How Much Power Is Enough When Selecting an AC Supply

Operating active electronic devices as part of a measurement may require an AC power source. It should be stable and dependable, but it must also provide enough voltage and current to properly operate the device under test (DUT). An earlier blog, “Essentials of DC Power Supplies and Loads,” examined DC power supplies. But AC power supplies may also be needed for testing, and understanding their use can ease the task of finding one for a measurement application.

While precisely controlled AC electricity may not be needed when testing DUTs powered by DC power, an AC supply can help simulate real-world operating conditions. For certain DUTs, such as electric motors, a well-controlled AC power supply can mimic the conditions in which the DUT will operate, including power supply line fluctuations, peaks, and surges. AC power lines transfer electricity around the world, albeit at different frequencies for different regions.

AC electrical networks transfer power typically as sinewaves at a fixed frequency for one region. Europe, Africa, and Asia operate AC electrical networks at 50 Hz while the United States runs AC at 60 Hz. Japan uses both 50 and 60 Hz, for various parts of the country. AC power supplies can be selected by several specifications, such as voltage, current, and frequency. A supply can provide current across a range of AC voltages, with maximum current at a high voltage determining the highest output-power level. Power can be supplied in single-phase or three-phase form to suit the needs of a DUT and test application.

AC power supplies can be compared by electrical capabilities, such as maximum root-mean-square (RMS) voltage, maximum RMS current, maximum frequency, and maximum power in volts-amperes (VA) at a given frequency. They can also be considered by physical attributes, such as the size and weight needed to fit an application. Electrical parameters can be further defined in terms of accuracy and stability as well as resolution or how precisely a parameter, such as voltage or frequency, can be adjusted. Voltage accuracy of better than 1% is not unusual while voltage resolution of 0.5 V or better enables fine tuning. Additional consistency parameters include how closely the AC line and load are regulated, with a value of 0.2% for either parameter signifying respectable regulation for an AC power supply.

Sampling AC Supplies

Test applications requiring a single-phase AC power source can benefit from the wide-ranging versatility of the Chroma 61504 programmable single-phase AC power supply, rated for maximum output of 2000 VA. The rack-mountable unit tunes voltages as high as 300 VAC with 0.1-V resolution at AC frequencies from 15 to 1000 Hz. Frequency accuracy is 1% or better across that wide range, improving to typically 0.3% at 50 or 60 Hz. Maximum current is 16 A at 150 VAC and 8 A at 300 VAC, with peak current reaching as high as 96 A at 150 VAC and 48 A at 300 VAC.

When a DC supply is needed, the same source can provide it by means of the unit’s AC + DC output mode. The unit delivers as much as 1000 W DC power, with maximum DC amperage of 8 A at 212 VDC and 4 A at 424 VDC. The single-phase AC supply is programmable via GPIB, RS-232, USB, or Ethernet interconnections and can generate a wide range of noise power waveforms using harmonic and interharmonic distortion at bandwidths as wide as 2400 Hz. Based on pulse-width-modulation (PWM) waveform generation, model 61504 operates with single-phase inputs from 90 to 250 VAC at frequencies from 47 to 63 Hz. It measures 5.25 × 19.0 × 22.42 in. (133.35 × 482.6 × 569.5 mm) and weighs 44.05 lbs. (20 kg).

For single- and three-phase AC power testing, try the Chroma 61509, with voltage coverage of 0 to 175 VAC in single-phase mode and 0 to 350 VAC in three-phase mode. It matches the 2000 VA output rating of the model 61504 but when in three-phase mode, delivers three times as much (2000 + 2000 + 2000 = 6000 VA) output power. When even more output power is needed, two units can be connected in parallel in a master-slave configuration to produce 6000 + 6000 = 12,000 VA power. The model 61509 is larger than the single-phase model 61504, at 8.72 × 16.73 × 26.77 in. (221.5 × 425 × 680 mm) and weighs 121 lbs. (55 kg). But the larger size provides more power as well as more measurement capability. Model 61509 features DC output capability (about 75% of the full AC output-power rating) and an AC frequency range of 15 to 2000 Hz. It is a low-noise supply, with total harmonic distortion (THD) of less than 0.3% for AC outputs at 50 and 60 Hz.

The 5U rack-mountable model 61509 incorporates a 5.7-in. color LCD screen to simplify local operation and all the computer interfaces of the model 61504 (GPIB, RS-232, USB, and Ethernet) for ATE setups. Its “softpanel” graphical user interface (GUI) eases simulation of power-line noise and disturbances and simplifies measurements involving AC voltage and frequency slew rate. The GUI supports simulation of harmonic distortion based on as many as 50 orders of harmonics and can create various forms of interharmonic distortion based on known AC power-line frequencies, such as 50 and 60 Hz.

Multimode Operation

The combination of single- and three-phase modes is also available in power supplies from California Instruments, such as the AST1503 Asterion™ programmable power source. This compact supply fits a 2U rack-mount enclosure and provides single- and three-phase AC, DC, and AC + DC outputs at levels to 1500 VA. It is capable of 2.5 A current per phase in three-phase mode at 200 VAC and 7.5 A in single-phase mode at 200 VAC. It can operate at voltages to 400 VAC with the corresponding reduction in current capacity  equaling its maximum 1500-VA output rating.

For more power and a much larger size, there’s the California Instruments MX45-3PI high-power AC and DC power source, which can operate in single- and three-phase AC modes and deliver as much as 45 kVA output power. It operates at AC voltages from 0 to 300 VAC and frequencies from 16 to 819 Hz, with frequency adjustable to 0.01-Hz resolution. THD is less than 0.5% for AC waveform frequencies to 66 Hz and 1% or better for AC waveform frequencies to 500 Hz. It handles as much as 300 A total three-phase current at voltages to 150 VAC and 150 A three-phase current at voltages to 300 VAC. The power source generates a wide range of user-defined output power waveforms at high power levels but requires generous floor space in a test setup, measuring 50.00 × 28.75 × 34.50 in. (1270 × 731 × 876 mm) and weighing 600 lbs. (272 kg). When less AC output power is required, turn to the California Instrument AST18K3, a three-phase member of the Asterion AC + DC power supply family that delivers 6000 VA output power per phase or 18,000 VA AC power to a three-phase output  at AC frequencies from 16 to 1200 Hz.

This is a glimpse of several AC power supplies having sufficient output power for feeding a wide range of DUTs, including electric motors. These supplies combine generous power capabilities with wide voltage ranges and versatile functionality with most of the computer interfaces needed for ready use in an ATE system. For those who need additional information on these and other AC power supplies please visit the Axiom Test Equipment website at www.axiomtest.com.