STANDARD TEST - Capacitance-to-ground (CTG)
What does it tell you?
The capacitance-to-ground (CTG) measurement is indicative of the cleanliness of the windings and cables. As dirt and contaminants build up on windings and cables, CTG values increase. An increasing trend showing rising CTG values indicates that the motor needs to be cleaned.
Why is this important?
Any two conducting materials called plates, separated from each other by a dielectric material form a capacitor. Dielectric material is anything that is "unable to conduct direct electric current". A cable or motor winding surrounded by insulation provides one conductor and the dielectric material. The stator core and motor casing iron form the second plate.
Normally, when the outside of the insulation is clean and dry, it is not a good conductor. When dirt, moisture and other contaminates begin to cover the stator windings inside the motor, they cause the outer insulation surface areas to become conductive. Since this surface is in contact with the ground, it allows an AC current path to ground. Cables in the power circuit are also subjected to the same effect, when moisture penetrates the outer casing. Over time, CTG values steadily increasing indicate an accumulation of dirt and that cleaning is necessary. This can be correlated with decreasing RTG Values.
Dirt and contamination also reduce a motor's ability to dissipate the heat generated by its operation, resulting in premature aging. A general rule of thumb is that motor life decrease by 50% for every 10 degrees C increase in operating temperature above the design temperature of the insulation system. This holds true with the motor operating at or above a 75% load. Heat raises the resistance of conductor materials and breaks down the insulation, providing paths for unwanted current to flow to ground. If capacitance is higher than normal, a low RTG reading is an indication that such a path already exists.
