The present disclosure relates to rotating electrical machine control devices that control an alternating current (AC) rotating electrical machine via an inverter that carries out conversion between direct current (DC) power and multi-phase AC power.
Inverters using semiconductor switching elements are often used for conversion between DC and AC. Pulse width modulation is known as a modulation method that is used to convert DC power to AC power. In pulse width modulation, pulses are generated based on the relationship in size between the amplitude of an alternating current waveform (e.g., an AC voltage waveform) as a command value and the amplitude of a triangular wave (including sawtooth wave)-shaped carrier waveform. There is a case where pulses are directly generated by digital computations rather than by comparison with a carrier. In that case as well, there is a correlation between the amplitude of an alternating current waveform as a command value and the amplitude of an imaginary carrier waveform. Pulse width modulation includes sinusoidal pulse width modulation (SPWM: sinusoidal PWM), space vector pulse width modulation (SVPWM: space vector PWM), etc. In these modulation methods, a carrier is determined according to the control period of a control device such as, e.g., the computation period of a microcomputer or the operation period of electronic circuits. That is, even if multi-phase AC power is used to drive an AC rotating electrical machine etc., the carrier has a period that does not directly correlate with the rotational speed and the rotation angle (electrical angle) of the rotating electrical machine. Neither the carrier nor the pulses that are generated based on the carrier are synchronous with rotation of the rotating electrical machine. Accordingly, modulation methods such as sinusoidal pulse width modulation and space vector pulse width modulation are sometimes called “asynchronous modulation methods.”
On the other hand, modulation methods using pulses that are synchronous with rotation of a rotating electrical machine can be called “synchronous modulation methods.” For example, rectangular wave modulation (single-pulse modulation) in which a single pulse is output in every period of the electrical angle of a rotating electrical machine is a synchronous modulation method. A plurality of pulses may be output in every period of the electrical angle as long as the pulses are synchronous with rotation of a rotating electrical machine. Multi-pulse modulation in which a plurality of pulses are output in synchronization with rotation of a rotating electrical machine can be performed as a synchronous modulation method. 5-pulse modulation, 7-pulse modulation, 9-pulse modulation, etc. can be used as the multi-pulse modulation.
These modulation methods are selected according to operating conditions such as required torque, rotational speed, etc. of a rotating electrical machine. In some cases, the modulation method is switched if the required torque or the rotational speed changes during operation of the rotating electrical machine. As described in Japanese Patent Application Publication No. 2013-132135, a transient current may flow when the modulation method is switched from an asynchronous modulation method to a synchronous modulation method (e.g., paragraphs 0025 to 0035 etc.). In Japanese Patent Application Publication No. 2013-132135, a certain amount of time is allowed before determining whether output of a rotating electrical machine should be limited or not, so that the output of the rotating electrical machine is not limited when a measured current is larger than a threshold for overcurrent determination due to such a momentary transient current. Semiconductor switching elements forming an inverter are required to be resistant to this transient current. However, selecting semiconductor switching elements having such characteristics prevents reduction in size of the inverter and hinders cost reduction. It is therefore preferable to reduce the maximum value (absolute value) of an alternating current that increases due to the influence of such a transient current.