INTRODUCTION

DC Motor is an electric motor that runs on direct current electricity. It works by converting electric power to mechanical work. It is accomplished by the application of forcing current through a coil and producing a magnetic field that spins the motor. Today, this application which relates the magnetic fields and current is known as Biot-Savart Law.

There are few basic components in dc motor in order to make DC motor works. The components are stator, armature, brush, commutator, magnet, winding, coil and the casing. These components are place as the picture above.

The stator is the stationary part of the motor. This includes the motor casing as well as two more magnet pole pieces. There are two static stators in the motor which create North Pole and South Pole respectively. The function of the stator is to induce magnetic fields while current is forcing through the coils. The magnetic fields will be generated while current is running connecting the north and south pole of the stator.

The armatures are the mechanical or rotary part of the motor. There are six armatures as stated in the picture above. Armatures are going to have North and South Pole while the current flow through the coils. As a result, the North pole with attracted to the South Pole of the stator and vice versa. The changing of the pole in armatures is control by the commutator and the brush of the motor.

The armature consists of windings (generally on a core), the windings being electrically connected to the commutator. The position of the brushes, commutator contacts, and armature windings are such that when the power is applied, the polarities of the energized winding and the stator magnet(s) are misaligned, the armature will rotate until it is almost aligned with the stator’s field magnets.

As the armature reaches alignment, the brushes move to the next commutator contacts, and energize the next winding. The rotation reverses the direction of current through the rotor winding, leading to a flip of the armature’s field and drives it to continue the rotating until the power is cut-off.

TYPES OF DC MOTORS

Brushed DC
The brushed DC motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary permanent magnets, and rotating electrical magnets.

Advantages
• Low initial cost
• High reliability
• Simple control of motor speed
Disadvantages
• High maintenance
• Low life-span for high intensity uses
Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.

Brushless DC
Brushless DC motors use a rotating permanent magnet in the rotor, and stationary electrical magnets on the motor housing. A motor controller converts DC to AC. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor.

Advantages
• Long life span
• Little or no maintenance
• High efficiency
Disadvantages
• High initial cost
• More complicated motor speed controllers.

Stepper DC
A stepper motor (or step motor) is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely, without any feedback mechanism. Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and several electromagnets on the stationary portion that surrounds the motor, called the stator. To make the motor shaft turn, first one stator is given power, which makes the rotor gear's teeth magnetically attracted to the stator's teeth. When the gear's teeth are thus aligned to the first stator, they are slightly offset from the next stator. So when the next stator is turned on and the first is turned off, the rotor gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a "step." In that way, the motor can be turned to a precise angle.

Advantages
• Very precise rotation
• Huge torque
Disadvantages
• High cost
• More complicated motor speed controllers.
• Slow speed

THEORY

According to Faraday's Law, if B is the flux density of a constant magnetic field and conductor is moved at this velocity V, and EMF is generated in the conductor such that:


E = B x V

If the conductor is part of a complete electrical circuit with a resistance R, then the EMF will produce a current in the conductor such that:
I = E/R = B x V/R

If a conductor of length L carrying a current I is placed into a magnetic field B, a magnetic force B is created such that:
F = BLI sin A

* where A is the angle between B and I, since the force F is perpendicular with both B and I

Obviously, to create an efficient DC motor, we need more than just a single current-carrying wire in a magnetic field. We also need to have a different structure that allows the motor to run continuously.

A viable motor consists of a coil in a static magnetic field of flux density B. The current is conducted through sliding contacts (commutator brushes) connected to the current source. The brushes ride on the ends of the coil wires, thus conducting current through the coil. Figure 2, 3, 4 and 5 illustrates the coil of the single-coil motor in a static magnetic field of flux density B at angular positions of 0 degrees, 90 degrees, 180 degrees, and 270 degrees.


At 0 degrees, the ends of the coils are contacting the "brushes", and current flows through the coil. The current will interact with the magnetic field B in the coil segment AB to generate a downward, force, as shown in the figure. The current I in coil segment CD will also interact with the magnetic field to create a upward force, F. The forces F generated by both coils are of the same magnitude but of opposite directions There are no force generated by the current in segment AC since the current is parallel with the magnetic field B.




The coil will rotate until it reaches the 90-degrees position, where ends of the brushes slide off the brush ends. Current will be cut off and no force will be generated by the coil. However, the inertia of the coil will keep it rotating in the forward direction, past the 90-degree position, and the brushes will again touch the other end of the other coils.



In the 180-degees position, current will again flow in the coil segments AB and CD, although now in opposite directions. The current flowing in coil segment AB will now generate an upward force, while the current in coil segment CD will produce a downward force. Again, there are no forces generated by coil segment AC since the current is parallel with the magnetic field B.




At the 270-degree position, again, no current flows in the coil, and the coil continues to rotate only due to its own inertia. Past the 270-degree position, the coil will return back to its original position, and will continue to rotate until the current source is turned off.






To increase its effectiveness, more coils are added to the motor, to increase the total force generated by the current in the coils, and create a continuous rotation.

AC AND DC MOTOR DIFFERENCES

DC MOTOR
DC motors are smaller in size compare to the AC motors. This making the DC motors more suitable to be used in small appliances that using small amount of power and where the speeds need to be controlled. The speeds of DC motors are easy to be control, simply by introducing a resistance. The levels of the speeds are based on the value of the resistances, the higher the resistance is, the slower the speed will be produced. DC motors also produced a stable and continuous current. The current produced is usually in small amount and not good in producing power over long lengths. DC motors have been proven that their can’t be used to generate electricity because the power was lost as the electric was transmitted. In producing the current, Brush DC motors use rings that conduct current while the Brushless DC motors use a switch, both are for forming the magnetic drive that power up the rotor.


AC MOTOR
AC power gets its name from the fact that it alternates in power. Alternate current or AC alternates between positive and negative and the number of times it does per minute is called cycles. Depending on the number of poles in the motor, the speed is constant. The amount of power given off by an AC motor is determined by the amount of power needed to operate the system. Today, there are many type of the AC motors each is used differently based on what type it is. The commonly used AC motors are single phase AC motors and polyphase AC motors. Single phase AC motors are known as general purpose motors. This is because they work well in many different situations. These AC motors work great for systems that are hard to start because they need a lot of power up front. Three phases AC motors or also called polyphase AC motors, are the AC motors that usually found in industrial environment. These motors also have high starting power built that transmit lower levels of overall power.

APPLICATIONS

Expert engineers were responsible for handling all applications for DC motors used in many different industries. Industrial applications use dc motors because the speed-torque relationship can be varied to almost any useful form for both dc motor and regeneration applications in either direction of rotation. Continuous operation of dc motors is commonly available over a speed range of 8:1. Infinite range (smooth control down to zero speed) for short durations or reduced load is also common.


Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting). This application is useful in electric battery powered road vehicles.




Other application is Dynamic braking (dc motor-generated energy is fed to a resistor grid) or regenerative braking (dc motor-generated energy is fed back into the dc motor supply) which can be obtained with dc motors on applications requiring quick stops, thus eliminating the need for, or reducing the size of, a mechanical brake.



Dc motors feature a speed, which can be controlled smoothly down to zero, immediately followed by acceleration in the opposite direction without power circuit switching. And dc motors respond quickly to changes in control signals due to the dc motor's high ratio of torque to inertia. This application is useful in low-inertia motors for leg carriage and cut-board cut-off machines.