When using digital control a pulse width modulated

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Unformatted text preview: ts as a 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 motor. 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.

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