History and development:

Fig:DC Motor

The principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire.
The first commutator-type direct-current electric motor capable of a practical application was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837.

Working Principle :

Electrical motors operate on the principle that two magnetic fields within certain prescribed areas react upon each other. All electric motors use electromagnetic fields to create torque.
The direct current (DC) motor is one of the first machines devised to convert electrical power into mechanical power. Permanent magnet (PM) direct current convert electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly, the other field is produced by an electrical current flowing in the motor windings. These two fields result in a torque which tends to rotate the rotor. As the rotor turns, the current in the windings is commutated to produce a continuous torque output. The stationary electromagnetic field of the motor can also be wire-wound like the armature (called a wound-field motor) or can be made up of permanent magnets (called a permanent magnet motor).

The Armature:

Fig:Armature

The Armature consist of a number of coils mounted on a shaft with the coil ends soldered to specific Commutator bars. The armature is actually an iron cored coil ,the core being made of laminations ,laminations are separated by fine coating of varnish on their surfaces.
The armature constructed with slots into which the armature coils are placed the active part of the armature coil is its horizontal section that fits into the slot. Sometimes a single conductor is used for each slot and in some motors two are used . the iron core of the armature is cylindrical, with the armature coil wound around the core and fitted into the slots on the outer horizontal surface. When current flows through the armature produces a magnetic field around it. The armature, then is an electromagnet. The armature coil are wound so that when one side of the coil under South Pole of the field coil , the other side is under the North Pole. It is a reaction between armature magnetism and the field’s magnetism that produces rotation of the armature Here field is another magnet which partially surrounds the armature . this magnet can be a permanent type or another electromagnet.

The Commutator:

In either style (wound-field or permanent magnet) the commutator. acts as half of a mechanical switch and rotates with the armature as it turns. The commutator is composed of conductive segments (called bars), usually made of copper, which represent the termination of individual coils of wire distributed around the armature. The second half of the mechanical switch is completed by the brushes. These brushes typically remain stationary with the motor's housing but ride (or brush) on the rotating commutator. As electrical energy is passed through the brushes and consequently through the armature a torsional force is generated as a reaction between the motor's field and the armature causing the motor's armature to turn. As the armature turns, the brushes switch to adjacent bars on the commutator. This switching action transfers the electrical energy to an adjacent winding on the armature which in turn perpetuates the torsional motion of the armature.

Permanent magnet (PM) motors are probably the most commonly used DC motors, but there are also some other type of DC motors(types which use coils to make the permanent magnetic field also).DC motors operate from a direct current power source. Movement of the magnetic field is achieved by switching current between coils within the motor. This action is called "commutation". Very many DC motors (brush-type) have built-in commutation, meaning that as the motor rotates, mechanical brushes automatically commutate coils on the rotor. You can use dc-brush motors in a variety of applications. A simple, permanent-magnet dc motor is an essential element in a variety of products, such as toys, servo mechanisms, valve actuators, robots, and automotive electronics. There are several typical advantages of a PM motor.. Furthermore, since the motor's field, created by the permanent magnet, is constant, the relationship between torque and speed is very linear. A PM motor can provide relatively high torque at low speeds and PM field provides some inherent self-braking when power to the motor is shutoff. There are several disadvantages through, those being mostly being high current during a stall condition and during instantaneous reversal. Those can damage some motors or be problematic to control circuitry. Furthermore, some magnet materials can be damaged when subjected to excessive heat and some loose field strength if the motor is disassembled.

Geared DC Motors:

The term gear motor is used to define a motor that has a gear reduction system (or gearbox) integrally built into the motor. The gearbox increases the torque generating ability of the motor while simultaneously reducing it's output speed

H Bridge Motor control :

Fig:Motor driving circuit and Data Table for Motor

VccSometimes the rotation direction needs to be changed. In normal permanent magnet motors, this rotation is changed by changing the polarity of operating power (for example by switching from negative power supply to positive or by interchanging the power terminals going to power supply).

The TTL output from the electronic interface is not able to drive the motor. Hence this direction changing is typically implemented using relay or a circuit called an H bridge

For the motor to run in one direction one terminal of the motor should get supply and the other is grounded .if the condition is reversed motor will run in the opposite direction. Hence we have provided two data bits for each motor ;10 for clockwise direction and 01 for anti-clockwise direction of motion.