SOLUTION

A DC motor is any of a class of rotary electrical machines that converts direct current electrical energy into mechanical energy. Controlling the speed of DC motors is a crucial matter since different machines and equipment have different aspects and output speed or torque requirements in which prospect. Speed Control may require to be very precise and over wide ranges as well. So before implementing the proposed system an engineer needs to understand its operation and according to that how the speed of the drive wit] be controlled needs to be decided by him with proper care.

Generally, the rotational speed of a DC motor is proportional to the voltage applied to it, and the torque is proportional to the current. Speed control can be achieved by variable battery tapings, variable supply voltage, resistors, or electronic controls. The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both. This is commonly done with a special set of contactors (direction contactors). The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device made of thyristors, transistors or, formerly, mercury arc rectifiers.

Speed control over a wide range both above and below the rated speed:
The attractive feature of the dc motor is that it offers a wide range of speed control both above and below the rated speeds. This can be achieved in dc shunt motors by methods such as the armature control method and field control method. This is one of the main applications in which dc motors are widely used in fine speed applications such as in rolling mills and in paper mills.

High starting torque:
DC series motors are termed as best suited drives for electrical traction applications used for driving heavy loads in starting conditions. DC series motors will have a starting torque as high as 500% compared to normal operating torque. Therefore dc series motors are used in the applications such as electric trains and cranes.

Accurate steep less speed with constant torque:
Constant torque drives are one such drive that will have motor shaft torque constant over a given speed range. In such drives, shaft power varies with speed.

• Quick starting, stopping, reversing, and acceleration
• Free from harmonics, reactive power consumption, and many factors which make dc motors more advantageous compared to ac induction motors.