Inrush Current Measurement

Worst case inrush current occurs at high line, when the customer energizes the product at peak input line voltage. This occurs at the 90° and 270° points along the input line waveform (assuming a sinusoidal input wave). The only way to test this reliably is either to make and time the connection electronically, or to synthesize a waveform that starts at zero and steps to the peak high line condition. The latter method affords the most control. The amplifier used must satisfy IEC Standard 4-11 which states that a generator used for the purpose of inrush current testing must not be taxed to more than 75% of it’s current rating at any time during the test. Generators capable of 500 Amperes or more output current may be taxed to a higher percentage of their maximum. In any case, the output voltage loop rise time of the generator (as it is programmed to transition from zero volts to 375 volts) must be less than 5 microseconds. The output voltage will rise from zero to its maximum magnitude within 5 microseconds and trace out a sinusoidal output from there. The power supplies input current is measured and recorded during this time. Typically as digital storage oscilloscope is used to measure the voltage drop across a current shunt. The scope is triggered from the low-level oscillator signal driving the output waveform.

Pacific Power Source Corporation offers a general-purpose ac power source that can be programmed with the necessary waveform for inrush current testing and measurement. This line of power sources is called the TMX Series Power Sources. Some special considerations must be taken into account when measuring inrush current with these power sources.