JP2644222B2 - Induction motor torque control method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a torque control method for an induction motor, and more particularly, to a torque control method suitable for controlling an induction motor such as a rolling mill that performs an overload operation. .

[Conventional technology]

As a motor for driving a rolling mill, a thyristor-leonard-driven DC motor has been used. However, there are problems in terms of maintainability and environmental resistance, and application of an AC variable-speed drive is being promoted. Recently, a system for driving a large-capacity induction motor by a vector control cycloconverter has been put to practical use. In this system, as an operation method at the time of overload operation of an induction motor, a method of limiting a current command value so that a stator current does not exceed a predetermined value as described in Japanese Patent Application Laid-Open No. 55-46893. Was taken.

[Problems to be solved by the invention]

During steady overload operation of the induction motor, when the output voltage of the cycloconverter is saturated, the motor current may not be a sine wave and torque ripple may occur.
The above-mentioned conventional method does not take this into consideration.

An object of the present invention is to provide a torque control method for an induction motor in which torque ripple does not occur during steady overload operation of the induction motor.

[Means for solving the problem]

The above purpose is to calculate the maximum allowable torque current from the output voltage set value of the cycloconverter for controlling the voltage and frequency, the exciting current of the induction motor, and the primary frequency command, so that the current command during overload operation is This is achieved by limiting this when the value is equal to or more than that value.The terminal voltage of the induction motor is calculated from the excitation current and torque current commands of the induction motor and the primary frequency command of the induction motor, and the value is equal to or greater than a predetermined value. Is achieved by limiting the torque current command.

[Action]

When the voltage of the induction motor rises above the output voltage of the cycloconverter due to the steady overload operation, the output voltage of the cycloconverter is saturated, and a sinusoidal voltage cannot be supplied to the motor, thereby causing torque ripple. To prevent this, the maximum allowable torque current during steady overload is calculated as described above, and the current command value during overload is limited so as not to exceed the maximum allowable value, so the motor voltage is limited. The occurrence of torque ripple can be prevented. Alternatively, the terminal voltage of the induction motor is calculated to limit the value to a predetermined value or less, so that the occurrence of torque ripple can be prevented.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. The AC power from the AC power supply 1 is converted in frequency and voltage by a frequency converter (cycloconverter) 2 and is supplied to a cage induction motor 3. The speed generator 4 detects the rotation speed of the induction motor 3, and outputs the output of a speed command circuit 5 for commanding the rotation speed of the induction motor 3 to a speed control circuit 6.
A value corresponding to the difference between the two inputs is output as a torque current component command signal _Iq0 of the induction motor 3. Limiter circuit 7
Limits the torque current command I _q0 , and its limit value is usually set to a value determined from the allowable current value of the thyristor. In the present invention, the limit value is further set by an output signal of a motor current calculation circuit 9 described later. Lower the limit value to limit the current. Works like that. The current component detecting circuit 18 detects a torque current component and an exciting current component of the primary current detected by the current detector 15, and the exciting current component is supplied to the current control circuit 20 together with the exciting current command I _d0 from the exciting current command value 10.
And the output corresponding to the difference is the current pattern circuit
Sent to 14. The torque current component detected by the current component detection circuit 18 is the torque current command I from the speed control circuit.
It is input to the current control circuit 19 together with _q0 , and an output corresponding to the difference is also applied to the current pattern circuit 14. The output of the slip frequency and speed generator 4, which is calculated by the slip frequency calculation circuit 11 are added by the adder 12, the primary frequency command omega ₁₀ next to the induction motor 3, the oscillator 13 having a frequency proportional to the command omega ₁₀ Outputs a sine wave signal. This output signal becomes a position (phase) signal of the secondary flux linkage of the induction motor 3. The current pattern circuit 14 commands the instantaneous value of the primary current (sine wave) of the induction motor 3 in accordance with the signals from the second and third current control circuits 19 and 20, based on the signal of the oscillator 13. The current control circuit 16 outputs a signal corresponding to the deviation between the current command given from the current pattern circuit 14 and the current detection value 15 and outputs the signal to the cycloconverter 2 via the gate control circuit 17.
Control thyristor gates. Note that the current pattern circuit 14 and the control circuit 16 are provided for three-phase AC. Motor current calculation circuit 9, which is a feature of the present invention, the setting circuit 8 for setting the primary frequency command signal omega ₁₀ and the maximum output voltage of the cycloconverter 2 from the adder 12 and the output signal I _d0 of the exciting current command circuit 10 The maximum permissible torque current of the motor is calculated from the output signal v _S0, and if the torque current command value during overload operation exceeds the permissible value, the limiter circuit 7
Limits the command value, and by this operation, prevents the steady voltage of the electric motor 3 from exceeding the allowable maximum output voltage of the cycloconverter 2, thereby preventing the occurrence of torque ripple.

The basic operation of this embodiment uses a control method of the induction motor 3 called vector control, and separates the primary current of the motor 3 into an exciting current component and a torque current component to make them non-interfering, and controls each of them independently. Thus, the torque of the electric motor 3 is controlled, and the speed response performance can be made equal to or higher than that of the DC motor.

Next, the contents of the present invention will be described. A method of calculating the maximum allowable torque current _{Iqm in} the motor current calculation circuit 9 will be described. When the induction motor is controlled by the vector, various quantities of the induction motor are converted from the stator coordinate system (α-β) to the rotational coordinate system (d
-Q), the voltage equation on the d and q axes and the magnetic flux linkage equation are expressed by the following equations (1) and (2).

When the condition φ _2q = 0 of the vector control is satisfied,
The d-axis primary voltage V _1d and the q-axis primary voltage V _1q are expressed by the following equations.

_{V 1d = L s αpi 1d +} (M / L r) pφ 2q + r 1 i 1d -ω 1 L s αi 1q ... (3) V 1q = L s αpi 1q + r 1 i 1q -ω 1 L s αi 1d + ω in _{(r 2 / L r) φ} 2d α = 1-M 2 / (L s L r) - 1 (M / L r) φ 2d ... (4) However _{pφ 2d = M / L r)} r 2 i 1d is there. Equations (3) and (4) are voltage equations for the induction motor when vector control is performed. Although the terminal voltage of the motor can be calculated using the equations (3) and (4), in a steady state, the transient term including the operator p in the equations (3) and (4) is zero.
V _1d , V _1q Becomes Terminal voltages V ₁ at this time induction motor is determined by substituting the value of the expression (5) to the following equation: Where r ₁ is the primary resistance, L _s α is the leakage inductance, L _r is the secondary inductance, M is the mutual inductance, φ _2d is the secondary flux linkage of the d-axis component, ω ₁ is the primary angular frequency, and i _1d is d The axis component primary current (excitation current), i _1q is the q-axis component primary current (torque current). Therefore, when each constant (motor constant, excitation current, torque current, and operating frequency) is given, the terminal voltage of the motor can be calculated.

Incidentally, the terminal voltage required for the induction motor 3 is supplied from the cycloconverter 2. The maximum output voltage V _sm of the cycloconverter is determined from the input voltage of the cycloconverter, the power transformer, and the like. Therefore, when V _sm is given, equation (7) is written as follows.

However, V _1d and V _1q are the values of equation (5). From equation (7), V _sm
Is given, the maximum allowable torque current I _qom can be obtained.
That is, equation (5), the quadratic equation for i _1q from (7) is as ^{_{follows: (r 1 2 + ω 1}} 2 L s α 2) i 1q 2 + 2r 1 ω 1 (M / L r) φ _2d i _1q + ω ₁
² (M / L _r ) φ _2d ｛L _s αi _1d +2 (M / L _r ) φ _2d ｝ + r ₁ ² i _1d ²
−V _sm ² = 0 (8) From this time, I _qom when V _sm is given is Becomes However ^{_{A = r 1 2 + ω 1}} 2 L s α 2 B = r 1 ω 1 (M / L r) φ 2d C = ω 1 2 (M / L r) φ 2d {L s αi 1d + (M / L _r ) φ _2d ｝ ＋ r ₁ ²
i _1d ² −V _sm ² .

Therefore, the motor current calculation circuit 9 calculates the maximum allowable torque current from each command signal (output voltage set value _Vso torque current command _{Iq0 and} primary angular frequency command ω ₁₀ ) based on equation (9). Calculate the command I _q0m . In FIG. 2, the primary angular frequency omega ₁₀ and the maximum permissible torque current command I
_Shows the relationship of _q0m . Maximum permissible torque current I _Q0m decreases with an increase of the primary angular frequency command omega _10, also tends to magnitude due to the characteristics of the excitation current command I _do change. Therefore, the torque current command I _qo during overload operation limits the torque current command I _q0 switches the current Rimitsuta value of Rimitsuta circuit 7 when the has fallen maximum permissible torque current command I _Q0m more. As a result, the torque current i _1q is controlled according to the torque current command _Iqo , so that the motor voltage does not exceed the output voltage of the cycloconverter even in overload operation. For this reason, the motor voltage is always controlled to a sine wave, and the occurrence of triples due to saturation of the output voltage of the cycloconverter can be prevented. The same effect can be obtained even when the current is limited by a lower limit value of the overcurrent limit value of the torque current and the limit value determined from the above-mentioned maximum allowable value. In the above description, the maximum allowable torque current _Iq0m was calculated from each command signal. However, the same effect can be obtained even if the relationship between each primary frequency and the maximum allowable torque current is _tabulated in ROM and used according to equation (9). Is obtained.

FIG. 3 shows another embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The difference from FIG. 1 is the motor voltage calculation circuit 21. This current of the electric motor 3 (torque current component i _q and the exciting current component i _d) and the motor voltages V ₁ calculated from the command signal of the frequency whose value is a predetermined value (output voltage maximum value V _sm of cycloconverter)
An output signal is generated in the case described above. When the torque current of the electric motor 3 increases due to the overload operation, the steady voltage of the electric motor 3 increases accordingly. Then, the motor voltages V ₁ In motor operation circuit 21 is the above-mentioned (6)
The limiter circuit 7 is operated when the value is equal to or more than the maximum output voltage V _sm of the cycloconverter, and the torque current command I _qo is limited. for that reason,
Since the motor the motor voltages V ₁ simultaneously torque current decreases the 3 drops is obtained substantially the same effect as described above.

FIG. 4 shows another embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.
The difference from FIG. 1 is that a PWM inverter 30 for converting DC to AC is used as a frequency converter, and a triangular wave generating circuit 31 for outputting a triangular carrier wave for the control thereof, an output signal of the first current control circuit 16 and a triangular wave. A comparator 32 is provided for comparing the output signal of the generation circuit 31 and generating a PWM signal for turning on and off the elements of the inverter 30. In this case, V _so is a value set by the input voltage setting circuit 33 of the inverter 30. In this configuration, the inverter 30 is vector-controlled, and excellent speed response performance of the induction motor can be obtained. In addition, the occurrence of torque ripple when the motor 3 is overloaded by the inverter 30 can be prevented as in the case of FIG.

〔The invention's effect〕

According to the present invention, when the torque current command of the induction motor is equal to or more than the maximum allowable value during overload operation, or when the terminal voltage of the induction motor is changed to the frequency converter (cycloconverter)
When the output voltage exceeds the allowable output voltage, the torque current is limited, so that the output voltage of the cycloconverter can be set to the maximum, the power factor of the cycloconverter can be increased, the converter capacity can be reduced, and the occurrence of torque ripple can be prevented. The effect is that it can be prevented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a maximum allowable torque current characteristic with respect to a primary frequency,
FIG. 3 and FIG. 4 each show another embodiment of the present invention. 2… Cyclo converter, 3… Cage induction motor, 4…
Speed generator, 5: speed command circuit, 6: speed control circuit, 7
... Limiter circuit, 8 ... Cycloconverter output voltage setting circuit, 9 ... Motor current calculation circuit, 10 ... Exciting current command circuit, 12 ... Adder, 13 ... Oscillator, 14 ... Current pattern circuit
18 ... current component detection circuit, 19 ... second current control circuit, 20 ...
Third current control circuit, 21 ... motor voltage calculation circuit.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Takahashi 5-2-1 Omikamachi, Hitachi City Inside the Omika Plant, Hitachi, Ltd. (56) References JP-A-60-200791 (JP, A)

Claims (2)

(57) [Claims] A torque current command is calculated from a detected value of a rotational speed of an induction motor and the command value, an exciting current command is set by an exciting current setting means, and a slip frequency calculated from the torque current command and the slip frequency are calculated. A primary frequency command of the induction motor is calculated from the detected value of the rotation speed, and a primary current of the induction motor becomes a composite value of the torque current command and the excitation current command by controlling frequency conversion means for driving the induction motor; and In a torque control method for an induction motor that controls the frequency to be the primary frequency command, a limiter circuit that limits the torque current command, and an arithmetic control circuit that calculates a limit value of the torque current command are provided, The primary frequency command, the exciting current command, and the output voltage set value of the frequency conversion means are calculated by the arithmetic control circuit. Calculate the maximum allowable torque current to prevent the terminal voltage of the induction motor from exceeding the maximum allowable output voltage of the frequency conversion means, and further calculate the torque current command value so as not to exceed the maximum allowable torque current. A torque control method for an induction motor, characterized by operating a limiter circuit. 2. A torque current command is calculated from a detected value of a rotational speed of an induction motor and the command value, an exciting current command is set by an exciting current setting means, and a slip frequency calculated from the torque current command and the slip frequency are calculated. A primary frequency command of the induction motor is calculated from the detected value of the rotation speed, and a primary current of the induction motor becomes a composite value of the torque current command and the excitation current command by controlling frequency conversion means for driving the induction motor; and In a torque control method for an induction motor that controls the frequency to be the primary frequency command, a limiter circuit that limits the torque current command, and an arithmetic control circuit that calculates a limit value of the torque current command are provided, The arithmetic control circuit converts the primary frequency command, the exciting current command, and the torque current command into an induction motor. Calculating the terminal voltage, further the calculated torque control method for an induction motor terminal voltage, characterized in that the actuating said limiter circuit so as not to exceed the maximum allowable output voltage of the frequency converting means out.