Online Fault Analysis Of DC Motors

ONLINE FAULT ANALYSIS OF DC MOTORS David L. McKinnon, PdMA Corporation, Member, IEEE Abstract-Over the last 20 years, Current Signature II. DISCUSSION Analysis (CSA) has become an established tool for online fault analysis of AC Induction motors. Presently, very little A. Turn-to-turn Short research has been performed using current signature analysis on DC motors. This paper is a brief introduction to online fault diagnosis of DC motors using current signature Many turn-to-turn or commutator bar-to-bar faults analysis. occur from carbon dust build up. Carbon dust from the brushes builds up on the commutator creating a short Index Terms-Current Signature Analysis, DC Motors, between commutator bars. To simulate the worst case of Armature, Field, Motor Testing, and Measurement. this fault condition, two wires that terminated on adjacent commutator bars were shorted together on the I. INTRODUCTION armature of the DC motor. The motor was then run and a current signature analysis in both the time and frequency This research initiative was undertaken to further domains was performed. develop online fault detection of DC motors using Figures 1 and 2 show a comparison of the current current signature analysis in both the time and frequency signatures in the time domain of a no fault condition to a domains. These faults include differential current, turn-to-turn short (faulted condition). In the no fault shorted armature windings, shorted field windings, and condition, there is no modulation of the 120 Hz carrier off magnetic neutral plane brush positions. frequency. In the fault condition shown in Figure 2, the To detect the various faults in DC motors, we must waveforms have a modulation of the 120 Hz carrier of develop a methodology to properly differentiate normal approximately 17 Hz. operating conditions from those of fault operating conditions. The first step is to establish a baseline of normal operating conditions. Once a baseline of normal operating conditions is established, a method of differentiating fault operating characteristics from baseline characteristics must be developed. The primary differentiating methodology used in our study was a visual comparison of fault operating conditions to the baseline condition. For our study, a deterministic fault condition is considered one in which there is an obvious visual or numerical change in either or both the time or frequency domain. Visual changes may include variations in the waveforms in the time domain or the number of peaks, their amplitude, or their location in the frequency domain. For our purposes, numerically deterministic changes are those that exceed the measurement error sensitivity of the equipment in use by more than a specified amount beyond the maximum Figure 1 - No fault - full load, full speed, and brushes at zero magnetic measurement error. For example, if the error sensitivity neutral axis. of the equipment is 1% of reading, and the specified change is 1%, the minimum fault differential required would be 2%.
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Figure 4 - Turn-to-turn short - full load, full speed, and brushes at zero. Figure 2 - Turn-to-turn short - full load, full speed, and brushes at zero. This fault condition is further noticed in the frequency B. Coil Group Short spectrums shown in Figures 3 and 4. The frequency spectrum shown in Figure 3 is the no fault condition. Figure 5 shows a fault operating condition in which Figure 4 shows the frequency spectrum of the turn-to- an entire coil group is shorted. Notice this increase in turn fault condition. Notice the dramatic increase in the modulation as compared to the turn-to-turn short shown harmonics throughout the spectrum. in Figure 2.

Figure 3 - No fault - full load, full speed, and brushes at zero. Figure 5 - Coil group short - full load, full speed, and brushes at zero.
Figure 6 shows the frequency domain of the coil group fault operating condition. There is a significant increase in the harmonics throughout the spectrum.

Figure 6 - Coil group short - full load, full speed, and brushes at zero. Figure 8 - Coil to coil short - f... [download for more]