JPH02197295A - Current detector for inverter device - Google Patents

Abstract

PURPOSE:To detect motor current by sample holding a voltage detected through a voltage detector such as a shunt resistor arranged at a DC circuit section based on a signal fed from a logic circuit for generating a voltage combination signal to be fed to a motor thereby detecting the motor current. CONSTITUTION:Relation between motor current and current iDC at a DC circuit section is utilized. In an AND logic, 12-a produces an 'H' output only when phase (u) is 'L', phase (v) is 'H' and phase (w) is 'H'. Consequently, an analog switch 11-a is turned ON and a signal produced by inverting the voltage of the product of iDC and a shunt resistor is sample held in a capacitor 13-a. In such circuitry, a voltage proportional to iu is detected through the capacitor 13-a, a voltage proportional to iv is detected through a capacitor 13-c and a voltage proportional to iw is detected through a capacitor 13-e.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a current detection device for an inverter device that converts direct current to alternating current.

2. Description of the Related Art Inverter devices that convert direct current voltage to alternating current voltage have been greatly developed mainly for controlling the speed of motors. Among them, inverters with an output of several tens of W or more adopt a pulse width modulation control (hereinafter abbreviated as PWM control) method to reduce loss in the inverse converter. This conventional method is explained with reference to FIG. 2. It is a control method in which a voltage to be output (shown by a dotted line in the figure) is changed into a pulsed voltage (shown by a solid line in the figure) and applied to the motor. FIG. 2 is a model of one of the three phases, and in the case of three phases, the other two phases are shifted by 120 degrees and output.

By the way, the inverter for controlling the speed of an induction motor applies the number of cycles and voltage to the motor, but it also has a function that monitors the motor load status and issues an alarm or trips in the event of an overload, as well as a function to control the power output due to overcurrent. It has the function of warning and tripping to prevent the switching element from being destroyed.

These are performed by detecting the current flowing to the motor or the current in the DC circuit of the inverter device.

When these are shown in the circuit configuration diagram in Figure 3, 1 is an AC power supply;
Reference numeral 2 denotes a forward conversion section (hereinafter referred to as converter section), which consists of a diode and a capacitor, and rectifies and smoothes alternating current voltage into direct current voltage. Reference numeral 3 denotes an inverse conversion section, which is composed of switching elements such as power transistors and power MOSFETs. 4
is the motor.

5 is a frequency command, and 6 is a control circuit section that converts the output frequency and voltage into a PWM control signal and outputs the signal to the gate circuit section 7. The gate circuit section 7 supplies base current or gate voltage to the power switching element. 8 is a current detection section of the DC circuit, and 9 is a motor current detection section. In either case, the detected signals are transmitted to the control circuit section 6.

A method of detecting current in the above configuration will be described below.

First, the current detection section 8 of the conventional DC circuit can be divided into two types: one uses DC and -CT, and the other uses a shunt resistor and detects the voltage across it to detect the current flowing into the DC circuit. It can be broadly classified. In either case, the current was not detected as a motor current and was merely used as overcurrent protection for the power switching element.

Further, the motor current detection section 9 uses a D
It can be roughly divided into methods using C-CT and CT (converter).

Although the method using CT has the advantage of simple circuit configuration,
During low frequency output, the magnetic circuit of the CT is saturated, so a CT with a large volume must be used, and when stopping the motor by applying DC voltage to the motor 4, the magnetic circuit of the CT is saturated.
Since the current flowing through is direct current, C which detects alternating current
It has a defect that cannot be detected by T. The method using DC-CT has the advantage of being able to detect DC current at low frequencies as mentioned above, but because the detection voltage of the Hall element/Hall IC is low, it is necessary to amplify the detection current with an amplifier such as an operational amplifier. , there were problems with offset and drift, making the detector expensive.

Problems to be Solved by the Invention As described above, the conventional techniques have the drawbacks of requiring a large CT or using an expensive C-CT.

Means for Solving the Problems The present invention detects motor current by using the detected voltage of a voltage detecting means such as a shunt resistor provided in a DC circuit section, and uses the detected voltage to generate a combination signal of voltages output to the motor. The generated logic circuit signal is sampled and held to detect the motor current.

Effects According to the above configuration, the current of each phase is detected by the combination of the current iDc of the DC circuit section and the logic signal of the output voltage terminal.

Example An embodiment of the present invention will be explained with reference to FIG.

1 is an AC power supply, 2 is a converter section consisting of a diode and capacitor, 3 is an inverse conversion section consisting of a power switching element, 4 is a motor, 6 is a control circuit section, and 7 is a switching element of the inverse conversion section 3. In the gate drive circuit section, 9 is a shunt resistor for current detection, and 10 is an operational amplifier that functions as a signal inverter together with two resistors. 11-a
~11-f are analog switches, and 12-a~12
-f is 3-man AND logic IC, 13a, 13c.

13e is a capacitor, and analog switch 118~
11-f constitutes a sample hold circuit.

In general, in the case of a three-phase inverter, the inverse converter is composed of six switching elements as shown in Fig. 3. , W terminal, and if we separate the U terminal,
Switching element Q1 in Figure 3 is turned on and Q2 is turned off.
If so, the positive DC voltage of the converter section 2 is applied to the motor. This state is designated as "H". On the other hand, Q
When Q1 is OFF and Q2 is ON, a negative DC voltage is applied to the motor. This state is designated as "L".

In any case when the motor is running, rH.
or "L", and if this is three-phase, the terminal voltages to the motor will be more than eight combinations. In other words, the U terminal.

■If the combination of terminal and W terminal voltages is [(u-phase terminal voltage), (v-phase terminal voltage), (W-phase terminal voltage)], then [
]L-L-Ll, L-L-H], [L・H-L], [L
-H-H], [H-L-L].

[H-L-H] , [H-H-L] , [H-H-H]
becomes. Let's examine the internals of each. First, the direction of current flowing from the inverter side to the motor side is "+", and conversely, the direction of current flowing from the motor side to the inverter side is "-".
'', and if the U-phase current is iu, the v-phase current is iv, and the w-phase current is iw, as is well known, iu+iv+iw=0 holds true. If the instantaneous current values are iu=+5A iv--3A iw=-2A, then iu+iv+iw-+5-3-2=0 holds true. Focusing on the terminal voltage at this time for the U phase, when [H-L-Ll and [L-L-Ll], the
4. When Q6 is ON, U phase (7) Ql is ON, Q
When 2 is OFF, it is [H-L-Ll, Ql is OFF, and Q2 is ONL.

When it is [L-L-Ll]. First, [H・L-L
In the case of l, from Fig. 4(a), 5A for Ql, 3A for Q4,
2A flows through Q6, and +5A flows through the DC circuit. (
The direction in which current flows from the converter section to the inverter section is defined as "+" in the DC circuit. ) then [If L-L-Ll,
From Figure 4(b), the flywheel diode of Q2 has 5
A, 3A in Q4.

2A flows through Q6, and no current flows through the DC circuit.

Similarly, when [H-H-Ll and [L-H-Ll], V.

The W terminal is Q3 in Figure 3. Q6 is ON, Q4. Q5 is O
FF, [When H-H-Ll, Ql is ON, Q2 is OFF, [When L-H-Ll, Ql is OFF, Q! 2 is ONL. [At the time of H-H-Ll, from Figure 4 (C), 5A is applied to Ql, and 3A is applied to the flywheel diode of Q3.
, 2A is flowing through Q6, which means that 2A is flowing through the DC circuit. Next, [When L-H-Ll, see Figure 4(d)
It can be seen that 5A flows into the Q2 flywheel diode, 3A flows into the Q3 flywheel diode, and 2A flows into Q6, and the DC circuit flows 13A, that is, 3A flows backward from the inverter side to the converter section.

Similarly, [H-L-HJ, [L-L-HJ] are shown in FIG. 4(e), FIG. 4(f), FIG. 4(g), and FIG.
.

The cases of [H-H-HJ and [L-H-HJ are illustrated, and the currents of the DC circuits are +3A and -2A-OA, respectively.
, -5A. The above eight combinations of three-phase terminal voltages and the current in the DC circuit at that time (indicated by ioc, the direction flowing from the converter section to the inverter section is "+", and the opposite direction is "-") are shown below. The following table is obtained.

This is a summary of the relationship between current and three-phase current, and these are collectively expressed as follows.

1. When all three phase terminal voltages are the same (all rH, or "L"
) Motor current does not flow in the DC circuit.

2. Among the three-phase terminal voltages, a phase current with a voltage different from the other two phases flows in a DC circuit, and the voltage of the different phases is
When it is "H", the direction of the current in that phase matches the direction of the current in the DC circuit, and when it is "L", the sign is opposite.

The present invention utilizes the relationship between the motor current and the current 1oc in the DC circuit section, and in the configuration shown in FIG.
Among the D logic ICs, 12-a has the U phase as "L", the ■ phase as "HJ, and the W phase as r" (the output becomes "H" only when it is J, turning on the analog switch 11a and shunting the IOC. A signal obtained by inverting the voltage multiplied by the resistance is sent to capacitor 1.
Hold the sample in 3-a. Hereinafter, the six combinations are as explained in detail in the above operation. With such a circuit configuration, a voltage proportional to iu is detected at the capacitor 13-a, a voltage proportional to iv is similarly detected at the capacitor 13-c, and a voltage proportional to iw is detected at the capacitor 13-e.

Incidentally, in recent inverters, the terminal voltage is often determined by a microcomputer, and the AND logic 1c may be configured in the microcomputer. Further, a similar function can be obtained by using a DC-CT instead of a shunt resistor.

As described in detail, by using the present invention, motor current can be detected with an inexpensive circuit configuration, and an inexpensive and highly reliable inverter can be provided.

[Brief explanation of the drawing]

13-c, 13-e... Capacitor. Name of agent: Patent attorney Shigetaka Awano and one other person Figure 4 is a circuit diagram of the inverter and a detailed explanatory diagram of the current value of PWM control. 2...Forward conversion unit, 3...Inverse conversion unit, 4
...Motor, 9...Shunt resistor (current detection means), 11a to 11-f...Analog switch, 12-a to 12-f... AND logic IC, 13'-at procedural amendment (method) Name of the invention indicated in the May 23, 1989 case Relationship with the person who corrects the current detection device of an inverter device Case Patent application
Colonel Tokoro 1006 Kadoma, Kadoma City, Osaka Prefecture
Name (582) Matsushita Electric Industrial Co., Ltd. Representative Akio Tanii 4 Agent Address 1006 Oaza Kadoma, Kadoma City, Osaka Prefecture 571 No. 4, Matsushita Electric Industrial Co., Ltd.

Claims (1)

[Claims] A current detection means is provided between the forward conversion section and the inverse conversion section of the inverter device, and the voltage generated by this current detection means is sampled by a signal of a logic circuit that generates a combination signal of terminal voltages of each phase to be applied to the motor. A current detection device for an inverter device that holds and detects the motor current.