JPS631393A - Vector control system for ac motor - Google Patents

Abstract

PURPOSE:To prevent an AC motor from damaging by detecting the switching time point of a torque polarity, and operating it so that a torque current component is zero, i.e., only with a magnetizing current component for a predetermined time from this time point to suppress the rise of a motor voltage. CONSTITUTION:A controller 16 presumes a load torque, takes a difference between the presumed value and a load torque corresponding amount iT, compares the difference with a predetermined value to detect a switching time point from a driving mode to a control mode. and outputs a signal which becomes 'H' for a predetermined period from the detection time point. Thus, since the output of ASR 6 is held at zero (iT=0) only during this period, a motor current is only iM during this period (specified exciting current value), and even if a motor power factor becomes zero, the motor voltage does not exceed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is directed to converting the current vector of an AC motor driven through a current source inverter consisting of at least a term and an inverter into a motor magnetic flux calculated by a calculator. The present invention relates to a vector control method for an AC motor that is controlled by dividing it into a magnetizing current component parallel to the vector and a torque current component perpendicular to the vector, and particularly to improvements thereof.

[Conventional technology]

FIG. 4 is a vector diagram showing current vectors.

That is, for example, the primary current (stator current) of an induction motor
11 can be taken as a space vector that rotates around the rotor axis. In this primary current vector equation, a component in the same direction as the magnetic flux vector j is called a magnetizing current or exciting current IM, and corresponds to the field current of a DC machine. Also, the component in the direction perpendicular to this is called the torque current 17,
Corresponds to the armature current of a DC machine. If these components in an induction motor can be controlled independently of each other, an induction motor that is cheaper and more robust than a DC machine can exhibit variable speed control performance equivalent to that of a DC machine. Controlling an induction motor based on this principle is called vector control of an induction motor. Note that vector quantities are indicated by arrows (→), but this distinction is not made unless it is particularly necessary. In order to enable such vector control of an induction motor, it is necessary to create a current whose command value is given in the form of two mutually orthogonal components, with the axis of the rotating magnetic flux vector (magnetic flux axis; M axis) as a reference. Since it is necessary to convert the vector into a vector based on a non-rotating fixed axis (α-axis), the position ψ of the magnetic flux axis (M-axis)
It turns out that we have to detect. Hiko, this stator axis (α axis) is generally set at one winding axis of the stator.

FIG. 5 is a block diagram showing a conventional example of such a vector control system. In the figure, 1 is a rectifier, 2 is an inverter, 3 is an induction motor, and 4 is a speed detector (
PG), 5 is an acceleration/deceleration pattern generator, 6 is a speed adjuster (
AS fl), 7 is stator 1st current (primary current) calculator, 8
is a current regulator (ACR), 9 is a phase shifter, 10 is a current detector, 11 is a divider, 12 is a vector calculator, 13 is an angle calculator, 14 is an angle adder, and 15 is a pulse distributor. be.

The forward converter 1, the inverse converter 2, and the DC reactor DCL constitute a current source inverter, and the induction motor 3 is driven by the inverter.

The rotational speed N (or rotor angular frequency ω2) is detected via a speed detector (pulse generator) 4, and the output of an acceleration/deceleration pattern generator 5 that generates a speed command value N from the speed setting value is detected by a speed regulator. (ASR) (Input to 5. ASFt6 performs adjustment calculation so that the deviation between the two becomes zero, and the output is the torque current command value IT"
becomes. The stator current calculator 7 calculates this torque t flow command value I.
Based on T'', the magnetizing current command value given via the vector calculator 12, M+ and K, the stator current command value i,'
, that is, 11'''-f10.*)!, (=.l}) is calculated and fed to the current regulator (ACR) 8,
8 performs a predetermined adjustment operation to make the stator current actual value i, obtained through the AC current transformer ACCT and the current detector 10, match the command value i, 10 ignition control is performed.

On the other hand, torque current command value tTI which is the output of AsFL6
is input to the divider 11, and the slip frequency ω5■ is obtained by dividing the magnetic flux command value separately given by N. The vector calculator 12 calculates this slip frequency ω
. The angle calculator 13 receives the output IMIIT from the vector calculator 12.
Calculate the angle β of the current vector from the magnetic flux axis by
The angle adder 14 calculates these angles β, ? Find the angular position of the current vector by adding This output is given to each switching element in the inverter 2 via the pulse distributor 15, thereby controlling the angle (frequency).

[Problems that the invention attempts to resolve]

By the way, the operating modes of the electric motor include a drive (power running) mode and a braking (regeneration) mode. FIG. 6 is an explanatory diagram for explaining the operating modes of the electric motor, with (a) showing the drive mode and (b) showing the braking mode.

In the former case, the rectifier 1 performs a forward conversion operation, and the inverter 2 performs a reverse conversion operation, and the power flow is as shown by the thick arrow.

On the other hand, in the latter case, the rectifier 1 performs an inverse conversion operation and the inverter 2 performs a forward conversion operation, and the power flow is opposite to that in the case (a).

At the same time, the load torque changes and the inverter 2 changes as shown in Fig. 6 (
Let us consider the case where the driving mode as shown in (a) changes to the braking mode as shown in the figure.

At this time, when the motor voltage is vM, the DC voltage Ed on the inverter side is Ed-(3V7/π) VMωSφ ・・・・・・(
1). In other words, changing from drive mode to braking mode means that the electric motor power factor (cosφ
) advances from the delay and changes by K, and in this process the motor power factor ωSφ-0 is passed. If this movement is relatively slow, the output of the ASR6 will change accordingly, so it will not be abnormal. However, if this movement is steep or is repeated frequently, the AsR
The movement of the output signal (iT') of No. 6 cannot follow this, and in the worst case, when this commands (!T') max, the electric motor force may become zero.

Figure 7 shows an equivalent circuit of an induction motor. In the same figure,
Lσ is the primary leakage reactance, R1 is the primary resistance, LM
is the excitation reactance, R2' is the secondary resistance, and when the motor force is zero, the motor input current is only the excitation current (-), which flows through the excitation reactance LM.

This current is constantly controlled to a specified value, but if the motor power factor approaches zero while AsR6 is commanding (i7'') max as described above, the specified value becomes: A current having a magnitude several times as large as IM will flow through the magnetizing reactance LM, and this current causes the motor voltage VMt- to increase significantly.

FIG. 8 shows the operation during torque switching, and FIG. 9 shows the relationship between exciting current and motor voltage. In addition, Fig. 8 (a) shows the inverter side DC voltage Ed, and the figure (opening) shows the DC current Id,
The figure (no) shows the waveform of the motor voltage vM, and the figure (2) shows the waveform of the speed N. When switching torque, the motor voltage vM
It can be seen that the value becomes larger as shown in the same figure (No.). Also,
If the motor voltage when the excitation current IM becomes vM', this and the motor voltage v when the specified excitation current flows
As is clear from FIG. 9, the ratio VM'/VM with M takes a value of 2 to 5, indicating that a voltage as large as 2 to 3 times the rated voltage is generated.

This voltage increase not only adversely affects the insulation of the motor, but also causes overvoltage breakdown in the commutating diodes (D) of the current source inverter as shown in FIG. 10, to which the motor voltage is directly applied. At this time, the voltage applied to diode D is represented by the sum of the commutating capacitor voltage and the motor voltage. Incidentally, the above-mentioned problems also occur when transitioning from braking mode to driving mode.

Therefore, it is an object of the present invention to provide a control system that can suppress an increase in motor voltage and prevent failure of the motor and inverter even when sudden or frequent torque switching is performed.

[Failure to solve the problem]

A system for vector-controlling an AC motor via a current-source inverter is provided with a detection means for detecting a switching point in torque polarity, and a time limit means for validating this detection force for a predetermined period of time.

[Effect]

From the time when the torque polarity detected by the detection means switches, for a predetermined time determined by the time limit means, the torque current component is zero, that is, the motor is operated with only the magnetizing current component, thereby suppressing the increase in motor voltage. and prevent damage to the equipment.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the present invention.

As is clear from a comparison with FIG.
The feature is that 6 is provided.

Specifically, as shown in FIG. 2, this control circuit 16 includes adders 16a, '+6e, 16f, integrators 16b,
ISd, coefficient unit 16C, comparators ISg, ISh,
Monostaple multivibrator (also abbreviated as Monostaple) 16i, 1<Sj and OR gate IS
It is composed of k, etc.

The part consisting of 16a to 16e constitutes a well-known load torque estimating circuit, and the speed simulation value is calculated by applying the output 17'' from the ASR 6 shown in Fig. 1 to the integrator 16bK with a time constant T. Take out N', and this is P in Figure 1.
The load torque is estimated by operating the coefficient unit 16c and the integrator ISd so as to match the actual speed value N given via G4. In addition, K + l
K2 is a coefficient, TJ is the sum of the mechanical time constants of the motor and load, and the load torque estimation signal TL' is the integrator 16.
It can be obtained as the output of d.

Therefore, this output TL' corresponds to the load torque II!
lT* is given to the adder 16f with the polarity as shown in the figure, the difference between the two is taken, and this is compared with a predetermined set value in the converters 16g and 1611 to detect the change point or switching point of the torque polarity. can do. The monoste 16i, 16j waits for a certain time T from the point of change in torque polarity.
Outputs a signal B that is at the "H" level by 1. In the speed regulator (ASR) 6, the switch SW is closed only during the period when the signal B is at the H# level, thereby holding the output at zero.
is set to the minimum value at which the operating mode of the inverter will end up changing.

The above situation is illustrated in FIG. 3. In other words, the switching point from the drive mode to the braking mode is detected by the converter 16g, for example, as shown in FIG.
As a result, the output B of the monoste 161 becomes ''H II for a certain period T1 as shown in the same figure (f).
As the output of As R6 is held at zero (lT" = 0) during this period as shown in the same figure (g), the motor current is only LM (specified excitation current value) during this period, and therefore the motor power factor is Even if the voltage becomes zero, the motor voltage does not become overvoltage as shown in Figure 8 (C), and this is shown in Figure 8 (
The difference is obvious when compared with (c). Note that FIGS. 3(a) to 3(b) are the same as those in FIG. 8, so their explanations will be omitted.

The above describes the case where the electric motor shifts from the drive mode to the braking mode. However, when the electric motor shifts from the braking mode to the drive mode, the comparator 16h and monoste 16
j, etc., in the same manner as above. Furthermore, although the induction motor has been described above, the present invention can be similarly applied to a synchronous motor.

〔Effect of the invention〕

According to this invention, even if sudden and frequent torque switching occurs when vector controlling an alternating current machine, the point in time is quickly detected and the output of the speed regulator is held at zero. ．． It is possible to suppress transient overvoltage in the electric motor relatively easily, and this brings about the advantage that it is possible to prevent insulation deterioration of the electric motor, damage to the commutation electrode in the inverter, etc.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of this invention, and FIG.
Figure 3 is a block diagram showing a specific example of the control circuit shown in Figure 3. Figure 3 is a time chart for explaining the operation of Figure 1. Figure 4 is a vector diagram for explaining the vector control method. Figure 5 is a vector diagram for explaining the vector control method. A configuration diagram showing a conventional example of the control system, Fig. 6 is an explanatory diagram to explain the operating mode of the motor, Fig. 7 is a circuit diagram showing an equivalent circuit of an induction motor, and Fig. 8 shows operation when torque is switched. FIG. 9 is a graph showing the relationship between excitation current and motor voltage, and FIG. 10 is a configuration diagram showing the configuration of a general current source inverter main circuit. Symbol explanation 1... Rigid converter (rectifier), 2... Inverse converter (inverter), 3... AC motor, 4
... speed detector (PG), 5 ... jerking speed pattern generator, 6 ... speed regulator (AsR
), 7...Stator current (primary current) calculator, 8
... Current regulator (ACR), 9 ... Phase shifter, 10 ... Current detector, 11 ......
Divider, 12... Vector operator, 13...
...Angle calculator, 14...Angle adder, 15
......Pulse distributor, i 6-----Control circuit, 16a, 16e, L6f...Adder, 16
b, 16d... Integrator, 16c... Fl. Equipment, 1 6g, i 6h... Converter, 16i+i6j... Monoste, 16k.
...or gate, sw...switch. Agent Patent attorney Akio Namiki Agent Patent attorney Tsuke Matsuzaki Figure 1 Speed test Figure 2 Figure 16 LSI colloquial Figure 3 Figure 4 Figure β Figure 5 Figure 9 Figure 10,? ．． isoverter

Claims (1)

[Claims] 1) The current vector of an AC motor driven through a current source inverter consisting of at least a forward converter and an inverse converter is divided into a magnetizing current component parallel to the motor magnetic flux vector axis and a torque current perpendicular to this. In a vector control method for an AC motor, the vector control system includes a forward converter control system, an inverse converter control system, and a speed regulator that provides a torque current command value to both of these control systems. A detection means for detecting the time point at which the torque polarity is switched; and a time limit means for validating the detected force for a predetermined time; A vector control method for an AC motor characterized by holding the output of a speed regulator at zero.