Written-Pole ® Technology
The application of SPPS Written-Pole® technology to single-phase electric motors yields many advantages when compared to limitations imposed by conventional motor technologies. SPPS Written-Pole® single-phase motors receive their exceptional operating characteristics from their unique patented design and construction that incorporate many of the strengths offered by existing technologies while overcoming their weaknesses.
ConstructionThe main elements of a SPPS Written-Pole® single-phase motor are illustrated in Figure 1. While SPPS Written-Pole® adopt construction techniques similar to those of conventional induction motors, it is their innovative use of materials and concepts that sets them apart.
Stator ConstructionThe stator of a typical SPPS Written-Pole® single-phase motor will be immediately recognizable to anyone familiar with induction motors. The stator lamination stack is constructed using low-loss electrical steel laminations with the latest oxide coatings. A single or three-phase winding is installed in the stator using inverter duty grade copper wire. The windings are similar in design and function to those used in a conventional induction or synchronous motor. When connected to input utility power, the current in the windings produces a rotating magnetic field, which interacts with the rotor to apply rotational force to the shaft. A unique feature of SPPS Written-Pole® single-phase motors is the use of a concentrated excitation winding located at one or two points of the stator. The excitation winding is contained within the stator structure and is located between the main stator windings. Designed to produce a magnetic field powerful enough to fully magnetize the portion of the rotor's magnetic layer that is immediately across the air gap from it, this winding is used to maintain the correct pole geometry in the rotor. Energy required for operation of the excitation winding is magnetically coupled through the stator core from the adjacent stator windings, eliminating the need for an external energy source.
Rotor ConstructionThe rotor of a typical SPPS Written-Pole® single-phase motor is a combination of induction, hysteresis and permanent magnet technology. The basic platform consists of a conventional steel shaft inserted into lamination stack containing a high resistance rotor cage. The rotor laminations are constructed using the same low loss, electrical grade steel found in the stator laminations. The high resistance cage is a key factor in limiting the starting current of SPPS Written-Pole® single-phase motors and provides considerable induction torque during the initial stage of starting. The total cross sectional area and resistivity of the rotor cage are selected so as to provide high slip, high power factor starting characteristic. While this configuration would be detrimental to the operating efficiency of a conventional induction motor, the synchronous mode of operation used in SPPS Written-Pole® single-phase motors eliminates induced currents in the rotor bars and resulting electrical losses. The continuous layer of semi-permanent magnetic ferrite material covering the rotor lamination stack is one of the unique features of SPPS Written-Pole® single-phase motors. The ferrite material used in the rotor is similar in many ways to the magnetic material used in conventional permanent magnet synchronous motors, enabling it to maintain its magnetization throughout the normal range of operation. However, the unique properties of this proprietary material reduce the strength of the magnetic field required to magnetize or reorient the material to a level low enough that it becomes practical to re-magnetize the rotor while it is rotating. This can be accomplished without losing the properties required for normal operation and has the added benefit of increasing the amount of hysteresis torque available during starting.
OperationSPPS Written-Pole® single-phase motors employ three modes of operation based on the rotational speed of the machine. The diagram below illustrates the three modes of operation along with their relationship to the rotational speed of the motor.
Start ModeIn Start Mode, a SPPS Written-Pole® single-phase motor produces large amounts of hysteresis and induction torque, which begin to accelerate the motor to its rated speed. Hysteresis torque is developed when the magnetic fields produced by the stator current are sufficiently strong to magnetize the ferrite material on the rotor producing useful torque. The magnitude of the input starting current and induction torque produced in this mode are determined by the properties of the rotor cage. The application of SPPS Written-Pole® single-phase technology provides designers with considerable freedom in choosing these properties, because SPPS Written-Pole® single-phase motors are able to generate synchronous torque over a broad speed range, unlike conventional synchronous motors that rely on induction torque to accelerate the machine to synchronous speed. As a result, the cage resistance can be optimized to achieve the desired starting characteristics without concern over the availability of induction torque at higher speeds. This process is a trade-off between gentle, low current starts versus fast abrupt high, current starts. The actual starting currents, times and acceleration rate depend on the model and application with most SPPS Written-Pole® single-phase motors optimized for lower starting currents. The approach yields several long-term benefits, including a gentler starting ramp that protects the connected load from damaging acceleration and mechanical shock. The lower starting current also reduces the temperature rise in the stator windings, permitting more frequent starts and restarts than are possible with conventional motors, when connected to similar high inertia loads.
Transition ModeAs the SPPS Written-Pole® single-phase motor accelerates towards its rated speed, it enters the Transition Mode, during which the excitation winding begins to influence the magnetic geometry of the rotor. The rotational speed at which the motor switches to Transition Mode depends on the model and application, but is nominally in the range of 80 to 90% of normal synchronous speed. Upon entering the Transition Mode, a SPPS Written-Pole® single-phase motor becomes electrically synchronous, allowing it to produce synchronous torque even though it has not attained true synchronous speed. As the electrical current in the excitation winding varies with one complete positive negative cycle, an alternating magnetic field is produced which induces a complete pair of north and south poles on the ferrite magnetic layer on the surface of the rotor. These poles are at the proper combination of electrical and mechanical phase angles to produce torque, regardless of the rotor speed or previous pole pattern. As a result, the magnetic pole pattern created by the excitation winding rotates in exact electromagnetic synchronization with the rotating fields produced by the stator windings even though its mechanical rotation is not synchronous with the stator fields. The design of the excitation circuit controls the phase angle between the excitation current and the current in the stator winding, which determines the torque angle between the rotor poles and the stator's rotating fields. This phase angle can be used to maximize the available torque in the Transition Mode or provide for leading power operation at rated load. The ability to operate as a synchronous motor over a wide range of speed enables a SPPS Written-Pole® single-phase motor to attain mechanical synchronization over a period of seconds or minutes, dramatically enhancing the machine's ability to start high inertia loads. Since SPPS Written-Pole® single-phase motors do not rely on induction torque to achieve near synchronous speed prior to making the transition to synchronous operation, the motor's stating characteristics can be optimized without sacrificing steady state performance and efficiency.
Run ModeA SPPS Written-Pole® single-phase motor enters Run Mode upon reaching its rated synchronous speed. Since operation of the excitation winding is not required in this mode, it is turned off and the motor continues to operate as a permanent magnet synchronous motor until power is removed from the input contactor. Another example of the enhanced operation provided by SPPS Written-Pole® single-phase motors is their capability to recover from momentary overloads. If sufficient torque is applied to the output shaft causing the motor to stall, it re-enters the Transition Mode and attempts to reaccelerate the load back to synchronous speed. In the event that the load torque continues to exceed the capability of the machine, standard overload devices disconnect power from the motor, preventing damage to the motor or driven load.
Re-start OperationAnother important feature of the Written-Pole motor is that it can be reconnected to input electrical power at any time following loss of input power, unlike conventional electric motors which require a delay to minimize the generation of large torque transients resulting from the phase shift between the rotor and input power supply. The SPPS Written-Pole® single-phase motor continues to deliver smooth, even torque and draws only normal starting currents when power is reapplied, regardless of the phase of the rotor field relative to the stator field. The standard configuration supplied by the factory provides for automatic restarts whenever power is reapplied to the input contactor. The motor will resume operation either in the Start Mode or Transition Mode depending on its speed when the power is reapplied.
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 TechnologyThe 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® AdvantageWritten-Pole® motors 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 degAn 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.
Written-Pole®ApplicationsSingle Phase Power Solutions, LLC, under an agreement with Precise Power Holdings Corporation, produces and sells large horsepower Written-Pole motors up to 100 hp. These motors are used in applications were stationary combustion engines have been used in the past. 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. 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 TechnologyA 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 John Roesel and Ronnie Barber, 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.
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