A brushless dc motor system combined with a thyristor bridge inverter

Claim 2

A brushless DC motor system having a polyphase stator winding; a rotor having DC magnetic poles and located within an AC revolving field generated by said polyphase stator winding; a bridge-type polyphase thyristor inverter provided with natural commutation function and having an AC output connected to input terminals of said polyphase stator winding and a DC input connected to a DC voltage source, said inverter being composed of a plurality of main thyristor units each of which is in each arm of said bridge inverter; angular position-detecting means mounted on an axial extension of said rotor to generate electric signals in the form of an electric pulse train for representing angular positions of said rotor; and a forced commutation device connected with said bridge inverter to be controlled by said electric signals from the angular position-detecting means to effect a forced commutation for the thyristor bridge inverter at a time between the beginning time of a natural commutation of the thyristor bridge inverter and the zero level of commutating voltage.

Claim 3

A brushless DC motor system according to claim 1 in which said angular position-detecting means are composed of first and second detectors, said first detector-generating gate pulses at the beginning time of the natural commutation of said invertor, said second detector-generating control signals at the time between the beginning time of the natural commutation and the zero level of the commutating voltage.

Claim 4

A brushless DC motor system according to claim 1 in which said angular position-Detecting means for the rotor consists of a single detector which generates electric signals in the form of a pulse train which includes pulses corresponding to the angular positions of the rotor, said pulses being directly fed to the gate electrodes of the main thyristor units of the inverter at the beginning time of natural commutation thereof and being fed through a phase shifter to the forced commutation device, said phase shifter causing the pulses from the angular position detector to lag by a predetermined electrical angle which is included between the beginning time of natural commutation and the zero level of the commutating voltage for the natural commutation.

Claim 5

A brushless DC motor system according to claim 3 comprising, means for detecting the amplitudes of DC input currents of the bridge inverter; a gate circuit connected between the output of the phase shifter and the input of the forced-commutation device; a comparator for receiving the outputs from said amplitude detecting means, and from a reference voltage source and generating electrical signals responsive to the differences between said outputs from the amplitude-detecting means and the reference voltage where the inputs for the inverter exceed a predetermined value; whereby when said gate circuit is simultaneously supplied with the electrical signals from the comparator and the phase shifter the function of the forced commutation device begins to start.

Claim 6

A brushless DC motor system according to claim 1 in which said forced commutation device being composed of a capacitor unit connected between the DC inputs of the bridge inverter and normally charged from the bridge inverter inputs; a first auxiliary thyristor unit having a conductive polarity to discharge the electric charges normally charged on said capacitor unit, an inductance coil connected in series with said first auxiliary thyristor unit, said inductance coil and capacitor forming an oscillation circuit when said first auxiliary thyristor unit turns on; a diode bridge having the same number of phases as those of the bridge inverter and each phase of DC inputs thereof being connected to the corresponding output phase of the inverter; a second auxiliary thyristor unit connected between one of the DC output terminals of said diode bridge and one of the terminals of said capacitor unit and having a conductive polarity so as to make the forced commutation currents for the thyristor bridge inverter flow from said one of the DC output terminals of the diode bridge to said one of the terminals of the capacitor unit; a third auxiliary thyristor unit connected between the other terminal of said capacitor unit and the other output terminal of said diode bridge and having a conductive polarity so as to make forced commutation currents for the thyristor bridge inverter flow from said other terminal of the capacitor to the other output terminal of the diode bridge; said first auxiliary thyristor becoming conductive every time when any commutation is effected between two main thyristors in the thyristor bridge, said second and third auxiliary thyristors becoming alternately conductive respectively, every time when said first auxiliary thyristor becomes conductive.

Claim 7

A brushless DC motor system according to claim in which said capacitor unit in the forced commutation device is connected through diode means between said DC inputs of thyristor bridge inverter, said diode means permitting to conduct the charged currents for said capacitor unit where said inverter functions as the inverter itself, but not permitting to conduct the currents through the forced commutation device where said electric motor functions as regenerative generator and thereby said inverter is converted into a bridge rectifier.

Claim 8

A brushless DC motor according to claim 5 in which a separate DC voltage source for ensuring the minimum charged voltage values on said capacitor unit, where the amplitudes of the DC input voltage for said thyristor inveRter fall beneath a charged voltage level on said capacitor unit insufficient to obtain the forced commutation, being connected through a resistor having a relatively high resistance value across the capacitor unit.