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Five mistakes engineers make when choosing power supplies

Written on October 27, 2016 at 6:38 am, by

The power supply may be one of the least-considered components of an electronic system. After all, how hard can it be to find the right power source for your system? You figure out how much current you need at the voltage your system will operate at, find a model that can supply that voltage and current in a catalog or on a website, then make the purchase.

asterion

Asterion’s ix2 current doubling technology allows Asterion power sources to linearly increase the output current and maintain maximum power output over a wider output voltage range. See #2.

Actually, it’s not that simple. There are many other things that you need to think about choosing and using a power supply. Here are five common mistakes that engineers make when choosing and using a power source for their projects:

  1. Not buying a supply with enough output power. While you certainly don’t want to buy a supply with too much excess power output, trying to save money by buying a supply with just enough power output isn’t a good idea, either. Buying an undersized power supply won’t save you money if you have to replace it with high power supply at some later date. To avoid this, consider future as well as current needs and buy a supply with at least 25% capacity above current needs.

  2. Not buying a supply with enough output current. Even though a power source might be able to supply a certain amount of power, it’s not a given that it can supply every voltage/current combination. For example, a 1,500 W DC power supply with a 400 VDC range may only be able to supply 3.75 A maximum across the entire range.

    AMETEK’s Asterion power sources overcome this limitation by using iX2 current-doubling technology. iX2 current-doubling technology allows Asterion power sources to linearly increase the output current and maintain maximum power output as the output voltage drops from the maximum output voltage to half that value. No other power source can deliver full output power over such a wide voltage range. iX2 current-doubling technology eliminates the need to buy multiple sources or overpowered sources to run tests at low line voltages.

  3. Not considering startup conditions. Many AC-powered products, such as switching power supplies and electronic lighting ballasts, draw high start-up currents to charge capacitive circuitry. Excessive inrush currents not only cause lights to flicker, but can also damage the power supply. Depending on what you’re powering, the power source may have to be able to supply this amount of inrush current.

  4. Not paying enough attention to wiring. Once you’ve selected the appropriate power supply for a system, you need to connect it to the system. If you’re supplying high currents, make sure that you select power cable large enough to handle that current. Refer to the National Electrical Code for these values.

    One rule of thumb is to keep the leads as short as possible. This will minimize the voltage drop in the power cables. When tolerances are tight, consider using a feature called remote sensing. AMETEK power sources with this feature allow you to sense the voltage with a second set of wires at the power input to a system and regulate the output voltage based on this value.

  5. Poorly designed rack mounting. Because many of AMETEK Programmable Power’s power sources are designed to supply relatively high power, they are often mounted in a 19-in. rack. One is a lack of cooling. Always ensure that the equipment in a 19-in. rack is properly ventilated to keep operating temperatures within specification.

For more information on choosing and using AC and DC power sources, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Power Source Multi-Box Configurations Meet a Variety of Needs

Written on October 25, 2016 at 11:17 am, by

Many of AMETEK Programmable Power’s AC power sources are designed to work as both standalone units and in multi-box configurations. The California Instruments iX Series AC/DC power sources, for example, includes independent 5 kVA power modules that can be combined into a number of configurations. You might use a single unit as a high-power, single-phase system or configure three units to form a medium-power, three-phase system. This modularity allows you to build a power system that meets your specific needs.

To do this with the ix Series, you need to purchase the Multi-Box option (-MB). This option includes the additional test and calibration required for a multi-box system, as well as the additional cabling needed to configure a multi-box system.

Standard three phase iX configuration

The standard, three-phase iX series configuration uses a single master unit with a three phase controller to drive two additional slave units. Figure 1 shows a standard 15003iX configuration, which provides a total power of 15 kVA and can be used in either single-phase or three-phase mode. Also available is the 30003iX configuration. This configuration can supply up to 30 kVA, but operates in three-phase mode only.

multi-box-fig1

As shown in Figure 1, the master provides phase A output, while the slaves are used to provide output for phase B and C. Neither slave unit has a controller since the master unit controls all three via the system interface. This connection consists of a ribbon cable that connects all three units together.

More controllers means more flexibility

In addition to the 15003iX or 30003iX configurations, you can specify a system to have three single-phase, 5 kVA units with controllers in the slave units. In this configuration, the controller in the master unit still drives the two slave units through a system interface connection, and users can control all three phases from a single front panel and operate in a phase locked mode.

When operating in phase-locked mode, the slave unit controllers must disabled, but the additional cost for the controllers units is offset by enhanced flexibility. For example, a 15003iX-MB system in which each unit has a controller allows you to configure the system in the following ways:

  • 15 kVA 3 ø system
  • 15 kVA 1 ø system
  • 10 kVA 1 ø system and (1) 5 kVA, 1 ø system
  • 5 kVA, 1 ø systems

Three-phase systems may be broken up into individual single-phase AC/DC sources to be used in different test stands. It is even possible to combine several single phase units into a higher power single-phase system by paralleling the output of two or three 5 kVA units to create a 10 kVA or 15 kVA single-phase system.

Single phase systems only require a single controller. If multiple units are used in parallel, the master unit’s controller is used while slave controllers are disabled, and you control the system with a single front panel. The following configurations are possible with the 15001iX-MB:

  • 15 kVA 1 ø system
  • 10 kVA 1 ø system and (1) 5 kVA, 1 ø system
  • 5 kVA, 1 ø systems

When used in a multi-box configuration, the California Instruments iX series allows you to configure an AC/DC power system that meets the needs of a wide array of applications and eliminates the need to purchase many different AC or DC power sources. For more information on configuration options, download Application Note 118, “Multi-Box iX Series Configurations.” To discuss your application needs, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Maximizing UPS Battery Life

Written on August 8, 2016 at 12:26 pm, by

GUPS_Main_ImageThe Elgar GUPS Series of uninterruptible power supplies are ruggedized on-line UPSs that accept a broad range of worldwide utility and military AC input power. Without operator intervention, they automatically select the appropriate input power ranges to accommodate global operation. The batteries used in the GUPS Series units are a spiral wound, valve-regulated, lead-acid batteries. While the unit itself maximizes battery life with automatic, microprocessor-controlled equalization and temperature compensation during charging, there are steps that users can take to extend UPS battery life even more.

What shortens UPS battery life?

Sulfation, a natural electrochemical reaction in lead acid batteries, is the primary culprit, but other phenomena also shorten UPS battery life. These include:

  • nonoperational discharge,

  • cell impedance,

  • operating and storage temperature,

  • the number and depth of discharges and charger characteristics.

Sulfation. In normal operation, a chemical reaction between the sulfuric acid electrolyte and the lead plates in a battery forms lead sulfate crystals. These crystals behave like insulation, hindering the battery’s ability to accept a charge. Sulfation also causes an increase in cell impedance. The rate of sulfation increases when the battery is exposed to higher temperatures, when the battery is stored for a long time without a recharge, and when the battery is stored in a discharged state.

Non-operational discharge. Even when powered down, the GUPS Series units draw a small current (300-500 µA). Users can remove the battery drawer from the GUPS chassis, but the batteries will continue to self-discharge when not in use. If the GUPS is stored without being recharged after the battery is used, the additional self-discharge will damage the battery

Cell impedance. Sulfation causes an increase in cell impedance, and eventually this increase in impedance will reduce the battery’s output voltage full charge. Sooner or later, the batteries will be unable to power the GUPS.

Operating and storage temperatures. Higher operating and storage temperatures reduce battery life by increasing sulfation rates.

Number and depth of discharges and charger characteristics. When operating a GUPS at higher temperatures, the charge voltage must be temperature-compensated by reducing the voltage to avoid an over voltage. At lower temperatures the charge voltage must be temperature-compensated by increasing the charge voltage to prevent undercharging. Although these charger related parameters are outside an end user’s control, they are taken into account in the GUPS system design.

How to make batteries last longer

While it is impossible to eliminate sulfation or self-discharge, there are some things that users can do to improve battery life, including:

  • recharge a discharged battery before storing it,

  • store the battery at a cool temperature, and

  • charge a battery for 72 hours prior to using it after it has been in long-term and/or high-temperature storage.

Another thing that users can do is to control operating temperatures, When and where possible, reduce the operating temperature will reduce sulfation, thereby extending battery life.

Summary

  • Keep the battery charged and cool.

  • Remove the battery drawer from the GUPS chassis if you need to store the batteries for more than 30 days.

  • Do not store the battery drawer in a discharged state.

  • Recharge the battery after use and before storing.

  • Store at lower temperatures. A fully charged battery drawer can be stored for 10 months at 10º C, but at 40º C this is reduced to 1.5 months.

  • Before use after long-term storage, charge the batteries in the chassis for 72 hours.

Finally, consider placing the GUPS batteries on a cyclic life-extension maintenance schedule where the chassis-in-use is swapped out with a stored battery drawer, at intervals frequent enough to minimize the effects of long-term storage, storage at high temperatures, and self-discharge.

For more information on the GUPS Series of uninterruptible power supplies, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Introduction to IEC 61000-4-11, Part II – AC source requirements

Written on August 2, 2016 at 7:04 am, by

In Part I, we introduced you to the concept of testing equipment for immunity to voltage dips and short power interruptions in accordance with IEC 61000-4-111. In addition to specifying the test waveforms, the standard also specifies AC source requirements for full compliance testing.

Most of these requirements are easily met by the California Instruments iX Series AC/DC power sources. Some, however, are not trivial and warrant more attention:

  • 500 A EUT inrush current capability

  • 1 to 5 msec rise and fall time at source output

  • Current capability at reduced voltage levels

The inrush current requirement of 500 Amps however is not practical, as it would raise the price of the AC source considerably. Since most EUTs don’t draw this kind of inrush current, sizing an AC source for this current level is impractical. Instead, the standard allows the source to measure the peak inrush current to the EUT and verify that it does not exceed the capabilities of the AC source. Our instrument control software provides a “peak inrush current pre-test” option in the IEC 61000-4-11 test window for this purpose. If the peak inrush current of the EUT, as defined in the IEC standard, exceeds the AC source capability, a warning is issued to the operator.

To fully meet the AC source qualification for IEC 61000- 4-11 testing, option -EOS can be added to the iX Series AC/DC power source to ensure output rise and fall times between 1 and 5 micro seconds. This option also ensures the source’s ability to deliver higher rms currents at reduced voltage levels for constant power products. Specifically, 5001iX with -EOS option meets the 23 A at 70% and 40 A at 40 % of Unom requirement called out in the IEC 61000-4-11 standard.

Pass/fail criteria

The pass/fail criteria in IEC 6100-4-11 is rather vague. It says,

“The test results shall be classified on the basis of the operating conditions and functional specifications of the equipment under test, as in the following, unless different specifications are given by product committees or product specifications.

a) Normal performance within the specification limits.

b) Temporary degradation or loss of function or performance which is self-recoverable.

c) Temporary degradation or loss of function or performance which requires operator intervention or system reset.

d) Degradation or loss of function which is not recoverable due to damage of equipment (components) or software, or loss of data.

As a general rule, the test result is positive if the equipment shows its immunity, for the duration of the application of the test, and at the end of the test the EUT fulfills the functional requirements established in the technical specification.”

When using the iX Series to perform these tests, you can use the source to measurement measure the load current of the device under test to determine if it is still operating after applying the voltage dips and interruptions. Note, however that this measurement doesn’t really determine if the unit is really functioning, only drawing power. For example, if you’re testing a microprocessor-controlled device, the processor may have locked up or rebooted during the test. In this case, you may want to run some kind of functional test to determine if the device has passed or failed the test.

For more information on IEC 61000-4-11 testing, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Reference

  1. IEC 61000-4-11:2004, Electromagnetic Compatibility (EMC) – Part 4-11: Testing and Measurement Techniques – Voltage Dips, Short Interruptions, and Voltage Variations Immunity Tests. International Electrotechnical Commission. https://webstore.iec.ch/publication/4162.

Introduction to IEC-61000-4, Part I

Written on July 25, 2016 at 1:14 pm, by

Mains voltage dips and short interruptions can be caused by a wide variety of phenomena and can cause equipment to operate unreliability, and in some cases, can damage the equipment. Faulty loads on an adjacent branch circuit, for example, can cause a circuit breaker to trip, and high-power loads such as welders, motors and electric heaters can cause voltage variations. Natural events, such as power lines downed by storms or lightning strikes, may also disrupt mains power.

If the fault is in the power distribution grid, an automatic recovery circuit may cycle open and closed several times within a short period attempting to clear the fault. This will likely result in a sequence of short voltage interruptions as seen by downstream loads.

Voltage variations are typically caused by high power loads that have continuously varying power requirements. These voltage changes can affect the operation of nearby electrical and electronic equipment and sometimes even damage it.

To ensure that products can withstand these interruptions and voltage variations and operate safely and reliably, you must test them under a variety of conditions. IEC 61000-4-11, Electromagnetic Compatibility (EMC) – Part 4-11: Testing and Measurement Techniques – Voltage Dips, Short Interruptions, and Voltage Variations Immunity Tests, aims to standardize the way companies test how their products react to power line variations. Although it is a European standard, manufacturers use it worldwide for design verification testing.

IEC 61000-4-11 was first published in 1994 (Edition 1.0) and amended in 2000. Its scope includes electrical and electronic equipment with an input current rating not greater than 16A per phase. It is one of the required tests in the Generic Residential, Commercial and Light Industrial immunity standard EN50082- 1: 1997 and is also under consideration to be included in the Generic Industrial immunity standard EN50082- 2. It is also included in several product-specific standards such as the EN 61326-1 which went into effect in mid 1999 and covers testing instruments, data acquisition and control systems.

IEC 61000-4-11, edition 2.0, was released in March 2004. Edition 2.0 references IEC 61000-2-8, Environment – Voltage dips and short interruptions on public electric power supply systems with statistical measurement results. It differs from the first edition in the following ways:

  • Preferred test values and durations have been added for the different EMC environment classes 1, 2, 3 and user class X.

  • The recommended voltage dip and interruption durations are shorter.

  • A new test level of 80% of Unom was added for voltage dips test. The 70% test level remains as well, however, and is still used in all product standards.

  • Voltage variations are now done using an abrupt voltage change from Unom instead of a voltage sweep. The change from the reduced level back to Unom is still a sweep however.

  • All durations for voltage variations are now expressed in no. of cycles of the fundamental AC frequency instead of seconds. The number of cycles for 50 Hz and 60 Hz is chosen so that the time intervals are the same.

  • Tests for three phase systems have been specified.

Figures 1 through 3 show timing diagrams for various power disturbances specified by IEC 61000-4-11. Figure 1 shows a 40 % voltage dip. Figure 2 shows a voltage interruption. The duration of either is defined in number of cycles of the fundamental frequency. The actual change in voltage may occur at a set phase angle, e.g. 0° or 90°.

figure-1

Figure 1. 40% voltage dip waveform specified by IEC-61000-4-11.

figure-2

Figure 2. Voltage interruption waveform specified by IEC-61000-4-11.

Figure 3 shows the timing for a voltage variation. This waveform is different from the voltage variation waveform in edition 1 and better simulates the effects of motor loads starting up on the mains voltage.

figure-3

Figure 3. Voltage variation waveform specified by IEC-61000-4-11.

In Part II, we will discuss test levels, the pass/fail criteria for IEC 61000-4-11 testing, and test generator requirements. For more information on IEC 61000-4-11 testing, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Frequently-Asked Questions about the Sorensen SG Series Power Supplies

Written on July 18, 2016 at 6:37 am, by

The Sorensen SG Series is one of the most popular high power programmable DC power supplies on the market. The SG Series is designed for exceptional load transient response and low noise output. With a full 15 kW available down to 20VDC output in a 3U package the SG Series leads the industry in power density. The power density is enhanced by a stylish front air intake allowing supplies to be stacked without clearance between units.

In addition to having the highest power density in the industry, the Sorensen SG Series has a low noise output and exceptional load transient response.

In addition to having the highest power density in the industry, the Sorensen SG Series has a low noise output and exceptional load transient response.

Here are some frequently-asked question about the Sorensen SG Series:

  • How do I choose a circuit breaker or wire size for connecting the AC input power to my SGA / SGI power supply?

For 5 kW – 15 kW output SGA/SGI models, we recommend using a 100 A circuit breaker or fuse. For 20 kW – 30 kW output SGA/SGI models, we recommend a 200 A circuit breaker or fuse. If you are unfamiliar with high power, electrical AC connections, contact your facilities manager or an electrician for assistance.

  • Can I power-up my SGA/SGI power supply using single-phase AC input power?

No. SGA/SGI power supplies require 3-phase AC input power. See the Operation Manual for more details.

  • Can I change the AC input voltage required by my SGA/SGI power supply?

No. SGA/SGI power supplies’ AC input power cannot be changed once a unit is built. Each supply uses input filter boards designed to work with a particular AC input voltage. See the Operation Manual for more details.

  • What is the connection orientation or phase rotation of my input AC line phases and where do I connect neutral?

SGA/SGI power supplies do not require a specific phase rotation for input AC lines. Neutral is not required or used and should never be connected. See the Operation Manual for more details.

  • What is the slew rate of my SGA/SGI power supply?

SGA/SGI power supplies have a typical slew rate of 100 ms. Slew rate is defined as the time it takes the output to change from 5-95% of full scale. SFA slew rate is 250A/ms rise, 200A/ms fall at full load (minimum) 400A/ms typical.

  • When I have the SGA/SGI front panel switch off, the top of the unit gets warm. Is this a problem?

No, this does not indicate a problem. The front panel switch on SGA/SGI power supplies is a soft enable/disable shutdown and not a circuit breaker. If you use only the front panel switch to turn of the power supply, portions of the internal circuitry remain live. The heat that you notice is generated by this live power feeding the soft-start circuit in the SGA/SGI, which limits the inrush current at power up. Removing external power from the SGA/SGI supply via an external contactor or circuit breaker will eliminate this heating effect.

  • I want to parallel multiple SGA/SGI power supplies, but did not receive any parallel cables, should this have been shipped with the units I purchased?

Parallel cables are not part of the standard model ship kit. Parallel cables, part number 890-453-03, are sold separately as accessories. Please contact Ametek Programmable Power Sales for a quote or order details.

  • Must I connect remote sensing to operate my SGA/SGI power supply?

No. Remote sensing is not required for operation, but it is recommended and needed to achieve specified load regulation. See the Operation Manual for more details..

  • What maintenance is required for my SGA/SGI power supply?

We recommend that you perform a visual inspection and verify the calibration annually. We also suggest cleaning the supplies at least once a year. See the Operation Manual for more details.

For more information on the Sorensen SG Series, contact AMETEK Programmable Power by sending an e-mail to sales.ppd@ametek.com or phoning 800-733-5427.

Generate Fast Transient Tests with an Electronic Load

Written on June 23, 2016 at 8:30 am, by

To ensure that automotive electronics can withstand the voltage transients induced on the power bus, companies use expensive arbitrary waveform generation and coupling networks during the design phase. For production test, however, a simpler and less expensive means for simulating bus voltage variations and transients is required. For many transient tests, all you need is a switching power supply and an electronic load.

Here, we will show how to use an economical electronic load (eLOAD1 ) and SMPS DC supply to create a typical automotive power bus voltage transient for an automotive ECU test stand. In particular, this setup will generate a 12V to 4.5V transition in less than the 15 ms, a common specification for automotive testing. In addition, we will show how to configure a test system able to test multiple units under test (UUTs), each drawing up to 60 A.

A block diagram of the test system is shown below. To supply power, the test system uses a programmable supply set at 14 VDC and operating in constant voltage mode with a current limit set to the sum of the max current for the UUTs. To generate the transient, the system uses a programmable electronic load to pull the bus voltage down to the levels required by the test.

Screen Shot 2016-06-23 at 11.28.43 AM

To demonstrate the feasibility of this concept we connected a DLM40-15 power supply and an SLM 60-60-300 electronic load as shown in the block diagram. The electronic load is used in constant voltage (CV) mode. Using the load in this mode works because the load sets an impedance low enough to cause the power supply to change modes from constant voltage to constant current. In constant current mode, the power supply reduces its output voltage once the programmable current limit is exceeded. This method of generating transient voltages will work with any Sorensen DC supply that has automatic CV / CC mode crossover control.

For this demonstration, we used a fixed 1 ohm power resistor to simulate a single UUT load of 14 A at 14V. At the beginning of the test, the electronic load (in CV mode) was set at >15V, so it was effectively out of the circuit. At t=1, the load was switched to V=4.5V. At that point, the power supply switched to CC mode and the output dropped to the required level. In this test, the voltage dropped to 4.5 VDC in about 4 msec, well below the 15 ms specification.

Scaling up

To test more than a single UUT, we need a supply capable of supplying more current and a load that can dissipate more current. For example, to simultaneously test four UUTs, each drawing 60A, we had to consider the following:

  • UUT load impedance = 14V/60A = 0.233 ohms.
  • At 4.5V, IUUT = 4.5 V / 0.233 ohms = 19.3 A.
  • The current flowing in the electronic load is Iload = Imax – IUUT = 60 A – 19.3 A = 40.7 A.
  • The maximum dissipation, Pdiss = Iload X 4.5V = 183 W per DUT

To test 4 UUTs we would need an electronic load that could dissipate 4 x 183 W or 732 W. The power supply for this test system would need to deliver 240 A at 14V, or about 3,400 W. A good choice for the power supply would be the SGA 15X267 (15V at 267 A max), and a good choice for the load would be the SLH 60-240-1200 (60V, 240A, 1200W) max ratings. For additional current and power margin the SLH 60-360-1800 could be substituted.

For more information, download the application note, “Generating Fast Transient Voltage Test Profiles.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.

Understanding AC Power Source Measurements, Part 4: Analog or Digital?

Written on June 23, 2016 at 8:27 am, by

I-IX_SII_Main_Image

The California Instruments iX Series are AC sources that use a digital measurement system.

Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test, but some AC sources, such as the California Instruments I-iX Series II, perform measurements as well. In this series of blog posts, we’ll discuss various aspects of making measurements with your AC power source:

  • Part 1 describes the benefits of using sources for measurement and how to make voltage and current measurements.

  • Part 2 describes how to make frequency and power measurements.

  • Part 3 describes how to make power factor and crest factor measurements.

  • In Part 4, we’ll talk about how AC power sources perform measurements.

Analog or Digital?

Measurement systems can be implemented in one of two ways, analog or digital. Analog systems use one or more RMS converters to determine the RMS level of the voltage and current signals. For true power measurement, an analog multiplier is used which multiplies voltage and current and feeds the output into an RMS converter.

The California Instruments iX Series uses a digital measurement system.More often than not, analog measurement systems use a single RMS converter in combination with a multiplexer. The multiplexer routes the required signal to the RMS converter. The RMS converter output is digitized using a high resolution, but slow, sample rate A/D converter. No high-speed A/D conversion is required with this approach. This significantly reduces cost and required board space.

AC power sources with analog measurement systems generally cost less and provide very accurate readings. The California Instruments RP Series is an example of an AC power source with an analog of measurement system.

A digital measurement system, on the other hand, digitizes voltage and current signals in realtime using high-speed analog-to-digital converters (ADCs). To make true power measurements, each channel (voltage and current) must either have its own ADC or a sample and hold circuit to ensure voltage and time measurements are made at the same time. If not, calculations for power and power factor will be inaccurate.

The instrument stores the measurements in memory and processes them to produce true RMS, power, power factor and crest factor readings. Digital measurement systems use a digital signal processor (DSP) in order to perform the required calculations fast enough.

The California Instruments iX Series are AC sources that use a digital measurement system. To ensure that the measurements are highly accurate, the measurement system use a pair of 18-bit sigma-delta analog-to-digital converters to digitize voltage and current values.

While digital measurement systems are generally more expensive than analog systems, they have capabilities not found in analog systems that justify the higher cost. Since the time domain information for the voltage and current is available, the DSP can apply digital filtering and FFT algorithms to provide harmonic analysis of both voltage and current. In addition, the time domain waveforms can be displayed on the front panel of the AC Source or on a PC, providing the operator with more details on the voltage and current than would be possible with an analog measurement system.

For more information, download the application note “Understanding AC Power Source Measurements.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.

Understanding AC Power Source Measurement, Part 3: Power Factor and Crest Factor

Written on April 26, 2016 at 7:19 am, by

Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test, but some AC sources, such as the California Instruments I-iX Series II, perform measurements as well. Part 1 describes the benefits of using sources for measurement and how to make voltage and current measurements. Part 2 describes how to make frequency and power measurements. In Part 3, we’ll discuss how to make power factor and crest factor measurements using an AC power source.

Power factor

Power factor is the ratio between true and apparent power. When the voltage and the current are both sinusoidal, the power factor is equal to the cosine of the phase angle between them. More often than not, however, the current contains higher order harmonics and determining the power factor is more complex. To determine the power factor, the AC source must have the ability to measure RMS voltage, RMS current and true power in order to accurately calculate power factor.

Since the true power can never exceed the apparent power, the power factor is always less than or equal to one. While the power factor does not provide any information about the phase shift between the voltage and current, a sign is often used to indicate whether the current is leading or lagging the voltage.

Note that the power factor is a ratio between two measurements, one of which (VA) is the product of two other measurements. This has an impact on the accuracy with which power factor can be measured. If the power level of the EUT is low, percent of scale errors in the current and power measurements will impact the accuracy of the calculated power factor. Published accuracy specifications for power factor measurement may only apply above a certain current or power level.

Crest factor

Crest factor is the ratio between the peak current and the RMS current drawn by a load. To measure crest factor, the AC source measurement system must be able to measure the true RMS current as well as the peak current. Some AC Sources such as the California Instrument 5001iX offer both single shot peak current and repetitive peak current modes. Single shot peak current mode is more suitable for determining an EUT’s inrush current and should not be used to determine crest factor. The repetitive peak current is normally used to calculate the crest factor.

These two waveforms both have an rms current of 5 A, but their crest factors are very different.

These two waveforms both have an rms current of 5 A, but their crest factors are very different.

When the load current is purely sinusoidal, the crest factor is equal to the square root of two. Any distortion of the AC current will often result in higher crest factors. High crest factor put more stress on AC source because the source must provide the large instantaneous currents associated with the distorted current waveform.

Most AC Sources have a maximum crest factor specification. The ac power source measurement range for crest factor specified generally also applies to the RMS current measurement. Most RMS current measurement systems have a limited crest factor range. This means that the RMS current measurement accuracy specifications may not apply if the current being measured has a crest factor that exceeds this rating.

For more information, download the application note “Understanding AC Power Source Measurements.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.

Understanding AC Power Source Measurements, Part 2: Frequency and Power

Written on March 31, 2016 at 7:23 am, by

Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test, but some AC sources, such as the California Instruments I-iX Series II, perform measurements as well. Part 1 describes the benefits of using sources for measurement and how to make voltage and current measurements. In Part 2, we’ll discuss how to make frequency and power measurements.

Frequency

Frequency is measured at the output of the AC source based on the voltage signal. For AC sources that use a direct digital synthesizer (DDS) to generate the output reference clock, the frequency accuracy of AC source’s output often exceeds the accuracy of the frequency measurement.

Real and Apparent Power

Apparent power (VA) can be calculated by multiplying the RMS voltage with the RMS current. Thus, an AC source that does not offer this function but does measure RMS voltage and current can still provide VA readings when used in a test system. The test program can do the calculation.

Measuring true power, however, is not this simple. Unless the load presented by the unit under test (UUT) is purely resistive, the voltage and current will not be in phase, and the true power is less than the apparent power. Only a true power measurement will provide an accurate measurement of the power being dissipated by the UUT.

True power measurement requires the integration of the instantaneous voltage and current product over time. The result of this calculation is shown in the figure below. Notice that the instantaneous power dips below zero for some period of time. Since many UUTa are not purely resistive, true power is an important measurement function.

Screen Shot 2016-03-15 at Tue, Mar 15 - 7.06PM

For more information, download the application note “Understanding AC Power Source Measurements.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.