening of the switch for providing a reverse bias on the power silicon controlled rectifier as the second electrical signal for rendering it nonconductive, and means for disabling the turn-off delay means in response to the normally open start switch being closed and for initiating the running of the second predetermined period of time in response to the motor start switch being opened;
• and
• wherein said gating on means includes a time delay capacitor, a timing resistor connected between the electrical current providing means and the time delay capacitor for charging the time delay capacitor, and means for discharging the time delay capacitor to the control gate of the commutating silicon controlled rectifier in response to the voltage across the time delay capacitor reaching a predetermined value, said discharging means including a unijunction transistor and a load resistor, said unijunction transistor having its emitter connected to the time delay capacitor and one of its bases connected to the control gate of the commutating silicon controlled rectifier, said load resistor being connected to said one base of the unijunction transistor, a control gate of the commutating silicon controlled rectifier and to the ground terminal for keeping the control gate voltage low until the unijunction transistor is turned on.