Pulse to Pulse Surge Tests When Windings Are Different; assembled motors, concentric windings etc.

Motor windings and coils are not always physically identical. An example is concentric wound coils where the size of the coils are different. Another example is when coils are connected such that the stator is not symmetrical from an inductance point of view.

Assembled motors (with the rotor installed) may have stators that are lap wound and electrically symmetrical. However, the rotor may cause the inductance in the stator phases to be different because of the transformer action between the stator and rotor.

To address these situations the Pulse to Pulse Surge Test was designed.

Summary of what the P-P surge test is used for

  1. Concentric wound stators with a %WD above expectations
  2. Assembled motors (rotor installed) with rotor influence on the stator inductance that creates different surge test waves in the 3 phases
  3. Single phase motors
  4. Any test where there is nothing to compare the standard surge test to
  5. Any time the surge test results are questionable or in the “gray zone”
  6. To find the Inception Voltage in Partial Discharge (PD) Measurments

How the P-P surge test works

Good pulse to pulse surge test.

Good pulse to pulse surge test.

Instead of rapidly going to the surge test design voltage and recording the surge wave after the voltage is achieved, the P-P surge test raises the surge voltage in small steps up to the design test voltage. At each step a surge wave is recorded, and the difference between this wave and the wave from the previous step is calculated. This means that the winding under test is compared to itself as the surge test voltage is raised. Since the comparison is to itself, it does not matter that the phases are different, or that there is rotor influence in an assembled motor. Hence for an assembled motor the rotor does not have to be turned during the test.

When the Pulse to Pulse surge test is completed the P-P %WD numbers for all voltage steps are stored, and the highest P-P %WD displayed. With the iTIG III surge tester the results from all the voltage steps are available in a bar graph.

If there is an arc or flash over at some voltage step, the P-P %WD will be significant for that step because the wave frequency is different from the previous step. For information on failure limits see Pass/Fail Guidelines.

Failed pulse to pulse surge test.

Failed pulse to pulse surge test.

Since the P-P surge test is looking for a change during the voltage ramp, it may not see hard shorts in the windings if the insulation around the hard or welded short is stable and strong. In such a case there is no change or arc during the voltage ramp. Accurate micro ohm winding resistance measurements or inductance measurements may be used to find such faults.

The other alternative for assembled 3-phase motors is to turn the rotor between each surge test so that all 3 waves line up. If they cannot be made to line up properly, there is a failure. Turning the rotor may not always be possible or practical making the P-P surge test a valuable tool.

Bottom line: The Pulse to Pulse surge test should be used in combination with accurate winding resistance measurements any time it is uncertain if the standard surge test is a pass or fail.