Low Core Flux Test (EL-CID)
The conventional method for testing for imperfections in the core insulation of motors and generators has been the rated flux test. This test requires high power levels for the excitation winding to induce rated flux in the core area behind the winding slots. The alternative low-flux test described in this section has been performed successfully across the world. Its main advantage is that it requires much smaller power supply for the excitation winding. Only 3 to 4 percent of rated flux is induced in the core. In reality, the power supply can be obtained from 120-V ac wall socket source. Also, the time required to perform this test is much shorter.
The electromagnetic core imperfection detector (EL-CID) identifies faulty core insula- tion. It is based on the fact that eddy currents will flow through failed or significantly aged core insulation even if the flux is a few percent of the rated flux. A Chattock coil (or Maxwell’s worm) is used to obtain a voltage signal proportional to the eddy current flow- ing between the laminations. The solenoid coil is wound in a U shape.
Figure 16.12 illustrates how the coil is placed to bridge the two core teeth. The fault cur- rent IF is approximately proportional to the line integral of alternating magnetic field along its length l (Ampere’s law). Thus, if the effects of the field in the core are ignored, the volt- age output in the Chattock coil is proportional to the eddy current flowing in the area encompassed by the coil (the two teeth and the core behind them).
The excitation winding that generates the test flux in the core induces an additional volt- age across the coil due to the circumferential magnetic field. A signal processor receives the output voltage from the Chattock coil. It eliminates the portion generated by the excitation winding and gives a voltage proportional to the eddy current (Fig. 16.13).
The output milliamperes of the signal processing unit are proportional to the voltage in the Chattock coil generated by axial eddy currents. High milliampere readings are normally caused by faulty insulation in the core or interlamination shorts at the core surfaces. A read- ing higher than 100 mA indicates significant core plate shorting.
The Chattock coil is moved along the teeth of the core, and the current readings are recorded. Areas where the readings exceed 100 mA should be marked with a nonconduc- tive substance and examined for defects.
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