Current analysis of DC motors can provide health assessment of the DC armature while the motor is running. Armature-related anomalies can include commutator bar defects, riser defects (very common) and shorted turns or coils in the armature circuit. The fundamental analysis of these anomalies starts with understanding that the bar, riser or winding with a fault passes under and out of the carbon brush face at a rate of the shaft speed times the number of brushes passed in a rotation. This causes a modulation of the DC armature current. A DC motor turning at 2,000 rpm with four brush holders has a brush passing frequency of 8,000 rpm, or 133.33 Hz. Trending of the brush passing frequency amplitude is highly recommended for early identification of developing armature circuit anomalies.
For a detailed description of online DC motor testing techniques, view: http://www.pdma.com/pdfs/Articles/Online_Fault_Analysis_of_DC_Motors.pdf
For more information on electric motor maintenance and reliability, visit the PdMA Corporation Web site at www.pdma.com.