1. Field of the Invention
The present invention relates to motor control devices that drive and control a motor, and to motor drive systems.
2. Description of Related Art
In general, in motor drive systems that drive a permanent-magnet synchronous motor (hereinafter referred to simply as the “motor”), flux-weakening control is performed by using a negative d-axis current in order to prevent an induction voltage (in other words, electromotive force) from rising excessively when the motor is rotating at high speed.
The induction voltage Vo generated by the rotation of the motor, an inductance of the motor, and an armature flux linkage is generally given by formula (1) below. Assuming that the induction voltage Vo is kept at the voltage limit Vom by flux-weakening control gives formula (2) below. Solving formula (2) for a d-axis current gives formula (3) below.
                              V          o                =                  ω          ⁢                                                                      (                                                                                    L                        d                                            ⁢                                              i                        d                                                              +                                          Φ                      a                                                        )                                2                            +                                                (                                                            L                      q                                        ⁢                                          i                      q                                                        )                                2                                                                        (        1        )                                                                    (                                                                    L                    d                                    ⁢                                      i                    d                                                  +                                  Φ                  a                                            )                        2                    +                                    (                                                L                  q                                ⁢                                  i                  q                                            )                        2                          =                              (                                          V                om                            ω                        )                    2                                    (        2        )                                          i          d                =                              -                                          Φ                a                                            L                d                                              +                                    1                              ω                ⁢                                                                  ⁢                                  L                  d                                                      ⁢                                                            V                  om                  2                                -                                                      (                                          ω                      ⁢                                                                                          ⁢                                              L                        q                                            ⁢                                              i                        q                                                              )                                    2                                                                                        (        3        )            
In the formulae above, ω represents the rotation speed of the motor, Ld represents the d-axis inductance, Lq represents the q-axis inductance, Φa represents the armature flux linkage ascribable to the permanent magnet, id represents the d-axis current, and iq represents the q-axis current.
In general motor drive systems, flux-weakening control is performed by calculating a flux-weakening current (a specified d-axis current value for flux-weakening control) to be followed by the d-axis current id according to formula (3) above.
FIG. 8 shows an example of the configuration of a motor drive system that calculates a flux-weakening current according to the formula (3) above. The motor drive system shown in FIG. 8 is a motor drive system that performs sensorless control. In this motor drive system, the axes estimated, for control purposes, as corresponding to the d-axis and the q-axis are referred to as the γ-axis and the δ-axis, respectively, and vector control is performed in such a way that the γ-axis coincides with the d-axis.
In FIG. 8,
θe and ωe represent the estimated rotor position and the estimated rotation speed, respectively,
iu and iv represent the detected U-phase current and V-phase current, respectively,
iγ and iδ represent the γ-axis current and the δ-axis current, respectively, based on θe, iu, and iv,
ω* represents the specified motor speed value,
iγ* and iδ* represent the specified γ-axis current value and the specified δ-axis current value, respectively,
vγ* and vδ* represent the specified γ-axis voltage value and the specified δ-axis voltage value, respectively, and
vu*, vv*, and vw* represent the specified three-phase voltage values based on θe, vγ*, and vδ*.
In a case where flux-weakening control is performed in the motor drive system shown in FIG. 8, iγ* corresponding to the d-axis current represents the flux-weakening current. The magnetic flux controller provided in the motor drive system shown in FIG. 8 calculates the value of the right side of formula (3) above by substituting ωe and iδ* for ω and iq in formula (3). In this way, iγ* corresponding to the flux-weakening current is calculated.
There have conventionally been proposed various methods for performing flux-weakening control. For example, there have been disclosed a method for calculating the flux-weakening current based on the battery voltage and the required torque, a method for calculating the flux-weakening current based on the battery voltage and the rotation speed, and a method for correcting the starting rotation speed of flux-weakening control according to the battery voltage.
In addition, there has been disclosed a method for calculating the flux-weakening current (the specified d-axis current value for flux-weakening control) according to formula (4) below. This method exploits the fact that the voltage drop (ωLqiq) attributable to the q-axis inductance can be assumed to be equal to the value obtained by subtracting the voltage drop attributable to the resistance from the d-axis voltage. The use of formula (4) makes the flux-weakening current independent from the q-axis inductance. This advantageously eliminates the need to take the influence of magnetic saturation into consideration, for example.
                              i          d                =                              -                                          Φ                a                                            L                d                                              +                                    1                              ω                ⁢                                                                  ⁢                                  L                  d                                                      ⁢                                                            V                  om                  2                                -                                                      (                                                                  v                        d                                            -                                                                        R                          a                                                ⁢                                                  i                          d                                                                                      )                                    2                                                                                        (        4        )            
On the other hand, to achieve high-efficiency operation by making effective use of reluctance torque, the specified d-axis current value for achieving high-efficiency operation usually needs to be calculated constantly. Such calculation increases the calculation load. Furthermore, it will take much time to adjust parameters needed to perform such calculation, and the values thus calculated will be affected by the parameter error.
As an effective technique for solving these problems, position sensorless vector control for permanent-magnet synchronous motors based on maximum torque control axes (a dm-axis and a qm-axis, which will be described later) has been disclosed (a description thereof will be given later). Although this vector control using the maximum torque control axes also needs flux-weakening control according to the rotation speed, a method suitable for flux-weakening control based on the maximum torque control axes has yet to be proposed.
In a case where control axes, such as maximum torque control axes, are displaced from the d-axis and the q-axis, it is impossible to perform satisfactory flux-weakening control with a conventional flux-weakening control method using formulae (3), (4) or the like. In this case, weakening magnetic flux becomes too small or too large unless the flux-weakening current is calculated by a calculation method suitable for the control axes other than the d-axis and the q-axis. Too small weakening magnetic flux produces variations in speed (which is caused by a cycle where a shortage of source voltage leads to a shortage of produced torque, which causes a reduction in the rotation speed of the motor, which causes a reduction in the motor induction voltage, which increases the current to be supplied to the motor, which increases the produced torque, which causes an increase in the rotation speed of the motor, which causes a shortage of source voltage again). On the other hand, too large weakening magnetic flux increases losses.