DC Motor Application Considerations II

Vibration: Rotor vibration can be a significant source of audible noise. Vibration and noise increases with speed. Even a slight imbalance in the rotor can cause major vibrations at speeds of 10,000 rpm.

Brush Options: Copper-graphite type brushes tend to be both audibly and electrically noisier than precious metal systems. Graphite based brushes are capable of withstanding considerably higher current densities, however, and they are often required in an application for that reason. Where a choice is available for a specific application and audible noise is important, precious metal brushes are the better choice.

Electromagnetic Interference (EI) DC motors are a source of electrical as well as audible noise. EMI (electro-magnetic interference) can be radiated by motor terminals and lead wires and may cause problems with other components in the vicinity of the motor. It is also possible for spikes to be coupled onto data lines or output lines from encoders. The result can be false data or encoder information. There are a number of methods that can be used to minimize EMI in motor applications. Like many other considerations in DC motor applications, each has its advantages and disadvantages and must be evaluated within the context of the application.

Motor Commutation: The most common source of EMI problems is the commutation of motors. At each commutation point, when the brush breaks contact with a commutator segment, the energy stored in the motor winding as a magnetic field causes an arc or voltage spike between the brush and
the commutator segment. This occurs not only during normal commutation but also in situations where the brushes "bounce" on the rotating commutator. Coreless DC motors are typically less electrically noisy than iron core DC motors because of the lower armature inductance which in turn reduces the level of the arc energy. This arcing can be further mitigated with the addition of a capacitor ring, which serves to dissipate the energy through a capacitor and resistor series back into the motor coil.