pairs with dead band. Using PWMs, the dc bus is synthesized to provide 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, then the motor runs in the forward direction. To reverse the direction of rotation, the voltage supplied to the main winding should lead the voltage supplied to the start winding.
|Phase voltages when the motor is running in forward and reverse direction.|
This H-bridge inverter method of controlling a PSC type motor has following disadvantages.
The main and start windings have different electrical characteristics. Thus, the current flowing through each switch is unbalanced. This can lead to the premature breakdown of switching devices in the inverter.
The common point of the windings is directly connected to the neutral power supply. This may increase the switching signals creeping into the main power supply, and may increase the noise emitted onto the line. In turn, this may limit the EMI level of the product, violating certain design goals and regulations.
The effective dc voltage handled is relatively high due to the input-voltage doubler circuit.
Lastly, the cost of the voltage doubler circuit itself is high due to two large power capacitors.
A better solution to minimize these problems would be to use a three-phase inverter bridge, as discussed in the next section.
Option #3: Using a Three-Phase Inverter Bridge
The input section is replaced with a standard diode-bridge rectifier. The output section has a three-phase inverter bridge. The main difference from the previous scheme is the method used to connect the motor windings to the inverter. One end of the main and start windings are connected to one half bridge each. The other ends are tied together and connected to the third half bridge.
|Control using a three-phase inverter bridge.|
With this drive topology, control becomes more efficient. However, the control algorithm becomes more complex. The winding voltages, Va, Vb, and Vc, should be controlled to achieve the phase difference between the effective voltages across the main and starting windings, in order to have a 90-degree phase shift to each other.
In order to have equal voltage-stress levels on all devices, which improves the device utilization and provides the maximum possible output voltage for a given dc bus voltage, all three inverter-phase voltages are kept at the same amplitude, as given by:
| Va | = | Vb | = | Vc |
The effective voltage across the main and starting windings as given by:
Vmain = Va-Vc
Vstart = Vb-Vc
The direction of rotation can be easily controlled by the Vc phase angle with respect to Va and Vb .
Figures on page 87 show the phase voltages Va, Vb, and Vc, the effective voltages across the main winding (Vmain) and starting winding (Vstart) for forward direction and reverse directions respectively.
Using the three-phase inverter control method on a 300W compressor gave a power saving of 30 percent compared to the first two methods.