Embedded Systems October 2000 Vol13_11

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SPECTRA • FIGURE 1 1.----- -=- 9 Volt Battery FIGURE 2 -=- Battery ) ~ Stator ·····', / ••••• Phase Windings [()J Motor FIGURE 3 Brushless three-phase motor ", '-';~:"'.- '~' , •• --- Arc Magnets Brushl ess permanent magne t motors differ from brush motors in that the windings are located o n the stator and do not move, while th e magnets are found on the rotor, The windings are distributed in multiple phases, usually three, Each phase is separated by 120 electrical degrees, One elect.ri- cal connection exists to each phase, Brush motors swi tch phases in and o ut mechanicall y, and because the device features numerous phases, the motion is relatively smooth, A similar technique is possible on a brushl ess motor, but because the re are only three phases, thi technique pro- •••••• Rotor duces large torque pe rturbations at each t.ransitio n. Therefo re, on a brushless motor, the commutation is done electronical- 150 OCTOBER 2000 Embedded Systel11s Programming .' ~ ~ '" .', ":. given period of time, The greater the inductance, the less total cUlTent will flow in a given period of time. Techniques Fo r voltage control t amp abound, but PWM is the most fre- quently used, PWM controls the aver- age voltage to tile windings, as wi til the lamp in Figure 1, and, therefo re, the average clllTe nt to the windings, In a brush perma ne n t magnet motor, torque is a linear function of CUlTenL In Figure 2, we see a typical brush permanent magnet motor driver, !L's often called an H-bridge because of the arrangement of tile switches and the motor, The four switches in the diagram control both the direction of curre nt thro ugh th e moto r, and, because they are pulse width modulat- ed, the average voltage across the motor, T he one important thing to remember is tllat one never has an upper and lower switch closed in tile same leg at the same time, This would result in a hort circuit. Whe n we use a brush permanent magne t motor, we can control the volt- age across the moto r (and tllerefOl-e the current through the motor and its torque) with PWM., We control the commutation with brushes, Brushless motors

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