Motor Brush Facts and Maintenance
Gregory Mecomber, IDCON INC
What is a Motor Brush?
A motor brush is not a brush at all in the traditional sense. It is actually a carbon or graphite cube, commonly held in place by a spring, that acts as a conductor between the electrified stationary and rotating parts of a motor. The whole brush assembly is made up of a carbon block, one or more shunts, a spring and a holder. The carbon blocks are easily replaceable and are therefore intended as a wear part to prevent damage to more costly motor components. More than one motor brush is usual.
AC motor speed is determined by the phase frequency and ultimately the variable frequency drive. Most AC induction motors do not use brushes, however, there are a few styles such as repulsion, synchronous and universal motors that do.
Motor brushes come in various sizes and can differ between manufacturers. Most will have a size chart available and you simply have to measure the thickness, width and length in millimeters and cross reference those measurements with the size chart to find the correct part number.
Some symptoms of poor brush condition can be not getting the speed and torque that the motor did when it was new, burning smell and frequent large sparks; small infrequency sparks can be expected and can sometimes be visible through the air intake vents. If you witness your brushes sparking excessively or any other of these symptoms, the best solution is to remove the brush, clean any carbon or copper residue and replace with a new brush.
Motor brushes will have to be replaced periodically. Inspect the brushes for signs of cracking, breaking, burning, discoloration of the spring. Spring discoloration is often a sign of overheating or that the shunt is damaged.
Check for signs of copper dust – It could result in the following issues and actual accelerate further wear on the motor components. Grooving is a smooth slot in your commutator which can also be a result of using an improper brush type. Threading is the creation of fine lines upon the commutator most likely from copper dust or particles stuck within the carbon block.
Copper drag is a buildup of copper within the grooves of the commutator and can result in shorted segments and loss of performance. Flash over is caused by excessive copper drag to the point that it results in shorting between the brushes themselves.
Brushless DC motors are the most common type of motor used in automation and motion control systems. Brushless motors use permanent magnets instead of brushes to operate. Hence many will have a sticker warning the user to “Do Not Strike” since that is the best way to demagnetize a natural magnet.