Composite control for soft start and dynamic braking of a three-phase induction motor

A soft start control for a three phase induction motor adaptable to be modified so as to include a braking function comprising:

• a source of three phase power having a first, second and third supply line connected to said motor;
• a three phase half-wave voltage controller including a silicon controlled rectifier connected in each of said supply lines;
• a selector circuit providing for operator selection of RUN and BRAKE modes;
• a motor contactor having a motor contact in each supply line for connecting said three phase supply to said half-wave voltage controller;
• a first, second and third phase control circuit means for respectively controlling the firing angles of the silicon controlled rectifiers in the half-wave voltage controller;
• each said phase control circuit means including an inverter, an integrator, a comparator, and a trigger circuit;
• each said inverter being responsive to a respective supply line source of power and furnishing its output signal to said respective integrator;
• each said comparator comprising an operational amplifier having an inverting and a noninverting input and responsive to output signal from said integrator;
• each said integrator providing a sawtooth wave-form to the noninverting input of the comparator in said phase;
• each said trigger circuit comprising an oscillating operational amplifier having an innverting and noninverting input and responsive to output signal from said comparator;
• the output of the comparator in each phase being connected to the inverting input of the oscillating operational amplifier in the trigger circuit of said phase;
• a first circuit means providing a fixed voltage on the noninverting input of the oscillating operational amplifier in the first phase trigger circuit;
• a second circuit means providing a fixed voltage on the noninverting inputs of the oscillating operational amplifiers in the second and third phase trigger circuits;
• and
• a first torque control circuit means for supplying a common positive ramp signal to the inverting input of each of said comparators;
• whereby said trigger circuits provide trigger signals for controlling the silicon controlled rectifiers in each of the supply lines to provide a soft start for said motor;
• and
• a first multicontact connector accepting said common positive ramp signal on two of its contacts and having a first jumper for connecting said common positive ramp signal to a third of its contacts and having said second circuit means providing a fixed voltage on the noninverting inputs of the oscillating operational amplifiers in the second and third phase trigger circuits connected to a fourth contact and having a second jumper for connecting said fourth contact to a fifth contact of said multicontact connector and additionally having circuit operating voltages and circuit ground connected to remaining contacts.

A control circuit for a three phase induction motor comprising:

• a source of three phase power having a first, second and third supply line;
• a three phase voltage controller having silicon controlled rectifiers connected in said supply lines for connecting said source of three phase power to said motor;
• a silicon controlled rectifier connected across two of said supply lines;
• means for providing a positive going ramp signal of adjustable slope and adjustable initial level;
• means for providing a constant level signal;
• circuit means for providing a fixed D. C. voltage of preselected duration;
• a first, second and third phase control circuit means for controlling the firing angles of said silicon controlled rectifiers, each of said phase control circuit means comprising:
• an inverter circuit responsive to the voltage on a respective one of said supply lines, furnishing as its signal output a rectangular wave synchronized with the zero crossings of that respective supply line voltage;
• an integrator, driven by said inverter circuit and furnishing as its output signal a modified negative-going sawtooth signal having a constant positive level during the positive alternation of said respective supply line voltage and a negative-going ramp signal from said constant positive level during the negative alternation of said respective supply line voltage;
• a comparator, responsive to output signals from said positive going ramp signal and said modified negative-going sawtooth signal and furnishing as its output a negative, variable pulse-width rectangular pulse;
• and
• a trigger circuit responsive to output signal from said comparator and furnishing high frequency oscillations as its output for a time interval related to said negative, variable pulsewidth rectangular pulse;
• said first, second and third phase control circuit means being thus responsive to said positive ramp signal and said modified, negative going sawtooth signal for controlling the firing angles of said silicon controlled rectifiers to provide a soft start for said motor;
• and
• a trigger circuit responsive to said fixed D. C. voltage of preselected duration and furnishing high frequency oscillations as its output for gating said silicon controlled rectifier connected across two of said supply lines to its on state during said preselected duration;
• said first phase control circuit means being responsive to said constant level signal for controlling the firing angle of said silicon controlled rectifier in said first supply line to provide a dynamic brake for said motor.