Drive Profiles for Motors

Drive Profiles: Operating conditions other than torque and speed also affect service life. The application may require frequent starting and stopping or reversals of direction. Both situations result in periods of high current density and a resulting shortening of service life. A similar effect is seen in applications where pulse width modulated (PWM) drives are used. If the PWM frequency is too low, the motor is constantly accelerating and decelerating with an accompanying increase in current density. As a general rule, PWM frequencies of 20 kHz or higher are recommended for ironless core motors.

Environmental Considerations: Environmental conditions can have a profound effect on motor service life. One good example is the rapid drying and wear of graphite-based brushes in a vacuum or very dry environment. Very warm and dry conditions also hasten the breakdown of bearing and commutator lubricants. The ambient temperature has a cumulative effect on the motor's operational temperature and can lower performance by limiting the operational temperature range of the motor. External cooling by contact, air or forced air can produce significant gains in motor performance. At the opposite extreme, very cold conditions increase the viscosity of lubricants and cause the motor to run at a higher current.

Shaft Loading: Shock loads and vibration contribute to the tendency of the brushes to "bounce" on the commutator, thus causing arcing and accelerated electro-erosion. Shock loads and vibration also accelerate bearing wear. Excessive axial or radial shaft loads decrease the life of bearings, sometimes significantly. In continuous duty applications with low radial shaft loads, sintered bearings are the inexpensive choice. For increased radial loads, ceramic bearings are available. Ball bearings are typically specified when the application calls for higher radial and axial shaft loads.