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PREPARED BY:
Mr. RAHUL PARMAR
Mr. SUSHIL RATHOD
Mr. HARSHAD PATEL
Mr. CHETAN KOTHIYA
Mr. VIKRAMSINH MALIVAD
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SPEED CONTROL OF DC MOTOR USING PWM TECHNIQUE
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.
GOAL
“To explain PULSE WIDTH MODULATION technique in brief.”
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.
1. 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.
2. Sawtooth PWM
The simplest analog form of generating fixed frequency PWM is by comparison with a linear slope waveform such as a saw tooth. As seen in Fig 1.2 the output signal goes high when the sine wave is higher than the saw tooth. This is implemented using a comparitor whose output voltage goes to logic HIGH when ne input is greater than the other. Other signals with straight edges can be used for modulation a rising ramp carrier will generate PWM with Trailing Edge Modulation.
3. 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.
There are many ways to generate a Pulse Width Modulated signal other than fixed frequency sine sawtooth. For three phase systems the modulation of a Voltage Source Inverter can generate a PWM signal for each phase leg by comparison of the desired output voltage waveform for each phase with the same sawtooth. One alternative which is easier to implement in a computer and gives a larger modulation depth is using space vector modulation.
4. Modulation Depth
For a single phase inverter modulated by a sine-sawtooth comparison, if we compare a sine wave of magnitude from -2 to +2 with a triangle from -1 to +1 the linear relation between the input signal and the average output signal will be lost. Once the sine wave reaches the peak of the transgle the pulses will be of maximum width and the modulation will then saturate. The Modulation depth is the ratio of the current signal to the case when saturation is just starting. Thus sine wave of peak 1.2 compared with a triangle with peak 2.0 will have a modulation depth of m=0.6.
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PRESENTED BY
D.RAMAKRISHNA
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INTRODUCTION
Today’s industries are increasingly demanding process automation in all sectors. Automation results into better quality, increased production an reduced costs. The variable speed drives, which can control the speed of A.C/D.C motors, are indispensable controlling elements in automation systems. Depending on the applications, some of them are fixed speed and some of the variable speed drives.
The variable speed drives, till a couple of decades back, had various limitations, such as poor efficiencies, larger space, lower speeds, etc., However, the advent power electronic devices such as power MOSFETs, IGBTs etc., and also with the introduction of micro -controllers with many features on the same silicon wafer, transformed the scene completely and today we have variable speed drive systems which are not only in the smaller in size but also very efficient, highly reliable and meeting all the stringent demands of various industries of modern era.
Direct currents (DC) motors have been used in variable speed drives for a long time. The versatile characteristics of dc motors can provide high starting torques which is required for traction drives. Control over a wide speed range, both below and above the rated speed can be very easily achieved. The methods of speed control are simpler and less expensive than those of alternating current motors.
There are different techniques available for the speed control of DC motors. The phase control method is widely adopted, but has certain limitations mainly it generates harmonics on the power line and it also has got p .f when operated lower speeds. The second method is pwm technique, which has got better advantages over the phase control.
In the proposed project, a 5 H.P DC motors circuitry is designed, and developed using pulse with modulation (PWM).The pulse width modulation can be achieved in several ways. In the present project, the PWM generation is done using micro- controller.
In order to have better speed regulation, it is required to have a feedback from the motor. The feedback can be taken either by using a tachogenerator or an optical encoder or the back EMF itself can be used .In present project, we implemented the feedback by using the EMF of the armature as the feedback signal.
The project proposed is a real time working project, and this can be further improvised by using the other safety features, such as field current, air gap magnetic flux, armature current, etc.,
1. DC MOTOR
1.1 INTRODUCTION TO SPEED CONTROL:
Speed control means intentional change of drive speed to a value required for performing the specific work process. This concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the shaft.
Any given piece of industrial equipment may have its speed change or Adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism and other mechanical devices. Historically it is proved to be the first step in transition from non adjustable speed to adjustable speed drive. The electrical speed control has many economical as well as engineering advantages over mechanical speed control
The nature of the speed control requirement for an industrial drive depends upon its type. Some drives may require continues variation of speed for the whole of the range from zero to full speed or over a portion of this range , while the others may require two or more fixed speeds
1.2 CLASSIFICATION OF DC MOTORS:
DC motors are classified into three types depending upon the way their field windings are excited. Field windings connections for the three types Of DC motors have been shown in figure
1.3 SPEED CONTROL OF DC MOTORS:
The DC motors are in general much more adaptable speed drives than AC motors which are associated with a constant speed rotating field. Indeed one of the primary reasons for the strong competitive position of DC motors in modern industrial drives is the wide range of specified afforded we know the equation
N= K (E_b/ϕ)
=K (V-Ia Ra / ϕ)
Where V=supply voltage (volts)
Ia = armature current (amps)
Ra=armature resistance (ohms)
Φ=flux per pole (Weber)
E_b=back emf(volts)
This equation gives two methods of effective speed changes.i.e.
The variation of field excitation, if this causes in the flux per pole Φ and is known as the field control.
The variation of terminal voltage (V).this method is known as armature control.
1.4 SPEED CONTROL OF SHUNT MOTOR
1.4.1 FLUX CONTROL METHOD:
It is known that Nα1/ Φ by decreasing the flux, the can be increased and vice versa. Hence, name flux or field control method.
The flux of DC motor can be changed by changing I_sh with help of a shunt field rheostat. Since I_sh in relatively small, shunt field rheostat has to carry only a small, so that rheostat is small in size. This method therefore very efficient in non-interpolar machines the speed can be increased by this method in the ratio 2:1 any further weakening of flux Φ adversely affect the communication
And hence puts a limit to the maximum speed obtainable with this method in machines fitted with interlopes in ratio of maximum to minimum speeds of 6:1 is fairly common.
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provide meminor project report on speed control of dc motor using pwm technique...............
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