Method and system for starting a motor-commutated SCR inverter

A starting system for an interter-motor system wherein a synchronous motor, having a stator and a rotor, is normally driven by the output a-c voltage developed by an inverter, which has a series of motor-commutated SCRs that are gated into conduction in predetermined pairs and in a prescribed sequence to conduct current to the motor, from an applied d-c bus voltage, and in response to received bus current, translated to the inverter over a d-c bus and a series-connected filter inductance from a controlled rectifier bridge to which is applied a-c line voltage, said starting system comprising:

• means for sequentially gating the pairs of inverter SCRs at an increasing frequency to current pulse energize the motor to effect step-by-step rotation thereof at a faster and faster rate, the motor current being reduced to zero at the termination of each energizing pulse by controlling the rectifier bridge to force commutate the conducting pair of SCRs,
• the bus current being decreased to zero, to force commutate the conducting SCRs, by controlling the rectifier bridge so that it will rapidly dissipate the energy that builds up and becomes stored in the filter and motor inductances when the bus current flows therethrough, thereby minimizi:ng the decay time of the bus current to zero to facilitate faster gating on of the next pair of SCRs in the sequence;
• a synchronizing circuit, operative after the motor has reached a preset speed, for sensing only the motor voltage and for developing synchronizing pulses in response to that sensed motor voltage,
• said synchronizing circuit including a rectifier for rectifying the motor voltage, an integrator for integrating the rectified motor voltage, a comparator for comparing the integrated voltage with a reference voltage representing a preset threshold level, a synchronizing pulse being produced each time the integrated voltage integrates up to the reference voltage and reaches the threshold level, and a reset circuit for feeding each synchronizing pulse back to the integrator to effect reset thereof to zero in order to start a new integration cycle, a constant volt-time integral thereby being obtained between successive synchronizing pulses as determined by the value of the reference voltage;
• and means for utilizing each synchronizing pulse to control the rectifier bridge to reduce the bus current to zero to force commutate the conducting pair of inverter SCRs and to then gate on the next pair of SCRs in the prescribed sequence and supply current to the motor through that next pair of SCRs, thereby effectively synchronizing the gating of the inverter SCRs to the rotor position while at the same time force commutating the SCRs,
• a constant ratio of the inverter output voltage relative to the inverter frequency being obtained as a result of the constant volt-time integral.