The Importance Of Surge Pulse Repetition Rate
Most high voltage winding analyzers made by Electrom Instruments have 50Hz or 60Hz surge pulse generators. The surge pulse repetition rate typically varies from one brand of tester to another. The repetition rate for other testers can be as low as 1Hz or less. Surge tests with a high repetition rate are an advantage because they are superior at detecting weaknesses in insulation, as explained in greater detail below.
The science behind why surge testers with higher repetition rates detect more insulation weaknesses than those with lower repetition rates at a given voltage, has to do with ionization of gases in voids in the insulation. Microscopic voids exist in all insulation systems. A higher repetition rate creates a higher level of ionization in these gaseous voids.
For example, normal air is a very good insulator. Ionized air on the other hand, is not, it is a good conductor. Consequently, when the gas in a void is ionized, it takes a lower voltage drop across the void to cause an arc between turns adjacent to the void. When rotating machinery is running, the dielectric strength of the insulation surrounding the void must be strong enough to prevent an arc.
Ionization of a gas dissipates very rapidly when the fast-changing electrical field creating the ionization is removed, i.e. after the surge pulse has passed. Higher surge pulse rates therefore maintain a higher level of ionization. As insulation is weakened, or is damaged somehow, a high frequency surge tester will find more insulation weaknesses, or find them at a lower voltage. This is confirmed in lab tests, and also reported by many customers who have testers operating at different surge pules rates.
Electrom Instruments’ proprietary Quick Surge© technology uses high frequency surge pulses. It not only finds more insulation weaknesses, but automatically gets to the surge test target voltage quickly. This may seem like an easy task, but requires complex algorithms since every load or DUT reacts differently to a surge pulse. A different internal discharge voltage is required to reach the target voltage in each DUT. The result of Electrom’s technology is a very quick set of 3 surge tests for a 3-phase motor.
Compromises must sometimes be made. The higher the surge test voltage, the longer it takes to charge the discharge capacitor in the surge pulse generator, or the bigger and heavier the power supply has to be. Therefore, lower surge test pulse rates are sometimes used in exchange for a tester with lower weight, a smaller physical size and consequently lower cost. Portability, size and weight are obviously important.