Herman vanEijkelenburg No Comments Blog

Power Compliance Testing of Aviation AC Products

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.

Test Standards

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.

Table 1 - AC Power Compliance Standards

Table 1 – AC Power Compliance Standards

Maximum Voltage

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.

Table 2: Available Test Software for Avionics Compliance Testing

Table 2: Available Test Software for Avionics Compliance Testing

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.

Conclusion

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.

Herman vanEijkelenburg No Comments Blog

Power Compliance Testing of Aviation DC Products

Critical Power Immunity Testing of DC Powered Equipment

Reliable operation of on-board electrical equipment under all circumstance is a critical requirement to ensure the safety of passengers and crew. This is true for both AC powered and DC powered equipment. With the continued growth of on-board electrical systems, the amount and types of equipment that needs certification and compliance testing for power anomalies increases every year. In this month’s blog we will discuss compliance testing of DC powered products but much of the same issues apply to AC powered products which we will cover in a future blog post.

Test Standards

The requirements that electrical equipment has to meet are controlled by standardization bodies or governments. For commercial aviation, the governing body is the US based Radio Technical Commission for Aeronautics or RTCA for short. (www.rtca.org).

The RTCA is a private association and a private public partnership between a large group of companies involved with aviation technologies.  RTCA publications cover many aspects of safety and design but the one of interest for this month’s blog is the DO160 standard.  This standard covers many aspects of equipment operation and verification, including Section 16 which deals with power immunity.

DO160 Rev G, section 16

Currently at revision G, the DO160 standard is constantly updated to cover new developments and technologies that find their way in new airplane designs. Section 16 of the standard covers environmental conditions and test procedures for power input.

RTCA/DO160 Test Standard

Figure 1: RTCA/DO160 Test Standard

Recent additions to Section 16 along these lines have been the inclusion of wild frequency for AC powered equipment and a new 270Vdc bus voltage to support the increasing electrification of modern and ever larger air frames.

For DC testing, requirements and test procedures are included to cover both 28Vdc powered and 270Vdc powered equipment.

Other Test Standards

Some aircraft manufacturers issue their own test requirement standards to their vendors and subcontractors. These proprietary test standards are often based on DO160 but expanded or modified in specific ways to meet the OEM’s requirements. Some examples of proprietary test standards are listed in the table below.

Aviation Power Test Standards

Table 1: Proprietary Test Standards

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 DC test requirements, careful consideration should be given to the following DC source requirements:

  • Required maximum DC output voltage. This is often considerably higher than the nominal test voltage.
  • Voltage slew rate. DC transient testing typically requires voltage slew rates that conventional DC power supplies cannot support. AC sources with DC mode however support high voltage slew rates as they are designed to support high frequency AC and have no output capacitance.
  • Maximum DC 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 power source specifications.

Maximum Voltage

All loads must be capable of riding through DC voltage transients without any disruption in operation. Voltage transients can be caused by power cycling of other loads on the same DC bus or by transfers between different DC buses.

For example, Section 16.6.1.4c addresses the need to test for these voltage surges that can occur during normal operation.  To support these tests for 270Vdc powered equipment, the power source used must be capable of outputting 400Vdc for 30 msec.  That means a 300V dc power supply would be ok to power the EUT but not to perform compliance testing to DO160. For abnormal operation test conditions, this requirement increases to as much as 425Vdc, beyond the range of even some AC and DC source models.

Voltage Slew Rate

Using the same Voltage Transient example, the required slew rate to run this test is much higher than most DC power supplies can handle.  The voltage rise time from 270Vdc to 400Vdc must occur in 1 msec – requiring a voltage slew rate of (400 – 270) / 0.001 = 130kV/s.  The voltage fall time must be less than 5 msec or 26kV/s.  These slew rates are supported on an AC and DC capable programmable source like the Pacific Power AFX series, but not on a regular programmable DC only power supply.

Figure 1: DO160-Table 16-3 DC Rise and Fall Times

DC Current

Another consideration is the current demanded by the EUT, especially at lower test voltage such as 14Vdc or 28Vdc.  Most AC sources with DC mode have higher voltage ranges as they need to support AC voltage output to at least about 150Vrms. That means voltage ranges are typically 212Vdc at their lowest and full power is not available at 14Vdc or 18Vdc.  This means the power source must be oversized to support the maximum required DC current. Fortunately, this is issue applies to 14Vdc and 28Vdc load testing only, not 270Vdc.  These lower dc input loads typically are fairly low power so DC current requirements are low.  For example, a 6kVA 3600AFX in DC mode can deliver 62.5Adc at 28Vdc or 1750W.  (29% of the source’s full power rating).

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.

Aviation Test Standards Table

Table 2: Other Aviation Test Standards

Why does it matter?

Understanding the requirements for the test equipment used to perform compliance testing to avionics DC power test standards is important for some of the reasons outlined here.  Additional considerations apply for AC testing which we will discuss in an upcoming blog entry. Always discuss your test requirements with our application engineers so they can help you determine the best power source available.

Conclusion

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.

Herman vanEijkelenburg No Comments Blog

GPIB or LAN with LXI Interface Considerations

Remote Control

Computer control of test equipment – including AC and DC power sources – is a critical requirement for building automated test systems (ATE) and to a lesser extend engineering development lab experiments. Key requirements for this type of computer control are:

  • Reliable communication to reduce or eliminate communication errors
  • Data transfer speeds
  • Industry standards to support long term use of ATE systems, especially in defense applications
  • Equipment interoperability to allows ATE system maintenance and upgrades
  • Cost and availability

IEEE488 Standard

Historically, the IEEE488 interface bus (aka GPIB) has been the de-facto interface standard for this purpose. Established in the late 1960s by Hewlett-Packard – currently Keysight – as the Hewlett Packard Interface Bus or HPIB, this standard was eventually adopted as an IEEE standards and is commonly referred to as GPIB for General Purpose Interface Bus.  Although it initially saw some use in early computer and peripheral applications, it was quickly replaced by faster and less expensive interface standards in the computer space so “GP” is somewhat of a misnomer these days.

It did thrive in the test equipment space however due to it’s tightly controlled technical specification which made it possible to mix test equipment from many different manufacturers and its support of specific ATE or instrumentation specific functions like SQR and serial or parallel polling of instruments.

Applicable GPIB Standards

The original HP specific standard has evolved through a series of IEEE and European IEC standard efforts over the years culminating in the 2004 version. Over objections from HP, it now also includes the HS-488 higher speed data transfer mode contributed by National Instruments in an effort to address the relatively low data transfer speed of the original HPIB interface.

The current 2004 IEEE/IEC standard IEC 60488-1, Standard for Higher Performance Protocol for the Standard Digital Interface for Programmable Instrumentation – Part 1: General,[10] replaced both older IEEE 488.1/IEC 60625-1, and IEC 60488-2,Part 2.

GPIB Pro’s and Con’s

There are benefits and drawbacks to GPIB, specifically:

Benefits

  • Proven, reliable communication interface dedicated to test equipment computer control applications.
  • Has been supported for long periods of time (45+ years)
  • Native hardware handshaking and hardware control for Remote/Local, Serial Parallel Poll and Service requests provided higher level of control than serial based interfaces like USB or LAN that require these functions to be added in software layers.

There are also obvious drawbacks to consider:

Drawbacks

  • The GPIB interface is a parallel bus which requires 24 signals including ground and shield and special shielded cabling and connectors. These cables are expensive and fairly limited in length. The required Centronics style 24 pin connector is large by today’s standards posing problems for 1U full or half rack size test equipment as the GPIB connector takes up a significant portion of the available rear panel.
  • The number of instruments supported is theoretically limited to only 30 but in practice far fewer as the total cable length to all instruments connected is limited as well.
  • The cost of the required GPIB controller and associated cabling is very high compared to modern interfaces like USB and LAN.
  • The cost of implementing a fully compatible GPIB talker/listener in piece of test equipment is considerably higher than that of USB or LAN, adding to the cost of the test equipment. For less expensive test equipment, this cost adder can be considerably compared to the cost of entire instrument.
  • Many GPIB controllers were based on computer bus standards such as ISA, PCI or PCIe which have since disappeared. This can make it more difficult to get GPIB controllers in the future as new computers increasingly rely on serial type interface like USB, Thunderbolt and LAN.
  • Sole source for PC based GPIB controllers. Most manufacturers of GPIB controllers other than National Instruments have disappeared due to the dwindling market for these controllers.

A typical PCI bus based GPIB controller is shown in Figure 1 below.

PCI Bus GPIB Controller

Figure 1: PCI Bus GPIB Controller

USB and LAN as alternatives?

From a cost point of view, using USB or LAN in place of GPIB for instrument control seems like an obvious choice. However, several of the functions provided by GPIB for control and management of the test equipment are missing and have to be implemented in some other way.  This means a simple USB or LAN approach is not a true replacement in ATE applications.

Obvious benefits of USB and LAN of course jump out immediately:

  • Must faster data transfer speeds. Even accounting for the parallel nature of GPIB which carries one byte at a time versus one bit at a time and allowing for HS-488 8x theoretical speed increase over basic GPIB, modern USB-3 and LAN speeds of 5 Gb/sec… and 10Gb/sec respectively provide data rates that are orders of magnitude higher.
  • USB and LAN are ‘essentially’ free on any PC or other computer device. The cost of USB and LAN on the test equipment itself is small as well compared to the required GPIB talker/listener logic and connector.

There are some drawbacks however to consider as well:

  • Lack of ATE specific functions like SRQ, ATN, Device Trigger and Parallel or Serial Poll on USB and LAN vs GPIB standard often requires additional protocol layers to be added, most of which are not standardized.
  • USB is a point to point interface, not a star topology so communicating with multiple instruments requires multiple USB ports or hubs
  • LAN is a shared interface so data transfer rates are affected by the number of instruments connected to the same segment or computer.
  • Connecting test equipment to a company network exposes them to the potential risk of unauthorized remote access. In the case of a power source, this can be a great risk to the safety of employees working on the ATE systems. Using an isolated, dedicated local LAN segment is the best way to mitigate this. Some test equipment like Pacific’s AFX series provides user access control provision to ensure security on networks.

Speed and Latency Benchmarks

Several benchmarks have been done to compare overall throughput between GPIB, LAN and PXI.  While PXI offers many speed advantages, the small form factor and severe lack of power and cooling as required for programmable power sources renders it unsuitable for power test equipment.

Some results are shown in Figure2 below. Note that data transfer rates and latency are only one aspect of overall ATE performance and other factors such as long-term support, cost and ease of integration should be weighed as well when selecting an interface solution.

Speed Bench Marks (© National Instruments)

Figure 2: Speed Bench Marks (© National Instruments)

LXI Standard

The LXI standard was developed by a consortium of test equipment manufacturers to address some of the issues associated with using LAN in ATE systems to control test equipment. This LXI standard which stands for “Lan eXtension for Instrumentation” has been in place since 2005 and is actively supported by the LXI consortium. (http://www.lxistandard.org/ )

The LXI standard for LAN equipped instrumentation helps reduce the time it takes to set up, configure, and debug test systems. LXI is an open, accessible standard based upon Ethernet that identifies specifications and solutions related to the functional test, measurement and data acquisition industries. Here are some key benefits of using LXI to build Test Systems:

  • Leverages the telecommunication industry infrastructure
  • Lowers test system cost using ubiquitous and inexpensive LAN components
  • Simplifies system integration
  • Provides high performance
  • Ensures broad instrument availability
  • IVI Driver requirement standardizes look and feel between products
  • Built-in Web Server for remote access and control of device

LAN or LXI Instruments?

While many test instruments feature a LAN interface, not all are also LXI compliant. Selecting an LXI compliant piece of test equipment provides easier integration and greater assurance of inter-operability, similar to that of GPIB instruments.

One of the many advantages of LXI instruments is the required web server that allows monitoring and often full control over the test instrument without the need to develop any software. A sample program and monitor screen for the Pacific Power AFX series of AC and DC power sources is shown in Figure 3 below.

AFX Series web interface - Programming & Measurements

Figure 3: AFX Series web interface – Programming & Measurements

Why Does it Matter?

The GPIB bus standard is getting very old and support for it is diminishing. Many new instruments no longer offer a GPIB interface.  This trend is likely to continue as faster, less expensive interfaces used in TI and the Computer industry have far more funding and development resources devoted to them.

Pacific Power sources offers the GPIB interface as an option on all of its power sources.  On new generation models like AFX series, LXI/LAN is included as a standard interface along with USB.

Conclusion

When selecting a programmable power source, either AC and or DC, consider the availability of LXI when planning use in ATE systems now of in the future.  Of source, combinations of several bus solutions like GPIB, USB and LAN are possible but for reasons of cost, ease of integration and long term support need to considered carefully.

Herman vanEijkelenburg No Comments Blog

AC Input Power Requirements for Power Sources

Power Conversion

Programmable AC power sources used for development and test applications convert locally available utility power to specific precision AC or DC output formats needed to test or control units under test. This is called “solid state power conversion” as active electronic circuitry is used rather than rotary converters or voltage only transformer.  This has numerous advantages for test applications:

  • Conversion of both voltage and frequency at the same time
  • Galvanic isolation between the grid and unit under test as the power source output can be floated
  • Available phase conversion between single, split or three phase grid to either single, split or three phase as needed

Regardless of the test objective, solid state power converters take utility power as input and convert to the desired output power voltage, frequency and phase configuration.

Single Phase AC Input

Single phase AC input configurations are by far the most convenient as any lab or factory floor will have single phase power outlets.  In countries where 230Vac or 240Vac grid voltage is standard, this provides for reasonable available input power to support requirements of up to 3000W.

For countries like the US or Japan where AC line voltage is only 120Vac or 110Vac, far less power can be drawn for a standard AC outlet.  A typical 120Vac outlet only supports 10A so best case, 1200VA is available. This ignores possible low line voltage conditions that may reduce input power further. There are 120V, 20A outlet versions available in the US but there are not very common and use a different pin orientation so standard modular line cord plugs won’t work with these.

Single Phase Grid Connection

Figure 1: Single Phase Grid Connection

For power output requirements higher than 1000W in the US, a split phase 240Vac or three phase 208V will be needed.

Three Phase AC Input

Three phase power is typically used for higher power and industrial applications. Factory floors and power test labs typically have three phase outlets available.  For office building, three phase power is used for lighting which is a big consumer of power so three phase power may be available in the building but three phase outlets may not.

There are three common three phase configurations available in the world:

  • 208Vac three phase Wye Japan
  • 208Vac three phase Wye US
  • 400Vac three phase Wye     Europe, Asia
  • 480Vac three phase Delta US

Higher voltages may exist in some countries (Canada) and Delta/Wye transformer may be used to change from Delta to Wye or vice-versa.

Three Phase Delta Grid Connection

Figure 2: Three Phase Delta Grid Connection

Delta or Wye Input?

Not all higher power AC or DC power sources have the same three phase input configuration.  Pay attention to the type of three phase voltage configuration supported by the power source you are considering.  If the input configuration is a Delta, the power source can be used with either a Delta or a Wye grid configuration.  The neutral connection is not needed when connecting to the grid.

If on the other hand the power source needs a neutral connection (Wye input supported only), they cannot be used with a Delta grid. These types of input design often are impacted when driving grossly unbalanced three phase loads as there can be a substantial amount of Neutral current flowing. The performing load unbalance compliance testing, try to avoid such power sources

Figure 4: Typical Three Phase Voltage Configurations used in the USA

Figure 3: Three Phase WYE vs DELTA Configurations

Of course, equally important is paying attention to AC input voltage range. It has to match what is available at the location where the equipment will be used.

While power factor corrected single phase input power sources often have a wide AC input voltage range, three phase products typically do not as the wide input current range requirement associated with that would be difficult or costly to implement.

Some products however use AC input transformers which may support multiple voltage input transformer taps allowing re-strapping for different locales in the world.  The drawback to this is that such power sources are typically quite a bit larger and heavier as a result.

AC Input Current

Input current requirements are determined by several factors for both Single or Three phase input configurations:

  • Input voltage range
  • Output power rating
  • Power factor
  • Efficiency
  • Overload Operation

All these factors determine what the AC input VA will have to be to support maximum rated output power of the programmable source.  For example, if we have a 2kVA source connected to a resistive load, the max. output power will be 2000W as well as 2000VA. Let’s assume the input specification is as follows

  • AC Voltage Input range: 230Vac ± 10%
  • Current: 15A
  • Power Factor: 8
  • Efficiency: 82%

Since the AC input voltage is 230V nominal, we have to allow for operation down to 230V * 0.9 = 207Vac worst case. Not all power grids in the world are stable and low line brownout conditions can be quite common.

To get the required 2000W output, the input power required from the grid will be determined by the following formula:

Pin = (Pout / PF ) / Eff

For our example, this equates to:

Pin = (2000) / 0.8 ) / 0.82 = 3048 VA

At a worst case low line input voltage of 207Vac, this will require 3048 / (230* 0.9) = 14.724 A.

For a three phase power source input, the relationship between required input power and current is calculated in a similar way but the actual current needs to be divided over three phase or by √3. Thus, a 10kVA input power requirement on a 3 phase 208V grid connection would result in

((10000 / (208*0.9)) / √3 = 30.84 A RMS per phase.

This illustrates the impact of both power factor and efficiency on AC input currents and associated facility breaker and input wire sizing. In this single phase input example, the 15A input current is generally available on standard 230Vac outlets in Europe and other countries.  Once we need more than 2000W output power, we would either have to select a more efficient or better input power factor AC sources or consider using three phase input power. Let’s look at the two specifications that have the biggest impact on required AC input current, power factor and efficiency.

Inrush Current

While we covered required RMS input current to support full rated output power, we also need to pay attention to initial inrush current when a power source or power supply is first turned on. Since most input circuits consist of a bridge rectifier and bulk storage capacitor, initial current peaks may be high if the input capacitor is fully discharged.  This may be true of power factor corrected input designs as well.

To prevent nuisance circuit breaker tripping due to excessive inrush currents, make sure the power source is equipped with a soft-start circuit.  Such as circuit uses a current limiting resistor or thermistor to limit the peak inrush currents while the bulk storage capacitors on the DC bus charge up.  Once charged, this resistor is either bypassed or remains in a low impedance state if a thermistor is used.

Pacific models with power ratings of 4500VA or higher all have soft-start circuits built-in. On lower power models, it is not always required but can be offered as an option.

Power Factor

Higher power factors can be obtained by selecting AC power sources with power factor correction. There are two methods commonly used:

  • Passive PFC
  • Active PFC

Passive PFC uses a line input inductor to compensate for any input inductance result in power factors that can be as high as 0.85. On three phase AC input designs, where the power factor without PFC can be as low as 0.6, the use of zig zag transformer can produce similar results.

To get any higher, active power factor is required.  For single phase AC input configurations, active power factor correction can reach up to 0.98 or 0.99 power factors under full load conditions.  For three phase AC input PFC configurations, a PF of 0.95 or 0.96 is feasible.

Note that input power factor specifications for power factor correction apply typically under full load. At less than full load, power factors will be lower but since the output power is lower anyway, this is not an issue.

For higher power requirements, choosing an active power factor corrected programmable AC source like the Pacific AFX Series can save on facility wiring and installation costs.

Efficiency

Efficiency is not as easy to improve as power factor.  Since AC power sources need at least two power conversion stages to allow for frequency conversion, getting high overall efficiency is not trivial. Even if both power stages were 95% efficient, the overall efficiency would still only be 90%. If active PFC in used, this adds a third stage – let’s assume also 95% – and overall efficiency would be 85%.

The example of 82% we used in this blog is not uncommon for AC power sources.  DC power supplies use fewer power stages and can be found with efficiencies in the low 90’s.

Why Does it Matter?

Understanding the AC input requirements of the power test equipment you plan to use will allow you to select the best configuration equipment that meets your objectives. Bringing in three phase grid power can be a costly proposition. At the same time, single phase grid power is typically limited in the amount of current and thus power is can provide.  This is particularly true for the US, Mexico or Japan where single phase grid voltage is only 120Vac or 100Vac.  The higher the power source’s efficiency and the higher its input power factor, the less input power is required to reach the desired output power demands. This also applies to three phase input power sources as it can avoid the need to upgrade your existing three phase power drops.

Conclusion

When selecting a programmable power source, either AC and or DC, consider the AC input configuration and specifications carefully to help you choose the best solution.

Herman vanEijkelenburg No Comments Blog

Understanding Three Phase Voltage

Single Phase AC Voltage

Most of us are familiar with the single phase voltage in our homes provided by the local utility. For the US, this is typically 120V. For single phase voltage, the voltage is expressed as a Line to Neutral voltage between two power conductors (plus a safety ground).  The neutral conductor is normally at ground potential while the Line conductor is a sinusoidal AC voltage with an RMS value of 120Vac. That means the peak of the AC voltage alternates from +169.7Vac to -169.7Vac every 16.667 msec on the US 60Hz grid frequency. For many other countries, these nominal values are 230Vrms @ 50Hz (20 msec) instead.

Figure 1: Single Phase 120Vrms Sinusoidal Voltage Waveform

Figure 1: Single Phase 120Vrms Sinusoidal Voltage Waveform

Power Limited

Single phase voltage can deliver only so much power as all power has to be delivered using the line and neutral conductors. This is no problem for home use but for industrial use, more current may be required to run machines, motors, lighting and other high power loads.  For these situations, it is often desirable to raise both the voltage and the current to deliver this higher power. One option is to use two phase as found in some US home to run electric dryers.  This is called a split phase connection where two 120Vrms phases are 180° phase apart, providing twice the 120VLN or 240V Line-to-Line voltage.  This doubles the available power. Split phase is not commonly used in Europe or Asia as the normal single phase grid voltage is already 220VLN to 240LN.

Three Phase AC Voltage

Taking this one step further, high power loads are typically powered using three phases. This distributes the current over three instead of one set of wires, allowing for smaller and thus less expensive wiring.  The three voltage sources are phase shifted 120° with respect to each other to balance the load currents.  This is illustrated in Figure 2.

Figure 2: Three Phase Voltage Waveforms with different rotations

Figure 2: Three Phase Voltage Waveforms with different rotations

The 120° phase shift between each waveform can be done in one of two phase rotations – A -> B -> C or A -> C -> B. Phase rotation does not affect most loads except for three phase AC motors which will turn in opposite direction if the phase rotation is changed.  Changing phase rotation can be accomplished by swapping any two of three phase connections.  When using a programmable AC power source like the AFX Series, phase angles for phase B and C can be programmed to either 120° and 240° or 240° and 120° respectively to change phase rotation.  The AFX also allows phase imbalances to be programmed to study the effect of phase variations on a unit under test.

Caution when Determining Line to Line Voltages

While the ‘normal’ three phase delta and wye voltage relationship is easily captured in a simple formula, this applies only with equal line to neutral voltages, perfect phase balance and sinusoidal voltages.  For this ideal case, the relationship between Line to Neutral RMS voltage and Line to Line RMS voltage can be expressed by the following formula:

Three-phs-voltage-Formula

This relationship between line to neutral and line to line voltage is shown in the phase diagram of Figure 3.

Figure 3: Three Phase Phasor Diagram

Figure 3: Three Phase Phasor Diagram

Figure 4 below shows two typical examples of three phase utility grid voltage configurations used in the United States.  Europe and Asia typically use 220/380V or 230/400V configurations instead.  The 120VLN per phase is equivalent with the 208VLL vector sum:

VLL = 120VLN * 1.732 = 207.84VLL

Note that the 480V delta grid configuration has no neutral connection and is referred to as a 3Wire + Ground Delta connection. To simulate this type of grid with an AC power source, the 3 phase load is connected as a delta between the three output phases only with no connection to the neutral output terminal.

Figure 4: Typical Three Phase Voltage Configurations used in the USA

Figure 4: Typical Three Phase Voltage Configurations used in the USA

This ratio of √3 is important when using a programmable three phase AC power source as all T&M style AC power sources are only programmable in Line to Neutral voltage.  Thus, if any of the stated conditions are not true, you cannot just rely on this formula to determine Line to Line voltage:

  1. Identical VLN voltages on all three phases
  2. Balanced Phase angles on phase B and C
  3. Low distortion, pure sinewave

A small phase shift on one or more of the three phases can have a significant impact on VLL voltages which results in load current imbalance as well.

A distorted voltage caused by a non-linear load on one of more phases can also throw off the Line to Line voltages.

Why Does it Matter?

Programmable three phase AC power sources have adjustable phase angles and often support arbitrary waveform capability. That means the relationship between line to neutral and line to line voltage is not necessarily ‘fixed’.  As a rule, all three phase programmable AC power sources are programmed in Line to Neutral RMS, regardless of the load type (Delta or Wye).  As such, it may be necessary to actually measure the resulting Line to Line voltage as calculating it is not valid if these conditions are not met.

Conclusion

When testing three phase loads, pay close attention to voltage and phase parameters when making assumptions about the Line to Line voltages applied to the unit under test.

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Constant Power Voltage Ranges, not all are created equal

Power Supplies and Output Power rating versus Voltage

As a rule, power supplies are rated for maximum power output only at their maximum output voltage. That means that as the programmed output voltage is decreased, the power output capability decreases proportionally. For example, a 10kW rated power supply with a 0 to 100V voltage range can deliver 100A at 100V or 100 x 100 = 10kW but only 50 x 100 = 5kW at 50V output as the current remains limited at 100A max.  This is true for both DC power supplies (DC output) as well as AC power sources (AC output). Such power supplies/sources are called ‘point rated’ as they provide the maximum rated output power only at the maximum voltage setting or one set point.

More recently, manufacturers of T&M power supplies and sources have started offering models that are not point rated but rather offer maximum output power over a portion of their output voltage range.  Such power supplies or sources are referred to as Constant Power (CP) voltage range models. For the case of AC power sources, let’s take a closer look at what this means.

Constant Power Voltage Range AC Source

A constant power voltage range source allows operation at maximum output power VA and Watt over a portion of its voltage range. This means the maximum available current at each programmed voltage set point increases as the voltage setting decreasing, thus maintaining a fixed V x I = P power profile. What this means is that the maximum power output set point is no longer restricted to the highest voltage setting.

Constant Power Voltage Range Comparisons

Although AC or DC power source models from different vendors may all claim Constant Power mode voltage range capability, that does not mean they are all created equal. This is best illustrated by comparing the technical specifications for similar output power rated AC source models side by side using the same 300Vac voltage range as is done in the table below.

Make
Power Rating
Brand A
45kVA
Brand B
45kVA
Pacific Power
45kVA
AC Voltage Range 0 – 300Vac 0 – 300Vac 0 – 300Vac
Max. Current 62.5A rms 75A rms 125A rms
CP Mode Range 80% – 100% 67% – 100% 40% – 100%
Max. Power @ 230Vac/Phs 14,375 VA 15,000 VA 15,000 VA
Max. Power @ 115Vac/Phs 7,187 VA 8,625 VA 15,000 VA

Table 1: Constant Power Voltage Range Comparison for different brand power sources

Note that all models reviewed have constant power voltage mode over some of their output voltage range but the range of it is rather limited for some.  The AFX Series is the notable exception, supporting constant power operation all the way from 100% to 40% of its 300Vac voltage range or down to 120Vac.  The same applies for its DC mode were full power is available from 100% to 40% of its 425Vdc range (425Vdc – 170Vdc).

The two graphs below (Figures 1 & 2) illustrate the significantly larger usable operating range of the Pacific Power AFX Series AC source compared to the other brands.  Figure 1 shows available max. load current as a function of programmed output voltage.

AFX vs Competition Current vs Voltage

Figure 1: Available Current per phase for 45kVA AC power sources from different vendors

A similar comparison can be made by plotting available power as a function of programmed output voltage as shown in Figure 2. Again, the usable range of the Pacific AFX Series is considerably larger.

AFX vs Competition Power vs Voltage

Figure 2: Available Power per phase for 45kVA AC power sources from different vendors

Why Does it Matter?

An increasing number of products support a wide input voltage range, also referred to as universal input range. To develop and test these products, they must be evaluated under both low voltage and high voltage input conditions, including under and over voltage stress testing, often beyond published specification.  This requires a power source with a wide voltage and current profile over which it can deliver full power.  It also means switching between a high and low voltage range is not acceptable as this invariably requires the output to the unit under test to be interrupted for some period of time during range change.

Have a wide constant power voltage range as found on the AFX Series eliminates the need to oversize the programmable power source used.  The AFX Series from Pacific Power meets these requirements better than its competition.

Conclusion

While product features like constant power mode voltage range capability may seem similar between equivalent power rated power sources from various vendors, it often pays to evaluate in greater detail the detailed technical specifications before making a selection. Such differences in specs can have significant impact on the usability of the power source for a range of situations.  The extent of the available constant power voltage range is one such example.

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What is the difference between Inrush Current and Peak Current?

What is the difference between Inrush Current and Peak Current?

While inrush current has a peak current value, the term “Inrush Current” is commonly used to describe the current that is required to energize an AC powered device or product when first applying voltage and power to it.  This is especially true for inductive loads such as transformers, Inductors and electric motors. It also applies to AC/DC power supplies that use a simple rectifier/capacitor input stage.  These initial currents can surge and be quite a bit higher than the normal operating current or what is called “steady state” current. An example of an electric motor inrush current is shown in Figure 1.  It shows the peak current for the first half cycle as being close to 30 amps and then decaying over subsequent half cycles as the motor spools up.

Motor Inrush Current

A different example of inrush current is an AC/DC input stage that uses a rectifier, capacitor circuit where the capacitor needs to be charged up to its nominal voltage as shown in Figure 2.  In both cases, it is apparent that the inrush current is considerably larger than the steady state current.

Retifier Capacitor Inrush Current

Peak current on the other applies to all AC currents, either inrush or steady state. An AC current waveform has an RMS value representing the effective or DC equivalent voltage but it also has a peak value, both positive and negative peaks where the current reaches it maximum and minimum value during each cycle.  The absolute ratio between the RMS value and the peak value is called crest factor (CF).  For a sinusoidal current as encountered with a resistive load, the crest factor will be the square root of 2 or ~1.4142 to 1. This crest factor or ratio is shown in Figure 3.

Sine Wave Crest Factor

Other wave forms have different crest factors as shown in Table 1 below for some typical other AC wave forms.

Crest Factors for Different Wave Types

Why Does it Matter?

When using an AC power source to determine the required inrush current for a unit under test, it is important to note that the AC source should be capable of delivering significantly more current for a short period of time than is required to run the unit under test in a steady state condition.  In the case of Motors and Inductors, the inrush current can be 10 to 30 times the nominal current. For toroidal inductors, this value may be up to 50 times nominal.

Source current limitation may be both in terms of RMS current rating and peak current rating.  For motors and inductor loads, the crest factor of the inrush current is only 1.414 so if the source can support the RMS current, the peak value will be supported as well.  For rectified AC input equipment, the current crest factor is generally much higher than 1.414, up to 2 or 3 to 1 so not only the RMS rating but also the peak current rating must be considered.  Most available AC power sources will support current crest factors from 2.5 to 4 at max RMS current output.

Current Limiting Effects

If the source is unable to deliver the required inrush current, it may still be used for testing normal operation but the required inrush current cannot be determined as the power source will go into current limit – either RMS or Peak or both – and limit the voltage in doing so. This means the unit under test will typically still start up or turn on but not as fast as it would when operated from the utility.

AC Source Voltage Distortion

High peak currents and distorted current wave forms also impact the AC power source distortion as they work against the output impedance of the power source.  The lower the power source’s output impedance, the less this effect will be.  Figure 4 shows the effect of highly distorted current on output voltage distortion.  As the current reaches its peak near the top of the voltage waveform, the voltage is pulled down causing some flat topping to occur.

ProgZ=0 vs ProgZ = 1

To alleviate this effect, a programmable output impedance feature may be offered on some AC source models that allows the output impedance the be reduced. See our expert blog post called “What is Prog Z on an AC power source and why does it matter” at https://pacificpower.com/2017/03/programmable-z-ac-source-work-matter/

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How Fast Can your Power Source transition between AC and DC modes?

What are voltage modes?

While all programmable AC power sources offer AC output mode, some also provide DC output mode. This may be useful for applications where only DC is required to power an EUT.  However, a growing number of power related products are coming on the market than can operate from both AC grid power and direct DC power.  This often eliminates a AC to DC conversion stage which reduces losses and improves energy efficiency. Such EUTs must not only be testing in both AC mode and DC mode, they also must be qualified for their ability to transfer between AC and DC power input without any undesirable effect on their output. For that purpose, the programmable power source used must offer AC+DC output mode in addition to Mode and DC Mode.  Not all power sources can support this but the AFX Series from Pacific Power does.

Why Does it Matter?

Most AC power sources only offer AC output and those that offer either AC or DC output cannot be used as there is a considerable amount time the output is dropped when switching between AC and DC modes or vice versa.  An alternative used by design and test engineers was to construct a power transfer switch between two lab supplies, one AC source and one DC supply. Such a method can obtain better transfer characteristics but transfer switches can be failure prone and still exhibit a certain amount of cross over delay, also known as dead-time as the two supplies cannot be shorted together under any circumstance. See Figure 1.

Figure 1:  Electronic Transfer Switch Setup

Figure 1: Electronic Transfer Switch Setup

Power Source with native AC+DC Mode

A far better solution is to use an AC power source capable of AC+DC and DC+AC output modes.  This allows transfers between AC and DC input power the be accomplished with zero cross over time as no switching between output modes or two power supplies is required at all. A scope capture of the input power to the unit test in AC+DC mode operation is shown in Figure 2 below.

Figure 2: Scope Capture of alternating AC and DC input

Figure 2: Scope Capture of alternating AC and DC input

Programming Fast Transition Times

To ensure the transition between AC input and DC input occurs instantaneously, we have to program a small transient program that alternates between 230Vac and 325Vdc for our example. This can be done using the TRANSIENT screen of the AFX power source or if more convenient, using the built in web server and a browser or the user’s smartphone, tablet or PC. Figure 3 shows the Web interface transient setup.

Figure 3: Two step transient program

Figure 3: Two step transient program

By programming the transition from AC to DC mode to occur at the 90° point of the sine wave, we can cause the AC to DC transition to occur at the positive peak of the AC waveform, causing a seamless transition as shown in Figure 4.

Figure 4: AC to DC Transfer at 90°

Figure 4: AC to DC Transfer at 90°

Conclusion

The new AFX Series is one of the most versatile and compact programmable AC & DC power source on the market that should be part of every power design and test engineer’s tools kit.  To learn more, check out the full application note on “AC+DC Mode Power Converter Testing”.

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Why do AC Sources have voltage distortion, what causes it and why does it matter?

What is Voltage Distortion?

Voltage distortion occurs when a AC output is purely sinusoidal but has small levels of voltage harmonics in addition to the fundament sinewave frequency. This low level of voltage distortion is often difficult to see on an oscilloscope and requires a power analyzer or harmonics analyzer to quantity. Distortion is typically express in a % based on the ratio between RMS amplitude of the higher harmonics versus the RMS amplitude of the first harmonic or fundamental (h=1).

The lower the Vthd %, the better the programmable AC power source.

 

What Causes It?

There are several possible factors that can contribute to voltage distortion, some of which having to do with the effect of non-linear loads. However, to compare the relative performance of a programmable AC source, it is best to use a pure resistive load so effects of load on the power source’s performance is neutralized. That only leaves the inherent voltage distortion of the AC power source.
For switch mode AC power sources – aka Class D amplifiers – the biggest contributor to distortion is the cross over distortion that occurs at each voltage zero crossing. Since a switch mode AC source switches its H-bride FETs or IGBT’s between on and off state, there can be no overlap between switching cycles between the positive and the negative half of the H-Bridge. If there was, a direct current patch between the positive and negative DC rail of the H-Bridge would be created causing excessive current to flow through power devices. This condition is often called shoot-through and invariably causes the H-Bridge or power amplifier output stage to fail. To prevent this, a certain amount of so called ‘dead-time’ is needed between turning one side of the H bridge off and turning the opposite site on. This dead time causes the output sine wave to stop just short of reaching zero, causing a momentary disruption as the voltage jumps from one side to the other. This necessary phenomenon is called cross over distortion and illustrated in Figure 1. Such a non-linear step in voltage contains a large number of harmonics components, contributing to the overall voltage THD.

Figure 1: Cross over distortion

Figure 1: Cross over distortion

 

Why Does it Matter?

Since a programmable AC power source is used to design, test and verify AC powered products, a highly distorted AC sine wave will negatively impact the unit under test. Even a few percent distortion can affect the load if it is sensitive to higher frequencies can the line frequency. Since the switch mode amplifier dead time is typically fixed, the impact of it will increase as the programmed output frequency is raised as it becomes a larger fraction of the decreasing sine wave period. Thus, all PWM AC power sources have higher distortion specs at higher frequencies. For avionics applications where frequencies can be thousands of Hertz, high levels of THD at such frequencies is typically unacceptable.

 

Linear AC Power Source.

One of the best performing programmable AC power sources are linears. As these are not Class D but rather Class AB amplifiers, they are zero biased and require no dead time. Thus, their distortion specs are significantly lower than those of switch mode (PWM) type power sources.

For example, the table bellows shows the voltage distortion specification for two equivalent power AC power sources, a 115ASX PWM switch mode unit and a 115AMX linear unit.

 

Table 1: Comparison of V THD Specs on Switch Mode vs. Linear AC power sources

Table 1: Comparison of V THD Specs on Switch Mode vs. Linear AC power sources

 

Selecting the best programmable AC power source for your application includes paying attention to voltage distortion specifications.

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What Is Programmable Z on an AC Source, How Does It Work and Why Does It Matter?

What Is Programmable Z?

Hate to disappoint, but programmable Z on an AC power source has nothing to do with getting sufficient sleep. Instead, the Z refers to the output impedance of the power source. A low AC source output impedance means there is little or no voltage drop at the output of the power source when the load current increases. This is obviously a good thing, as it means the voltage will not change under changing load conditions. Load regulation is good in this case. If the output impedance is high, there will be measurable voltage drop at the output of the power source and thus at the load with an increase in load current. Load regulation will be poorer. Generally, the output impedance, or Z, of a power source is determined by the output stage of the DC/AC inverter (single-phase source) or inverters (multi-phase source). It is generally also true that the higher the power rating of the AC source, the lower the output impedance has to be to deliver sufficient current without significant voltage drop.

As we have seen in previous expert blogs, the load regulation feedback loop of a programmable power sources helps keep the output impedance low. A source with a low output impedance is sometimes referred to as a “stiff” source. But what if we want to simulate to a “softer” source or – conversely – study the effect of a stiff source on, say, peak inrush current of our unit under test? To do so, we would need an AC source with programmable output impedance, also known as programmable Z or prog-Z.

The example below shows the difference in current, both RMS and peak, for the same load when using a prog-Z = 0.0 Ohm (off) setting versus a prog-Z = 1.0 Ohm setting. Note the flat topping that occurs with a higher Z setting, indicating a “softer” source. This concept can be used to simulate various utility connection impedance points.

Figure 1: Effect of Prog-Z Settings on Voltage and Current Waveforms

How Does It Work?

Figure 2: Prog-Z Current Feedback Block Diagram

Programmable impedance relies on feeding back signal to the output inverter’s control loop that is proportional to the load current. By summing this signal with the error signal that is used to maintain output voltage, the amount of voltage change is now a function of both programmed set value as well as load current. This concept is illustrated in Figure 2.

At low load currents, there is little boost from this current feedback. At higher load currents, the amount of feedback increases, effectively boosting the output voltage to simulate a lower output impedance. Since the level of compensation can be programmed, the output impedance is now programmable.

Why Does It Matter?

Lower output impedance results in higher peak current delivery to the load and less voltage distortion, especially at higher output frequencies where the output inductance of the source’s output filter can result in relative high output impedance.

Due to the “real time” nature of the feature, the programmable output impedance provided by Pacific Power Source UPC controllers supports sub-cycle response times to load induced current demands. The same prog-Z can also improve voltage transient response by requiring less load regulation adjustment than would otherwise be needed.

An additional benefit of prog-Z is the ability to compensate for line losses by programming a “negative” impedance in various test applications.

 

Herman vanEijkelenburg No Comments Blog

What is Load Regulation on an AC Source, How Does It Work and Why Does It Matter?

What is Load Regulation?

Load regulation ensures programmed output voltage remains constant despite load variations It is calculated using the following formula:

AC Load Regulation Formula

Where VLmin is the voltage at no load and VLmax is the voltage at maximum rated load. It is typically specified in % of full scale voltage. The lower the load regulation spec %, the better. In an ideal case, load regulation would be 0% so changes in load level would have no effect on the output voltage of the supply. That is obviously not practical but load regulations in the fractional percentile are feasible with good regulation loop design.

How Does It Work?

Programmable impedance relies on feeding back signal to the output inverter’s control loop that is proportional to the load current. By summing this signal with the error signal that is used to maintain output voltage, the amount of voltage change is now a function of both programmed set value as well as load current. This concept is illustrated in Figure 2.

Simple Error Amplifier Schematic

Figure 1: Simple Error Amplifier Schematic

At low load currents, there is little boost from this current feedback. At higher load currents, the amount of feedback increases, effectively boosting the output voltage to simulate a lower output impedance. Since the level of compensation can be programmed, the output impedance is now programmable.

Why Does it Matter?

Without good load regulation, output voltage will sags with increases in load or surge when a load is suddenly removed. Test results under such conditions may not be repeatable. How can you ensure that was it programmed is indeed applied if the power supply has poor load regulation.

Good Load regulation applies to both DC supplies and AC sources but is particularly important for AC power sources as output frequency can have a big impact on load regulation. In today’s world of switch mode power sources, output filters are required to mitigate output switching noise. These output filters contain series inductance which increases output impedance as the frequency increase. Thus, where load regulation may be ok at 50Hz, at 400Hz or 800Hz, it may not be.

To overcome this, good feedback loop design that incorporates frequency compensation must be used. Another refined it to incorporate current feedback input into the feedback look that results in better load compensation.

Historically, power supplies have relied on analog circuits like the one shown in Figure 1.In modern AC power source designs like the all-digital AFX Series® from Pacific Power Source, load regulation feedback loops are implemented in the digital domain. This results in several improvements such as:

  • No more reliance on analog components like capacitors and resistors with finite accuracy tolerances
  • Ability to incorporate programmed output parameters like frequency and load current in the feedback loop algorithms
  • Ability to adapt feedback loops to varying dynamic load conditions as loop parameters can be changed on the fly as needed to maintain optimal load regulation.

When selecting an AC source or DC power supply, don’t just look at the load regulation specification number, also consider the technology used.