Today’s electronic products must work under all types of conditions, not just ideal ones. That being the case, AC sources used in test applications must not only supply a stable source of AC, they must also simulate power-line disturbances and other non-ideal situations.

Fortunately, today’s switching AC power sources are up to the task. They offer great specifications and powerful waveformgeneration capabilities that allow users to more easily generate complex harmonic waveforms, transient waveforms, and arbitrary waveforms than ever before. Some can even provide both AC and DC outputs simultaneously and make measurements as well as provide power. This level of flexibility is making it easier to ensure that electronic products will work under adverse conditions.

When choosing an AC source, make sure to consider the following criteria:
• Current requirements for your device under test
• Worst-case input current (including transient demands,
such as inrush)
• Crest factor of your load’s current
• Power factor
• Regulation and distortion
• Response time and slew rate
• User and test-system interfaces
• Facility requirements

Current Requirements
When selecting an AC source for your test application, you must consider both much current your unit under test (UUT) will draw. Be sure to include inrush current and transient currents that may occur during intentional input voltage swings and during different modes of operation your device may use.

Worst-Case Input Current
Rectifier-type power supplies and motors are notorious for drawing high inrush currents. These devices have inrush currents anywhere from two to ten times the nominal run current, and they will draw this current anywhere from a few cycles to several seconds.

The response of the AC power source to inrush current is dependent on the method that the source uses for currentlimiting. AC power sources are designed to protect themselves from excessive loads current by either folding back the voltage (current limiting) or shutting down the output (current-limiting shutdown) and in many cases, this is user selectable.

In some instances, it may not be practical to have an AC source that can supply the full inrush current demanded by the load. If the test does not require the stress test from this current, it may be possible to use the current-limiting foldback technique for these tests. AC motors can draw up to seven times the normal operating current when first started. How long the motor will draw this current depends on the mechanical load and the
motor design.

For loads such as motors and rectifier-type power supplies, an AC source that is folding back its output voltage to limit current will result in a longer start up time for the device under test. A source that is not capable of supplying the proper level of voltage and current may remain in the current-foldback state too long, causing the device under test to not start correctly or shut off altogether.

If you must measure the inrush current or your test call for supplying the full inrush as part of the test, you will need an AC source that can supply the full peak inrush current, so that the source never reaches the foldback state.

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