Abstract: A system for automatically detecting whether a power converter or inverter is correctly connected to a three-phase AC power source consisting of first, second, and third phases, the system comprising: a zero-crossing comparator (1) for determining whether the phase of the first-phase voltage (current) is in a positive half cycle or a negative half cycle and generating a first discrimination signal (C.sub.U); a zero-crossing comparator (2, 3) for determining whether the phase of the second- or third-phase voltages (current) is in a positive half cycle or a negative half cycle and generating a second discrimination signal (C.sub.V or C.sub.W); and a D-type flip-flop (5) which detects, in response to the first discrimination signal, that the second discrimination signal is in a predetermined half cycle.

Abstract: A phase-locked loop circuit adapted to produce a stable output signal having the same frequency and phase as those of an input signal. Phase comparator means measures the time difference between the rising edge of the input signal and the falling edge of the feedback signal, and calculator means executes a phase control calculation twice per cycle on the basis of the measured time difference. Then a variable-frequency oscillator circuit produces, in accordance with the result of such calculation, an output signal whose frequency and phase are locked to those of the input signal.

Abstract: A parallel operating system for A.C. output converters of high frequency PWM inverters of an instantaneous current valve control type is provided with a current minor loop for controlling the instantaneous value of the output current. The current of each inverter minor loop is adapted to be supplied with a command value from a voltage major loop and a command value corresponding to the load current to be borne by each converter derived from the load current, as the command value for the current to be output by the converter for maintaining the output voltage at a sine wave.

Abstract: A digital rotation detecting apparatus of the invention, which is adapted to detect a phase angle .theta..sub.r of a resolver by using a multiple frequency clock of the excitation frequency for the resolver and a clock of a voltage control oscillator, is detectable of an instantaneous value of the phase angle .theta..sub.r at a desired time without delay, is also detectable directly of the rotational speed, and requires no low frequency excitation because the excitation frequency for the resolver needs only to be constant.

Abstract: A digital phase-shifting circuit employed for a thyristor converter, which operates on a polyphase power supply. In the digital phase-shifting circuit which is simply constructed as a whole, a counter portion consists of an N-bit binary counter and a divide-by-six ring counter, and a comparator portion consists of a common comparator which compares data bits and simply constructed two-bit comparators that are provided for each of the phases.