How a Surge Tester Works
The Electrom winding analyzer, or “surge tester”, is an advanced high voltage source- and measurement unit constituted by a programmable pulse train and advanced pattern analysis software. The instrument takes normal 115 or 220V AC power and transforms it to the higher voltage required for the surge test. The higher voltage is rectified into a DC voltage which charges up a large discharge capacitor. The other side of the discharge capacitor is connected to the test load, the DUT, through the surge tester’s output leads. The DUT is also connected to ground in order to have a complete electrical circuit. In the circuit there are switches that are open during the charging phase of the discharge capacitor. These switches are IGBTs or SCRs. The switches close some time after the discharge capacitor is at the desired voltage. The closing speed is very high resulting in a pulse with a rise time of approximately 100nsec.
The easiest way to think about what happens next is to envision the switches opening again, trapping the pulse energy in a tank circuit between the tester’s capacitors and the inductive DUT. An oscilloscope is connected to the tank circuit to capture the wave created by the surge pulse. As the voltage oscillates in the tank circuit it decays to zero volts because of impedance in the circuit. The scope sends the wave to a computer for in-the-loop processing and the wave is immediately displayed on a monitor. The whole process of circuitry response, wave decay and processing is complete within milli seconds, upon which, successive pulses may be generated and analyzed in quick succession. This is a very simplified explanation of the how the complex hardware and software of the Electrom surge tester works.
A lead switching matrix in the surge tester connects the output leads to provide a complete electrical circuit. For a 3-phase stator this is usually between two phases. One phase is energized, the other phase, or other two phases are grounded. The result of the test is displayed on the screen. Next, one of the other phases is energized, and finally, the third phase is energized which results in 3 unique waves, or signatures, on the surge test screen. Each step in the sequence can be done manually one at a time, or by a push of a button activating the matrix, all in an automatic sequence.
The 3 surge waves are compared and a percent wave difference (%WD also referred to as %EAR by some) is calculated for each wave pair by the surge tester. The WD or EAR is calculated somewhat differently by different surge tester brands, but the result is similar. The general approach is to calculate the difference between many points along a wave pair (points on the two waves with the same x-axis position), add up all the differences and divide by an average. Electrom Instruments uses a root mean square (RMS) equation to calculate the %WD. If the %WD is too high, the surge test failed. For information on failure limits see Pass/Fail Guidelines.