Ed Blackmond via EV wrote:
Another way to approach it is to use the switching to build an AC rather than DC battery pack. By switching between different series/parallel combinations it is possible to create a multi-level converter. This can create very good sign waves without a frequency component at a PWM frequency. Create three strings operating 120 degrees out of phase from each other and the battery circuit becomes a very nice variable frequency three phase drive. Running the strings in phase and synchronized to the utility grid and the battery circuit becomes a charger. With sensors monitoring all the cells, they can be switched in and out of the circuit keeping all the cells at the same state of charge. One circuit integrates the function of battery cell protection, motor drive, and charger.
This approach is common in very high power converters (like utility company inverters that are handling megawatts).
A 3-phase inverter can intrinsically generate a 2-level AC output without PWM because it can connect windings in series or parallel. For example, a 300v DC source can apply 0v +100v +200v +100v 0v -100v -200v -100v to each winding of a wye-connected motor. This waveform already has a fairly low harmonic content.
If you add the ability to switch the DC source voltage in 2 or 3 series/parallel steps, the inverter can deliver a pretty good sinewave without high frequency PWM or fast devices.
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