Surge Arresters – Protection

As the rated voltage and rated power of the electric motors increase, so does its surge impedance. Common sources of voltage surges are line discharges, on-off and on-off maneuvers, and high current loads on the same circuit. The level of insulation of rotating machines is much lower than that of other types of electrical equipment.

The level for a 5kV transformer immersed in oil, for example, is 60kV, whereas for a nominal 4kV motor it is of the order of 13kV. This level will be equal to the peak value of the applied voltage wave during one minute in the applied voltage test, that is, twice the nominal voltage + 1kV.

An alternating current machine winding can be considered as a small transmission line, with reflection and wave refraction at the terminals and surge impedance. The greater promotion from the point of view of the impulse tension usually occurs in the first turns of the winding, because the outbreak is being diminished along the winding. The electric motors have surge impedance in the range of 150 to 1500 ohms approximately.