BY

A.ANANTH

R.RADHIKA

B.USHA RANI

SPEED CONTROL OF DC MOTOR USING P.W.M TECHNIQUE

The aim of development of this project is towards providing

efficient and simple method for speed control of DC motor using

pulse width modulation technique. The modulation of pulse width

is obtained using two 555IC Timers.

To get desired modulation of pulse width as output, we

have fabricated astable multivibrator and monostable multivibrator

circuit using two 555IC Timers. The width of the pulse is changed

by varying the control voltage of the monostable circuit.

Pulse Width Modulation (PWM) is a common technique for

speed control which can overcome the problem of the poor starting

performance of a motor.

ABSTRACT

Introduction to PWMWhy only PWMBlock diagram of PWMIntroduction to 555 timer IcAstable multivibrator Mono stable multivibratorCircuit diagramConclusionsReferences

Contents

Dc motor

Applications of PM DC

Speed –torque relation

Chopping techniquevo=vs

Where, =ton/T

Modulation means to vary something. Pulse Width

Modulation means to vary the width of pulses to obtain

desired output voltage.

• PWM applies a pulse train of fixed amplitude and

frequency, only the width is varied in proportion to an

input voltage.

The duty cycle, the ratio of the time tON during which

the output is high to the total time period T is given as,

% duty cycle, D =ton/ T * 100

Introduction to P.W.M

A PWM circuit works by making a square wave with a variable on-to off

ratio, the average on time may be varied from 0 to 100 percent. In this manner, a

variable amount of power is transferred to the load. The main advantage of a

PWM circuit over a resistive power controller is the efficiency, at a 50% level,

the PWM will use about 50% of full power, almost all of which is transferred to

the load, a resistive controller at 50% load power would consume about 71% of

full power, 50% of the power goes to the load and the other 21% is wasted

heating the series resistor.

Instead of supplying a varying voltage to a motor, it is supplied with a

fixed voltage value (such as 12V) which starts it spinning immediately. The

voltage is then removed and the motor 'coasts'. By continuing this voltage on/off

cycle with a varying duty cycle, the motor speed can be controlled.

Why only P.W.M

Block diagram of P.W.M

These ic’s introduces time delays from few microseconds to several hours.

Requires about 5v to 18v power supply. Depending on the manufacturer, the standard 555 package

includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8) It needs two or three external components to produce an oscillator capable of operating at a frequency from 1Hz to 500kHz. When it oscillates at a frequency less than 1Hz, the circuit is called a Timer or Delay.

Introduction to 555 TIMER IC'S

Astable multivibrator:- producing a square wave. No stable states. Free running multivibrator.

Monostable multivibrator:- producing a single pulse when triggered. One stable and one quasi-stable state. One shot multivibrator.

Applications of 555 ic

Astable multivibrator

In this circuit there is a

diode in parallel to R2, as

to bypass the resistance

of R2 while capacitor is

charging.

Astable multivibrator operation vc =vf+(vi-vf)e

t/

TC=0.69(RA+RB)C

TD=0.69(RB)C

T=TC+TD

T=0.69(R1+2R2)C

F=1/T

F=1.44/(R1+2R2)C

For 549hz frequency

Assume R1=10R2, C=.01F

R2=22K R1=220K

monostable multivibrator

Mono stable operation Vc=Vf+(Vi-Vf)e-t/

Substituting vf=vcc

Vc=2/3vcc , vi=0, we get,

T=1.1RaC

considering timing component as 2.42ms,

T=1.1RaC

Ra=0.00242/1.1*0.01*10-6

Ra=220kΩ

Circuit diagram

OUTPUT WAVEFORMS

Astable M/V:-Pin 2:-Across

Capacitor

Pin 3:-o/p

Monostable M/V:-Pin 2:-input

Pin 3:-o/p

Across capacitor:-

Voltage (v) Speed (R.P.M)

3 550

5 990

9 1820

11 2200

12 2390

Results

From the project work, following points can be concluded.

It fulfils all the requirements for its application.

The motor responds to the average value of the pulses

and not to the individual pulses as the chopper works at

high frequency.

Changing the duty-cycle of the pulse by changing the

potentiometer changes

the average voltage level.

It is possible to improve overall performance of the

chopper drive

CONCLUSION

Power Electronics by p.s. Bimbhra 3rd Edition. A Textbook of Electrical Technology Vol. 2 – Theraja Power Electronics Handbook - Devices_ Circuits_ and Applications_ 2nd Edition_ Muhammad

H Rashid http://en.wikipedia.org/wiki/Pulse-width_modulation http://www.ecircuitcenter.com/Circuits/pwm/pwm.htm http://www.jmcproducts.com/products/pwm_fans/pwm_faq.shtml http://www.eleinmec.com/article.asp?28 http://pcbheaven.com/wikipages/PWM_Modulation/

References