May 02, 2005
5 Motor Testing and CBM
In order to fully understand how different technologies detect problems within the electric motor insulation system, we must understand how these technologies work. Over the next few weeks, we will discuss how electrical insulation works and what is occurring during equipment operation, failure and how the different technologies detect this failure. In effect, the strengths and weaknesses of each technology used to analyze electric motors.
VP, Electrical Reliability Group
T-Solutions, Inc.
Insulation Analysis: Inter-Turn and Ground Wall
In order to fully understand how different technologies detect problems within the electric motor insulation system, we must understand how these technologies work. Over the next few weeks, we will discuss how electrical insulation works and what is occurring during equipment operation, failure and how the different technologies detect this failure. In effect, the strengths and weaknesses of each technology used to analyze electric motors.
What few realize is that most electrical insulation testing is performed at the atomic level.
The electrical insulation circuit is modeled as a series of parallel RC (resistance and capacitance) circuits between conductors and conductors and ground. As changes occur in the insulation system, the values of R and C change. The values of the insulation in each phase are the sum of the turn to turn and coil to coil RC values of each phase. Insulation to ground values are the sum of the insulation between conductors and ground for the complete circuit.
The capacitance of the electrical insulation is a direct function of the generation of dipoles within the insulation system. As a field is generated across an atom, or molecule, of a dielectric (electrical insulation), it will polarize, meaning that the ‘electron orbit’ of the atom will shift slightly, making one side of the atom more positive and one more negative.
As current passes through conductors near electrical insulation, the insulation reacts by polarizing the atoms (dipoles) within the insulation. As the dipoles polarize, there is less leakage (capacitance) between the conductors and ground. This also occurs in the insulation system between conductors when there is a difference in potential. In a good insulation system, the polarization of the insulation system occurs in a larger number of atoms. Once the potential is removed, the atoms return to their original state.
The effect can be observed when a DC potential, such as an insulation to ground tester, is applied across the insulation system. The test result (Meg-Ohms displayed) increases as the DC voltage is applied, in a good system. If you remove the potential, energy remains stored in the insulation system until it is released through gradual leakage to ground or touching the conductors to ground. If you were to touch the conductors following charging with an insulation tester, you might get a shock!
Think of the atoms of the material of the insulation system as rubber bands. Applying the DC potential across the bands causes them to stretch. The energy is held until it is let go, such as a path is allowed, and the energy returns to its rested state.
To be continued….
