Bill Martin, Senior Applications Engineer, AMETEK Programmable Power
The proliferation of renewable-energy applications is driving an increasing need to test batteries, fuel cells, solar panels, and other devices that produce renewable DC power. An electronic load—typically programmable and designed to draw various and specified amounts of power from a source—is the ideal instrument to characterize DC-output devices under test in the lab or on the production floor. When selecting an electronic load, you will want to consider several options, including power, voltage, and current ratings; cooling method; operating modes; calibration techniques; transient response time; computer-control interfaces; and protective features.
One of the first choices you will need to make is to determine your voltage, current, and power ratings, including some subtleties such as whether you need a load that can sink high currents at very low voltages. Based on your power rating, you will need to choose between an air-cooled and a water-cooled load. Air-cooled loads offer advantages with respect to flexibility—they are self-contained and can be moved throughout your facility without regard to plumbing. Water-cooled supplies are smaller and less costly than air-cooled supplies for a given power rating, and they don’t burden your facility’s HVAC system with extra heat do dissipate. A 1kW air-cooled load probably won’t unduly burden your HVAC system, but a 50kW air-cooled load very well might.
The exact power level above which a water-cooled load is the preferred solution depends on a variety of factors, including space, your application, and your facility. AMETEK Programmable Power’s electronic loads employ field-effect transistors (FETs), and as a rule of thumb, you can use about 50% of the capability of each FET in your electronic load with air cooling, vs. 85% for a water-cooled load, resulting in approximately 35% savings in the number of FETs you need for a given power level, reducing your costs and space requirements. For a 7.5kW rating, a water-cooled load can cost half as much as an air-cooled load.
However, water-cooled loads lack the flexibility of a self-contained air-cooled unit, and if you don’t already have water-cooling infrastructure, you will need to install a chiller and associated plumbing, which could be a difficult and costly task depending on the layout of your facility. You will be locating the chiller at some available location outside your facility and will need bring in the water to the location of your load or loads. If you do buy a chiller, plan for growing requirements for water-cooling in the future.
Next, determine what operating modes you will need. In constant-current mode, the electronic load will maintain a programmed constant current regardless of input voltage (within the load’s specifications); in constant-voltage mode, the load will sink sufficient current to maintain a constant voltage on its input. Some loads also offer constant-resistance and constant-power modes. In the former mode, the load will maintain a constant resistance by sinking a current linearly proportional to the input voltage. In the latter, the load will adjust its current in response to a change in voltage to maintain a constant power level.
Your application might also call for a load that can sink full rated current at a very low input voltage, as illustrated in Figure 1. You will also want to make sure the load includes your preferred computer-control interfaces, including GPIB (IEEE-488.1 or IEEE-488.2), RS-232, Ethernet, or USB as well as support for SCPI commands.
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Figure 1. Some electronic loads can draw full rated current (in this case 60A) at very low voltage levels (in this case less than 0.5V for a load rated at 120V)
Make sure the electronic load you choose offers the types of programming your application requires, whether via the instrument’s front panel or via a remote interface. Figure 2, for example, shows current vs. time for a dynamically programmed stepping mode, where the varying current levels over time can simulate battery drain for testing portable battery-powered products. Still other modes include pulse and toggle modes (Figure 3), which can help test the transient recovery time of a constant-voltage power supply. And finally, a continuous transient mode (Figure 4) generates a repetitive pulse train to test a power supply’s load regulation.
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Figure 2. An electronic load can simulate battery drain for testing battery-operated products
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Figure 3. Pulse (top) and toggle (bottom) modes can help characterize a device under test’s transient response.
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Figure 4. A continuous transient mode can test a power supply’s load regulation
You will also want to choose an electronic load with robust protection features, including overvoltage, overcurrent, overpower, overtemperature, and reverse-voltage protection as well as a remote sense fault feature to determine whether remote sensing leads are properly connected. In addition, electronic loads can offer fast response times, but sometimes fast response times can lead to the occurrence of instability—due, for example, to the inductance of long leads between the load and device under test. A load with oscillation protection will offer adjustable bandwidths to eliminate such instabilities or will shut down when detecting an oscillation that exceeds a preset oscillation threshold. This feature can protect both the load and the device under test. Finally, note that in electronic loads incorporating FETs, the FETs can be subject to cascading failures. Look for a load with individual FET protection to avoid this issue.
Air and Water cooled Loads
AMETEK Programmable Power offers two series of programmable electronic loads: the air-cooled PLA Series, with standard ratings from 800W to 7.5kW, and the water-cooled PLW Series, with standard ratings from 6kW to 36kW. Custom configurations are available with significantly higher ratings. Air-cooled loads above 7.5kW and water-cooled loads above 36kW come in a cabinet.
Both series offer constant-current, constant-voltage, constant-resistance, and constant-power modes as well as fast response times. The loads can switch from one current level to another in 50 μs, regardless of whether the load is changing from 1A to 4A or from 10A to 500A. They offer several programming modes that can be initiated via the front panel or remotely via SCPI commands over the computer-control interface.
Other features of both series include a 0 to 10V analog programming input and user-friendly front-panel controls. Both support ultralow-voltage operation and can sink full rated current at 1% of rated voltage. RS-232 and GPIB interfaces come standard; Ethernet and USB are optional. Both series also offer individual FET protection and oscillation protection.
In addition, a closed-case calibration feature on both series allows the user to perform a complete calibration through software without opening the unit or adjusting a potentiometer. The user needs a DMM; a 0.01Ω, 0.005% precision shunt resistor; and a power supply capable of matching the load’s rated voltage and current. This calibration feature eliminates the recurring costs and downtimes associated with sending a unit back to the factory or employing an expensive third-party calibration service.
Specific standard models in the air-cooled PLA Series offer standard voltage ratings of 60V, 120V, 400V, and 600V with standard current ratings up to 1,500A. Standard power ratings include 800W, 1.5kW, 2kW, 2.5kW, 3kW, 4kW, 5kW, and 7.5kW versions. Customized versions offer voltage ratings up to 1,200V and power ratings greater than 30kW.
Like the PLA Series, models in the water-cooled PLW Series offer standard voltage ratings of 60V, 120V, 400V, and 600V with standard current ratings up to 1,500A. Standard power ratings include 6kW, 9kW, 12kW, 18kW, 24kW, and 36kW versions. Custom systems are available with ratings exceeding 180kW. Significantly larger requirements can often be addressed by using multiple individual loads in parallel.
PLW Series loads include an anti-condensation circuit to eliminate internal condensation. The unique design also offers a water system shutdown when power is removed, eliminating the chance of condensation resulting from continued water flow during emergency shutdown.
Note that the PLA Series and PLW Series use air and water, respectively, to carry away excess heat. Alternatively, a regenerative instrument can transfer some excess power to the grid. This approach, however, has its own drawbacks. A regenerative load would lack the fast transient response of the PLA and PLW Series. And a system that provides power to the grid raises concerns with respect to standards and safety. For example, a grid-connected regenerative system must meet anti-islanding requirements—that is, it must shut down within a specified time should the grid go down, and power companies are concerned about transients as customers alternate between regenerative and nonregenerative operation. A regenerative approach can be beneficial in applications that generate high power nearly continuously, and AMETEK Programmable Power offers the capability on its four-quadrant AC power sources for applications such as grid simulation for photovoltaic inverter test. But for applications served by the PLA and PLW Series, regeneration adversely affects transient performance and imposes unnecessary costs related to standards compliance and related concerns that generally make the approach impractical.
Electronic loads serve a variety of markets in addition to renewable energy, including defense/aerospace, avionics, industrial, and automotive applications. They enable test and validation of devices including batteries, ultracapacitors, DC power supplies, battery chargers, UPS systems, fuses and circuit breakers, and power-electronics components. They can be found in an ATE rack or cabinet or on a benchtop. AMETEK Programmable Power’s PLA and PLW Series offer a high power density and performance as well as cooling and operating-mode options to serve fuel-cell manufacturers and users, battery manufacturers and users, solar-cell users, power-supply manufacturers, and system integrators. AMETEK Programmable Power’s sales channel stands ready to serve all these industries and application areas.