DC Motor Application Considerations

Audible Noise: 

Audible noise is a concern in some types of motor applications. In many medical applications like infusion pumps or prosthetic devices, the patient can be very sensitive to the noise disturbance. Good design practice requires that the noise be limited as much as possible. In large machines, the combination of hundreds of DC motors and gears operating simultaneously can be very loud and distracting to the employees who have to work in close proximity to the machine.

Quality Components: 

Probably the best method of insuring low audible noise in motors is to specify quality components. Motors using cheap or poorly fitted bearings are more likely to be noisy. Poorly designed or loose fitting brush sets can contribute to audibly noisy commutation. Manufacturers of inexpensive, high volume motors cannot reasonably be expected to concern themselves with quiet operation beyond some minimum standard, and the use of such motors in applications where quiet operation is important should be considered carefully. The designer must consider whether low cost takes precedence over quiet operation in the priorities of the customer.

Bearing Choice: 

The use of ball bearings without preload is a potential source of audible noise. Where the specific application permits, ball bearings should be preloaded. This means that the balls will not be able to move axially in the race and cause the minute intermittent rattling that can sometime be associated with unpreloaded ball bearings. Smaller ball bearings can be sensitive to heavy shaft loads. They are easily damaged during press fitting added components and by short radial or axial overloads. Care should be taken not to exceed the shaft loading ranges specified in the datasheets. A damaged ball bearing can be a significant source of audible noise and can effect motor life. Sintered sleeve bearings are a very good choice when limiting audible noise if the application does not require the motor to endure significant continuous or intermittent changes in the shaft loading characteristics. The shaft of the motor actually rides on a thermodynamic film of lubrication and the reduced friction can limit audible noise. If the bearing is overloaded radially, however, this film breaks down and the shaft will grind the bearing down causing audible noise and reducing the operational lifetime of the motor.