TAILIEUCHUNG - Brushless Permanent Magnet Motor Design- P7

Brushless Permanent Magnet Motor Design- P7: You've just picked up another book on motors. You've seen many others, but they all assume that you know more about motors than you do. Phrases such as armature reaction, slot leakage, fractional pitch, and skew factor are used with little or no introduction. You keep looking for a book that is written from a more basic, yet rigorous, perspective and you're hoping this is it. | Motor Drive Schemes 177 Figure Clocked tum-OFF PWM waveforms. discharge. Later when the current decays to I a switch closes and the inductance charges until the next clock pulse appears. Once again the switching frequency is fixed by the clock frequency. Important aspects of this PWM scheme include Current control is not as precise here since there is no fixed tolerance band that bounds the current. The frequency at which switches change state is a fixed design parameter. Acoustic and electromagnetic noise are relatively easy to filter because the switching frequency is fixed. This PWM method has ripple instability that produces subharmonic ripple components for duty cycles below 50 percent Kassakian Schlecht and Verghese 1991 Anunciada and Silva 1991 . While this instability does not lead to any destructive operating mode it is a chaotic behavior that reduces performance. The predominant current ripple occurs at one-half the switching frequency. Dual current-mode PWM This PWM method was developed by Anunciada and Silva 1991 to eliminate the ripple instability present in the previous two methods. Their scheme combines the clocked turn-ON and clocked turn-OFF methods in a clever way. For duty cycles below 50 percent the method implements stable clocked turn-ON PWM whereas for duty cycles 178 Chapter Seven above 50 percent the method implements stable clocked turn-OFF PWM. As illustrated in Fig. this method has two clock signals where the turn-OFF clock is delayed one-half period with respect to the turnON clock. Operation is determined by logic that initiates inductor charging when the tum-ON clock pulse appears or the current reaches 1 and initiates inductor discharge when the turn-OFF clock appears or the current reaches . As shown in the figure the method smoothly moves from one mode to the other. This scheme has all the attributes of the two previous PWM schemes except for the ripple instability. Furthermore this scheme reduces to hysteresis PWM if the

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