New lightweight alternator for the standby market

Cummins Generator Technologies, the alternator manufacturer based in Stamford, UK, has produced a new range of its Stamford brand alternators targeting the 7.5 to 62.5 kVA power range. At the official launch, at the Middle East Electricity 2016 show in March, Cummins will bring to market what it calls the most compact and lightweight industrial alternators of their kind, aimed primarily at the global standby market.

Customer response

Cummins is basing its buyer appeal on a survey of customer needs and wishes that resulted in a re-engineered machine that could be coupled to its prime mover 20 % faster than previous models, and with specially designed supports and SAE options to make it compatible as a replacement for virtually any other alternator on the market. It is also said to be lighter and more compact than the company's previous alternators, and readily convertible between 50 and 60Hz.

In terms of reliability it has been designed to perform for a minimum of 20 000 hours, with all components subjected to 9000 hours of highly accelerated life testing, and is compliant with global codes and standards, including IP23 protection as standard, UL certification and IEC/NEMA compliance. To date under test conditions the new range has proved reliable during more than 20 000 hours of operation.

Options include auxiliary winding for improved motor starting capability, and accessories including space heaters, and diode failure detector.

Scott Strudwick, a sales director at Cummins commented: "We recognise that reliability is a priority for the standby market, alongside the delivery of a product which is both compact and lightweight. We have therefore worked with our customers to develop a technological solution that will support their long-term business growth ... [enabling us] to deliver a product range which is durable, reliable and easy to assemble."

Other features

Stamford alternators are available with bearing and stator winding resistance temperature detectors (RTDs) for protection against thermal overload. They exhibit salient pole rotor construction, which when combined with a specially designed fan, ensure optimum air flow to maximise cooling. And a variety of analogue and digital automatic voltage regulators (AVRs) designed to achieve maximum performance is available.

Wire wound v bar wound

Cummins produces two brands of alternator, the AvK brand which is aimed mainly at applications in, for example, the oil and gas extraction business which prize robustness and reliability above electrical performance, and the Stamford brand which is more tailored to applications across the power generation range.

Across these two ranges both wire wound and bar wound alternators are available, the choice depending on the specific requirements of the application.

In a wire-wound design, the alternator windings are usually pre-formed wire bundles, which are inserted into the stator, often by machine processes. In a bar-wound alternator the copper windings of the stator are composed of individual copper bars. This leads to performance differences. For a similar winding size, the physical characteristics of the more compact wire- wound machine result in lower harmonic distortion in the voltage produced. On the other hand the rigidity of a copper bar produces a design that is more easily braced for mechanical stresses than is a wire-wound machine, and while its insulation characteristics are better it is more difficult to cool. To summarise, bar-wound alternator designs typically offer greater mechanical strength than wire-wound designs and greater dielectric strength, but they provide poorer performance in terms of voltage waveform quality, motor-starting performance and short circuit performance.

The bottom line is that in general for similar-sized machines, the bar-wound machine is best suited to medium-voltage applications and cases where the loads subject the alternator to severe mechanical stresses, while the wire-wound machine will provide the best service in applications where motor-starting capability and the best resistance to waveform distortions are important.

Author: Leonard Sanford

(Originally published in MPS February 2016)