What If The Recommend Surge Test Voltage Cannot Be Reached

The surge tester is rated based on the maximum voltage it can generate internally (12kV for example). This rated voltage is not the highest voltage generated in every device under test (DUT). The voltage that results in the DUT from any surge pulse voltage applied by the surge tester depends on the characteristics of the DUT. For example: 12kV may be generated by the surge tester, but 9kV may be the voltage reached in a large motor.
This means that if a motor is “too large” for the tester, the desired test voltage will not be reached. It also means that if the inductance in the DUT is “too low”, such as in a Surge Test of Large Motorsingle low inductance form coil, the desired surge test voltage will not be reached. Click here for information on low inductance surge tests of coils and DC motor armatures.

The energy required to reach a given surge test voltage in a DUT increases with several factors including the line to line operating voltage, HP or kW, frame size and number of poles. The capacitance of a motor plays a significant part in DUT peak voltage as motor capacitance must be overcome by the energy available from the surge tester. This is one reason why frame size matters. Likewise, when motors are tested from a motor control center, the capacitance in the power cables is added and the voltage reached in the motor will be less.

The energy available from the surge tester is:   
energyavail-formula

Surge Test of Large Vertical MotorIf a surge tester cannot reach the desired surge voltage in a DUT, options for getting to a higher voltage are to use a tester with higher charge capacitance, higher max output voltage or both. A power pack can be added to any iTIG II surge tester taking the max voltage to 18kV, 24kV, or 30kV. Power packs have their own output leads but are controlled by the surge tester. Electrom Power Packs can be added at any time, they are calibrated independently and can be used with multiple iTIG II surge testers.

Surge Test of Large Generator RotorAnother alternative for getting to a higher voltage in a large DUT is to break the DUT down into individual phases and even individual groups if this is possible. The smaller the section that is energized by the surge tester, the higher the voltage will be. Portions of the DUT that are not being energized should be shorted to ground in this case.

The final alternative is of course to test to whatever voltage the surge tester can get to. Although one will not know the condition of the motor at the voltage recommended in the standard, useful data will be recorded and hopefully at a voltage higher than the peak voltage seen by the motor while operating (Peak = RMS voltage x 1.41). This is important if one wants to find weak insulation before it degrades to the point where turn to turn failures occur at the operating voltage of the motor.

For guidelines on what size motors Electrom Instruments’ surge testers can handle, please contact us.