DE3490167T1 - Inverter device - Google Patents

Description

description
Title of the invention
Inverter device,
Technical area

The invention relates to an inverter device which protects an AC motor driven by an overcurrent.
10

Background prior art

To an AC motor such as an Induk-λ tion motor, which is driven by an inverter device, to protect against an overcurrent, was uses a protection system which uses an overcurrent relay which has an anti-time limit characteristic as shown in FIG.

In FIG. 1, reference numeral 1 denotes a commercially available AC power source, 2 an inverter device, 3 an overcurrent relay having an anti-time limit characteristic which includes an output contact 3a, and 4 an induction motor.

The anti-time limit characteristic overcurrent relay 3 which is connected in series with the primary winding of the induction motor 4, provides the overcurrent of the induction motor 4 fixed and the output contact signal S of the overcurrent relay 3 is applied to the inverter device 2 to output of the inverter device 2 so as to

Anti-time limit overcurrent interruption characteristic too is obtained so that as the current increases, an operating time is shortened.

Generally, an induction motor includes a self-cooling fan so that it is cooled when it is turned the rotor turns. However, the self-cooling effect is reduced when the induction motor is at a variable Speed by means of an inverter device operates, particularly at lower speed operation in a low frequency range; and therefore become reduces the overcurrent resistance data of the induction motor. Hence it was with the one described above Protection method in which the overcurrent relay with the anti-time limit characteristic is used with a predetermined current, it is difficult to: accurately protect the induction motor.

Disclosure of the invention

This invention aims to obviate the above-described problem in connection with a conventional one Protection system. In particular, the invention seeks to provide a device that can operate at any frequency of operation of an AC motor such as an induction motor having an anti-time limit characteristic corresponding to the frequency, thus the induction motor to be fully protected against overcurrent.

Brief Description of the Drawings 30

Fig. 1 is a diagram showing the principle of a conventional device;

Fig. 2 is a diagram showing the essential parts of an apparatus according to this invention;

Fig. 3 is a diagram showing the whole arrangement of the apparatus of the present invention.

FIG. 4 is a flow chart showing the operation of a microcomputer shown in FIG.

Best mode for carrying out the invention

An embodiment of this invention will now be discussed in connection with FIGS. 2 and 3 described.

First, the principle of the invention will be described.

In Fig. 2, reference numeral 5 denotes a current transformer (hereinafter referred to as "CT5") which is suitable is to detect a direct current, which is a reference current for three-phase alternating currents contained in a Induction motor 4 flow. 6 is a microprocessor for operation. 7 is a read-only memory (hereinafter referred to as Designated "R0M7"). The remaining components are the same as shown in FIG. 1 and their description will be omitted. ■

A current signal SN, which is detected by the CT5, is applied to the microprocessor 6, where there is one Time integration is subjected. If the current value integrated in this way coincides with a value, which is supplied by the R0M7, the microprocessor 6 outputs a signal SS to interrupt the output of an inverter circuit 8 at. Operating frequencies are assigned addresses in the R0M7. According to an operating frequency signal

SF applied the ROM7 to the microprocessor 6 a value according to the overcurrent resistance data of the induction motor 4, which for each operating frequency stored in this way is provided. Therefore, the overcurrent protection of the induction motor 4 can be achieved with great accuracy.

A special embodiment of the invention will now be made with With reference to FIG. 3. In Fig. 3, the reference numeral denotes 1 a commercial AC power source, 2 the inverter device according to the invention / 4 a Three-phase induction motor operated by the inverter device, 5 a current transformer (CT) which serves as a detector device for the detection of a current in a direct current bus 9, 8 a conventional inverter circuit comprising a transistor and a diode, 10 a converter for converting the alternating currents (three-phase alternating currents) the power source 1 into a direct

I · ι

current, 11 a smoothing capacitor and 12 a direct

Current detector circuit in which the current flowing through the CT5 is detected, averaged and subjected to a level conversion will. The current treated in this way is applied to an operating point setting unit 13.

Numeral 14 denotes an analog-to-digital converter for converting an analog signal into a digital signal. 15 denotes a frequency setting circuit for setting the operating frequency of the induction motor 4 a desired value. 16 denotes a control circuit comprising a one-chip microcomputer with the microprocessor 6 and the ROM7 shown in Fig. 2, where for example, the microcomputer is of the type M5L8049-XXXP made by Mitsubishi Denki Kabushiki Kaisha in Japan will.

~ _ G> J ^

Numeral 17 denotes a modulator circuit for generating a pulse width modulation signal PWM corresponding to the output of the control circuit 16. 18 denotes a protection circuit for supply a predetermined interrupt signal SS in response to an output interrupt instruction from the control circuit 16.

Numeral 19 denotes a base drive circuit for controlling the inverter circuit 18, the Base driver circuit provides a signal for controlling the base current of the transistor. 20 denotes three-phase bus lines.

The operation of the device will now be described with reference to FIG described. The control circuit 16 operates in accordance with a program as shown in FIG. It means that after the start (30) an operation (31) for setting a flag is carried out only once every sixteen times.

Next, an operation (32) for determining what is the induction motor? running current. This Detecting or reading operation (3 2) is carried out with reference to the signal detected by the CT5 as described with reference to FIG. One Operation (33) for averaging the currents obtained by sixteen read operations is carried out. Then an operation to subtract a current value that is 5% of the average current value is performed, from the average current value in order to carry out a protection operation in the case where the Current value is equal to or greater than 105% of the current value which is a reference value of the protection operation.

A decision operation (35) for determining whether the result of the subtracting operation (34) is negative or not, will be executed. In the case where the result is negative, a decision operation (36) becomes Determining whether the content of the integration error is zero or not is carried out. If it's not zero, it will a decision operation (37) is performed to determine whether or not a flag is available. If it is available, an operation (38) is performed to subtract one from the content of the integration counter. Then all operations are finished (100).

In the case where the result of the subtracting operation (34) is positive, a decision operation (39) carried out to determine whether the result is 100% of the reference value or not. If it's not more than Is 100%, an operation (40) for reading the operation frequency becomes SF performed and an operation 41 to determine whether the integration counter is an addition or perform subtraction. The choice of addition or subtraction depends on the operating frequency SF. As the frequency SF decreases, the tendency to choose increases

adding to it, even if

the current value is small L.

In the case of addition, the result of the decision operation (42) becomes an operation (43) of referring to the Data of the ROM7 executed. As a result of the decision operation (39), when it is determined that the current value is more than 100%, an operation (44) becomes the reference

on the data of the ROM7 *

leads. After the operations

(43 and 44) referring to the data of the ROM7 becomes a Operation (45) for determining whether or not there is a flag is carried out. If there is no label, will

3A90167

the operation ended immediately (100). If a license plate

i I

If there exists, an operation (46) for adding the data of the ROM7 to the content of the integration counter is carried out and an operation for determining whether or not a carryover takes place at the integration counter as a result of the addition operation. If the carry over doesn't takes place, the operation is terminated (100). When the carryover takes place, a mutual lock-inhibiting operation is performed (48) is performed, and to ensure again, an operation (49) is performed to determine whether or not a carry takes place in the integration counter is carried out. If there is no carry over, will an operation (50) for releasing the above-mentioned mutual Lock inhibition performed and the operation terminated (100). When the carryover takes place, will a protection operation (51) is carried out.

The operation (31) of setting a flag once every sixteen times is initially carried out as above has been described. As the control circuit 16 performs a number of control operations in addition to controlling the modulator circuit 17, such a main flow for overcurrent protection is executed only once every sixteen times.

In the device described above, the CT5 is on the DC bus line 9 is provided in the inverter device 2 and therefore the transistors in the inverter circuit 8 are protected. However, it is evident that the CT: "on the three-phase bus line 20 between the Inverter circuit 8 and the induction motor 4 could be provided.

As can be seen from the above description, the

-y-

Anti-time limit over current interruption characteristic can be obtained in accordance with the operating frequency of the AC motor according to the invention. Hence
the inverter-operated AC motor can be fully protected.

Claims (3)

Patent claims: 1. An inverter device in which an inverter circuit outputs an output signal from a commercial power source converts AC power into a variable frequency and variable voltage, so that an AC motor , is operated at a variable speed, characterized in that the inverter device comprises: a detector means for determining a current value applied to the AC motor and a control device to which a detector output signal the detector device and an operational frequency signal of the AC motor applied and which has an anti-time limit interruption characteristic for each of the operating frequencies, corresponding to the operating frequency, with an output interrupt instruction signal of the controller is used to interrupt the AC power. 2. Inverter device according to claim 1, characterized in that the detector device is provided in a circuit which supplies an alternating current to the inverter circuit. 3. Inverter device according to claim 1, characterized in that the control circuit comprises a microcomputer which contains a microprocessor for operation and a read-only memory (ROM) in which a current value corresponding to overcurrent resistance data of the AC motor for each operating frequency get saved.