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How To: Selecting Suitable AC Sources & Loads



August 19, 2015

Alternating-current (AC) power sources and loads can bring tremendous versatility to test systems and measurement applications, helping to precisely control the energy required for testing. Of course, selecting an AC power supply for different types of applications can be challenging. But the task can be made easier by better understanding key performance parameters that differentiate AC power supplies and loads, and matching AC source and load capabilities to meet current and future measurement requirements.

 

In a typical test environment, an AC power source will supply a repeatable, precisely controlled, low-distortion sinusoidal voltage to a device under test (DUT). The AC source must be capable of providing the amount of power required by the DUT. The power source should be accurate, consistent, and reliable, since characterization of the DUT essentially begins with the AC power that the power source supplies. It should provide current and voltage in the ranges required for testing. It should also provide control of transient responses, harmonics, and other forms of distortion needed to properly simulate the operating conditions of a DUT. An AC load serves as a termination point for the energy being fed from an AC power source, helping to properly set up the AC power source for both steady-state and transient power conditions.

 

Performance parameters for an AC power source include its input power requirements, current range, and input voltage range. Key performance parameters include its output voltage range, voltage accuracy, frequency, crest factor, power factor, total harmonic distortion (THD), response time, and slew rate. In general, the requirements for testing a DUT will determine the recommended ranges of these different AC power source parameters in a test application.

 

In addition to its levels of steady-state current, an AC power source must be capable of supplying considerably higher levels of transient and inrush currents, such as the current drawn by an electric motor when it is first turned on. For a given AC power source, for example, the inrush current level may be many times the nominal root-mean-square (RMS) current rating for that source. Although the inrush current duration requirements for a particular DUT may be quite brief, in the range of microseconds, a DUT may also require testing with inrush current lasting several seconds. So, a suitable AC power source for testing that DUT should be capable of providing a required steady-state RMS current level as well as considerably longer-duration high transient current levels.

 

An AC power source with fast response times will be capable of operating effectively in systems dealing with changes of voltage, frequency, and load. AC source response times are typically measured in microseconds, and a source with fast response time will be capable of simulating real-world operating conditions in a test system, such as voltage fluctuations and changes in current frequency.

 

Crest factor is the ratio of the peak current amplitude to the root-mean-square (RMS) AC amplitude. When a DUT requires input current with high crest factor, an AC source must be able to transfer current quickly to the load, and this requires low output impedance and high peak instantaneous current capability. An AC source with high crest factor will be capable of delivering extremely narrow pulses to a load. Power factor is the cosine of the phase angle between the voltage waveform and the current waveform. It is essentially the ratio of the real power being delivered by a supply to the apparent power capabilities of the supply. Power factor is a dimensionless value from -1 to +1 with the positive values representing more in phase voltage and current waveforms from the AC power source.

 

An AC power supply’s input range will determine suitable geographic application areas for the unit, with wide input voltage and frequency ranges permitting use around the world. A “universal” AC power source can be used around the world, including with much different input power/frequency requirements in Europe and the United States (US). For example, in the US, AC power sources are designed for operating at a voltage frequency of 60 Hz. AC power supplies with a wide input voltage range, such as 85 to 265 V, and wide frequency range, such as 47 to 63 Hz, enable operation almost anywhere in the world. Very high power AC supplies >2000W may require three phase input power.

 

Accuracy and control are important to any measurement application, and an AC power supply is typically specified in terms of its voltage accuracy and total harmonic distortion (THD). Ideally, both specifications should be as tight as possible, with voltage accuracy of +/-0.1% considered excellent and THD of 0.5% or less considered very good. Voltage accuracy will be dependent upon an AC source’s operating frequency, so specifications should be checked to determine whether voltage output accuracy is referenced to the operating frequency required for a particular DUT.

 

Of course, no AC power supply offers an ideal combination of performance parameters, with some stronger in some areas than others. But the requirements of a particular DUT or a group of measurements can usually establish the performance levels needed from an AC power source for testing that DUT under the conditions that it will most likely experience in typical use. For those seeking additional information on characterizing the performance of an AC power supply, California Instruments (www.calinst.com) offers a four-page Application Note #106, “Understanding AC Power Source Measurements,” free of charge on its website.

 

In terms of some real-world examples, a number of excellent suppliers of AC power supplies can be found on the Axiom website (www.axiomtest.com), with their AC power supplies organized by their output capabilities: AC power supplies providing 3 kVA and less, AC power supplies capable of more than 3 kVA to 10 kVA, and AC power supplies that can generate more than 10 kVA. These AC power supplies are available in a variety of configurations, including rack-mount and portable units, from leading suppliers, including California Instruments, Chroma, Keysight/Agilent, Kikusui, and Pacific Power Source.

 

Looking For A Load

An AC electronic load is a suitable companion for an AC power source in a test system, with its capability of providing repeatable, precisely controlled load conditions for the supply and the system. In addition to providing the capabilities to simulate real-world load conditions in a test system, an AC power load can simulate extreme conditions, such as high crest factors and variable power factors, for stress testing of a DUT. As with an AC power source, specifying an AC power load is a matter of matching the performance parameters to the needs of a DUT or set of DUTs.

 

AC loads as well as AC sources are available with a number of different control interfaces, including GPIB, RS-232C, and Standard Commands for Programmable Instrumentation (SCPI), to ease integration into automatic-test-equipment (ATE) systems. In addition, AC loads can be supplied with built-in-test (BIT) functions for metering and monitoring performance, which helps to simplify the number of instruments needed in a test system. Loads are available with numerous automatic operating modes, such as constant current or constant voltage operation, to help when developing measurement routines for a given DUT.

 

As an example, California Instrument 3091LD is an AC electronic load that is ideal for providing the precisely controlled loads needed for testing AC power generation equipment, such as AC power supplies. The model 3091LD can be remotely controlled by means of GPIB or RS-232C interface and can be used to simulate various load conditions to 3000 W average power and 13 kW peak power, including conditions with variable power factor and high crest factor. It provides operating modes for constant current, constant voltage, and constant resistance.

 

For another electronic load example, the model 63803 combination AC and DC electronic load from Chroma is rated for 3600 W power, 36 A (as much as 108 A peak current), and 350 VAC. It also provides multiple programmable control buses and the operating flexibility to support testing over wide voltage and current ranges and high crest factor.

 

It should be noted that both AC power sources and AC loads are limited by their wattage specifications. Maximum voltage and maximum current are not available simultaneously as that combination will far exceed their wattage, or total continuous power capability. In the above example of the Chroma model 63803, 350VAC and 36 amps gives a wattage of 12600 watts, which is 3.5 times the rated continuous wattage specification of 3600 watts. At a maximum current of 36 amps the largest continuous voltage available is 3600W/36A = 100VAC at a maximum voltage of 350 VAC, the maximum continuous current available is 3600W/350VAC = 10.3 amps.



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