Written-Pole® technology is a revolutionary, yet fundamentally simple approach to enhancing the performance of electric motors and generators. The application of Written-Pole® technology frees engineers from one of the most basic constraints faced by designers of conventional induction and synchronous machines. Unlike conventional designs, where the magnetic poles are fixed by the geometry of the rotor cage and stator windings, Written-Pole® machines incorporate a proprietary concept that optimizes the magnetic geometry of the rotor for peak performance as the operating speed of the machine varies.
Limitations of Conventional Technology
The speed of an electric motor is directly proportional to both frequency and number of poles. Conversely, the output frequency of a generator is directly proportional to its speed and number of poles.
The requirement for smooth balanced output dictates that conventional electric machines must have an even number of magnetic poles with an equal number of north and south orientations. As a result, conventional electric machines are typically classified as 2, 4, 6, or …. pole machines, having either 1, 2, 3, or ….. pairs of north and south poles respectively. Deviating from this principle will produce an electric machine that is magnetically unstable at its rated speed and unsuitable for steady-state operation.
Written-Pole® technology frees designers from this constraint without contradicting the fundamental rules that govern the operation of electric machines. The technology makes it possible to produce a pole pattern that correlates directly to the rotational speed of the machine enhancing performance through a considerable speed range.
Written-Pole® motors and generators incorporate a continuous layer of magnetic material on the surface of the rotor which can be magnetized into any desired pole configuration using a high density winding contained within the stator windings. As the magnet material passes beneath this excitation winding, it is subjected to an alternating magnetic field produced by AC current flowing in the winding. The strength and orientation of this magnetic field controls the geometry of the magnetic poles induced on the rotor. If the polarity of the magnet material passing beneath the excitation winding does not match the polarity of the magnetic field produced by the winding, the polarity of the magnet is reversed to match the field produced by the excitation winding.
Since the power supplied to the excitation winding is a constant frequency 60 Hz, AC sinusoidal supply, the actual size and quantity of poles generated on the surface of the rotor are dependent on the rotational speed of the machine. Lower speeds result in a larger number of smaller poles with shorter spans, while higher speeds result in a smaller number of larger poles with longer spans. The concept is similar in principle to the idea of a continuous magnetic tape looping through a recording head.
Speed = 1600 rpm Poles = 4.5 Pole Span = 85 deg
An 1800-rpm, or 4-pole, electric motor operating at 1600-rpm requires 4.5 poles to maintain synchronous operation on constant frequency, 60 Hz input. Conversely, for an 1800-rpm generator to maintain its rated 60 Hz output at 1600 rpm also requires 4.5 poles. The ability to optimize the pole geometry while in motion allows Written-Pole® machines to accommodate significant variances in speed without compromising performance.
Upon attaining its rated speed, power to the excitation winding is removed allowing the machine to operate as a synchronous machine. Variation from the machine’s rated speed restores power to the excitation winding thereby ensuring that the pole geometry on the rotor remains matched to the rotating electromotive fields produced by the stator winding.
A truly revolutionary development, Written-Pole® technology allows designers of electric machines to attain new levels of performance for the first time in the history of electric machine design.
The application of Written-Pole® technology to electric motors yields many desirable characteristics including very low starting current requirements, high operating efficiency, unity power factor operation, instantaneous restart capability, the ability to synchronize under load, and the capability to start very high inertia loads without over-sizing.
When applied to electric generators, Written-Pole® technology allows for the regulation of output voltage and frequency over a considerable speed range without relying on external regulators or power electronics. Combining Written-Pole® motors and generators with an integral flywheel allows Precise Power Corporation to create a simple, rugged power protection unit that is ideally suited for rugged industrial and commercial environments where space and infrastructure costs are key considerations.
Written-Pole® technology is used in a family of single and three-phase motive power and power protection systems used by many customers including the US Weather Service, NASA, Federal Aviation Authority, US Air Force and many commercial and industrial customers.
Award Winning Technology
A recipient of the R&D 100 Award in 1994, Written-Pole® technology was recognized by R&D Magazine as one of that year’s most technologically significant innovations. The technology has also been recognized by numerous technical organizations including the Professional Society of Engineers in the United States.
Developed by Precise Power Corporation, a Bradenton, Florida based motor and generator manufacturer, with support from the Electric Power Research Institute and numerous electric utilities including Manitoba Hydro, this innovative concept is protected by numerous US and International patents.