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low pass filter so a PWM waveform of sufficient
frequency will generate a stable current in the motor
winding. The relation between average voltage, the
supply voltage, and duty cycle is given by: EQUATION 1:
VAVERAGE = D VSUPPLY
Speed and duty cycle are proportional to one another.
For example, if a BDC motor is rated to turn at 15000
RPM at 12V, the motor will (ideally) turn at 7500 RPM
when a 50% duty cycle waveform is applied across the
The frequency of the PWM waveform is an important
consideration. Too low a frequency will result in a noisy
motor at low speeds and sluggish response to changes
in duty cycle. Too high a frequency lessens the
efficiency of the system due to switching losses in the
switching devices. A good rule of thumb is to modulate
the input waveform at a frequency in the range of 4 kHz
to 20 kHz. This range is high enough that audible motor
noise is attenuated and the switching losses present in
the MOSFETs (or BJTs) are negligible. Generally, it is a
good idea to experiment with the PWM frequency for a
given motor to find a satisfactory frequency.
So how ca...
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This note was uploaded on 01/29/2014 for the course AA AA taught by Professor Aa during the Winter '10 term at ENS Cachan.
- Winter '10