JPH04105565A - Electronic thermal circuit of inverter - Google Patents

Abstract

PURPOSE:To miniaturize it equipping it with a detection circuit, which detects the voltage occurring across the shunt resistor for detecting the occurrent of an inverter, and a peak hold circuit, which converts the detection voltage in such a way that it can be regarded as equivalent to the current flowing in a three-phase induction motor. CONSTITUTION:The noise components of the current flowing in shunt resistor 7 are removed by a filter 9, and are amplified with an amplifier 8. next, the output of the amplifier 8 is input into a peak hold circuit 10, which is charged at the time constant about half as long as the minimum pulse width and is discharged at the time constant about ten times as long as the minimum pulse width, and after smoothing with a filter 23, DC voltage is gotten. Next, a comparator 12 compares this DC voltage with the voltage which decides the electric thermal level being gotten by a setter 15, a microcomputer CPU 14, an insulating amplifier 11, and a filter 16, and this is input into the CPU 14 after insulation with an insulating amplifier 13. When the input from the insulating amplifier 13 continues for a certain 67-107time, the CPU 14 stops the output to an inverter 5, and stops the output of the inverter.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an electronic thermal circuit for an inverter for the purpose of protecting a motor and a motor-driving innocoator from overheating.

2. Description of the Related Art A conventional electronic thermal circuit for an inverter will be described below with reference to the drawings.

Figure 3 is a block diagram of the electronic thermal circuit of a conventional inverter, in which 1 is a single-phase or three-phase AC power supply, 2 is a forward conversion section composed of diodes, 3 is a smoothing capacitor for the DC power supply, and 4 is various types. 5 is an inverse converter composed of power switching elements such as transistors and GTo/Mo5FETs, 6 is a three-phase induction motor, 7 is a shunt resistor for detecting excessive current in the inverse converter 5, 17 is a CT that detects the motor current, 8 is an amplifier that amplifies the output of CT 17, 9 is a filter that smoothes the output of amplifier 8,
22 is A that converts the output of the filter 9 into a digital value.
/D converter, 15 is a setting device that sets the level at which the electronic thermal of the inverter operates, 14 is a CPU that controls the operation of the inverter, 20.21 is an isolation amplifier that isolates the output of the CPU 14, and 18 is a positive side DC- A driver circuit 19 drives a switching element connected to the BUS line. A driver circuit 19 drives a switching element connected to the negative DC-BUS line. The arrow indicates the supply of power.

The operation of the electronic thermal circuit of the inverter configured as above will be described below.

AC power supply 1. Forward conversion unit 2. The DC voltage obtained by the capacitor 3 is input to a power supply circuit 4, where the DC voltage is insulated and divided and supplied as each power supply voltage of the circuit. Then CPU1
When the inverter output start command is given to the CPU 14
The PWM waveform outputted from the insulating amplifier 20.21 and the driver circuit 18.degree. 19 is input to the inverse converter 5, and the three-phase induction motor 111i6 rotates. The current flowing through the three-phase induction motor 6 is detected by the CT 17, amplified by the amplifier 8, smoothed by the filter 9, and obtained as a DC voltage. After the DC voltage is converted into a digital value by the A/D converter 22, it is input to the CPU 14, and the state in which the input to the CPU 14 is higher than the level at which the electronic thermal of the inverter operates, which is set in advance by the setting device 15, is constant. When the time continues, the CPU 14 stops the output to the inverse converter 5 and stops the output of the inverter.

Problems to be Solved by the Invention However, the above-mentioned conventional configuration requires a CT to detect the current flowing through the three-phase induction motor, making it difficult to miniaturize the inverter.

The present invention solves the above-mentioned conventional problems, and aims to provide an electronic thermal circuit for an inverter that does not use a CT.

Means for Solving the Problems In order to achieve the above object, the present invention includes a detection circuit that detects a voltage generated across a shunt resistor that detects an excessive current in an inverse conversion section, and a detection circuit that detects a voltage generated across a shunt resistor that detects an excessive current in an inverse conversion section, and a three-phase induction motor that detects the detected voltage. It is equipped with a peak hold circuit that converts the current so that it can be regarded as equivalent to the current flowing through the current.

Operation The present invention detects the current flowing through the three-phase induction motor without using a CT with the above-described configuration, so it is possible to provide an electronic thermal circuit that is free from practical problems without increasing the size of the inverter.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 shows the configuration of an electronic thermal circuit of an inverter in an embodiment of the present invention. Note that the same configurations as in the conventional example are given the same reference numerals as in the conventional example, and detailed description thereof will be omitted. In FIG. 1, 10 is a peak hold that holds the output peak value of the amplifier 8, 11 is an isolation amplifier that insulates the output of the microcomputer CPU 14, 16 is a filter that smoothes the output of the isolation amplifier 11, and 12 is the peak hold 10 and the filter. a comparator that compares the outputs of 16;
Reference numeral 13 indicates an isolation amplifier for insulating the output of the comparator 12.

The operation of the electronic thermal circuit of the inverter of this embodiment configured as described above will be described below.

It is assumed that the three-phase induction motor 6 is rotating in the same manner as in the conventional example.

The power factor of the current flowing through the shunt resistor 7 is improved as the motor load increases, so the peak value is equal to the motor current and the frequency is twice the carrier frequency of the PWM waveform that drives the inverse converter 5. It's a pulse. The noise component of the pulse is removed by a filter 9 and amplified by an amplifier 8. Next, the output of the amplifier 8 is input to a peak hold circuit 10 that charges with a time constant of about 1/2 of the minimum pulse width and discharges with a time constant of about 10 times the minimum pulse width, and filters the output of the amplifier 8. After smoothing at step 23, a DC voltage is obtained. The DC voltage thus obtained is transferred to the setting device 15° microcomputer CPU14. Isolated amplifier 11, filter 16
The voltage V that determines the electronic thermal level obtained by
It is compared with ref by the comparator 12, isolated by the isolation amplifier 13, and inputted to the microcomputer CPU14. When the input continues for a certain period of time, the CPU 14
Stop the output to the inverter and stop the inverter output. Furthermore, in Fig. 1, power for the electronic thermal circuit is separately supplied by the power supply circuit 4, but in order to simplify the configuration, it is possible to share the power supply with other circuits as shown in Fig. 2. . FIG. 2 differs from FIG. 1 only in the power supply system of the electronic thermal circuit of the inverter. In FIG. 2, the t source of the electronic thermal circuit of the inverter is shared with the power source of the driver circuit of the power switching element connected to the negative side DC-BUS line in the inverter 5, and in the power supply circuit 4, There is no need to separately supply power for the inverter's electronic thermal power.

As described above, according to this embodiment, the voltage across the shunt resistor is detected and the detected value is used to operate the electronic thermal inverter, eliminating the need for CT, which was conventionally required for electronic thermal. Become.

Also, there is no need to provide a separate power supply for the electronic thermal circuit (
It is also possible to downsize the power supply circuit.

Effects of the Invention As described above, according to the present invention, there is no need for a CT or a new power source, and the inverter can be downsized, so a small and inexpensive inverter can be provided.

[Brief explanation of drawings]

1 and 2 are block diagrams of an electronic thermal circuit of an inverter according to an embodiment of the present invention, and FIG. 3 is a block diagram of a conventional electronic thermal circuit of an inverter. 1... Single-phase or three-phase power supply, 2... Old converter, 3... Capacitor, 4... Power supply circuit, 5... Reverse Conversion unit, 6... Three-phase induction motor, 7... Shunt resistor, 8... Amplifier, 9, 16.23... Filter 10...
...Peak hold circuit, 11.13, 20゜21
...Isolation amplifier, 12 ...Comparator, 1
4...Microcomputer, 15...
・Setting device, 17...CT, 18.19...
...Driver circuit, 22...A/D converter. Name of agent: Patent attorney Haruaki Ogata 28 g

Claims (2)

[Claims] (1) Negative DC-B in motor drive inverter
A comparison circuit that compares the output of a current detector for overcurrent detection inserted into the US line with a reference voltage, an isolation amplifier that isolates the output of the comparison circuit, and an output of the isolation amplifier that continues for a certain period of time. An electronic thermal circuit for an inverter, which is equipped with a circuit that cuts off the inverter's output when a reference value is exceeded. (2) The electronic thermal circuit for an inverter according to claim 1, wherein the power source of the comparison circuit is shared with the power source of a driver circuit that drives a switching element connected to the negative side DC-BUS line.