Programmable AC laboratory power supplies current-regulated

The company ABB STOTZ-KONTAKT GmbH from Heidelberg develop, manufacture and sell products for electrical equipment and automation of buildings, machinery and equipment. The specialist for switching and control technology offers customers from industry and trade in addition to installation equipment and building systems also primarily encapsulated and non-encapsulated switches, circuit breakers/power switches and NH fuse load disconnectors. Twelve inspectors working in the company's own development laboratory prepare new products for the start of production and demonstrate to regulatory authorities compliance with statutory provisions as well as product compliance with various international standards such as IEC, CSA, UL and CCC.

"Our development laboratory is allowed to perform these tests themselves, because we are UL-accredited and thus meet the requisites for checking and certifying compliance with other standards", explains Simon Stephan from the laboratory for switching technology. "To this purpose, our laboratory is re-audited annually, which involves examining and checking testing services and processes as well as the technical equipment". Auditing the laboratory also requires appropriate power sources for the standards-compliant testing of such products as line safety switches and overcurrent cut-outs. The efficiency and accuracy of the current sources play an important role here, because high currents are required in part to check the behaviour of overcurrent relays, line safety and motor protection switches, thermal and electronic overload relays or small contactors at different loads.
 

Special device with flexible frequency range

Since 2013, ABB STOTZ-KONTAKT have been using a single-phase AC power source type EAC-S 9000 by ET System electronic for these safety-related testing tasks. The power supply specialist develops and manufactures AC and DC sources, laboratory power supplies, inverters, and electronic loads - and in addition to their standard product range, also mainly offers customer-specific modifications used to fulfil some very special requirements. ABB STOTZ-KONTAKT had such requirements when searching for a suitable AC source. The device was to have not only an accuracy of 0.2% but should emit currents up to 2000 amps and also mainly cover the frequency range between 50 Hz and 400 Hz.

The 400 Hz technology is becoming increasingly important, because compared to the established 50 Hz variant, the same current can be transferred with smaller converters or transformers and cable cross-sections can be smaller while maintaining the same power transmission. The thereby achievable saving in weight and space plays a role primarily in aircraft construction. The advantages of higher-frequency energy transmission are, however, also increasingly recognised in other areas, mainly in the rail technology and the shipbuilding industry.
This is why overcurrent relays or line safety and motor protection switches in 400 Hz technology will be increasingly in demand - a clear reason for ABB STOTZ-KONTAKT to expand their product range in this segment further and to acquire a suitable AC power source for the corresponding tests and inspections.
 

Special transformer enables high currents

The EAC-S 9000 offers a current adjustable over a wide range, from minimal power with 5 A at 160 V up to the full 2000 A at 9 V. These high currents, however, are not generated directly in the EAC-S 9000, because its maximum power is 60 amps. The emitted currents are fed to a special transformer first, which then converts the current to a different voltage and power level and in a second step then allows drawing the desired high currents with corresponding low voltages via different tappings. And it is with this special transformer that ET System electronic have ventured into uncharted technical terrain because hands-on experience with variable frequencies was really not available. Gripped by pure engineering ambition, the company here took the initiative even before the order was placed and commissioned a specialised transformer manufacturer to produce a traditional E-I transformer, which they used for initial experiments. These experiments, however, revealed that such an E-I transformer was unsuitable for the ABB requirements.

Thus, the developers decided to commission a second transformer based on a toroidal core because toroidal cores are generally less prone to saturation problems at higher frequencies than E-I cores. ET System electronic was able to find a transformer manufacturer daring enough to build a toroidal transformer for the frequency range of 50 Hz to 400 Hz. This was also a completely new challenge for this manufacturer because transformers are built for a fixed frequency of 50 Hz or 400 Hz, and the commonly used calculation software is tailored to one of these two frequencies as well. Overcoming the problems associated with the calculations and manufacturing of the variable frequency transformer was a tour de force in itself but six months later, the new toroidal transformer, a colossus with a weight of about 112 kilogrammes, was delivered to ET System electronic. Detailed tests confirmed that the new transformer fulfilled the requirements, although only within the boundaries set by physics: For example, only 800 A can be drawn with a frequency of 400 Hz because here too the toroidal core is limited by the boundaries of magnetic saturation and cannot transfer any higher currents.
 

Universal options for the test bench

The new AC source is used mainly in the context of new developments, but also serves to carry out tests for the production and other departments. Numerous tests can be carried out with the test bench of the development laboratory. For example, various currents are applied to the overcurrent relays or line safety and motor protection switches during thermal tests lasting hours, while thermal sensor measuring systems log the temperature progression on the housing. Duration tests, however, consist of applying the rated power to switches for several days to observe their behaviour at the trigger threshold. After applying the rated current, the test current is usually also increased to the trigger current at which the device depending on model or design must trip within a defined period of time. The AC source by ET System electronic by now has been in constant use for a year and the testers are more than satisfied with the device.

"Previously, we were able to experiment only up to maximal 100 A when using 400 Hz and the appropriate transmission ratios for the transformers had to be determined through testing", summarises Simon Stephan enthusiastically. "With our power supply by ET System electronic, we now have an altogether perfect solution for these tasks". The device is very accurate and reliable, and the entire technology including transformer and high-quality transducer are fitted in a compact enclosure". Since collaboration with ET System electronic also leaves no wish unfulfilled, Simon Stephan and his colleagues have recommended ET System electronic not only for other locations within the group, but already have also ordered a second power supply: A three-phase AC power source with 50 Hz which can emit currents from 90 mA to 1000 A with an accuracy of 0.2%. New technical challenges are associated with this tremendous bandwidth - but all those involved have little doubt that the developers of ET System electronic will overcome these problems as well and present another masterpiece of engineering in due time. Up to 2000 amps: Special version of the EAC-S 9000

Diagrams below: Test bench with motor and line safety switches

The company needed a reliably regulated power source which can supply three-phase AC current in a frequency ranging between 45 and 150 Hz up to 250 A in order to execute temperature rise tests on assemblies.

For temperature rise tests that are executed over several days, the power sources need to be very reliable and the voltage should remain under the voltage range considered normal for these units, ideally within the extra-low safety voltage range, in order to protect the operating personnel.

Test engineer Heiko Fischer found what he was looking for at ET System electronic: “Right from our first telephone call, we received reliable information and were told that adapting an AC power source, type EAC-3S unit, to our wishes would not be a problem. Their offer for a unit with an output voltage of 3x8 V AC at currents of up to 3x250 A was even cheaper than those from other suppliers, and when we went to visit the company, we obtained a very positive impression of the expertise and manufacturing quality at ET System electronic. We have now been using the units for some time and they have confirmed our excellent first impression. Our actions are also proof of this: we have just issued an order for our third unit from ET System electronic.

Diagrams below: ET System electronic unit in a Hager rack