Abstract:
A permanent magnet motor having a high speed ratio may remove the need for a gearbox or multiple windings in the permanent magnet motor. A gearbox or a multiple-winding configuration stator may thus be omitted.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a nonprovisional application which claims priority from U.S. provisional application No. 62/099,812, filed Jan. 5, 2015. 
    
    
     TECHNICAL FIELD/FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to electric motors, and specifically to AC permanent magnet electric motors. 
     BACKGROUND OF THE DISCLOSURE 
     Alternating current (AC) electric motors rely on alternating currents passed through induction windings within the stator to cause rotation of the rotor. So-called three phase AC motors include three matched sets of windings positioned radially about the stator. By supplying sinusoidal AC power to each of the sets of windings such that each set receives an alternating current offset by 120 degrees, a largely continuous torque can be imparted on the rotor as it rotates. 
     Unlike a brushed DC motor, output speed in an AC motor is controlled by the frequency of the current sent to the stator windings. In order to control output torque, and thus speed, a variable frequency drive (VFD) may be used to vary the current fed to the AC motor. Because the inductive reactance of the stator windings is proportional to the frequency applied to the windings, increased voltage is necessary to maintain a relatively constant current within the windings, and thus a relatively constant output torque up to the motor&#39;s rated speed. The rated speed generally corresponds with the rated voltage (usually equal to the supply voltage of the VFD). Past the rated speed, the speed of the AC motor may be increased through field weakening as discussed hereinbelow. This control algorithm is typically referred to as volts/Hz or V/Hz control. 
     In some electric motors, both the rotor and stator include coils. In such an induction motor, the magnetic field induced by the stator coils induces current within the rotor coils which, due to Lenz&#39;s law, causes a resultant torque on the rotor, thus causing rotation. 
     In a permanent magnet motor, on the other hand, the rotor includes one or more permanent magnets. The permanent magnets, in attempting to align with the magnetic field induced by the coils in the stator, cause a resultant torque on the rotor. By varying the orientation of the magnetic field, the rotor may thus be caused to rotate. In high-torque permanent magnet motors, such as for an internal rotor permanent magnet motor, multiple permanent magnets may be positioned on the exterior of the rotor. 
     VFDs typically use one of two control methods. In a Volts/Hz control scheme, the VFD varies the output speed of the motor by supplying AC power to the stator windings at a particular frequency and voltage. For a given desired torque, voltage is proportionally related to the frequency by a so-called “voltage-to-frequency” or “volts/Hz” ratio. As understood in the art, the impedance of an electric motor includes a static impedance and a back-EMF or counter-EMF. The static impedance is determined by the winding arrangement and construction of the motor. The counter-EMF is created by, in a permanent magnet motor, the current induced in the motor windings by the rotating magnetic fields of the permanent magnets. Accordingly, counter-EMF is negligible when the motor is at a standstill. As the speed of the motor increases, the counter-EMF likewise increases requiring additional drive voltage to be applied to the motor in order to maintain sufficient current through the windings of the motor. Traditionally, the drive voltage is supplied by the volts/Hz ratio, which is typically a constant through the normal operating range. Throughout this normal operation range, current is maintained in phase with the rotor. At a certain speed, the counter-EMF voltage reaches or exceeds the output voltage capability of the motor driver, at which point the phase of the current supplied to the motor is modified in order to, in a permanent magnet motor, inject negative polarity flux into the permanent magnets, effectively reducing their magnetic fields and thus the counter-EMF. By using closed-loop feedback, a VFD using volts/Hz can maintain motor speed in changing conditions. This simple form of volts/Hz may not allow accurate torque control. 
     With the rapid advancement in low-cost, high speed microprocessor technology, VFDs utilizing so-called vector control or field-oriented control (FOC) models have been used. In FOC, the current supplied to the phases of the AC motor is decoupled into torque and flux components acting on the rotor in a rotating reference frame. Thus, each of these components may be independently controlled. Current supplied to the phases of the motor are measured or derived and transformed into the torque-flux space (utilizing, for example, a Clarke/Park transformation), a closed-loop feedback model can be created to control each of these components continuously. The processor then back-transforms the torque and flux components into three phase currents. The three phase currents are fed to a three phase inverter which outputs pulse-width modulated signals to each set of windings in the motor. 
     In any AC motor, even under FOC, as the speed of the permanent magnet motor is increased, the voltage generated by the fixed magnetic field (EMF) increases proportionally. At a certain speed, the voltage generated by the motor exceeds the maximum voltage that can be produced by the drive that is controlling the motor. If operation above this speed is desired, it is necessary to modify the current vector applied to the motor to maintain the desired torque, and control the terminal voltage of the motor to a value less than the maximum drive output voltage. Control in this speed range is known as “field weakening control.” As the speed of the motor increases, the torque available decreases when in field weakening. At some speed, the available torque will not be enough to sustain operation of the motor. The ratio between this maximum speed and the rated or base speed of the motor is known as a speed ratio. Typically a permanent magnet motor may achieve a 3:1 speed ratio, an induction motor may achieve a 5:1 ratio, but typical electric motors may achieve between a 2:1 and 3:1 speed ratio. 
     SUMMARY 
     The present disclosure provides for a method for operating a piece of equipment driven by rotary power. The method may include coupling a permanent magnet AC motor to a shaft of the piece of equipment. The shaft may be adapted to be rotated directly by the permanent magnet AC motor. The permanent magnet AC motor may include a stator and a rotor. The rotor may include a permanent magnet positioned thereon. The method may include supplying alternating current to the stator. The method may include rotating the shaft. The method may include injecting a direct current in the opposite direction of a magnetic flux created by the permanent magnet and weakening the magnetic field of the permanent magnet. The method may include increasing the speed of the permanent magnet AC motor to a maximum speed based on a speed ratio of the permanent magnet AC motor, the speed ratio defined as the ratio between the maximum speed and a rated speed, the rated speed corresponding to a rated voltage of the permanent magnet AC motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  depicts a block diagram of a three phase permanent magnet AC motor controlled by a VFD. 
         FIG. 2  depicts a cross section view of a permanent magnet AC motor consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
       FIG. 1  depicts a block diagram of three phase AC motor  10  controlled by VFD  20 . VFD  20  may be positioned to output three phase AC power to the stator windings (not shown) of AC motor  10  in response to input parameters  22 . Input parameters  22  are at least one of torque demand, speed demand, and maximum drive voltage. 
     In the case of a permanent magnet motor, the interaction of current, flux, voltage, and speed may be defined by the model voltage equation as follows:
 
   v     s   =R   s   ·i   s   +l   s   · {dot over (i)}     s   +jω   0   l   s   · i     s + {dot over (φ)}   r   +jω   0 · φ   r ,
 
where  v   s  is the stator voltage vector, R s  is the stator resistance,  i   s  is the stator current vector, l s  is the stator leakage inductance,  φ   r  is the total rotor flux vector, and ω 0  is the synchronous frequency given by:
 
ω= P   p ×ω r ,
 
where P p  is the number of pole pairs per phase, and ω r  is the speed of the rotor. Total rotor flux  φ   r  may be given by:
 
                   ϕ   _     r     =         ϕ   _     pm     +         L   _     m     ·       i   _     s           ,         
where  φ   pm  is the permanent magnet flux (the reference frame is chosen such that the permanent magnet flux is entirely real), and  L   m  is the mutual stator-rotor inductance. As used in the equations, “_” indicates a vector quantity, and the “·” operator is the first order time derivative.
 
     Substituting the flux equation into the voltage equation, and the definition that  φ   m  is entirely real (direct or d-axis), the voltage equation evaluates to: 
                 [           v   d               v   q           ]     =         [             R   s     +     pL   d               -     ω   0       ⁢     L   q                   ω   0     ⁢     L   d               R   s     +     pL   q             ]     ⁡     [           i   d               i   q           ]       +     [         0               ω   0     ⁢     Φ   m             ]         ,         
where p is the d/dt operator, L d  is the total inductance in the d-axis, L q  is the total inductance in the quadrature or q-axis, each given by:
 
 L   d =( l   s   +L   dm )
 
 L   d =( l   s   +L   qm ),
 
where L dm  is the d-axis component of  L   m  and L qm  is the q-axis component of  L   m .
 
     Torque supplied by the motor may be described by:
 
 T   e =3 P   p ( φ   r   ×i   s ),
 
which, from the definition that  φ   m  is entirely real, can be expressed as:
 
     
       
         
           
             
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     Thus, speed can be expressed by the following equation: 
                   ω   .     r     =       1     (       J   m     +     J   L       )       ⁢     (       T   e     -       T   m     ⁡     (     ω   r     )       -       T   L     ⁡     (     ω   r     )         )         ,         
where J m  and J L  are the motor and load inertias respectively, T m (ω r ) is the motor loss torque as a function of speed, and T L (ω r ) is the load torque as a function of speed.
 
     The voltage and flux equations can thus be combined into the following extended state-space format: 
     
       
         
           
             
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     In some embodiments of the present disclosure, AC motor  10  may be a permanent magnet motor used to power another piece of equipment through rotary power provided by the output shaft of the motor. For example and without limitation,  FIG. 2  depicts drawworks  101 . One having ordinary skill in the art with the benefit of this disclosure will understand that drawworks  101  may be replaced by any other piece of equipment driven by rotary power. 
     Drawworks  101  is positioned on surface  30 . In some embodiments, surface  30  may be, for example and without limitation, the deck of a drilling rig for which drawworks  101  is being used. In other embodiments, surface  30  may be an undercarriage, frame, or skid upon which drawworks  101  is positioned. 
     Drawworks  101  may include drum  103 . Drum  103  may be adapted to have hoisting line  105  wrapped therearound and, as drum  103  is rotated, heave in or pay out hoisting line  105  depending on the direction of rotation of drum  103 . 
     In some embodiments, drawworks  101  may include one or more electric motors. In some embodiments as depicted in  FIGS. 1, 2 , a single electric motor  107  may be coupled to drum  103 . In some embodiments, drawworks  101  may include one or more electric motors positioned on each end of drum  103 . In some embodiments, electric motor  107  may include motor housing  109 , stator  111 , and rotor  113 . As understood in the art with the benefit of this disclosure, rotor  113  and stator  111  may be located within motor housing  109 . 
     In some embodiments, as depicted in  FIG. 2 , rotor  113  may be positioned within stator  111 , i.e. an interior rotor electric motor. Stator  111  may be fixed to the inner wall of motor housing  109 . Stator  111  may extend around the cylindrical interior of motor housing  109 . Stator  111  may include coils of wire positioned around the inner surface of stator  111 . The coils of stator  111  are positioned to, when AC power is supplied thereto, induce a continuously rotating electromagnetic field into the interior thereof. In some embodiments, in which electric motor  107  is a permanent magnet motor, the electromagnetic field may interact with the permanent magnet field of permanent magnets included in rotor  113 , thus rotating rotor  113 . 
     In other embodiments, not shown, one having ordinary skill in the art with the benefit of this disclosure will understand that rotor  113  may be positioned outside of stator  111 , an exterior rotor electric motor. The coils may thus be positioned on an exterior surface of stator  111 . In some such embodiments, rotor  113  may be formed as part of drum  103  which is positioned about stator  111 . 
     Rotor  113  may be coupled to shaft  115 . In some embodiments, as depicted in  FIG. 2 , rotor  113  may be a generally tubular member, with shaft  115  extending at least partially into the interior thereof. In some embodiments, as depicted in  FIG. 2 , shaft  115  may couple both to rotor  113  and to drum  103 . In other embodiments, shaft  115  may be made up of multiple shaft components. For example shaft  115  may include a motor shaft coupled to a drum shaft by a rigid coupler such as, for example and without limitation, a flange coupler. The flange coupler may, as understood in the art, be adapted to join the motor shaft and the drum shaft into a single, continuous shaft  115  adapted to be rotated by rotor  113  as it is rotated by electric motor  107 , and thus rotate drum  103 . 
     In some embodiments, electric motor  107  may be configured such that, in the field weakening regime, electric motor  107  may achieve a speed ratio of between 2:1 and 20:1. In some embodiments, electric motor  107  may achieve a speed ratio of between 6:1 and 15:1. In some embodiments, electric motor  107  may achieve a speed ratio of between 8:1 and 12:1. In some embodiments, electric motor  107  may achieve a speed ratio of generally 10:1. In some embodiments, the arrangement of the components of electric motor  107  may be modified by varying the arrangement of the permanent magnets, varying the distance between rotor  113  and stator  111  (“air gap”), modifying the winding arrangement of stator  111 , or combinations thereof. Because field weakening on PM motors is achieved by injecting a direct current (DC) in the opposite direction of the magnetic flux created by the permanent magnets to counteract (weaken) the permanent magnetic field, modification of the air gap, arrangement of permanent magnets, or winding arrangements may permit a higher DC to be injected. Thus, more of the permanent magnetic field is weakened, allowing a reduced counter-EMF. Thus, less voltage is required to maintain adequate potential difference to drive AC current through the stator windings, and thereby extending the speed range. 
     By utilizing an electric motor  107  with a high speed ratio, additional components such as gearboxes between the motor and the driven equipment may not be necessary. Additionally, electric motor  107  may be operated without including reconfigurable or multiple winding sets in stator  111 . For example, as understood in the art, a three phase motor may have windings arranged in a Wye or delta configuration. Additionally, windings at each pole in each of the phases may be connected in parallel or in series. In some motors, complicated switching apparatuses or multiple winding sets are used to allow the motor to be reconfigured. In embodiments of the present disclosure, such switching may be unnecessary due to the ability of electric motor  107  to operate with a high speed ratio achieved by an extended field weakening range. The ability to omit multiple winding sets, switching apparatuses, and gearboxes may, for example and without limitation, reduce weight, cost, and complexity of electric motor  107  and the equipment in which it is installed. Additionally, when used in adverse conditions, such as in the presence of flammable or explosive gases or liquids, the lack of electric switching equipment may increase the safety of the apparatus. 
     The foregoing outlines features of several embodiments so that a person of ordinary skill in the art may better understand the aspects of the present disclosure. Such features may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed herein. One of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. One of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.