Motor/generators used in hybrid powertrains typically require that three phase alternating current be fed to the stator windings of the stator. A power inverter includes switches that are moved between on and off positions to pulse the voltage to approximate a desired waveform, separately for each of the three windings. The motor/generator acts as a low pass filter of sorts, to filter the pulsed voltage waveform resulting in an essentially sinusoidal current waveform with small ripple component superimposed. The frequency of the switching can be modified, and the voltage can be held on for different time intervals, and/or the voltage can be held off for different time intervals to implement the desired modulation type (e.g., discontinuous pulse width modulated (DPWM), continuous pulse width modulated (CPWM), etc.).
Inverter switching losses are a significant percentage of the total energy losses for a hybrid electric vehicle. By decreasing the switching frequency, switching energy losses are decreased. However, as switching frequency decreases, the switching noise is generally considered to be more audible, whereas as switching frequency increases, the switching noise is generally considered to be less audible. Inverter control strategies have included masking the noise of the switching, such as at low frequencies, by ensuring that background noise is at a relatively high level. This has been done by limiting low frequency switching to periods when operating characteristics of the motor/generator, such as motor/generator torque level or motor/generator speed level, will ensure sufficient masking of the switching noise.
DPWM can provide essentially sinusoidal current waveforms while simultaneously minimizing inverter switching losses. This is achieved by adding an appropriate zero sequence voltage to each of the inverter phases, while maintaining essentially sinusoidally-shaped line-to-line voltage excitation to the motor/generator. The zero sequence voltage is selected such that each of the inverter phases will be saturated at either 0 or 100 percent duty cycle for one third of the motor fundamental electrical period. Switching losses for a particular phase are eliminated when operating with the 0 or 100 percent duty cycle. The resultant DPWM waveforms employ a single zero vector for each PWM period, as opposed to the two distinct zero vectors used for CPWM implementations. Accordingly, a DPWM waveform, with its less frequent switching, is generally noisier than a CPWM waveform with its more frequent switching. The DPWM type of waveform tends to minimize inverter switching losses while increasing current ripple and acoustic noise as compared with a CPWM type of waveform.