Embedded Systems September 2000 Vol13_10

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Don Morgan Field-Oriented Control Last month, we looked at how a three-phase motor works. Basically, it's a system in which an element (rotary or linear) can be moved by manipulat- ing an associated magnetic field . Not only can we move it, we can move it as we wish. The magnetic field that we control is in the stator of the motor. This is the shell of a rotary motor that surrounds the rotor (usually the element that moves). In a linear motor it is most likely to be the platform upon which the moving element rests. The mag- netic field in the rotor is set up either through induction, as with an induc- tion motor, or with the use of penna- nent magnets. For the rest of this col- umn, we will refer only to a rotary device but the same information may be u ed to control a linear motor. In a three-phase motor, the cur- rents are controlled in each of the phase windings in such a way as to establish a magnetic field in the rotor-if this is not a permanent mag- net motor-and cause the rotor to align with the field flux. Then, by properly controlling the currents in the stator field, we produce a vector that leads the shared magnetic field of the stator and rotor, which causes the rotor, and ultimately the shaft, to move. In a three-phase motor, the sta- tor is an electromagnet made with a winding for each phase on a soft iron casting. In each winding, current may flow in a forward (positive) or reverse (negative) direction; this results in six unique steps or pole alignments. We control the amount of current that flows by either pulse width modulation (PWM) or analog means. The resolu- tion of control actually depends on the resolution of the positioning feed- back device, the current feedback, and the update rate. The technique described is called field-oriented controL PWM versus analog A PWM-driven system controls the cur- rent in the windings of a motor by have just are on, the voltage drop across each device is only on the order of a couple of volts, so the power they dissipate is at a minimum. For th is reason, PWM drives can be small , an

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