01-03-2013, 04:48 PM
SPEED CONTROL OF DC MOTOR USING PWM TECHNIQUE
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ABSTRACT
The aim of development of this project is towards providing efficient and simple method for control speed of DC motor using pulse width modulation technique. The modulation of pulse width is obtained using dual timer IC - NE556.
There are several methods for controlling the speed of DC motors. One simple method is to add series resistance using a rheostat. As considerable power is consumed in the rheostat, this method is not economical. Another method is to use a series switch that can be closed or opened rapidly. This type of control is termed as chopper control. The PWM based chopper circuit smoothly controls the speed of general purpose DC motors.
To get desired modulation of pulse width as output, we have fabricated astable multivibrator and monostable multivibrator circuit using single dual timer IC – NE 556. The width of the pulse is changed by varying the control voltage of the monostable circuit.
Pulse Width Modulation (PWM) Basics
There are many forms of modulation used for communicating information. When a high frequency signal has amplitude varied in response to a lower frequency signal we have AM (amplitude modulation). When the signal frequency is varied in response to the modulating signal we have FM (frequency modulation. These signals are used for radio modulation because the high frequency carrier signal is needs for efficient radiation of the signal. When communication by pulses was introduced, the amplitude, frequency and pulse width become possible modulation options. In many power electronic converters where the output voltage can be one of two values the only option is modulation of average conduction time
Linear Modulation
The simplest modulation to interpret is where the average ON time of the pulses varies proportionally with the modulating signal. The advantage of linear processing for this application lies in the ease of de-modulation. The modulating signal can be recovered from the PWM by low pass filtering. For a single low frequency sine wave as modulating signal modulating the width of a fixed frequency (fs) pulse train the spectra is as shown in Fig 1.2. Clearly a low pass filter can extract the modulating component fm.
Regular Sampled PWM
The scheme illustrated above generates a switching edge at the instant of crossing of the sine wave and the triangle. This is an easy scheme to implement using analog electronics but suffers the imprecision and drift of all analog computation as well as having difficulties of generating multiple edges when the signal has even a small added noise. Many modulators are now implemented digitally but there is difficulty is computing the precise intercept of the modulating wave and the carrier. Regular sampled PWM makes the width of the pulse proportional to the value of the modulating signal at the beginning of the carrier period. In Fig 1.5 the intercept of the sample values with the triangle determine the edges of the Pulses. For a saw tooth wave of frequency fs the samples are at 2fs.
CONCLUSION
From the project work, following points can be concluded.
1. It fulfils all the requirements for its application.
2. The motor responds to the average value of the pulses and not to the individual pulses as the chopper works at high frequency.
3. Changing the duty-cycle of the pulse by changing the potentiometer changes the average voltage level.
4. It is possible to improve overall performance of the chopper drive