Embedded Systems October 2000 Vol13_11

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• SPECTRA Iy. The drive, or power stage, monitors the position of the rotor, usually with an encoder, and excites the appropri- ate winding to maintain a 90 degree commutation angle. Figure 3 shows a cross-section of a four-pole brush less motor. If you imag- ine the rotor making a comple te mechanical revolution of 360 degrees, you will notice that the flux from the magnets cycles twice, once for each north/ south pair. In othe r words, there are more elecu;cal cycles than mechanical cycles. In fact, the number of electrical degrees is equal to one half the number of poles on the motor multiplied by the number of mechani- cal degrees. Most bmshless perma- nent magnet motors have more than two poles. Figure 4 is a diagram of a typical power stage. It consists of 'ix witches configured much like the H-bridge de picted previously but with an extra leg for the third phase. Here, we also use PWM to control the average volt- age to each winding, and, therefore, the current through each phase. Again, one never has an upper and lower switch on at the same time. In this diagram, the voltage that we speak of is measured from the input to each phase to the center of the Wye. Thus, we can control the average voltage across each phase with PWM, as in the brush application. But how do we do the commutation? Commutating a brushless pennanent magnet motor Another techn ique exists that i more suitable for three-phase motors than the six-s tep method I described last month (p. 179). It's called sinusoidal commutatio n. Because a brush less motor can control current in multi- ple phases independently, it move the winding flux in very small II1cre- ments. To understand this, recall Kirchoff's law, which states that the sum of all currents, voltages, and flux- es in an electrical circuit must equal zero. That relationship is represented by the fo llowing formul ae: for current for voltage for flux A simple way to understand this is to imagine a water hose. The water moving into the hose, which we' ll call +water, must be equal in amount to the water coming out of the hose, ¥ Download Free software today, and configure your embedded design with your mouse! designers needing to integrate programmable logic along with high density Flash and SHAM into embedded designs. Unlike 311 other options, !'SOs form an inregrJted system level solution that saves time, power, and costs. Waferscale's OA" """'~.·" development/programming tools ... PSDsoft Express and the new PSDsofr 2000, provide an easy to lISC, point and cl ick L1ser intcrfaet: in whjch an engineer can accomplish in a couple of hours what may have, in the past, takcn severa.! days. Visil' Ollr website today to learn morc abollt EasyFLASH !'SOs and to download your free copy of PSOsoft Express! ~De/Jf::/olmlfml Kits $I (mlllg III $99 (IIXII/(Ib/e 1I0W~ www.waferscale.comlesp/flashisp.html ~==_ E r _ Waferscale-USA • Tel. 80().832-6974 • Fax 510-657·59 16 • Embedded Systems Programming OCTOBER 2000 151

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