Measuring Inrush current with TMX ac Power Sources

This document describes inrush current measurement considerations when using the 125TMX or 140TMX ac power sources.

Configuration:

These power sources use high voltage linear amplifiers to generate the ac output waveform. Each power source is equipped with two such amplifiers. There are two operating modes possible, designated 1 phase and 2 phase (traditionally referred to as “single phase” and “two phase”). In single phase operation, the two amplifiers are wired in parallel while two phase mode wires the amplifiers, via a relay contactor, in series. As a result, single phase mode offers double current capacity and two phase mode offers double voltage capacity.

Limitations:

Each amplifier includes a dynamic current limiting circuit which protects the output pass elements from excessive stress. This circuit allows for high instantaneous currents in the neighborhood of 60 Amps while reducing the output current into sustained overloads, exponentially to a final value of around 20 Amps. In addition to this time dependency, the current limit circuit also senses the voltage stress on the amplifier and “folds back” the current limit value accordingly. The above numbers refer to the circuits operation in the absence of foldback current limiting. Foldback current limiting will occur any time the amplifiers are operating in the second and third quadrants, that is, when they are passing positive current while maintaining a negative output voltage, or passing negative current while maintaining a positive voltage. In the extreme case, when the output voltage is close to either supply rail, the foldback current limit value can be as low as 2 Amps.

In two phase mode, miniscule imbalances in the current limit setting of the two amplifiers results in one of the amplifiers being driven fully into this foldback region, severely curtailing short circuit and inrush current capacity. Any fix for this limitation would involve synchronizing the current limiting activity of the two amplifiers, an easy job, but not implemented in the 125 or 140 TMX power sources.

The work-around for this “bug” is to simply avoid two phase mode when current overloads are expected, such as when measuring a products inrush current.

While single phase mode limits the output voltage to 150Vrms, a step up transformer or variac may be used to achieve the desired output voltage. The minimum turns ratio should be used in order to maximize available inrush current from the power source/transformer assembly.

Worst case inrush current occurs when the customer activates his or her product when the input line voltage happens to be at one of its peak values. This occurs at the 90° and 270° points along the input line waveform (assuming a sinusoidal input wave). The most convenient way to simulate this with the ac power source by setting the output voltage to zero volts and the “execution angle” to 90° or 270°, as desired (see manual), then setting the output voltage to the desired voltage (150V recommended. See above.), and then pressing the execute button. The output voltage will rise from zero to its maximum magnitude as quickly as possible and trace out a sinusoidal output from there.