JP2834184B2 - Overload detection device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overload detection device for a device that controls an AC motor with the output of a frequency conversion means such as an inverter or a cycloconverter.

[Conventional technology]

In an AC motor control device using an inverter as frequency conversion means, without using a mechanical thermal relay,
Japanese Patent Publication No. Sho 62-55379 discloses a method of indirectly assuming the temperature of an AC motor using a current detecting means and a microcomputer to prevent the AC motor from burning.

[Problems to be solved by the invention]

Conventionally, since the thermal time characteristic is single, if the combination of the inverter capacity and the AC motor driven by the inverter is not specified, the protection function may not work sufficiently. Was.

To explain the example in more detail, for example, it is assumed that a thermal time characteristic is set so that an inverter having a capacity of 180 KVA can protect an overload of a motor of 120 KW.
When controlling a 90 kW motor using this inverter, the 90 kW motor cannot be adequately protected with the thermal time characteristic adapted to a 120 kW load.

An object of the present application is to set a control device for an AC motor including a protection device for preventing the AC motor from burning by indirectly measuring the temperature of the AC motor using a current detection unit and a microcomputer. Thermal time characteristics
An object of the present invention is to provide a control device for an AC motor capable of performing overload protection in which a thermal time characteristic is more appropriately matched to an actual operating condition by appropriately selecting the operating condition.

[Means for solving the problem]

The present invention provides a speed setting means for setting an operation speed of an AC motor, and a frequency conversion means for converting a frequency of a power supply so that the speed of the AC motor approaches the speed set by the speed setting means and giving the frequency to the AC motor. And a current detecting means for detecting a load current of the frequency converting means, wherein the continuous operation in consideration of a cooling effect of the AC motor determined according to a speed set by the speed setting means. Storage means indicating a possible time, determining means for determining whether the AC motor is accelerating or decelerating or operating at constant speed, and indicating that the AC motor is accelerating. Reading the thermal time characteristic at the time of acceleration from the storage means when the operation is being performed, and reading the thermal time characteristic at the time of the constant speed operation during constant speed operation; And an abnormality detecting means for generating an abnormal signal when the integrated value exceeds the stored value read by the reading means. This is an AC motor control device.

[Action]

With the above configuration, by operating the setting means or the selection means, it is possible to set or select the thermal time characteristic that matches the characteristics of the AC motor which is the load of the frequency conversion means.

〔Example〕

Hereinafter, embodiments shown in the drawings will be described. This embodiment uses an inverter as frequency conversion means. Then, a value corresponding to the primary current supplied to the motor connected to the inverter device is calculated from the DC average current of the DC circuit unit inside the inverter device by using the inverter device operation unit in the inverter device, and further stored in the storage unit in advance. The stored thermal time characteristics of the motor are compared with the calculated integrated value of the temporary current, and the temperature rise of the motor due to overload or overcurrent is estimated, and the output of the inverter device itself is cut off to protect the motor. I do. That is, FIG. 1 is a block diagram for explaining the overall configuration of an inverter device incorporating an overload detection device, where T ₁ , T ₂ , and T ₃ are input terminals of a three-phase AC power supply, and CNV is a three-phase AC power supply. A converter section for converting power to DC power, and INV is an inverter section for converting DC power again to three-phase AC power, is composed of a combination of transistors or thyristors. RS and CB are a current limiting resistor and a smoothing capacitor respectively arranged in a DC circuit connecting the converter CNV and the inverter INV. CO
N is an inverter control circuit for mainly controlling the base circuit of the transistor of the inverter unit INV or the gate circuit of the thyristor, and is constituted by the following.
The ROM is a storage device and stores a control procedure of the inverter unit INV and various data necessary for control. The RAM is a temporary storage device and stores various intermediate data necessary for control. i / o1 and i / o2 are interfaces for converting various analog quantities to digital or digital quantities for analog. The CPU is an inverter operation unit, performs various operations in accordance with a program stored in advance in the storage device ROM, and issues a signal for controlling the transistor base circuit or the thyristor gate circuit of the inverter unit INV through the inverter face i / o2. Things. T ₄ , T ₅ , T ₆ are output terminals for extracting the output of the inverter INV, and IM is connected to the output terminals T ₄ , T ₅ , T ₆
It is a phase induction motor. SSC is a speed setting device as speed setting means for setting the operating speed of the motor IM, RAMP is a slow variable circuit that receives the output of the speed setting device SSC and softens the rapid change, and outputs it.i / o3 is an interface circuit It is. HSC is a setting means for setting the thermal time characteristic, and i / o4 is its interface circuit. Thermal time characteristic setting means
FIG. 2 shows an example of the HSC. The relationship between the primary current Im and time is k.
Can be set arbitrarily.

In the inverter configured as described above, when the operation of the inverter is started, the output frequency and the output voltage of the inverter INV are controlled according to a predetermined program, and the three-phase induction motor M starts. Also, when this speed command signal is input through the interface i / o3 by operating the speed setting device SSC, the inverter device operation unit CPU is required to realize the operating conditions set by the speed setting device SSC. Calculate various base signals or gate signals, and use interface i / o2
To the inverter unit INV, and drives the three-phase induction motor IM at the specified speed.

Continuing the description, RSH is a current detection resistor, which is provided in a DC circuit connecting the converter unit CNV and the inverter unit INV, and detects the amount of DC current consumed by the inverter unit INV. The voltage drop across the current detection resistor, which is proportional to the amount of DC current, is
Via the inverter control circuit CON. Various calculation programs for protecting the load device are stored in the storage device ROM in advance. Well, setting means
By operating the HSC, an arbitrary thermal time characteristic is written into the rewritable storage means RAM. As shown in FIG. 2 (a), this thermal time characteristic is such that the function k of the continuous operable time (heat allowable time) is inversely or substantially inversely proportional to the primary current Im (b) or (c). As shown in FIG.

The basic operation principle of the overload protection in the above configuration will be described. FIG. 2A is a diagram showing an example of thermal characteristics when the induction motor is driven by a commercial power supply. The horizontal axis represents the magnitude Im of the primary current flowing into the induction motor, and the vertical axis represents the continuous operation time ( In this case, the safe operation of the induction motor can be performed in the area A at the boundary of the line (a), and in the area B, the induction motor is overloaded and needs protection. is there. The temperature rise of the induction motor is generally the product of the square of the inflow current In ² and the function of time k, Im ² ×
It is proportional to k and the cooling coefficient representing the cooling effect. Then, the setting means HSC is operated to write the thermal time characteristic into the rewritable storage means RAM. The thermal time characteristics are determined so that the function k to the primary current Im ² as shown in FIG. 2 (a) is inversely proportional or almost inversely proportional. Now, assuming that the primary current of the induction motor is Im, the operation time is t, and the temperature rise is ΔT,
The temperature rise ΔT is Is represented by Here, _Ko is a proportionality constant that differs depending on the structure of the induction motor, and the cooling coefficient is included in this and becomes a function related to the operation speed (rotation speed). On the other hand, the function of the DC current Id flowing in the DC circuit between the converter CNV and the inverter INV and the primary current Im flowing in the induction motor is PWM.
In the case of an inverter device, it is as follows.

That is, the primary current Im of the induction motor is calculated from Equation (2) based on the value of the DC current Id detected at each sampling time using the current detection resistor RSH. This primary current Im
Is raised to the power n by an index n {n = 2} in FIG. 2 (a) obtained from a thermal time characteristic curve of the induction motor, and a function α of a cooling coefficient having a relationship as shown in FIG. Is multiplied by the sampling time (detection time) Δt, and a value Im ⁿ × α × Δt is repeatedly calculated for each sampling time Δt, and is added to the already calculated integrated value Ii. That is, the integrated value ^{Ii = Ii + Im n × α} × ΔT ...... concerning operating time within a certain time (3) is re-calculated every sampling time Delta] t.

In FIG. 3, the horizontal axis represents the output frequency of the inverter device (the rotational speed of the induction motor), and the vertical axis represents the function α of the cooling coefficient. As described above, the function α of the cooling coefficient takes a value from plus to 0 or further minus according to the operation speed of the induction motor. When the operation of the induction motor is continued as described above and the integrated value Ii exceeds the thermal time characteristic curve of the induction motor shown in FIG. 2, the temperature rise of the induction motor exceeds the allowable range, and the overload state occurs. Therefore, it is necessary to stop the operation of the inverter device and protect the induction motor.

Actually, the calculations of the formulas (2) and (3) are performed by using the inverter device calculation unit CPU in the inverter control circuit CON of the inverter device. The relationship between the cooling coefficient function α and the operating speed N of the electric motor IM (output frequency f of the inverter device) is also stored in the storage device RAM in advance by opening the setting means HSC as a numerical table. FIG. 4 shows a processing flow of the overload protection, and it is assumed that a processing program having the contents shown in this processing flow is stored in the storage device ROM in advance.

Now, the inverter device arithmetic unit CPU in the inverter control circuit CON reads and decodes the program in the storage device ROM, and first, as shown in FIG.
The voltage drop between the terminals of the current detection resistor RSH (the value of the DC current Id) is taken in through the step (a), and based on the DC current Id value, the primary current Im is calculated by using the equation (2). In addition, the current output frequency data of the inverter is
MP output or command value of inverter device (actually a temporary storage device that manages the output frequency of the inverter device)
(Data in the RAM), the data of the function α of the cooling coefficient corresponding thereto is read from the storage device RAM, and the integrated value obtained by performing the calculation of the equation (3) and performing the integration processing
Ii is stored in the storage area of the temporary storage RAM corresponding to the already determined primary current value class. Next, the inverter device arithmetic unit CPU reads out the function k of the heat allowable time for the primary current value from the table stored in the storage device RAM in advance, and compares the function k of the heat allowable time with the integrated value Ii already integrated. When the integrated value Ii is equal to or exceeds the function k of the heat allowable time limit, an abnormal signal is output and it is determined that the overload is abnormal. When determining that an overload is abnormal, the inverter device operation unit CPU stops sending the base signal or the gate signal to be supplied to the inverter unit INV through the interface i / o2, and stops the operation of the inverter unit INV.

The function k of the heat allowable time is compared with the integrated value Ii, and the integrated value Ii
If does not reach the function k of the heat allowable time limit, the inverter device operation unit CPU determines whether or not a certain time provided in advance for safety has elapsed. If this time has not elapsed, the sampling time Δt , And after this count-up, the processing following the reading of the DC current Id is continued again. If the integration process of the equation (3) is repeated during this certain period of time, an integration error accumulates, and a difference occurs with the function k of the actual allowable heat time limit. If there is no such value, this is for clearing each integrated value Ii stored in the storage area corresponding to each primary current value of the temporary storage device RAM to zero. After the storage area of the temporary storage RAM is cleared to zero, a fixed time timer for measuring a fixed time is also cleared to zero. After performing such initialization processing, the inverter device arithmetic unit CPU reads the DC current Id again and repeats the following operations. Thus, the inverter control circuit CON
If the overload protection of the motor is performed by interrupting the signal given to the inverter section INV and directly stopping the reconversion operation, the overload can be performed without any external processing device. A protective device can be configured.

5 and 6 show a further embodiment of the present invention. In this embodiment, as shown in FIG. 5, two thermal time characteristics are stored in the RAM as shown in (s) and (d). (S) is a thermal time characteristic when the motor IM is accelerating and decelerating, and (d) is a thermal time characteristic when the motor IM is operating at a constant speed. The time allowable characteristic of the time characteristic (s) is set to be larger than that of (d). This is because the current Im is generally larger during acceleration and deceleration than during constant speed operation, but the time is shorter.

The CPU fetches the output frequency command of the inverter INV commanded by the CPU at st1 in FIG. 6, and determines at st2 whether or not it has changed from the previous command value. As a result, if the frequency command value is the same as the previous sampling value, proceed to st3,
The thermal time characteristic (d) is selected, and if different, the process proceeds to st4, and the thermal time characteristic (s) is selected.

The determination of st1 and st2 may be made by sampling the output of the gradual variable circuit RAMP, or by sampling the output frequency command of the INV generated in the CPU.

The thermal time characteristic and the cooling coefficient may be stored as a table or may be stored as an arithmetic expression.

〔The invention's effect〕

According to the present invention, it is possible to set the thermal time characteristic to a value suitable for each of acceleration and deceleration and during operation, and it is also possible to set the thermal time characteristic according to the capacity of the electric motor.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention;
2 and 5 are diagrams showing thermal time characteristics, FIG. 3 is a diagram showing cooling characteristics of an electric motor, and FIGS. 4 and 6 are flowcharts used to explain the operation of the device of the present invention. SSC is speed setting means, INV is frequency conversion means, HSC is thermal time setting means, RAM is storage means, and CPU is abnormality detection means.

Continuation of the front page (56) References JP-A-62-224695 (JP, A) JP-A-2-133034 (JP, A) JP-B-62-55379 (JP, B2) (58) Fields investigated (Int) .Cl. ⁶ , DB name) H02H 7/08-7/20 H02P 5/408-5/412 H02P 7/628-7/632

Claims (1)

(57) [Claims] 1. A speed setting means for setting an operation speed of an AC motor, and a frequency conversion means for converting a frequency of a power supply so that the speed of the AC motor approaches the speed set by the speed setting means and giving the frequency to the AC motor. Means, and a control device for an AC motor having a current detection means for detecting a load current of the frequency conversion means, wherein a continuous operation considering a cooling effect of the AC motor determined according to a speed set by the speed setting means. Storage means for indicating an operable time; determining means for determining whether the AC motor is accelerating, decelerating, or operating at constant speed; and indicating that the AC motor is accelerating. Reading the thermal time characteristic at the time of acceleration from the storage means during the operation, and reading out the thermal time characteristic at the time of the constant speed operation during the constant speed operation; An overload detection device, comprising: an abnormality detection means for performing an integration process by taking in a detection value of a stage and generating an abnormality signal when the integration value exceeds a storage value read by the reading means. AC motor control device.