JP3424836B2 - AC motor current control device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to current control of an AC motor.
The present invention relates to a device , and more particularly, to a current control device that increases output torque of a motor in a high speed region.

[0002]

2. Description of the Related Art FIG. 1 is a block diagram of a speed loop of a conventional AC motor. In FIG.
Element 1 is the term of the transfer function of the velocity loop, kv1 is the integral gain, and kv2 is the proportional gain. The speed deviation is obtained by subtracting the feedback value fv of the actual speed of the electric motor detected by the speed detector or the like from the command speed vc,
(Proportional integration) control is performed and a torque command (current command) is obtained. In response to this torque command, the current limit circuit 2 limits the torque command. That is, element 1
If the absolute value of the torque command obtained by the speed loop processing by is smaller than the limit value of the limit circuit 2,
This torque command is directly applied to the torque command T to the current loop.
However, if the absolute value is larger than the limit value, the limit value is output as the torque command Tc. The torque command Tc is multiplied by a sine wave having a phase difference of 2π / 3 according to the electrical angle θ of the rotor by the elements 3R to 3T, and the torque command (current command) T of each phase of R, S, and T phases is obtained.
cR, TcS, TcT are obtained (the example shown in FIG. 1 is a three-phase AC motor).

Each phase torque command (current command) TcR, T
cS and TcT are input to the current loop circuits 4R to 4T for the respective phases, and the current loop processing is executed. Current loop is usually PI
Although it is controlled by control or the like, and FIG. 1 shows only a block diagram of the R-phase current loop, the configuration of the other phases is the same as that of the R-phase. The current loop processing for the R phase will be described. The torque command (current command) TcR to the R phase and the R phase current feedback value IRr detected by the detector for the actual current of the R phase are detected.
And the current deviation of the difference is integrated (element 5), and the integration gain k1
The value obtained by subtracting the value obtained by multiplying the R-phase current feedback value IRr by the proportional gain k2 (element 6) from the value obtained by multiplying by
Output as WM command. The same applies to the other S and R phases.

FIG. 2 is a block diagram showing the block diagram of the R-phase current loop in FIG. 1 including the electric motor. Elements 5 and 6 are the same as the elements 5 and 6 in FIG. It is a proportional term. Further, the element 7 is a term of the transfer function of the electric motor, R is the winding resistance of the R phase, and L is the inductance of the R phase winding. Note that the S phase and the T phase are similar block diagrams. The transfer function of the current loop is calculated as shown in the following equation 1. IRr / TcR = k1 / [LS ² + (R + k2) S + k1] (1) As can be seen from the transfer function of the current loop represented by the above equation 1, the characteristics of the current loop are affected by the integral gain k1 and the proportional gain k2. However, if these gains are too large, a phenomenon such as oscillation occurs and the band of the current loop is limited.

FIGS. 4 and 5 are views showing an example of the frequency characteristic of this current loop. The horizontal axis is the logarithmic scale of the command frequency (rotation speed), and the vertical axis of FIG. 4 is the gain of the transfer function of the current loop. And the vertical axis of FIG. 5 shows the phase delay. It can be seen that this example has a band of about 200 Hz. It usually has a band of 200 Hz to 300 Hz. Figure 5 above
As shown in, the phase delay occurs when the rotation speed of the electric motor becomes high. Therefore, since the command frequency is proportional to the rotation speed of the electric motor, currently, at the time of high speed rotation, phase advance control is performed to advance the phase of the command in order to prevent the power factor from decreasing due to the phase delay. That is, elements 3R to 3 in FIG.
The rotor electrical angle θ due to T is corrected for the phase delay ph (v) according to the actual speed of the electric motor, and the torque commands (current commands) TcR, TcS, TcT of the angular phases are obtained. That is, the phase advance control is performed by substituting [θ + ph (v)] in place of θ in FIG.

[0006]

As described above, the phase advance control can prevent the decrease of the current power factor due to the delay of the phase of the current loop. However, as shown in FIG. Since the motor current decreases during high-speed operation, the actual current of the electric motor during high-speed operation is smaller than the limit value of the limit circuit by the gain decrease, and the maximum output torque of the motor decreases.

Therefore, an object of the present invention is to provide a current control device for an AC electric motor which prevents a decrease in output torque in a high speed range.

[0008]

The present invention has solved the above problems by changing the limit value of the torque command according to the speed of the AC motor. In particular, it corresponds to the speed of the motor
The absolute value of the transfer function of the current loop is obtained according to the frequency of the current, and the limit value of the torque command is changed in inverse proportion to the absolute value. Alternatively, the limit value of the torque command inversely proportional to the absolute value of the transfer function of the current loop corresponding to the speed of the electric motor is stored in the storage means, and the limit value stored in the storage means is read according to the speed of the electric motor. , And controls the AC motor with the limit value as the limit value of the torque command.

[0009]

When the speed of the AC motor increases, the gain of the transfer function of the current loop decreases. However, the output torque of the motor is increased by changing the limit value of the torque command so as to compensate for the decrease in the gain. Prevent the decrease of.
In this case, depending on the frequency of the current corresponding to the speed of the motor
Then, the absolute value of the transfer function of the current loop is obtained, and the limit value of the torque command is changed in inverse proportion to the absolute value. Alternatively, the limit value of the torque command inversely proportional to the absolute value of the transfer function of the current loop corresponding to the speed of the electric motor is stored in the storage means in advance, and the limit value is read from the storage means according to the speed and the limit value is read. Limit the torque command with.

[0010]

EXAMPLE When S = jω (ω is the electrical angular velocity of the motor) is substituted into the transfer function of the current loop shown by the above equation 1, IRr / TcR = k1 / [-Lω ² + jω (R + k2) + k1] (2) ) If the absolute value G of the transfer function, G = | IRr / TcR | = k1 / [(k1-Lω 2) 2 + ω 2 (R + k2) 2] 1/2 ... (3) in addition, the electrical angular velocity of the electric motor ω is obtained by the following four equations, where v is the speed of the electric motor and P is the number of poles of the electric motor.

Ω = (v / 60) · (P / 2) · 2π (4) The integral gain k1 and the proportional gain k2 of the current loop are set values, and if the electric motor to be used is determined, the number of poles P Since the winding resistance R and the inductance L are determined, the absolute value G of the gain of the current loop can be obtained by detecting the speed v of the electric motor from the above expressions 3 and 4.

Therefore, the current limit value (torque limit value) I _lim of the limit circuit 2 is _calculated by the following five equations. I _lim = I _max · (1 / G) (5) Note that I _max is the maximum current value that the motor can output.

[0013] The above Equation 5 and Equation 3, as can be seen with reference to Equation 4, when the motor speed is low, the value of the absolute value G is substantially "1" current limit value of the gain becomes I _max . However, when the speed v (angular speed ω) becomes high speed, the value of the absolute value G of the gain decreases, and the current limit value I _lim calculated by the above equation 5 increases by this decrease amount. The maximum output torque of the electric motor never decreases. The relationship between the speed of the electric motor and (1 / G) is shown in FIG.

Therefore, by detecting the speed of the electric motor,
The current limit value (torque limit value) I _lim is obtained by performing the calculation of four expressions, or (current limit value (torque limit value) I _lim for speed is stored as a table from FIG. The current limit value (torque limit value) I _lim may be changed accordingly.

FIG. 3 is a flow chart of the speed loop processing for each predetermined cycle which is executed by the processor that controls the electric motor. First, the speed command vc is read, and a feedback value f of the actual speed of the electric motor detected by a speed detector or the like.
v is read (steps S1 and S2). Next, the current limit value I _lim is calculated by performing the calculations of the above expressions 3 to 5 (step S3). Instead of calculating the current limit I _lim, as shown in FIG. 6 described above, stores the current limit I _lim as a table for the motor speed, it is read the current limit value from this table Good. Next, based on the read speed command vc and the feedback value fv of the actual speed, speed loop processing is performed in the same manner as the conventional method, and the torque command is calculated (step S4). The absolute values of the torque commands are compared, and the smaller one is transferred to the current loop as the torque command Tc, and the speed loop process is ended (steps S5 and S6). In the current loop, the phase advance processing is performed from the detected electrical angle position θ of the rotor and the actual motor speed, and the torque command (current command) for each angular phase is obtained, and the current loop processing is performed in the current loop, as in the conventional case. Will drive.

[0016]

According to the present invention, when the AC motor rotates at a high speed, the gain of the transfer function of the current loop decreases and the maximum output torque decreases. Since it was adjusted to compensate for
The maximum output torque of the AC motor does not decrease.

[Brief description of drawings]

FIG. 1 is a block diagram of speed loop control of an AC motor.

2 is a block diagram of the current loop in FIG. 1. FIG.

FIG. 3 is a flowchart of a speed loop process executed by a processor that controls an AC electric motor according to an embodiment of the present invention every predetermined period.

FIG. 4 is a diagram showing a decrease in gain of a transfer function of a current loop with respect to a speed of an AC motor.

FIG. 5 is a diagram showing a phase delay due to a current loop with respect to a speed of an AC motor.

FIG. 6 is a diagram showing coefficients for current limit value correction according to an embodiment of the present invention.

[Explanation of symbols]

1 Transfer loop element of velocity loop 2 Current limit circuit 3R to 3T Elements for obtaining torque command (current command) for each phase 4R to 4T Current loop circuit for each phase

   ─────────────────────────────────────────────────── ─── Continued front page       (56) References JP-A-2-70280 (JP, A)                 JP 59-159687 (JP, A)                 JP-A-3-236925 (JP, A)                 JP 4-75498 (JP, A)                 JP-A-4-21388 (JP, A)                 Actual Kaihei 2-133197 (JP, U)

Claims (2)

(57) [Claims] 1. In the control of an AC motor for current control, an absolute value of a transfer function of a current loop is obtained according to a frequency of a current corresponding to a speed of the motor, and a torque command is inversely proportional to the absolute value. A current control device for an AC electric motor that changes the limit value. 2. In the control of an AC motor for current control, a limit value of a torque command inversely proportional to an absolute value of a transfer function of a current loop is stored in a storage means in accordance with a frequency of a current corresponding to a speed of the motor. A current control device for an AC motor, which reads a limit value stored in a storage unit according to the speed of the electric motor and uses the limit value as a limit value for a torque command.