The first approach is relatively easy as far as the power circuit and control circuit are concerned. On the input side, a voltage doubler is used and on the output side an H-bridge, or 2-phase inverter, is used as shown in Figure 2. One end of the main and start windings are connected to each half bridge and the other ends are connected together to the neutral point of the AC power supply, which also serves as the center point for the voltage doubler.
The control circuit requires four PWMs with two complementary pairs with sufficient dead band between the complementary outputs. PWM0-PWM1 and PWM2-PWM3 are the PWM pairs with dead band. The PIC16F72 does not have PWMs designed in the hardware to output the way we need. Therefore, the PWMs should be generated in firmware and output to the port pins. Using PWMs, the DC bus is synthesized to give two sine voltages at 90 degrees out of phase with varying amplitude and varying frequency according to the VF profile. If the voltage applied to the main winding lags the start winding by 90 degrees, the motor runs in one (i.e., forward) direction. To reverse the direction of rotation, the voltage supplied to the main winding should lead the voltage supplied to the start winding.
Figure 3 and Figure 4 show the main and start winding voltages in forward and reverse respectively.
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