Motor Service Life

Operating Point: In most applications, the torque and speed demands placed upon a DC motor determine its overall operational lifetime. As the torque requirements on the motor increase, the current through the armature increases proportionally, thus increasing the current density at the brush-commutator interface. High current densities promote electro-erosion of brush and commutator materials, a limiting factor in motor service life. In addition, high rotational speeds shorten motor service life by accelerating mechanical wear.

Although each application has its own specific requirements to be addressed, it is usually advisable to operate a DC motor with precious metal brushes and commutator continuously at no more than 1/3 of its rated stall torque. Motors with graphite on copper commutation systems should be run continuously at no more than 1/2 of the motor's rated stall torque. These recommendations attempt to maximize motor service life. Some applications may not require the maximum lifetime that the motor has to offer.

Rotor Inductance: One of the factors limiting brush and commutator life is the inductance of the motor armature. During commutation, when current flows through a particular coil winding there is storage of energy in the form of a magnetic field. When the motor commutates and the current flow is switched to another winding, the magnetic field collapses and the resulting discharge of energy causes an arc between the commutator and brush. This arcing accelerates electro-erosion and decreases motor life. One could, theoretically, reduce the armature inductance of the motor windings by decreasing the number of turns in each armature segment. This lowers the torque constant of the motor, however, which increases the motor current for a given torque and, therefore, increases the current density at the brush-commutator interface. This is not recommended. To reduce the affect of inductance and arcing on motor lifetime a capacitor ring is being mounted to the commutator. The ring provides the equivalent effect of each winding connected in parallel with a small capacitor and resistor. The collapse of the magnetic field during commutation then serves to charge the capacitor rather than creating an arc between brush and commutator. The stored energy is released and dissipated back into the next coil phase in the commutation sequence. This technique, while slightly increasing the electrical time constant of the motor, dramatically increases motor service life.