Page 59 - Energize April 2022

TECHNICAL

How to select an AC power supply

BY GRADY KEETON, AMETEK PROGRAMMABLE POWER

oday’s electronic products must work under all types of conditions, not just ideal as part of the test, you will need an AC
ones. That being the case, AC sources used in test applications must not only supply source that can supply the full peak inrush
Ta stable source of AC, they must also simulate power-line disturbances and other current, so that the source never reaches
non-ideal situations. the foldback state.
Fortunately, today’s switching AC power sources are up to the task. They offer
great specifications and powerful waveform-generation capabilities that allow users Crest factor
to more easily generate complex harmonic waveforms, transient waveforms and Crest factor is the ratio of the peak
arbitrary waveforms than ever before. Some can even provide both AC and DC outputs current amplitude to the rms amplitude of
simultaneously and make measurements as well as provide power. This level of flexibility is an alternating current or pulsating direct
making it easier to ensure that electronic products will work under adverse conditions. current waveform. For UUTs that draw an
input current with a high crest factor, it is
When choosing an AC source, make sure to consider the following criteria: important to select an AC source with low
• Current requirements for your device under test impedance and high peak instantaneous
• Worst-case input current (including transient demands, such as inrush) current capability. Low source impedance
• Crest factor of your load’s current facilitates the quick transfer of current to
• Power factor the load. High peak current is provided
• Regulation and distortion from these sources for pulse widths
• Response time and slew rate ranging from 60° to 30°. The narrower the
• User and test-system interfaces pulse width, the higher the crest factor
• Facility requirements capability of the high peak current source.
Switch-mode, or rectifier-type power
Current requirements supplies that are not power factor
When selecting an AC source for your test application, you must consider how much corrected, are an example of a UUT that
current your unit under test (UUT) will draw. Be sure to include inrush current and transient has a high crest factor. They draw current
currents that may occur during intentional input voltage swings and during different modes from a power source in narrow pulses at
of operation your device may use. the peaks of the voltage waveform. These
pulses can be from 3 to 4 times the value
Worst-case input current of the rms current.
Rectifier-type power supplies and motors are notorious for drawing high inrush currents. Many AC sources can only support
These devices have inrush currents anywhere from two to ten times the nominal run a crest factor of 1,414 (the peak of a
current, and they will draw this current anywhere from a few cycles to several seconds. sinusoidal current waveform). If your
The response of the AC power source to inrush current is dependent on the method source is not capable of supplying a load
that the source uses for current-limiting. AC power sources are designed to protect that exhibits a high crest factor, it may
themselves from excessive loads current by either folding back the voltage (current limiting) reduce its output voltage to unacceptable
or shutting down the output (current-limiting shutdown) and in many cases, this is user levels, have a distorted output, or shut
selectable. down completely. In either case, the UUT
In some instances, it may not be practical to have an AC source that can supply the full will not be tested properly. With a crest
inrush current demanded by the load. If the test does not require the stress test from this factor rating of up to 3,25:1 the AMETEK
current, it may be possible to use the current-limiting foldback technique for these tests. CSW Series AC source, for example, can
AC motors can draw up to seven times the normal operating current when first started. drive difficult nonlinear loads with ease.
How long the motor will draw this current depends on the mechanical load and the motor This translates into driving a rectifier for
design. instance, which has a 52 A peak current
For loads such as motors and rectifier-type power supplies, an AC source that is folding at 13 A rms at a 120/208 V three-phase
back its output voltage to limit current will result in a longer start up time for the device output.
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 Power factor
start correctly or shut off altogether. If your load has a low power factor,
If you must measure the inrush current or your test call for supplying the full inrush this will cause a derating of the output

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