JPH0270280A - Control method for electric motor - Google Patents

Abstract

PURPOSE:To prevent trouble of overheat and power source trip or the like by enabling a limit value of current, supplied to an electric motor, to be set to a value different in the time of fixed speed operation and in the time of acceleration and deceleration operation and preventing the electric motor from carrying its overcurrent. CONSTITUTION:In a current limiter arithmetic circuit 30, first a speed command V0 is time-differentiated (dV0/dt), when a calculated differential value is equal to zero, because an electric motor 10 is judged to be in fixed speed operation rotating at a fixed speed, a command of setting a current limit value to 100% the rating of the electric motor 10 is output to a current limiter 28. On the contrary, when the calculated differential value is judged to be not equal to zero, because the electric motor 10 is judged to be in acceleration and deceleration operation, a command of setting the current limit value to 200% the before said rotating is output to the current limiter 28. By the method, the electric motor 10 can be prevented from showing its trouble of burning and power source trip or the like due to a continued overcurrent while enabling a sufficient torque to be obtained of acceleration and deceleration.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method of controlling an electric motor, and is particularly suitable for use in detecting the speed of a DC motor using a speed detector and controlling the speed of the DC motor so as to reach a target speed based on the detected speed. This invention relates to an improvement in a method of controlling an electric motor.

[Conventional technology]

BACKGROUND ART Electric motors such as DC motors are usually used as prime movers for driven equipment and control equipment, but at that time, they need to rotate at a target speed while generating torque according to the load. In addition to direct current motors, examples of speed controllable motors include induction motors using inverters and the like. FIG. 6 shows a conventional example of a speed control system used to control the speed of an electric motor to a target speed. In the speed control system shown in FIG.
It rotates when supplied with power at a predetermined voltage. At this time, the number of revolutions per hour (speed) of the electric motor 10 is detected by a speed detector 14 such as a pulse generator. In the speed control system, an automatic speed regulator (Auto
On the input side of the natic 5peed regulator (hereinafter abbreviated as ASR) 16, the speed command ■. The detected speed fed back via the minor loop is compared at the addition point 18, and the difference between the speed command and the detected speed is determined. In this case, when the output of the adder 18 is positive, that is, when the detected speed is not in line with the speed command, the speed of the electric motor 10 is increased;
8 is negative, that is, when the detected speed is greater than the speed command, the speed of the electric motor 10 is decreased.
automatically adjusts the speed. Here, the output signal of the ASR 16 is limited by the current limiter 20 so that the maximum value of the current flowing through the motor 10 is below a predetermined value (hereinafter referred to as current limit value), and then the current command is Automatic current regulator (＾ut
o11atic Current Regulation
, hereinafter abbreviated as ACR) 22. In FIG. 6, reference numeral 24 is a current transformer for detecting the value of the current flowing into the electric IIO, and 26 is a second addition point for feeding back the detected current. Conventionally, the current limit value has been fixed and set to a value that allows current to flow up to 200% of the motor rating. That is, during normal constant speed operation, the current value of the electric motor 10 needs to flow up to about 200% of the motor rating when a large torque is required such as during acceleration or deceleration, so the current limit value is It was set to 200%. Such 2 during acceleration/deceleration operation
Since the current of approximately 00% continues only for a short period of time, problems such as overheating will not occur in the motor. Due to the above-described action, the speed control system
! 110, good speed control was performed during constant speed operation as well as during acceleration/deceleration operation. However, when controlling the speed of the electric motor 10 using the conventional speed control system, if the load becomes heavy for some reason during constant speed operation where the current value is normally small,
The current command input to the ACR 22 to compensate for the speed reduction due to the load may exceed 100% of the rating (rated load), and an overcurrent state may occur continuously in the motor 10. If such an overcurrent condition continues, the motor 10 may suffer from various problems, such as burnout due to overheating or tripping of the power supply, but in the speed control system, the current limit value is set to a value larger than the motor rating. There was a problem that this problem could not be dealt with because it was set fixed at (approximately 200%).

Problem 1 to be achieved by the invention The present invention has been made in order to solve the above-mentioned problems of the conventional art. Sometimes, even when the load is heavy and large torque is required, the current to the motor is kept within the current value that does not cause overcurrent.
An object of the present invention is to provide a method for controlling an electric motor that can prevent problems such as overheating and power trip caused in the electric motor due to overcurrent. [Means for Accomplishing the Object] The present invention provides a method for controlling an electric motor, in which the speed of the electric motor is detected, and when the speed of the electric motor is controlled so as to reach a target speed based on the detected speed, supply is supplied to the electric motor. The above-mentioned problem has been achieved by making it possible to set the current limit value to different values during constant speed operation and during acceleration/deceleration operation to prevent overcurrent of the motor.

[Operation and effects of the invention]

If a load higher than the rated load is applied to the motor during constant speed operation, if there is no equivalent restriction, a current higher than the rated current, that is, an overcurrent will flow through the motor. In conventional speed control systems, in order to prevent overcurrent from flowing into the motor, the current is limited from flowing beyond a predetermined value, but the limiting current value (current limit value) does not depend on the operating state. In addition, considering the acceleration and deceleration of the motor, a value larger than the rated current, e.g.
It was set to 00%. Therefore, although the conventional speed control system can deal with an excessive load exceeding the current limit value, it cannot deal with an overload that is close to the rated value for a long time. Therefore, as a result of various studies, the inventors decided to prevent overcurrent of the motor by making it possible to set the current value limited by the current limiter (current limit value) to different values during constant speed operation and during acceleration/deceleration operation. The present invention was created based on the discovery that this is a good idea. Therefore, according to the present invention, it is possible to flow the required current when accelerating or decelerating the motor, and even when the load is heavy during constant speed operation, the motor current can be suppressed within the rated current value to prevent overcurrent. You will be able to do this. Therefore, it is possible to prevent long-term overcurrent operation due to continuous overload of the electric motor during constant speed operation, and it is possible to prevent various malfunctions that occur in the electric motor, such as burnout due to overheating and tripping of the power supply. [Embodiment 1] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, as shown in FIG. 1, the present invention is applied to a speed control system of an electric motor 10. As shown in Fig. 1, this speed control system has an ASR1
6, a summing point 18, a second summing point 26, an ACR 22, a voltage generator 12, a current transformer 24, and a speed detector 14. Since it has the same functions and actions as the speed control system, detailed explanation thereof will be omitted. In addition, this speed control system limits the current supplied to the motor 10 from exceeding a permissible current value (current limit value), and also controls the current limit value during constant speed operation and acceleration/deceleration operation. Current limiter 28 set to different values with
And electric! In order to determine whether 1910 is in constant speed operation or acceleration/deceleration operation, and to prevent overcurrent of electric motor 10,
A current limiter calculation circuit 30 is provided for selecting and setting a current limit value corresponding to the operation. The effects of the embodiment will be explained below. Basically, the speed control system according to the embodiment detects the speed of the electric tR10 with the speed detector 14 and inputs the detected speed into the speed command, similar to the conventional speed control system shown in FIG. 6 above. The speed of the electric m10 is controlled by the ASR16 through feedback. In the conventional speed control system, the current limit value of the current limiter is fixed, but in the embodiment, the setting of the current limit value changes to a different value based on a command from the current limiter calculation circuit 30. That is, the current limiter calculation circuit 30 first receives the speed command V
, is differentiated with respect to time (dVo/dt), and as a result of the 0 judgment that determines whether the calculated differential value is equal to zero, if the differential value is equal to zero (dV o / dt = O), the motor 10 Since it is determined that the motor is rotating at a constant speed and is operating at a constant speed, the current limit value should be set to 100% of the motor 10 rating.
A command to set the current limiter 28 is output to the current limiter 28. On the other hand, the calculated differential value is not zero (dVo/dt≠0
), it is determined that the electric motor 10 is in acceleration/deceleration operation, and therefore a command is output to the current limiter 28 to set the current limit value to 200% of the rating. As described above, the setting of the current limit value of the current limiter 28 can be changed to a different value depending on the operation, so if speed control is performed using this speed control system, the electric motor can reach up to 200% of its rating during acceleration/deceleration operation. 10, sufficient acceleration/deceleration torque can be obtained. In addition, during constant speed operation, current of 100% or more of the rated current does not flow through the motor 10, so the current of 200% of the rated current does not flow continuously.
% or more, it is possible to prevent problems such as burnout and power trip due to overheating of the motor 1o due to continuous overcurrent. Next, Fig. 2 shows an example of how the current changes when a motor is controlled using a conventional speed control system with a fixed current limit value. Figure 3 shows an example of the change in current when the system controls the electric power @1°. In Fig. 2 and Fig. 3, acceleration is performed from time t1 to t2, operation is performed at a load within the rated value from time 12 to t3, and time t3 to t4.
The engine was operated at a load higher than the rated value at t4, and the speed was decelerated at 5 from time t4. From Figures 2 and 3, in both control systems, the current ranges from 100% to 200% during acceleration/deceleration t1 to t2, and the current ranges from 100% to 200% during normal constant speed operation from t2 to t3. Since it is below %, no problem will occur.
However, if the load becomes heavy for some reason during constant speed operation, in the conventional speed control system, the current value will exceed 10026 between time 13 and t4, as shown in Figure 2. The overcurrent continues. In contrast, when the speed control system according to the present invention is used, even when the load increases during constant speed operation as described above, the time t3 as shown in FIG.
The current value is limited to a maximum of 100% between That is, depending on the tension balance before and after the rolling line, etc., the electric current of the motor may be constantly operated in a negative region. In such a case, a relationship as shown in FIGS. 4 and 5 occurs, in which the plus and minus values in FIGS. 2 and 3 are reversed. In the prior art shown in FIG. 4, the normal current is negative, and the current during deceleration exceeds -100%. However, at time t3, if the forward tension of the motor increases for some reason and the generator is operated strongly, the current will exceed -1°0%. However, when the present invention is utilized, as shown in FIG. 5, it is possible to prevent overload operation due to negative current during constant speed, and also ensure a current of 1100% or more during deceleration. Therefore, if the present invention is adopted, the required current will flow during acceleration/deceleration operation, and even if the load increases during constant speed operation, the current value will be maintained at 100% of the rated value, and problems caused by continuous overcurrent will be avoided. It is understood that this does not occur. Note that, as in the embodiment, it is preferable for the current limit value to be limited by the present invention to be 200% for acceleration/deceleration and 100% for constant speed operation from the viewpoint of motor protection. However, this current limit value is not limited to this, and can be set as appropriate depending on the characteristics of the motor, operating conditions, etc. Further, the speed control system employing the present invention is not limited to that used for speed control of a DC motor, and the present invention can also be applied to a speed control system of a type of electric motor.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a speed control system according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of change in motor current value in a conventional speed control system, and FIG. FIG. 4 is a diagram showing an example of change in motor current value in the adopted speed control system. FIG. 5 is a diagram showing another example of change in motor current value in a conventional speed control system. FIG. 6, a diagram showing another example of change in motor current value in the adopted speed control system, is a block diagram showing the configuration of a conventional speed control system. 22... Automatic current regulator (ACR), 28... Current limiter, 30... Current limiter calculation circuit.

Claims (1)

[Claims] (1) When detecting the speed of an electric motor and controlling the speed of the electric motor so that it reaches a target speed based on the detected speed, the limit value of the current supplied to the electric motor is set for constant speed operation and acceleration/deceleration operation. A method for controlling an electric motor, characterized in that overcurrent of the electric motor can be prevented by setting different values at different values.