The application of Written-PoleŽ technology to electric motors yields many advantages when compared to limitations imposed by conventional motor technologies.Written-PoleŽ 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.

Construction

The main elements of a Written-PoleŽ motor are illustrated in Figure 1. While Written-PoleŽ adopt construction techniques similar to those of conventional induction motors, it is their innovative use of materials and concepts that sets them apart.

Stator Construction

    The stator of a typical Written-PoleŽ 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 produce a rotating magnetic field, which interacts with the rotor to apply rotational force to the shaft.

    A unique feature of Written-PoleŽ 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 Construction

    The rotor of a typical Written-PoleŽ 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 Written-PoleŽ 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 Written-PoleŽ 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 Written-PoleŽ 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.

Operation

    Written-PoleŽ 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 Mode

    In Start Mode, a Written-PoleŽ 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 Written-PoleŽ technology provides designers with considerable freedom in choosing these properties, because Written-PoleŽ 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 Written-PoleŽ 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 Mode

    As the Written-PoleŽ 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 Written-PoleŽ 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 Written-PoleŽ motor to attain mechanical synchronization over a period of seconds or minutes, dramatically enhancing the machine's ability to start high inertia loads. Since Written-PoleŽ 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 Mode

    A Written-PoleŽ 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 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 Written-PoleŽ 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 Operation

    Another 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 Written-PoleŽ 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.

More information on Motive Power Products

Written-PoleŽ is a registered trademark of Precise Power Corporation.