CTG is indicative of conductive moisture and dirt in and around the stator insulation system. As contaminants build up, the apparent conductive surface area of winding insulation of the motor grows, which is comparable to increasing the size of one plate of a capacitor as illustrated in Figure 8-21.
The value of the variable “plate” can be measured and trended over time as the change in capacitance-to-ground.
By using the ‘Relative Comparison’ analysis technique you would take the initial (baseline) readings, stored for later use, and compare them to any future values recorded. When a significant rise occurs it is seen in the trend data. An investigation into the cause may find:
Internal motor contamination
A problem with moisture infiltration into the motor
A problem with the circuit cables leading to the motor.
Capacitance-to-Ground CTG - Is a natural characteristic of a motor circuit which starts to increase when the motor circuit is affected by its environment, due to moisture ingress and conductive contaminant(s) accumulating on the surface of insulation materials providing a path to ground for leakage current. Increased capacitance due to the above plus insulation system deterioration are the two conditions needed for resistance-to-ground reading deterioration
Capacitance in the Conductor Path – Capacitance in the conductor path is performed in analysis of 3 phase motor winding condition by comparison of impedance and inductance readings with the rotor in two different positions 90º apart.
Readings changing in parallel and rotating in sequence with each other indicate no serious problem, but readings that “cross” and aren’t changing in parallel with rotor position and other readings may indicate a fault. Capacitance of the cables to the motor may be the problem especially in long runs to the motor. The following case study illustrates this point.
Capacitance-to-Ground Case Study #1:– What’s wrong according to the data in Figure 8-22? It would appear that Motors #2, #3 and #4 are grounded and may be contaminated as well with something providing a path to ground over their insulation systems. It is also possible that the Capacitance-to ground (CTG) reading may be affected by cable length if the motors are at progressively greater distance from the motor control center where the readings were taken.
Figure 8-22 - MCrA Tests of 4 Identical Motors
The next case study shows how it is possible to misinterpret data.
Capacitance-to- Ground Case Study #2 - A PdM Technician in a Hot Rolling Mill at a steel company noticed that capacitance-to-ground of some motors was higher than others. He assumed that since motors that had higher readings were at the “dirty” end of the run-out table that the problem was due to contamination. He had the worst ones pulled and cleaned. Capacitance did not change significantly. Further analysis showed that the motors with the longest cable runs had the highest capacitance-to-ground readings. Plot of all 145 motors showed trend (upward) consistent with the fact that the longer the cable run from MCC to motor, the higher the capacitance-to-ground. The analyst took this into account for later decisions.
Resistance-to-Ground – The final test completed in the suite of tests conducted on both AC and DC motors, which includes DA and PI tests is covered in Chapter 6 (Volume 2).