8131 Hysteresis Current Controller

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FIGURE 8.12 (a) Hysteresis current control and PWM output. (b) Ramp comparison control and PWM output.

The hysteresis current controller can be used in a three-phase PWM inverter, with each phase having its own PWM controller. If the actual current is higher than the reference current by the amount of half of the hysteresis band, the lower leg switch of the bridge inverter is turned on to reduce the phase current. The difficulty in three-phase hysteresis control is that there may be conflicting requirements of switch conditions for the phases based on the output of the hysteresis controller. The difficulty arises from the interaction of the phases of the three-phase system and the independent hysteresis controllers for each phase. The consequence of this difficulty is that the current does not remain within the hysteresis band. For example, a current-increase command in Phase a needs a return path through Phases b or c lower legs. If Phases b and c happen to have upper leg switches on during this instant, the current in Phase a will not increase to follow the command but will freewheel. In this case, it is possible that the current error of Phase a exceeds the

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O irr int

O irr int

FIGURE 8.13 Hysteresis and ramp comparison techniques.

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Current Error

PWM Signa)




Ha mp Comparison

FIGURE 8.13 Hysteresis and ramp comparison techniques.

hysteresis current band. Using the dq transformation theory, it is possible to first transform the three-phase currents into two-phase dq currents and then impose the hysteresis control in the dq reference frame.1

Another way of controlling the required stator current is to use a controller-based fixed-frequency ramp signal that stabilizes the switching frequency. The current error is first fed into a linear controller, which is typically of the proportional integral (PI) type. The output of the linear controller is compared with a high-frequency saw-tooth-shaped triangular signal to generate the PWM switch signals. If the error signal is higher than the triangular signal, the PWM output signal will be "1," and in the other case, the output will be zero. The control actions are shown graphically in Figure 8.12b. Stator voltages will vary in order to minimize the current error signal. Three identical controllers are used in three-phase systems.

The ramp-compari son method has a fixed switching frequency set by the saw-tooth wave frequency that makes it easier to ensure that the inverter switching frequency is not exceeded. There are more parameters to adjust in the ramp comparison controller, allowing greater flexibility of control compared to the hysteresis controller. The control parameters include gains of the linear controller, and magnitude and frequency of the sawtooth wave in the case of ramp comparison controller, whereas the only control parameter in a hysteresis controller is the width of the hysteresis band. The functional differences in the two methods are depicted in Figure 8.13.

The primary disadvantage of the ramp comparison controller is increase in response time due to transport delay. The situation can be improved by using a high gain proportional controller instead of a PI controller and by increasing the switching frequency of the sawtooth signal.

EV and HEV propulsion drives require accurate speed control with fast response characteristics. Scalar control methods for induction motors that are easier to implement are inadequate for such applications demanding high performance. The induction motor drive is capable of delivering high performance similar to that of DC motors and PM brushless DC motors using the vector control approach. Although vector control complicates the controller implementation, the lower cost and rugged

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