<P> Brushes are usually made of graphite or carbon, sometimes with added dispersed copper to improve conductivity . In use, the soft brush material wears to fit the diameter of the commutator, and continues to wear . A brush holder has a spring to maintain pressure on the brush as it shortens . For brushes intended to carry more than an ampere or two, a flying lead will be molded into the brush and connected to the motor terminals . Very small brushes may rely on sliding contact with a metal brush holder to carry current into the brush, or may rely on a contact spring pressing on the end of the brush . The brushes in very small, short - lived motors, such as are used in toys, may be made of a folded strip of metal that contacts the commutator . </P> <P> Typical brushless DC motors use one or more permanent magnets in the rotor and electromagnets on the motor housing for the stator . A motor controller converts DC to AC . This design is mechanically simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor . The motor controller can sense the rotor's position via Hall effect sensors or similar devices and can precisely control the timing, phase, etc., of the current in the rotor coils to optimize torque, conserve power, regulate speed, and even apply some braking . Advantages of brushless motors include long life span, little or no maintenance, and high efficiency . Disadvantages include high initial cost, and more complicated motor speed controllers . Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors . </P> <P> Other types of DC motors require no commutation . </P> <Ul> <Li> Homopolar motor--A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field . The name homopolar refers to the absence of polarity change . Homopolar motors necessarily have a single - turn coil, which limits them to very low voltages . This has restricted the practical application of this type of motor . </Li> <Li> Ball bearing motor--A ball bearing motor is an unusual electric motor that consists of two ball bearing - type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply . An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel . The direction of rotation is determined by the initial spin which is usually required to get it going . </Li> </Ul>

Explain and perform the testing of dc motor and their wiring connection