Programmable AC power sources seem like straightforward components that you can integrate into a test system, but make sure not to overlook critical details before issuing a purchase order.1. Choose an AC source that economically meet your input and output requirements.
First, on the input side, make sure your power-distribution system can provide the voltage and current levels within the ranges specified by your AC -source manufacturer. On the output side, ensure the source can meet the requirements of your test system and equipment under test (EUT). But don’t over-specify by choosing an expensive three-phase AC source that may be overkill for your application. For example, you might require a 480-VAC three-phase line-to-line output. You might find an AC source that delivers 400 VAC, seemingly short of your 480-VAC requirement. But keep in mind, most AC sources specify (and program) voltages Line-to Neutral, and 480 L-L is equivalent to 277 VAC L-N. You would not need a more expensive 600-VAC source—the lower-cost 400-VAC source would more than adequate for testing a device that requires three-phase 480-VAC line-to-line input.
Finally, keep in mind that your output-power requirements might change. You can economically adapt to higher output-power requirements by choosing sources that can be connected in parallel, which is often more cost-effective than swapping out an under-powered source for a higher power version. Following this advice will help you meet your requirements without overspending. But do not underspend either. Carefully consider all possible use cases of the system you are building before specifying your AC source.
2. Examine your potential use-cases before committing to a power source purchase.
Next, consider your requirements in detail. You may have a clear picture of your needs in terms of output voltage, current, and frequency ranges. But your specific use cases might require additional subtle specifications. Slew rate, for example, refers to how quickly a programmable AC power source responds to a change in programmed voltage or frequency. A fast slew rate might be critical if you are testing how well your equipment under test responds to rapid voltage or current changes. AMETEK Programmable Power’s CSW Series models offer impressive slew-rate specs.
Loads such as motors can draw high inrush currents—from two to as much as 10 times the normal rated current. If your EUT includes a motor or other device that draws high currents during startup and you need to measure the inrush current your equipment draws when powered up, you’ll need a power source that can support the expected inrush-current level.
In general, you’ll want to choose a source that provides a clean, well-regulated output. But you may also want to study and characterize how your equipment responds in a poor power quality environment. If so, consider selecting a source that can generate harmonic distortion and simulate outages lasting tens of milliseconds or tens of minutes. Looking at your likely use cases in advance can go a long way to avoiding problems later. And one of your use cases may require regenerative capability.
3. Determine whether you could benefit by using a regenerative power source.
If you are planning a renewable energy project, you may require a programmable AC power source to simulate the power grid. If you are using such a source to test photovoltaic, wind, or hydrogen-fuel-cell inverters, you can use a regenerative source to return energy that would otherwise be wasted back to the grid. AMETEK Programmable Power’s MX Series, for example, offers automatic crossover between source and sink power modes and can regenerate up to 100% of the rated output power back to the utility grid during sink-mode operation.
Regenerative operation does raise some issues however, including power-quality and safety issues related to islanding. Your regenerative power source can act as a distributed energy resource on the grid, and it must shut down within a specified time if the grid shuts down. Be sure that the regenerative AC source that you select has built-in anti-islanding protection.
4. Consider power factor and crest factor.
If your device-under-test has a low power factor you will need to derate a linear AC source. In contrast, a switching AC source need not be derated for power factor unless operating at a low-voltage setting. If your EUT has a high crest factor (the ratio of peak current amplitude to the RMS value), choose an AC source with low impedance and high peak current capability. We will explore this further in an upcoming blog post.
Carefully consider the options and tradeoffs in the categories presented here to ease the power source selection that will address your project requirements and avoid costly replacements. In short, do not overspend, but do make sure to specify all the programmable AC power source features you may need throughout the life of the test system you are developing.