What Is It Used For?
The surge comparison test finds shorts and insulation weaknesses in coils, windings, electric motors, generators, alternators and transformers. These faults are typically turn-to-turn, coil-to-coil, or phase-to-phase. Other problems found by the surge comparison test include wrong internal connections, wrong turn counts, and more for DC motors.
Weak turn-to-turn insulation leads to most winding failures. The surge comparison test is important for motor reliability and maintenance programs because it is the only test that can find weak insulation. For coil and motor manufactures, the surge comparison test is an important quality control tool and is especially powerful when combined with partial discharge measurements.
How Does A Surge Comparison Tests Work?
A set of fast rising pulses pass through the coil or motor. The operator sets the voltage of the surge test pulses based on standards and best practices. The test voltage ranges from the peak operating voltage of the motor to around 3.5 times the operating voltage of the motor. 2E+1000V where E is the operating RMS voltage of the motor is most common.
The surge pulses produce a decaying wave form by the oscilloscope channel in the tester. Each wave is compared to the wave from another coil or to the waves from the other motor phases. All wave forms display on the touch screen. (see picture)
The waves are nearly identical if the coils or windings are identical. If one has a fault or insulation weakness, the wave will have a different frequency from the others and appear separate. The iTIG II tester calculates the percentage wave differences (%WD) which is also called Error Area Ratio (%EAR).
For more details on surge comparison tests, see Surge Tests.
Pulse-to-Pulse Surge Test
The pulse-to-pulse surge test is used in applications with normal differences in the surge waves but when the tolerance for pass/fail is unknown. This is the case with some assembled motors and many concentric wound stators. It is also used when there are no other coils/phases to compare to.
For more information on pulse-to-pulse surge tests and pass/fail recommendations, see Surge Tests.
Are Surge Comparison Tests Destructive?
Surge comparison tests are not destructive. They are most often done at a voltage greater than the peak operating voltage of the motor but it is far below the design voltage of the insulation. Therefore, the energy involved in an arc is low. A good analogy is an arc from your finger to a door handle as a result of static electricity. The voltage involved is in the 12kV to 20kV range but the energy is low and thus harmless.
A low-energy arc in a winding from a surge test will not damage the insulation as long as the number of pulses used in the surge comparison test is limited and the test is done when over-voltage tests are recommended.
The Electrom iTIG II series of motor testers come with Surge Guard™ and Quick Surge™ technology to eliminate damage from arcs during a surge test. Click here for more details on surge tests and proper conditions for over-voltage testing.