Page 85 - Instrumentation and Measurement
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FIGURE 5.28 Torque-speed characteristics of d.c. motors.
The speed of a permanent magnet motor can be controlled by varying the current through the
armature coil, with a field coil motor by either varying the armature current or the field current though
generally it is the armature current that is varied. Thus, speed control can be obtained by controlling
the voltage applied to the armature. Rather than just try to directly vary the input voltage, a more
convenient method is to use pulse width modulation (PWM). This basically involves taking a
constant d.c. supply voltage and using an electronic circuit to chop it so that the average value is
varied (Figure 5.29).
FIGURE 5.29 PWM: (A) principle of PWM circuit and (B) varying the average armature voltage by
chopping the constant d.c. voltage.
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