FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1959
1959 - 0398.PDF
195 EXCITER COILS XVt-.vnONAL STATO" The simplicity of the Secsyn is evident from this slightly idealized "exploded" illustration SE C ST N AN IMPROVED ELECTRIC SYNCHRONOUS MACHINE SEVERAL years have now elapsed since the publication of theBekey-Robinson patent as a legal document. During thistime its principles have become familiar to only a small proportion of the world's users of electrical machinery, and even those most likely to welcome it have—not unnaturally, perhaps— been reluctant to adopt so unconventional a device. Now, how- ever, the Bekey-Robinson patent is being exploited by Jack and Heintz Inc. of Cleveland, Ohio, one of the leading American firms in the aircraft electrical field. They are to market machines using the principle under the name Secsyn (Stationary-Exciter-Coil SYNchronous). The Secsyn brushless generator is a salient-pole synchronous machine utilizing a conventional stator but incorporating a highly unconventional means of providing excitation to the rotating pole structure. Bekey-Robinson's design objective was to induce flux from stationary field coils into a rotating pole structure in a simple, efficient, lightweight manner. The simplest approach was to cause the field flux to link stationary excitation coils. To do so, however, necessitated auxiliary air gaps. Thus the problem was reduced to inducing field flux through the stationary excitation coils and auxiliary air gaps, without incurring a weight penalty in com- parison with generators using slip-rings to supply rotating field windings. The Rotor. A typical four-pole rotor is shown in the accom- panying illustrations. The rotor is essentially a tube supported by a "spider of spokes" on the shaft. These spokes, extending from the shaft to the north poles, are of magnetic material. The north poles appear as approximately hexagonal areas on the rotor surface. What appeared initially to be a problem in shaping the pole faces for optimum wave-form is now a basic advantage of the design. Flux distribution in the air gaps can be controlled very simply by shaping the pole faces, and the near-hexagonal shape was chosen because of good wave-form results and also for manu- facturing simplicity. Further development may produce even better results from other unusual shapes. Both north poles are surrounded by non-magnetic but electric- ally conducting material which forms a part of the damper cage. The remainder of the central portion of the rotor tube (equivalent to the width of the north poles) forms the south pole. At each end of the rotor tube is an outer ring to provide a path for the return of flux to the stationary field coils via the homopolar air gaps. While the rotor design shown is four-pole, the machine can be built in any configuration, including two-pole. The Flux Path. A flux is induced by each stationary exciter coil in the iron within the coil. The flux flows into the shaft axially, and then moves radially out through the magnetic spokes of the spider to the hexagonal north poles. It then continues radially across the main air gap to the stator teeth; peripherally around the stator yoke to the stator south-pole area; radially across the main air gap to the rotor south-pole area; longitudinally to the ring on each end of the rotor tube; across the homopolar air gaps to the soft-iron field yoke adjacent to the stationary exciter coils; axially into the coils; and, finally, across the homopolar air gaps into the shaft. By virtue of the unique magnetic field, state Jack & Heintz, the Secsyn combines the advantage of simple brushless construction with the weight and size advantages of conventional statically excited generators—proving that the hurdle of the auxiliary air gaps has been crossed. In fact, owing to the ideal proportions of these auxiliary gaps m the Secsyn, the mmf drop across the gaps is less than 10 per cent of the total excitation ampere-turns. The bobbin-wound field coils enclose a maximum flux-carrying area with no waste copper in end-turns, and their increased ampere- turns can be produced without additional excitation wattage. Thus, while the Secsyn inherently requires greater mmf (ampere- turns), the efficient, circular field coils (with a shorter mean turn length than possible in wound-rotor construction) make excitation power requirements rather less than that of conventional machines. Advantages. Four prime advantages, say the makers, accrue to users of the Secsyn brushless machines: long life and reliability, high operating speeds, high-temperature operation, and adapt- ability to any cooling method. The Secsyn machines are exhibit- ing a service life more than twice the conventional figure, because the design has: no brushes or rotating rectifiers; no rotating copper field windings which may have a tendency to shift, result- ing in winding failures as well as unbalance and subsequent bearing failure; and no seals for coolant in the rotor. It can be designed for operation at any speed up to, and possibly beyond, 100,000 r.p.m. The machine does not suffer from operating limits imposed by brushes and rotating rectifiers, rotating copper field winding which must be mechanically braced and seals for rectifier coolant. [Continued overleaf 1 North magnetic spoke 2 North pole - 3 South-pole area 4 Non-magnetic damper cage 5 Polar zone 6 Flux-return rings 7 Stationary exciter-coil. (Locates under flux-return ring of rotor tube; does not rotate, being fixed relative to stator) 6 Ceil cooling-air passages Basic construction of the fixed and rotating portions is depicted above. On the right is a diagram illustrating the flux-paths (arrowed)
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
