Testing Electric Street Light Components with LabVIEW-Controlled Virtual Instrumentation

Author(s):
Ahmad Sultan - Integrated Engineering Technology (IET)

Industry:
Industrial Controls/ Devices/ Systems

Products:
LabVIEW

The Challenge:
Automated testing of magnetic ballasts used in electric street lights.

The Solution:
Developing a PC-based virtual instrumentation system using SCXI and DAQ products controlled by LabVIEW.

Introduction
Our task was to develop an automated test system for magnetic ballasts used in high-pressure sodium (HPS) streetlights. Our client, who manufactures ballasts for the North American and international markets, believes that product development and quality assurance require thorough and complete testing of prototypes and production samples to verify compliance with national and/or international standards.
The test system needed to accommodate the following:

• Different types of core and coil ballasts, such as reactor, autotransformer, constant wattage autotransformer (CWA), and constant wattage isolated transformer (CWI)
• Operating voltages from 120 to 600 V and rated lamp wattage from 50 to 400 W
• Capacitors for wattage control and/or power factor correction
• Different lamp igniters
• Open-circuit, short-circuit, lamp-starting, and lamp running tests
  

At the ballast input and output ports, we needed to measure true rms values of current and voltage, true power, and the ratio of watts to volt-amperes (power factor, if the voltage and current waveforms are clean sinusoids). Because HPS lamps are nonlinear loads, we monitor current and voltage peak values and crest factors, along with total harmonic distortion.

System Integration Approach
With the tight budget of a growing company, establishing a test bench with the functionality we required using conventional test equipment becomes difficult. We implemented a virtual instrumentation approach to achieve project objectives within budget while maintaining flexibility for future needs.

Virtual instrumentation consists of using mainstream computers, off-the-shelf plug-in instrumentation boards, and software. Because the virtual instruments you create with these products are user-defined, not vendor-defined, you can tailor applications to meet your needs exactly. Some of the benefits of virtual instrumentation are ease of use, flexibility, and savings of time and money. We used LabVIEW software as the heart of the instrumentation system. BallastVIEW is the name of the LabVIEW application we wrote to acquire signals, process data, and present results to the user on the computer screen.

The instrumentation system hardware consists of:

• 486 DX2-66 PC (12 MB RAM, 340 MB hard drive) running Windows
• Variac (manually adjustable transformer) supplying AC power to the ballast under test through the system test fixture\
• System test fixture containing switches and wiring required for the different test configurations
• Transducers for sensing current and voltage signals (such as resistive dividers and current shunts
• Antialiasing RC filters, with components selected to avoid loading the board input amplifiers
• 5B signal conditioning modules, to amplify and isolate the filtered signals
• National Instruments Lab-PC+, installed in the PC, to digitize the conditioned signals
  

The cut-off frequency of the antialiasing filters was set to half the sampling frequency; the RC filters also serve to protect the electronics items from the high-voltage spikes generated when the igniter starts the lamp.
We configured the Lab-PC+ board for bipolar differential input (four channels). We set the sampling frequency to 7680 Hz/channel. Acquisition was software-triggered on the rising slope of the input voltage.

BallastVIEW Presentation
The LabVIEW screen on the next page is the front panel of BallastVIEW. It illustrates a stack of VIs representing an input AC power analyzer, an output AC power analyzer, a waveform graph, and a harmonic analyzer. The controls at the top of the screen are switches for controlling acquisition, metering, harmonic analysis, and program execution. The user can capture a single shot or continuously acquire signals.
For the power analyzers, the indicators (from left to right in each row) display the rms, maximum, minimum, peak average, and crest factor of each signal. The active and apparent power, and their ratios, are displayed in the right column.

The waveform graph displays the signals acquired by the data acquisition (DAQ) board. Because both voltage and current waveforms are displayed, the ordinate is labeled in relative units (PU). To find the true amplitude of a particular signal, multiply its measured value from the graph, in PU, by the respective base value from the PU Base table (to the right of the waveform).

The line spectrum, shown in the bottom right corner, displays harmonic magnitude in either peak volts/amperes or per unit values normalized to the fundamental component of the respective signal. Magnitude of harmonics can be checked by flipping the cursors of the harmonic magnitude indicator (bottom center). The user can window signals before applying the Fast Fourier Transform.

Example Results
The results presented in the BallastVIEW screen are test results for a 200 W CWI ballast. The output power analyzer indicates that the lamp is operating at rated lamp power. Lamp voltage and current are very close to the ANSI reference specifications (100 V and 2.4 A). Lamp current crest factor (CCF) is 1.6 (1.8 is the maximum permissible). The input power analyzer indicates that the ballast draws 2.037 A at rated input voltage. Ballast loss is approximately 39 W and the power factor is high (0.973 lagging). The waveform graph shows almost clean input voltage and current signals. Output (lamp) voltage is the square waveform of a typical arc in a high-intensity- discharge (HID) lamp, containing the full odd harmonics spectrum. The magnitude of the lamp voltage third-harmonic component is 39 percent of the fundamental. Total harmonic distortion (THD) of lamp voltage and lamp current are 33.84 percent and 3.73 percent, respectively.
We verified the credibility of this system by obtaining agreement with test results from an independent test laboratory, electric utility companies, and customers of the ballast company.

Conclusion
BallastVIEW measures and displays the electrical parameters required to test and develop ballasts and performs on-line waveform analysis. The result is a flexible, high-performance, easy-to-use, and cost-effective PC-based measurement system, which saved time in both product development and production testing.
An advantage of using LabVIEW is our ability to increase BallastVIEW functionality in the future, for example, by monitoring the ballast-lamp characteristic curves and compiling results. The core of the BallastVIEW program constitutes the cornerstone for testing other electrical products, such as transformers, rectifiers, inverters, and UPSs, as well as for power line monitoring.

For more information, contact:

Ahmad Sultan

Computer Solutions, Inc.

One Eastern Heights Plaza

7582 Currell Blvd, Suite 109

Woodbury, MN 55125

Tel: (612) 288-0092

Fax: (612) 730-4286

E-mail: ASultan@ComputerSolutions.com

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