JPH0767311B2 - Current and torque limiting circuit for AC motor drive inverter device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention controls the output frequency and output voltage of an inverter device to suppress the output current of the inverter device and the AC motor generated torque of the load within a fixed value. Current and torque limiting circuit.

[Conventional technology]

As a conventional speed adjusting circuit for an AC electric motor using this type of inverter device, one illustrated in the block diagram of FIG. 5 is known. In FIG. 5, reference numeral 1 denotes an inverter device, which is composed of a rectifier circuit that forward-converts an AC input voltage into a DC voltage, an inverter circuit that reverse-converts a DC output voltage of the rectifier circuit into an AC voltage, and a control circuit for both converter circuits. Reference numeral 2 is an induction motor driven by the inverter device.
FIG. 6 is a speed characteristic curve of the input current Im, the output torque Tm, and the load torque T _L of the electric motor 2. In FIG. 6, the load torque T _L is an example of the reduced torque characteristic,
The f on the horizontal axis is a frequency corresponding to the rotation speed of the electric motor 2 and is a value converted by the number of poles of the electric motor. Further, fr is the output frequency of the inverter device 1 and is the power supply frequency of the electric motor 2. As shown in Fig. 6, the load torque T _L is T
_When increasing from _L1 to T _L2 , the operating points of the torque Tm of the electric motor 2 and the current Im with respect to the frequency f both move on the characteristic curve with the power supply frequency as fr, and the points P ₁ (f ₁ , T
m ₁ ) and point Q ₁ (f ₁ ,, Im ₁ ) to point P ₂ (f ₂ , Tm ₂ ) and point Q
₂ (f ₂ , Im ₂ ), the motor ₂ decelerates from f ₁ to f ₂ at its corresponding frequency, its output torque increases from Tm ₁ to Tm ₂ , and its input current and therefore the inverter device The output current of unit ₁ increases from Im ₁ to Im ₂ .

[Problems to be solved by the invention]

However, in the conventional method, when the load torque increases, the operating point related to the input current and the output torque of the AC motor moves on the speed characteristic curve of the input current and the output torque corresponding to the inverter output frequency before the increase of the load torque. Then, while reaching a new operating point where both the input current and the output torque of the motor are increased while decelerating the motor, the motor is stabilized. The AC motor input current is the output current of the inverter device at the same time, and when the value is increased along with the load torque fluctuation as described above, for example, the allowable current as the inverter device shown by the current value Imu in FIG. If it exceeds, the operation of the AC motor by the inverter device cannot be continued. In view of this, the present invention suppresses the input current and the output torque of the AC motor by using the inverter output frequency and the correction operation that also includes the inverter output voltage as needed when the load torque changes, and enables the inverter operation to continue. It is an object of the present invention to provide a current and torque limiting circuit.

[Means for solving problems]

The inverter device output frequency is reduced and corrected so that the AC motor input current, that is, the difference current value between the inverter device output current and its allowable current, is set to zero, and when the reduction correction alone is insufficient, the inverter device output voltage is reduced. The correction means for suppressing the output current of the inverter device is also provided by using the boosting correction within the allowable value of. That is, in an inverter device for driving an AC motor in which a ratio of an output voltage and an output frequency is constant, a current detection circuit for detecting an output current of the inverter device, a detection value of the current detection circuit and an allowable output of the inverter device. In addition to calculating the current difference from the set value of the current, when the detected value of the output current becomes larger than the set allowable value, the proportional / integral calculation using the current difference as an input is performed, and the result is obtained. A current operation circuit that outputs a frequency reduction correction signal to the frequency operation circuit of the inverter device, and, if the reduction correction signal continues, an increase operation that increases the output voltage to output frequency ratio by a predetermined value is performed, and the result is described above. It outputs as a voltage increase correction signal to the output voltage-to-frequency ratio calculation circuit of the inverter device, and indicates the overexcitation within the allowable range for the AC motor. It is characterized in that comprising a voltage calculation circuit for.

[Action]

An induction motor will be described below as an example of the AC motor.
Generally, the number of revolutions at the stable operating point of an induction motor driven by an inverter or its corresponding frequency and the output torque of the electric motor are specified as a value at an intersection point on both speed characteristic curves of the output torque Tm and the load torque T _L , Further, the input current Im of the electric motor, that is, the output current of the inverter is specified as a value corresponding to the frequency at the intersection point on the speed characteristic curve of the current Im, and the torque Tm and the current Im of the respective speed characteristic curves. The starting point on the horizontal axis is the output frequency of the inverter corresponding to the synchronous speed of the electric motor.
It becomes fr and shifts according to the control state of the inverter. In addition, the output voltage of the inverter, that is, the voltage correction for appropriately increasing the voltage Vm in the low frequency region of the frequency fr is based on the constant control of the ratio Vm / fr of the primary input voltage Vm of the motor and the frequency fr. When the control is performed, the E / fr ratio can be controlled to be substantially constant with respect to the secondary induced voltage E of the electric motor, and the maximum value of the torque Tm can be substantially constant with respect to various values of fr. There is a proportional relationship between Tm and the secondary current of the electric motor, and thus the primary current Im is also in a substantially proportional state. Further, in the case of performing over-excitation within the allowable range of the electric motor by performing a voltage boost correction that gives a voltage Vm larger than the Vm value according to the specified value of the Vm / fr ratio, the torque Tm and the current Im are boosted. It increases compared to the case where no correction is made.

Therefore, if the load torque T _L has a speed reduction torque characteristic and the Im increases with the increase of the T _L and exceeds the specific upper limit current value Imu, the excess ΔI = Im− Imu
If the inverter output frequency fr is controlled to be reduced to zero, the Im is limited to Imu and the Tm is also limited corresponding to the Imu, and the rotation speed of the electric motor is reduced without stopping the electric motor. It becomes possible to continue the operation within the limit value of the input current and the output torque by decreasing the value. Further, if the load torque T _L has a constant speed-to-speed characteristic, it is difficult to limit the current Im to Imu only by reducing the frequency fr as in the reduction torque characteristic, but the fr It is possible to suppress the current Im to its allowable value Imu by reducing the motor slip frequency with respect to the frequency fr by using the boost correction control for the voltage Vm together after the passage of an appropriate time for the reduction control. According to the present invention, when the electric motor input current exceeds the upper limit set current due to the fluctuation of the load torque T _L , the current arithmetic circuit corrects the reduction of the inverter output frequency fr by the proportional / integral arithmetic result with the excess current as an input. At the same time, the voltage correction circuit performs boost correction control for the inverter output voltage having an appropriate time delay with respect to the frequency correction control, thereby improving the stability of the motor operation by the inverter when the load torque changes. Is.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is an explanatory view of a current limiting operation at the time of speed characteristic reduction torque characteristic load fluctuation, and FIG. 3 is a similar explanation of a current limiting operation at the time of constant torque characteristic load fluctuation. FIG. 4 and FIG. 4 are time variation diagrams of the current limiting operation corresponding to FIG. In FIG. 1, components having the same functions as those in the embodiment of the prior art shown in FIG. 5 are designated by the same reference numerals. FIG. 1 shows a circuit example according to the prior art shown in FIG. 5 in which a current / torque limiting circuit 7 is provided. The limiting circuit 7 is a current transformer for detecting the output current of the inverter device 1, that is, the input current Im of the induction motor 2.
CT, a current detector 3 that outputs a detection signal of an appropriate value upon receiving the output signal of the transformer, a voltage transformer PT for detecting the output voltage of the inverter device 1, that is, the input voltage Vm of the electric motor 2, It comprises a voltage detector 4 which receives an output signal of the transformer and outputs a detection signal of an appropriate value, a voltage operation circuit 5 and a current operation circuit 6. The current calculation circuit calculates a signal difference between the output signal of the current detector 3 and the set value signal of the allowable current Imu of the current Im, and when the current Im becomes larger than the allowable current Imu, A proportional / integral calculation is performed with the current signal difference as an input, and the result is output as a frequency reduction correction signal to the frequency calculation circuit in the inverter device. When the frequency reduction correction signal continues without disappearing, the voltage calculation circuit 5 has a value equal to or higher than the output voltage-to-output frequency ratio during the steady operation of the inverter device 1 with respect to the value after a certain period of time has elapsed. A value increase operation is performed, and the result is superposed on the output signal of the voltage detector 4 as a voltage increase correction signal ΔV for the output voltage-to-output frequency ratio operation circuit of the inverter device 1 and output to the induction motor 2. It is a command for overexcitation within an allowable range. Next, FIG. 2 (a) is a speed characteristic curve of the motor input current Im,
The characteristic curves Im ₁ and Im ₂ when the output frequency fr of the inverter device is changed to fr ₁ and fr ₂ , respectively are shown. Figure (b)
Shows the load torques T _L1 and T _L2 having the reduced torque characteristic on the speed characteristic curve of the output torque Tm of the electric motor 2. The torque characteristic curves Tm ₁ and Tm ₂ correspond to the inverter output frequencies fr ₁ and fr ₂ , respectively. As shown in the figure, when the electric motor 2 is operating with the output torque characteristic Tm ₁ load torque characteristic T _L1 and the input current characteristic Im ₁ , the stable operating points are points P ₁₁ (f ₁ , Tm ₁₁ ) and points Q ₁₁ (f ₁ , Im ₁₁ ), and at this time, the frequency fr ₁ remains unchanged and the load torque characteristics are changed from T _L1 to T
If the operating point is increased to _L2 , the operating point becomes point P ₁₂ (f ₂ , Tm ₁₂ ) and point Q.
₁₂ (f ₂ , Im ₁₂ ), the motor input current, that is, the output current of the inverter device, increases from Im ₁₁ to Im ₁₂ and exceeds its allowable value Imu, and as a result, stable operation of the inverter continues. Becomes impossible. However, when the input current Im exceeds the allowable value Imu, the inverter output frequency fr is reduced and corrected according to the excess value by appropriate timed characteristic control such as proportional / integral control, and the frequency is changed to fr.
_If it finally reaches fr ₂ from ₁ , the torque characteristic is
From Tm ₁ to Tm ₂ again, the current characteristics shift from Im ₁ to Im ₂ , respectively, so that the stable operating points are from point P ₁₂ and point Q ₁₂ to point P ₂₂ (f ₄ , Tm ₂₂ ) and point Q _{22 respectively.} (F ₄ , Im ₂₂ ).
The input current Im _{22 at the} operating point Q ₂₂ is obtained as a result of the output frequency reduction correction performed so that the excess current value becomes zero, and therefore its value becomes equal to the allowable current Imu. Next, FIG. 3 shows the electric motor 2 when the reduced torque characteristic load in FIG. 2 is replaced with a constant torque characteristic load.
3A and 3B show the movement pattern of the operating point, and FIGS. 3A and 3B are current and torque characteristic diagrams corresponding to FIGS. 2A and 2B, respectively. If the inverter output frequency is f _r1 , its output torque characteristic is Tm ₁ , its input current characteristic is Im ₁ and its load torque characteristic is T _L1 , the motor stable operating point is point P ₁₁ (f ₁ , T _L1 ). a point _{Q 11 (f 1, Im 11} ), respectively point involves the two operating point increases to T _L2 of the load torque characteristic _{P 12 (f 2, T L2} ) between the point Q ₁₂ (f _2, Im ₁₂ ) And move to. When the input current Im _{12 at the} operating point Q ₁₂ exceeds the allowable value Imu as shown in FIG.
The output frequency fr _{1 due to} the reduction correction control is performed, but the current difference between the currents Im ₁₂ and Imu continues without decreasing,
After the elapse of a certain time, the voltage correction circuit 5 also starts boost correction control for the inverter output voltage Vm. By the boost correction control, the electric motor output torque Tm and the input current Im increase as shown in FIG. (B) from the characteristic Tm ₂₁ to Tm ₂₂ and from the characteristic (A) from the characteristic Im ₂₁ to Im ₂₂ respectively. As a result, the rotation speed of the electric motor rises and the slip from the synchronous value decreases, and the operating point is the point P ₂₃ (f ₃ ,
T _L2 ) and point Q ₂₂ (f ₃ , Im ₂₃ ) are stable. The operating point Q
Current Im ₂₃ in ₂₃ is equal to are those obtained as a result of the boosting correction the allowable value Imu. If the current difference becomes zero only by the frequency correction before the elapse of the certain time for starting the boost correction, the boost correction cannot be performed. FIG. 4 is a time variation diagram of the output torque Tm and the input current Im of the electric motor due to the frequency fr correction when the reduction torque characteristic load increases and the restoration variation occurs, and FIG.
Figure (b) shows the torque Tm, and figure (c) shows the frequency.
This is for fr. Note variation pattern between time t ₂ and t ₃ relates said Im and Tm and fr is determined by the control characteristic of the control system overall and the inverter device 1 and the current calculation circuit 6, the time t ₄
For the subsequent steps, only the inverter device 1 is determined by the control characteristics.

〔The invention's effect〕

According to the present invention, regardless of whether the load torque characteristic of the AC motor driven by the inverter device is a constant torque characteristic or a reduced torque characteristic, the inverter output frequency correction and the output voltage correction are used together when the load torque changes. This makes it possible to suppress the inverter output current, that is, the electric motor input current and the electric motor output torque, to within the respective allowable values, and improve the stability of the AC electric motor operation by the inverter.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are explanatory views of a current limiting operation at the time of load fluctuations of a reduced torque characteristic load and a constant torque characteristic load, and FIG. FIG. 5 is a time variation diagram of the current limiting operation corresponding to FIG. 2, FIG. 5 is a block diagram of a prior art embodiment, and FIG. 6 is a speed torque curve of input current and output torque of the induction motor and load torque. 1 ... Inverter device, 2 ... Induction motor, 3 ... Current detector, 4 ... Voltage detector, 5 ... Voltage calculation circuit, 6 ...
Current calculation circuit, 7 Current / torque control circuit, CT
Current transformer, PT ... voltage transformer.

Claims (1)

[Claims] 1. An inverter device for driving an AC motor in which a ratio of an output voltage and an output frequency is constant, a current detection circuit for detecting an output current of the inverter device, a detection value of the current detection circuit and the inverter device. And calculating the current difference from the set value of the allowable output current, and when the detected value of the output current becomes larger than the set allowable value, the proportional / integral calculation using the current difference as an input is performed. A current operation circuit that outputs the result as a frequency reduction correction signal to the frequency operation circuit of the inverter device, and, if the reduction correction signal continues, an increase operation that increases the output voltage to output frequency ratio by a predetermined value is performed. The result is output as a voltage increase correction signal to the output voltage-to-frequency ratio calculation circuit of the inverter device, and the overexcitation within the allowable range for the AC motor is performed. Current and torque limiting circuit of the AC motor driving inverter apparatus characterized by comprising a voltage calculation circuit Ryosuru.