INEXPENSIVE d.c. motors are often used by model-makers, not only for model locomotives and racing cars but in robots of all kinds. They may also be used for driving non-mobile models made from anything from cardboard to Meccano. This project controls a small 6V d.c.motor, but can be used for 12V or highervoltage d.c. motors as well. The circuit controls both the speed and the direction of the motor. There is a wide range of cheap low-voltage motors available, running at maximum no-load speeds ranging from 3600 r.p.m. to 18,000 r.p.m. or more. Torque may be anything between 3g-cm for the cheaper versions and over 60g-cm for the slightly more expensive motors. Many applications require slower speeds with correspondingly greater torque, and there are several different plastic gearboxes available for the model maker to use. There are also motor-driven devices such as cooling fans and water pumps that can be controlled by this project. If controlling motors is of no interest to you, perhaps you might like to use the circuit for controlling the brightness of a filament This D.C. Motor Controller project It is possible to control the speed of a motor by simply wiring a resistor in series with it. Apart from the inefficiency of such a system, there is the need for a heavy-duty variable resistor (potentiometer). A typical d.c. motor requires several hundred milliamps to drive it. If the motor is taking, say, 300mA and the variable resistor is dropping, say, 3V, the power dissipated in the resistor is 0·9W. The typical variable potentiometer is rated at only 0·25W, so something much more robust is required, costing three or more times as much.Another factor is that motors do not run well on less than their full rated voltage. They fail to start turning when current is switched on and have a tendency to stall when extra load is applied.
We all know that ac motors are normally constant speed motors Vriable speed ac drives uses very complex circuitory because the ac motor speed is varied by the frequency variation.and is not quite easy.Stepper motor gives calculated angle of rotation they take number of steps in one revolution they canot be stopped inbetween the step. Also they are not constant running motors their use is limited to the area where you wanted a calculated steps to be taken such as priners, plotters, and other similar applications. Dc motors are widely used in industry where wide range of variable speed is required. Their application area is very wide : textile , petrochemical, medical electronics,consumer electronics, submarines,aircrafts, military equipments,setallites vehicles, rockets, trains,trams, non pollution vehicles,cars, robotics and the list is non ending. In a nut shell where a precission speed control is required, dc motor is required. The circuit designed using atmel 89c51 family of microcontroller the speed is controlled using pwm techniques pwm pulse is the on off ( low or high pulse) given to the motor driver circuit.