Patent Publication Number: US-8975842-B2

Title: Permanent magnet motor control

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate electric motors and, in particular, to controlling permanent magnet electric motors. 
     Permanent magnet motors typically have three windings on a stator and a permanent magnet on a rotor. The stator windings are typically powered from a three-phase converter that creates a balanced set of three phase currents. This arrangement of three stator windings powered by the three-phase current system generates a rotating field with a rotation speed proportional to the number of pole pairs and the frequency of the stator current. In a typical permanent magnet motor, the rotation speed of the rotor is determined by a speed sensor or derived from the signal from a position sensor. A rotor position sensor gives information about position of rotor magnets with respect to stator windings. The position of the rotor magnets is important for properly energizing stator windings with current to control torque. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Embodiments of the invention include a method of controlling a three-phase permanent magnet motor by generating two-phase control signals. The two phases are defined as a d-phase and a q-phase. The d-phase and q-phase have a d-phase winding and a q-phase winding, respectively, separated by ninety (90) degrees. The method includes generating a rotation speed value representing a rotation speed of the permanent magnet motor based on a q-current reference value and a q-current feedback value, the q-current reference value and the q-current feedback value corresponding to the q-phase winding. The method further includes generating a d-phase voltage change value based on a d-current reference value and a d-current feedback value, the d-current reference value and the d-current feedback value corresponding to the d-phase winding. The method includes generating a first d-phase voltage value based on the rotation speed value, the d-phase voltage change value, the d-current reference value and the q-current reference value. Finally, the method includes generating a first q-phase voltage value based on the rotation speed value, the q-current reference value and the d-current reference value. 
     Additional embodiments include a permanent magnet motor system which includes a permanent magnet motor having three windings corresponding to three phases and a motor control module configured to generate three winding current values to control current levels on the three windings of the permanent magnet motor to control a rotation of a rotor of the permanent magnet motor. The system includes a two-phase voltage control signal generator configured to generate a d-voltage control signal and a q-voltage control signal corresponding to a d-phase and a q-phase of the permanent magnet motor, the d-phase and q-phase representing a two-phase reference frame of the permanent magnet motor. The two-phase voltage control signal generator is configured to generate the d-voltage control signal and the q-voltage control signal based on a rotation speed value corresponding to a rotation speed of a rotor of the permanent magnet motor and based on a d-phase voltage change value, the two-phase voltage control signal generate configured to output the d-voltage control signal and the q-voltage control signal to the motor control module to generate the three winding current values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a block diagram of a permanent magnet control system according to an embodiment of the invention; 
         FIG. 2  illustrates a permanent magnet control system according to another embodiment of the invention; 
         FIG. 3  illustrates a flux position estimator according to an embodiment of the invention; 
         FIG. 4  illustrates a flow diagram of a method according to an embodiment of the invention; and 
         FIG. 5  illustrates annotations associated with a conventional permanent magnet motor. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Conventional permanent magnet motors use one or more sensors to detect a rotation speed of the rotor and rotor position. Embodiments of the invention relate to a permanent magnet motor. Embodiments of the invention relate to methods and systems for controlling a permanent magnet motor using two-phase control signals without measuring the rotation rate of the motor and without a rotor position device. 
       FIG. 1  illustrates a block diagram of a permanent magnet motor control system  100  according to an embodiment of the invention. The system  100  includes a permanent magnet motor  101  and a motor control module  102  that supplies three-phase current signals  103  to the motor  101  to control rotation of a rotor with respect to a stator. In embodiments of the invention, the three-phase permanent magnet motor  101  is controlled based on two-phase voltage control signals v dref  and v qref  generated by a two-phase voltage control signal generator  104 . 
       FIG. 5  illustrates the relationship between the physical three-phases of the permanent magnet motor  101  and the conceptual two-phases that are used to generate the three-phase control signals. Referring to  FIG. 5 , the permanent magnet motor  500  includes a permanent magnet  501  that rotates around an axis  502 . The permanent magnet  501  is surrounded by three coils  503 ,  504  and  505 , typically wound around, or mounted to, a stator (not shown). The three coils  503 ,  504  and  505  are energized by currents corresponding to three phases a, b and c, respectively. The rate at which the coils  503 ,  504  and  505  are energized determines a rotation speed omega of the permanent magnet  501 . The three coils  503 ,  504  and  505  are located one hundred twenty (120) degrees apart from each other and define a stationary, three-phase frame of reference. 
     In embodiments of the invention, control signals are generated based on a two-phase frame of reference. In  FIG. 5 , the two phases are identified as d-phase and q-phase, where d-phase lags q-phase by ninety (90) degrees. In a rotational frame of reference, a vector defining the d-phase extends along a longitudinal axis of the permanent magnet  501  and out from an “N” side of the permanent magnet. The permanent magnet  501  is illustrated as rotating in a counter-clockwise direction, and the vector defining the q-phase is ninety (90) degrees ahead of the vector defining the d-phase. From a stationary frame of reference, the coil  503  defines the q-axis (q s ) and the coil  506  defines the d axis (d s ) located at minus ninety (−90) degrees with respect to the q axis, where q s  and d s  indicate a stationary frame of reference. 
     Also illustrated in  FIG. 5 , ω represents an actual rotation speed of the permanent magnet  501 , ω est  represents an estimated, or calculated, rotation speed of the permanent magnet  501 , θ represents an actual position of the magnet  501 , relative to an axis A, θ est  represents an estimated, or calculated, position of the magnet  501 , d represents an actual d-phase vector in the rotational frame of reference, d est  represents an estimated, or calculated, d-phase vector in the rotational frame of reference, q represents an actual q-phase vector in the rotational frame of reference, and q est  represents an estimated, or calculated, q-phase vector in the rotational frame of reference. 
     A three-phase system may be transformed into a two-phase model based on mathematical algorithms implemented as logic and one or more processors executing instructions. An advantage of working with a two-phase system rather than a three-phase system is that a mutual inductance between the two orthogonal windings in a d-axis and q-axis is zero, which simplifies calculations. In addition, the number of voltage equations is reduced by a factor of ⅔. Also, when transformed into synchronous reference values, the values become DC quantities, which facilitates analysis and control with proportional-integral (PI) regulators. 
     Referring again to  FIG. 1 , the system  100  a d-current regulator  105  receives as inputs a d-current reference value i d-ref  and a feedback signal from the motor control signal feedback module  107  and outputs a d-voltage difference signal Δv d  to the two-phase voltage control signal generator  104 . A q-current regulator  106  receives as inputs a q-current reference value i q-ref  and a feedback signal from the motor control feedback module  107  and generates a rotation speed value ω. The q-current regulator  106  outputs the rotation speed value ω to the two-phase voltage control signal generator  104 . 
     The system  100  further includes a flux and current feedback module  108  that receives as inputs data regarding a power level on a DC bus  109  and modulation signals from the motor control module  102  and outputs to the two-phase voltage control signal generator  104  an estimated flux value representing an estimated flux generated by the permanent magnet motor  101 . The two-phase voltage control signal generator  104  generates the two-phase voltage control signals v dref  and v q-ref  based on the d-voltage difference signal Δv d , the rotation speed value ω and the estimated flux value. According to embodiments of the invention, the rotation speed of a rotor of the permanent magnet motor  101  is calculated by the q-current regulator  106 , and a separate detection mechanism, such as a sensor, counter, or other mechanism, is not needed to obtain the rotation speed value ω. 
       FIG. 2  illustrates a system  200  for controlling a permanent magnet motor in additional detail. The system  200  includes a three-phase permanent magnet motor  215  and a three-gate-switch based power section (inverter)  210  for providing three phase motor voltages to control currents i a , i b  and i c  to the three windings of the motor  215 . In one embodiment, the power section  210  is an insulated-gate bipolar transistor (IGBT) bridge implemented by IGBT devices operating as switches. The switches in the power section  210  receive as inputs a direct voltage on a (DC) bus  203  and pulse-width modulated signals from a pulse-width modulator  209 , and output to the motor  215  the three-phase voltage to control currents i a , i b  and i c . 
     In embodiments of the invention, the three-phase motor  215  is driven by a two-phase model. Accordingly, the pulse-width modulator  209  receives as inputs modulation signals v a-ref , v b-ref  and v c-ref  from a two-phase to three-phase converter  208 , also referred to as a 2-to-3 converter  208 . The 2-to-3 converter  208  receives as inputs stationary reference frame voltage signals v d-ref-s  and v q-ref-s , which are alternating current (AC) signals corresponding to the d-phase and the q-phase, respectively, of the two-phase model. The voltage signals v d-ref-s  and v q-ref-s  are generated by a rotator  207 , which receives as inputs rotating reference frame voltage reference signals v dref  and v qref , as well as an estimated rotor position signal θ est . The voltage reference signals v dref  and v qref  are output from a voltage feed forward calculator  206 . The voltage feed forward calculator  206  may correspond to the two-phase voltage control signal generator  104 , illustrated in  FIG. 1 . The voltage feed forward calculator  206  receives as inputs a d-phase voltage difference signal Δv d , a rotation speed signal ω, a q-current reference signal i q-ref , a d-current reference signal i d-ref  an estimated q-phase flux value λ q-est  and an estimated d-phase flux value λ d-est  and motor winding parameters—resistance and inductance. 
     The d-phase voltage difference signal Δv d  is generated by a d-current regulator  205 . The d-current regulator receives as inputs the d-current reference signal i d-ref  and a d-current feedback signal i d-fb . The q-current regulator receives as inputs the q-current reference signal i q-ref  and a q-current feedback signal i q-fb . The q-current reference signal i q-ref  is generated by a speed regulator  211 , which receives as inputs a reference rotation speed signal ω r-ref  corresponding to a rotational frame of reference, and a rotation speed feedback signal ω r-est  corresponding to the rotational frame of reference. By referring to the “rotational frame of reference,” it is understood that ω r-ref  and ω r-est  represent a rotation speed relative to a reference frame that rotates together with a rotor of the permanent magnet motor  215 . 
     In one embodiment, one or more of the speed regulator  211 , the q-current regulator  212  and the d-current regulator  205  is a proportional-integral (PI) controller. The rotation speed value ω that is input to the voltage feed forward calculator  206  is also provided to a flux position estimator  217 . The flux position estimator  217  receives as inputs the rotation speed value ω and an estimated q-phase flux value λ r-q . The flux position estimator  217  outputs a rotation speed feedback value ω est  to the speed regulator  211  and a rotor position feedback value θ est  to a rotator  213 . 
     The flux position estimator  217  is shown in additional detail in  FIG. 3 . The flux position estimator  217  modifies the rotation speed value ω generated by the q-current calculator  212  according to an estimated q-phase rotor flux λ r-q . The q-phase rotor flux λ r-q  may be proportional to an angle Δθ, or a difference between an estimated angle θ est  and an actual angle θ. As illustrated in  FIG. 3 , the q-phase rotor flux λ r-q  is input to an amplifier  304  to increase the value of the q-phase rotor flux λ r-q  according to the gain K1. The resulting signal is combined with the rotation speed value ω output from the q-current regulator  212  by a summing circuit  301  to generate an estimated rotor speed value ω est . In one embodiment, the q-phase rotor flux λ r-q  and the rotation speed value ω are combined according to the following equation:
 
ω est   =ω+K 1λ r-q   (1)
 
     An estimated angle of the rotor flux θ est  is calculated by integrating the estimated speed with an integrator  302  and feeding forward the q-phase rotor flux λ r-q  (represented by summing circuitry  303 ) multiplied by a gain K2 (block  305 ), according to the following equation:
 
θ est =∫ω est   dt+K 2λ r-q .  (2)
 
     The rotor speed  ωr-est  is estimated by filtering the estimated speed ω est  by a first order filter  216  with a time constant τ ω , according to the following equation:
 
 ωr-est =(1/(1 +sτ   ω ))ω.  (3)
 
     The voltage feed forward calculator  206  receives its rotor q-phase and d-phase flux values from a flux and current observer circuit  201 . The flux and current observer circuit  201  receives as inputs the estimated rotor speed value ω est , the estimated q-current value q est , the estimated d-current value i d-est , and q-phase and d-phase motor feedback voltages v d-fb  and v q-fb . The q-phase and d-phase motor feedback voltages v d-fb  and v q-fb  are generated by a motor voltage feedback circuit  202 , which generates the voltages v d-fb  and v q-fb  based on the modulation voltages v a-ref , v b-ref  and v c-ref , and based on the DC bus voltage u dc  of the DC bus  203 , measured across the capacitor  204 . 
     The d-current feedback signal i d-fb  and the q-current feedback signal  iq-fb  are output from an rotator  213 . The rotator receives as inputs the estimated angle of rotor flux θ est  from the flux position estimator  217 , as well as the AC q-current value i q-s  and AC d-current value i d-s . The AC q-current and d-current values i q-s  and i d-s  are output from the 3-to-2 phase converter  214 , which monitors the current lines  218  output from the three-gate switch  210  to the motor  215 . In one embodiment, the 3-to-2 phase converter  214  monitors only two of the current lines  218 , and calculates a current value of the third line based on the two monitored lines using an equation in which the sum off all three motor currents is zero. The 3-to-2 phase converter  214  converts the current signals i a  and i b  from a three-phase reference frame to a two-phase reference frame. 
     While interconnections of the system  200  have been described above, an operation of the system  200  is provided in more detail below. First, reference values are provided to the system  200  from an external source (not shown), such as a controller, processor or other system that calculates the reference rotation speed value ω r-ref  and d-current reference value i dref . The reference rotation rate value ω r-ref  is provided to the speed regulator  211 , along with the estimated rotation rate value  ωr-est . The speed regulator  211  adjusts an output current, corresponding to the reference q-current value i q-ref , until the estimated rotation speed  ωr-est  equals the reference rotation speed ω r-ref . 
     The reference q-current i q-ref  is provided to the q-current regulator  212 , along with a q-current feedback value i q-fb . The q-current regulator  212  may be a PI controller. The q-current regulator  212  adjusts the rotation speed value ω until the q-current feedback value i q-fb  equals the q-current reference value i q-ref . The q-current regulator  212  provides the rotation speed value ω to a voltage feed forward calculator  206 . 
     A d-current reference value i d-ref  and a d-current feedback value i d-fb  are provided to a d-current regulator  205 . The d-current regulator  205  adjusts an output signal corresponding to a d-phase voltage difference value Δv d  until the d-current feedback value i d-fb  equals the d-current reference value i d-ref . The d-phase voltage difference value is output from the d-current regulator to the voltage feed forward calculator  206 . 
     The voltage feed forward calculator  206  also receives as inputs the q-current reference value i q-ref , the d-current reference value i d-ref  and estimated d-phase and q-phase flux values λ q-est  and λ d-est . The flux and current observer  201  generates the estimated d-phase and q-phase flux values λ q-est  and λ d-est  based on the estimated rotation speed ω est , the d-current and q-current feedback values i q-est  and i d-est , and motor voltage feedback values v q-est  and v d-est . The motor voltage feedback values v q-est  and v d-est  are generated by a motor voltage feedback circuit  202 , which calculates the motor voltage feedback values v q-est  and v d-est  based on a DC bus  203  voltage value u dc  and modulation signal values v a-ref , v b-ref  and v c-ref . 
     In one embodiment, a nonlinear state and parameter observer algorithm is implemented as follows: 
     
       
         
           
             
               
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     Based on these equations, the closed loop observer algorithm for stator current and rotational components of back EMF can be formulated as follows: 
     
       
         
           
             
               
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     The above equations may be re-written in standard matrix notation according to known methods to facilitate gain selection of the flux and current observer  201 . 
     The voltage feed forward calculator  206  receives the d-phase voltage difference value Δv d  from the d-current regulator  212 , the rotation speed value ω from the q-current regulator  205 , the q-current reference value i q-ref  from the speed regulator  211 , the d-current reference value i d-ref , and the estimated q-phase and d-phase flux values λ q-r-est  and λ d-r-est  from the flux and current state observer  201  and generates a q-phase reference voltage signal v q-ref  and d-phase reference voltage signal v d-ref  based on the inputs. In one embodiment the voltage feed forward calculator  206  generates the q-phase reference voltage signal  vq-ref  and d-phase reference voltage signal v d-ref  based on the following formulas:
 
 v   {circumflex over (q)}   =Ri   {circumflex over (q)} +{circumflex over (ω)}( L   d   i   {circumflex over (d)} +λ r{circumflex over (d)} ) and
 
 v   {circumflex over (d)}   =Ri   {circumflex over (d)} −{circumflex over (ω)}( L   q   i   {circumflex over (q)} +λ r{circumflex over (q)} )+Δ v   d .
 
     Since the q-phase reference voltage signal v q-ref  and d-phase reference voltage signal v d-ref  are DC values, they must be transformed to AC values and converted to a three-phase reference frame to drive the motor  215 . Accordingly, the q-phase reference voltage signal v q-ref  and d-phase reference voltage signal v dref  are passed through a rotator  207 , a 2-phase to 3-phase converter  208  and a pulse-width modulator  209  to generate the pulse-width modulated signals that drive the three-gate switch  210 . 
       FIG. 5  is a flow diagram of a method according to an embodiment of the invention. In block  501 , the reference signals are obtained. In particular, the rotation speed reference value ω r-ref  is obtained and the d-current reference value i d-ref  is obtained. In block  502 , the reference values are provided to regulators to generate a rotation speed value ω and d-phase voltage difference value. The regulators receive as inputs a reference value and a feedback value, and adjust the outputs until the feedback value matches the reference value. In particular, embodiments of the invention include a speed regulator, d-current regulator and q-current regulator. The q-current regulator receives as inputs a q-current reference value i q-ref  and a q-current feedback value i q-est  and outputs a rotation speed value ω. 
     In block  503 , a d-phase voltage value and q-phase voltage value are generated based on the outputs of the regulators. In particular, a voltage feed forward calculator generates the d-phase voltage value and q-phase voltage value based on the outputs of the regulators and an estimated q-phase and d-phase flux value. In block  504 , the d-phase and q-phase voltage values are converted to three-phase modulation signals, or a-phase, b-phase and c-phase modulation signals. 
     In block  505 , the three-phase modulation signals are used to control a permanent magnet motor. For example, in one embodiment, the three-phase modulation signals may be provided to a pulse-width modulator, which may generate modulated signals to control a three-gate switch. The three-gate switch may be connected to a DC bus, and may generate three motor control signals, or three coil energizing signals, to energize the coils of the permanent magnet motor to drive the motor. 
     In block  506 , feedback signals are generated based on the three motor control signals. In block  507 , a flux of the permanent magnet is calculated based on the feedback signals. In one embodiment, a q-phase flux value and a d-phase flux value are calculated, and a q-phase flux position is estimated. The process is repeated at block  501 , and the feedback values and estimated flux values are used to generate the rotation speed value, d-phase voltage change value, d-phase voltage value and q-phase voltage value. 
     In embodiments of the invention, a permanent magnet motor is controlled without the need to sense or detect a rotation speed or position of the permanent magnet motor. Instead, the rotation speed is provided as a value output from a q-current regulator and a rotor position angle is generated in as shown in block diagram in  FIG. 3 . In addition, a q-phase flux and d-phase flux of the permanent magnet motor are estimated and used to generate a q-phase reference voltage and d-phase reference voltage. In addition, a flux position of a rotor of the permanent magnet motor is estimated and used to calculate a rotation speed estimate value of a rotating frame of reference, and a rotor position value. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.