JPH06106038B2 - Induction motor torque limiting method - Google Patents

Description

The present invention relates to a torque control method for an induction motor in which variable speed operation is performed via an inverter.

[Conventional technology]

In order to perform a stable variable speed operation in an induction motor that is generally driven through an inverter, it is necessary to keep the main magnetic flux φ in the air gap of the induction motor substantially constant. Side resistance and leakage reactance are ignored, the main magnetic flux φ has a constant ratio V / ω between the primary voltage V of the induction motor and the primary frequency ω, and thus the ratio between the output voltage and the output frequency of the inverter is constant. In that case, the generated torque T of the induction motor has a relationship of only the slip frequency Δω at the primary frequency ω, and the generated torque curve moves substantially parallel to the change of the primary frequency ω. What to do is as shown in FIG. That is, the generated torque curves when the primary frequency ω of the induction motor is ω ₁ , ω ₂ , and ω ₃ are represented by curves A, B, and C, respectively, and the torque decreases as the rotation speed decreases, for example. When a load having a so-called gradually decreasing torque characteristic is taken, the load torque curve D and the respective generated torque curves
The intersections a, b and c with A, B and C give the operating points of the induction motor at the respective frequencies ω ₁ , ω ₂ and ω ₃ .

[Problems to be solved by the invention]

Therefore, when the load changes and the load torque curve D shown in FIG. 4 moves in the conventional induction motor in which the variable speed operation is performed via the inverter, the load torque curve D after the movement and the torque curve generated by the electric motor, respectively. The new intersections with and are the operating points of the motor at the respective frequencies ω ₁ , ω ₂ , and ω ₃ . Now, assuming that the rated torque curve of the induction motor is represented by the A curve,
When the load torque increases, the slip of the electric motor increases, and its operating point becomes the intersection point a ₁ between the new load torque curve D ₁ and the generated torque curve A. In that case, the main magnetic flux φ of the electric motor is kept constant, so that the secondary current, and hence the primary current, increases to correspond to this. Therefore, in the conventional induction motor which is not provided with a means for limiting the generated torque, when the load torque increases remarkably, the generated torque required for the electric motor and therefore the primary current also increases remarkably, resulting in a severe overcurrent of the electric motor. In addition to inducing a load, the output current of the inverter that drives the electric motor increases, which causes a trip of the inverter, which may cause a stop of the operation of the electric motor.

The present invention solves the above-mentioned problems of conventional induction motors in which variable speed operation is performed via an inverter,
By detecting the increase in the generated torque of the electric motor required by the increase of the load torque and its temporal change amount by a simple means and controlling the primary frequency thereof via an inverter, the generated torque of the electric motor is always maintained, Another object of the present invention is to stabilize the operation of the electric motor by making the primary current equal to or less than a predetermined limit value, removing the cause of overload of the electric motor or trip of the inverter.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention includes an induction motor in which variable speed operation is performed via an inverter, and a torque of an induction motor that drives a load having a decreasing torque characteristic in which the torque decreases as the rotation speed of the motor decreases. In the control method, the primary current of the electric motor, which is pulsed through an appropriate voltage-frequency conversion means, is input to an appropriate arithmetic circuit to change the temporal change of the primary current from the number of the pulses within a certain period. The ratio is calculated, and when the primary current exceeds a predetermined limit value, the output frequency of the inverter is controlled so as to reduce the primary frequency of the electric motor in proportion to the rate of temporal change of the primary current. It is a thing.

[Action]

By configuring the torque control method for the induction motor according to the present invention as described above, in the induction motor in which variable speed operation is performed with the main magnetic flux kept constant, the generated torque is almost proportional to the primary current. , The primary frequency is reduced in proportion to the rate of increase of the load torque, and the primary current is kept below a predetermined limit value, whereby the torque generated by the electric motor can be limited below a predetermined value. .

〔Example〕

Next, the details of the present invention will be described based on the embodiments shown in the drawings.

As described above, it is necessary for the induction motor, which is operated at a variable speed via the inverter, to keep its main magnetic flux constant. Therefore, the ratio between the primary voltage V and the primary frequency ω is always constant. Is controlled so that the generated torque curves move substantially parallel to each other in accordance with the change of the primary frequencies ω ₁ , ω ₁ , ω ₂ . And ω _{3 as represented} by respective curves A, B and C. Therefore, when a load having a so-called diminishing torque characteristic in which the torque decreases as the number of revolutions decreases, the operating points of the electric motor are the intersection points a, b and C of the load torque curve D and the generated torque curves A, B and C, respectively. c.

Therefore, as shown in FIG. 3, when the induction motor 13 driven via the inverter 11 is stably operating at the intersection a between the generated torque curve A and the load torque curve D, the load is not satisfied for some reason. When the load torque curve changes from D to D _{1 due} to an increase in the torque required by the motor 13
The operating point of moves from a to a ₁ on the generated torque curve A, and the generated torque T of the electric motor 13 becomes the operating point a _m on the generated torque curve A.
It is possible that the maximum torque allowed by the electric motor 13 in this case, that is, the limit torque T _m represented by In that case, the generated torque T of the electric motor 13 is almost proportional to the primary current I from the condition that the main magnetic flux is constant, and the increase of the generated torque T appears as the increase of the primary current as it is.
This causes an overcurrent in the inverter 13 and the inverter 11, and causes an inverter trip in the worst case.

In order to prevent such a situation, when the generated torque T of the electric motor 13 exceeds a predetermined limit torque T _m , the electric motor 13
The primary frequency is lowered, and the generated torque curve of the electric motor 13 in that case is moved substantially in parallel so that the operating point of the electric motor 13 on the generated torque curve is at a position lower than the limiting torque T _m . That is, as illustrated in FIG. 4, the load torque increases, the torque curve D becomes D ₁ , the intersection with the generated torque curve A of the electric motor 13 moves from a to a _1, and the limit torque T of the electric motor 13 becomes T 1. _When the point a _m on the generated torque curve corresponding to _m is exceeded, the output frequency of the inverter 11 is reduced to reduce the primary frequency of the electric motor 13 from ω ₁ to ω ₂ , and in this case the operating point of the electric machine 13 is generated. By moving to the intersection b ₁ of the torque curve B and the load torque curve D _1, it is possible to operate at the point b _m or less representing the limiting torque T _m on the generated torque curve B. Furthermore the load torque curve D ₁ and the load torque is increased becomes D _2, intersections of the generated torque curve B in this case is limited torque thereon
When the point b _m corresponding to T _m is exceeded, the output frequency of the inverter 11 is reduced again and the primary frequency of the electric motor 13 is decreased from ω ₂ to ω ₃ , and in this case the generated torque curve C and the load torque curve of the electric motor 13 are reduced. By moving the operating point to the intersection C ₁ with D ₂ ,
To be operating below C _m points representing the limit torque T _m of a developmental torque curve C. By gradually decreasing the primary frequency ω of the electric motor 13 in this manner, the rotation speed of the electric motor 13 decreases, but the electric motor 13 is operated at an operating point that does not exceed the point representing the limiting torque on the generated torque curve at each time. To be able to.

In that case, in the torque limiting method for an induction motor according to the present invention, as shown in FIG. 3, the induction motor 13 is operated by a three-phase power source via the inverter 11 to perform variable speed operation.
A current transformer 12 is provided on the input side connection line of each of the
Is detected, the output of the current transformer 12 is converted into a DC voltage by a three-phase bridge rectifier 14 and a resistor 15 as illustrated in FIG. 2, and then the voltage-frequency converter is converted through an appropriate filter circuit 16.
It is input to 17 and pulsed, and the output of the converter 17 is input to an arithmetic circuit 18 for controlling the output frequency of the inverter 11 illustrated in FIG. Clock oscillator in arithmetic circuit 18
From the number of the pulses for each cycle T obtained by 19, the rate of change of the primary current I of the induction motor 13 with time, To calculate.

On the other hand, as described above, the torque T generated by the induction motor 13 in this case is almost proportional to the primary current I thereof. T = k ₁ I (2) Now, the output frequency of the inverter 11 and therefore the time of the primary frequency ω of the induction motor 13 In order to make the rate of dynamic change of the generated torque T of the electric motor 13 proportional to the rate of temporal change of the generated torque T, and hence the rate of temporal change of the primary current thereof, the gain with respect to the amount of change of the generated torque T of the electric motor 13 is increased. If k ₂ when is expressed as k ₂ is large enough to stably control the generated torque T according to the flywheel effect GD ² converted into the electric motor and the electric motor shaft, The relationship can be established. As a result, when the torque required by the load increases and the operating point on the generated torque curve of the electric motor 13 exceeds the point corresponding to the control torque T _m , the rate of increase in the torque, and thus the electric motor 13 The primary frequency is reduced according to the rate of increase of the primary current to move the generated torque curve in parallel so that the operating point on the generated torque curve becomes equal to or lower than the point corresponding to the limiting torque on the same curve. . That is, when the generated torque T and therefore the rate of temporal change of the primary current I is small, the rate of temporal reduction of the primary frequency of the electric motor 13 is reduced as shown in FIG. When the rate of temporal change of the primary current I is large, the rate of temporal reduction of the primary frequency of the electric motor 13 can be increased as shown in FIG. 5 (B).

Therefore, when the comparator 21 of FIG. 1 detects that the primary current of the electric motor 13 exceeds the current value I _m corresponding to the predetermined limiting torque T _m , the control circuit 20 inputs the primary current by the operation circuit 18. The output frequency of the inverter 11 is controlled by detecting the rate of temporal change of the current I and determining the required rate of temporal reduction of the primary frequency ω, and the torque curve generated by the electric motor 13 at the reduced primary frequency is controlled. The operation of the electric motor 13 may be continued at an operating point determined by the load torque curve and the load torque curve.

〔The invention's effect〕

The present invention provides a torque control method for an induction motor that includes an induction motor that is operated at a variable speed through an inverter as described above, and that drives a load having a gradually decreasing torque characteristic in which the torque decreases as the rotation speed of the motor decreases. In, the primary current of the electric motor pulsed through an appropriate voltage-frequency conversion means is input to an appropriate arithmetic circuit, and the rate of temporal change of the primary current is calculated from the number of the pulses within a certain period. When the primary current exceeds a predetermined limit value, the output frequency of the inverter is controlled so as to reduce the primary frequency of the electric motor in proportion to the rate of temporal change of the primary current. By doing so, when the operating point on the generated torque curve of the electric motor exceeds the point representing the allowable torque of the electric motor as the load torque increases, the inverter Control the output frequency of the motor to reduce the primary frequency of the electric motor, limit the torque so that the operating point on the generated torque curve at the frequency is equal to or less than the allowable torque point, and further, control the frequency of the inverter. Since it is possible to limit the torque of the electric motor in a short time regardless of the increase or decrease of the load torque in proportion to the rate of increase of the torque of the electric motor, There is an effect that stabilization can be easily performed.

[Brief description of drawings]

FIG. 1 is a schematic block diagram illustrating a circuit for controlling an output frequency of an inverter in order to realize a torque limiting method for an induction motor according to the present invention, and FIG. 2 is a primary current detection of the motor input to the circuit. FIG. 3 is a schematic circuit diagram illustrating the means, FIG. 3 is a schematic circuit diagram of the induction motor driven by an inverter, FIG. 4 is a torque-frequency characteristic curve of the induction motor, and FIG. 5 is related to the present invention. 3 shows the operating characteristics of the torque limiting method for an induction motor. 11 ... Inverter, 12 ... Current transformer, 13 ... Induction motor, 14 ... Three-phase Bridge rectifier, 15 ... Resistor, 16 ... Filter, 17 ... Voltage-frequency converter, 18 ... Arithmetic circuit, 20 ... Inverter control circuit, 21 ... comparator.

Claims (1)

[Claims] 1. A torque control method for an induction motor, comprising: an induction motor in which variable speed operation is performed via an inverter; and driving a load having a gradually decreasing torque characteristic in which the torque decreases as the rotation speed of the motor decreases. The primary current of the electric motor, which is pulsed through the voltage-frequency conversion means, is input to an appropriate arithmetic circuit to calculate the rate of temporal change of the primary current from the number of the pulses within a certain period, and Induction, characterized in that when the primary current exceeds a predetermined limit value, the output frequency of the inverter is controlled so as to reduce the primary frequency of the electric motor in proportion to the rate of temporal change of the primary current. Electric motor torque control method.