Time to Failure (Continued)
Motor Diagnostics and Quantum Mechanics – Part 15 DC Windings
Howard W Penrose, Ph.D.
ALL-TEST Pro, A Division of BJM Corp
Note: A detailed paper covering this lecture series can be found on ReliabilityWeb: http://www.reliabilityweb.com/art04/mca_concept.htm
As most may have realized, quite a bit of time has been spent on AC induction motors. For some reason, most research on testing has been focused on this subject and little on DC motor analysis. However, you are in luck, as we have spent time researching DC motor analysis.
For winding analysis, there are two components that we will focus on for DC machines. The first is the armature and the other wound component are the fields (series and shunt) and interpoles. So, we will break them down, covering first the fields, then the armature.
Series fields, labeled S1 and S2, are connected in-line with the armature and are made with a few turns of large wires. The good news is that the series fields rarely fail, nor do the interpoles. The shunt coils, on the other hand, generate a great amount of heat (high resistance and resulting I2R losses). The main cause of failure in fields and interpoles is thermal degradation, as they carry dominant DC voltage. However, there is a component of AC that is carried on the DC voltage that results from SCR switching, called the form factor. This is the remaining AC ripple that results when the AC voltage is chopped with the SCR circuit of a DC drive. Therefore, winding contamination will have some limited impact with capacitive faults. A majority of shunt coil faults are the result of poor ventilation in which the coils degrade from the near center of the coil out.
When trending the condition of DC motor shunt fields, the Fi and I/F are compared from test to test with a tolerance of +/- 1 digit change in each over time. The MCA method of test allows for the very early detection of faults that are detected later as a loss of speed and torque control in the DC motor. If possible, splitting the coils into two groups and comparing the Fi and I/F will allow for immediate fault detection.
The armature circuit is actually an AC circuit as current flows in one direction then the other as the commutator bars pass the brushes. Therefore, faults such as contamination will have a greater impact, as in AC motors. In the case of a DC motor, brush wear generates carbon dust, which is a dielectric. When the carbon dust accumulates within the armature, breakdown will occur in the same way as contamination in an AC motor. Therefore, there are several methods of reviewing the condition of the armature:
• The first method is used for trending and performing an evaluation of the condition of the insulation system. Testing is performed through the armature circuit twice, and the results are compared. Due to the change in circuit capacitance from carbon contamination between conductors, or on the surface of the coils, carbon contamination generates non-repeatable results in impedance, Fi and I/F. If caught early enough, low pressure, dry air, can be used to clean out the armature and correct the problem.
• If a commutator or armature short is suspected, all but two brushes 90 degrees apart from each other should be lifted. Using circuit impedance, line up the edge of a brush along the edge of a commutator bar and take a reading. Move through each brush and record the circuit impedance for the circumference of the armature. The impedance results should follow the same pattern, or a repeating pattern. A drop in impedance indicates a winding fault.
MCA is capable of evaluating the condition of DC motors. Early detection of a fault will allow for trending of the insulation failure over time.