Dynamic brake control

A method for achieving smooth dynamic braking in an AC motor system comprising a computer controlled full-wave rectifier and chopper and a chopper control firing circuit for regulating an AC line voltage to a DC voltage at the input terminals of an inverter, the chopper and inverter being energized at a base execution rate which is synchronized to the AC line voltage frequency, the direction of current through the inverter being reversible in order to dissipate regenerated electric power in a braking resistance, the method comprising the step of:

• providing an energy absorbing resistance;
• coupling the resistance in circuit with a controllable switching means to the input terminals of the inverter;
• generating switching signals during each cycle of the AC line voltage, the switching signals being generated at intervals corresponding to a binary weighted division of each cycle;
• and
• selectively applying the switching signals to the switching means for switching the resistance in and out of circuit with the inverter in a manner to maintain the DC voltage at a predetermined value.

A control system for an alternating current (AC) motor comprising:

• a converter for converting AC power having a predetermined AC frequency to direct (DC) power;
• a DC link for receiving the DC power;
• an inverter connected to the DC link for converting DC power to variable voltage, variable frequency power for application to the motor, said inverter being operable to transfer regenerated power from the motor to the DC link resistive means;
• time ratio control means connected in circuit with said resistive means and arranged to selectively couple said resistive means to said DC link for varying the effective resistance thereof;
• control means responsive to a motor speed command for selectively energizing said converter in synchronism with the AC power for establishing a selected voltage on said DC link, said control means further controlling said inverter to vary the frequency of excitation applied to the motor in a manner to force the motor to the commanded speed and being operative to control said inverter in a manner to transfer power from the motor to said DC link when motor speed exceeds the commanded speed;
• and
• means operatively connected in circuit with said control means and responsive to the voltage on said DC link exceeding the selected voltage for energizing said time ratio control means for selectively coupling said resistive means to said link in a manner to minimize any difference between said link voltage and said selected voltage, said time ratio control means being energized in synchronism with the AC frequency such that said resistive means is coupled to said DC link for time periods during each cycle of the AC frequency which are binary weighted time segments of each cycle.