Carbon brushes are electromechanical conducting devices that connect to moving parts to provide an electric current. They are typically used in motors, generators and alternators. Many carbon brush uses exist, but the three basic uses are in household applications that run on alternating current (AC), automotive applications that run on direct current (DC) and industrial applications that run on both AC and DC.
Household
Carbon brushes are a common component in household appliances such as power tools, gardening equipment and office equipment. Vacuum cleaner turbines, hair and hand dryers, and washing and drying machines are also common applications of this type of carbon brush, which is generally much smaller than its industrial or automotive counterparts. Micro-carbon brushes, for example, are precision parts designed for toys, electric razors, and audio and video equipment. In such small dimensions, carbon brushes are often placed inside tiny battery-operated mini-motors designed to transmit power silently and with the least amount of friction to moving parts such as wheels and razors.
Automotive
Cars generally use small and auxiliary carbon brushes for DC motors, starters and alternators. Alternators are devices that convert mechanical energy to electrical energy. They consist of a wire-coiled rotor that produces a magnetic field when a current runs through it. Automotive carbon brushes are used to supply the current to the wire coil in the alternator. Automotive carbon brushes can be found in most passenger vehicles, motorcycles, trucks and oil- and diesel-fueled cars. Even smaller, specially designed carbon brushes are used in steering wheels and contacts for airbags.
Industrial
Industrial carbon brushes are used on large commutator machines, in which the current between the rotor and external circuit is periodically reversed. These include AC and DC motors, slipring rotors, traction motors, turbogenerators, windmills, hydro power stations and steel, cement and paper mills. Industrial carbon brushes also can be used to power battery-driven cars, cable cars and trolley buses, and they are often used in construction vehicles such as forklift trucks and cranes. Brushes connect the generator to the rotor through the commutator, which has copper segments, or lands. The brushes bear the brunt of wear and tear between the rotating rotor and the non-moving generator while conducting electricity between them. Industrial brushes are specially designed to withstand extreme temperatures.
What is the purpose of carbon brushes in an electric motor
I just took apart our washing machine to replace the carbon brushes.
Was wondering why the machine stopped working when the brushes were worn down?
Also if the engine makes a wuring noise after you put it together, does that mean that the brushes have been put in the wrong way?
The brushes transfer the electricity from outside the motor to the spinning winding in the center of the motor. They undergo quite a lot of friction, and after a while wear out. Carbon is used as it is a reasonable conductor, and is soft enough to wear down instead of wearing down the "comutator" - the ring the brushes press against. Brushes are designed to be replaceable in large motors because of this.
When brushes are first put into a motor they won't be the exact same shape and size as the commutator (which may have had a small amount of wear) and thus won't make a perfect smooth contact. With time the brushes will wear to fit the commutator perfectly. The noise you hear is probably the brushes undergoing this initial shaping wear and will stop soon enough.
The purpose of carbon brushes is to reduce wear on the commutator, compared with what metal brushes would do. In many cases, carbon brushes can be made long enough that they will not wear out while a device would otherwise be usable; when they do wear out, they can be readily replaced. If one were to use metal brushes instead, then by the time the motor had been used enough that carbon brushes would have worn out, the commutator would probably be just about shot.
The situation is somewhat analogous to break pads versus rotors. One could construct brakes with steel pads and rotors, but rotors would wear out very quickly.
