Following up on last month’s blog posting, this time we will be looking at AC voltage range requirements needed to support avionics power compliance testing. If you missed last month’s blog entry, we encourage you to read it first for some background info. http://bit.ly/2xREMKo
For AC testing, requirements and test procedures found in Avionics test standards cover the following nominal AC Voltage levels:
- 200V/115V three phase, 400Hz and variable frequency
- 115V single phase, 60Hz, 400Hz and variable frequency
- 400V/230V three phase, 400Hz and variable frequency
- 26V single phase, 400Hz
Some standards include 235Vac as well but these nominal levels are less common.
Both private organizations, governments as well as aircraft manufacturers issue test requirement standards to their vendors and subcontractors. These test standards are often based on DO160 for commercial aviation but expanded or modified in specific ways to meet the OEM’s requirements. Some examples of test standards are listed in the table below.
Test Equipment Requirements
When dealing with DO160 or proprietary test requirements, the user must review the test equipment requirements called out in the standard. This will be different for AC or DC testing but often the same power source can be used if it is capable of both AC and DC output modes.
For AC test requirements, careful consideration should be given to the following AC source requirements:
- Required maximum AC output voltage. This is often considerably higher than the nominal test voltage.
- Frequency ranges. Both fixed and variable frequency power standards exist requiring up to 800Hz frequency programming.
- Maximum AC current supported. At lower than nominal input voltage, some EUT may draw more current which the power source used must be able to deliver.
The considerations we are reviewing in this blog are similar for both DO160 power testing and testing to any of these OEM test standards. Let’s take a closer looks at what this means for the AC power source specifications.
All loads must be capable of riding through AC voltage transients without any disruption in operation. Voltage transients can be caused by power cycling of other loads or by transfers between different AC generators.
These transients may require momentary AC output voltage levels well above the nominal AC voltage. Depending on the nominal value, this can have implications for the voltage range required.
For 26V ac nominal test voltages: Most AC power sources have a 135V or 150V voltage range. This will support testing at 26Vac including voltage transients. However, at 26V output, the AC power source has more limited power output as the current is limited. So while a 2kVA source may provide 2000/135 = 14.8A rms current at 135Vac, the same current at 26Vac only results in 26 x 14.8 = 385VA of power. That may not be enough to power the load being tested. Thus, the AC power source used may have to be oversized to meet these test requirements.
For 115V ac nominal test voltages: To power the unit under test at 115Vac, a 135V or 150V AC voltage ranges may seem adequate and will deliver close to full rated power from the AC source. However, AC voltage transients for 115V test requirement can be as high as 180Vac. That means the high voltage range will be needed to run these tests. As we saw earlier, operating at 180Vac on a 300V AC range will typically only provide limited output power. For a 5kVA rated AC source operating at 180V on a 300V range, the available power will only be 5000 / 300 = 16.67Arms * 180Vrms = 3000 VA or 60% of rated power. Before the transient is applied, the nominal test voltage will only 115Vac however so only 16.67 * 115 = 1917VA or less than 40% of rated power. This may be enough to power the unit under test in high voltage range but if not, the AC source must be oversized again.
For 230V ac nominal test voltages: Testing 230Vac nominal equipment presents a special challenge as AC transients can be as high as 360Vac. Most AC power sources have a 300V maximum AC output so they can’t be used as is. For these situations, Pacific offers transformer output options on most of its AC power sources that allow output voltages up to 1039VLL / 600V LN . Using a transformer coupled higher output range also limits the available output power at 230Vac. For example, with a 400V range, only 230/400 or 57% of rated power may be available at the nominal test voltage so the AC source must be sized accordingly.
Available Test Software
Many of the tests called out in these power test standards require extensive programming of voltage levels and durations. There are also several EUT performance tests that will require additional measurement equipment such as scopes, meters and power analyzers.
Developing these test routines in house can be a time consuming and costly endeavor. To support its aviation and test lab customers, Pacific Power has developed extensive libraries of pre-written test sequences that are available to run using our Test Manager Windows software. These test sequences covers all available public and most OEM test standards for AC and DC power testing. The table below lists available sequences and the latest standard revision level.
Why does it matter?
Understanding the requirements for the test equipment used to perform compliance testing to avionics AC power test standards is important for some of the reasons outlined here. Additional considerations apply for DC testing which were discussed in a previous blog entry.
Always discuss your test requirements with our application engineers so they can help you determine the best power source available.
Do your homework and consult with our product specialist and application engineers to make sure your test needs for AC and or DC power are covered before making a selection. Also consider the ability to expand the power level of your test equipment as the level of electrical power used on new airplane demands keeps rising.