JPH0734622B2 - Inverter ground protection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an induction heating device, in which a heating coil contacts a metal that is an object to be heated, which causes a ground fault or insulation of a part of an inverter main circuit. The present invention relates to a ground protection device for a ground fault accident due to deterioration or the like.

[Conventional technology]

When the heating coil of the induction heating device has a high temperature, the refractory material deteriorates over time, causing the conductor to come into contact with the metal material, which is the object to be heated, and the object to be heated is grounded. There is a possibility of a ground fault. Further, in the induction heating device inverter, a heating coil serving as a load and a resonance capacitor are connected in series, and when the two operate in a voltage resonance state with each other, the voltage of the heating coil is the normal circuit voltage of the inverter. It will have twice as much voltage.
Therefore, depending on the state of grounding such as incomplete grounding, an abnormal state of the voltage between the main circuit of the inverter and its load to ground occurs due to the superposition of high voltage due to the interruption of the arc current generated at the grounding point. May cause dielectric breakdown.

As a protective device for the ground fault as described above, one illustrated in the circuit diagram of FIG. 2 is known.

In FIG. 2, 1 is an insulating transformer that transforms the power supply voltage to a predetermined value and the secondary winding side of which is ungrounded, 2 is a rectifier using semiconductor elements such as power thyristors or diodes, and Cf is DC The smoothing capacitor 3 is an inverter using semiconductor elements such as power transistors or thyristors and diodes. Reference numeral 4 is a resonance capacitor, and 5 is a heating coil, both of which are connected in series and serve as a load of the inverter 3.

Next, the choke coil CH for blocking the high frequency voltage applied between the main circuit connected to the heating coil 5 and the ground, the current limiting resistance Rs, the DC power supply 6, and the energizing current thereof have exceeded the set value. A series connection consisting of a meter relay 7 having an alarm contact that operates in the case constitutes a detection and protection device of the ground in the main circuit, which is inserted between the heating coil 5 and the ground, and the ground is generated. Then, a direct current passing through the ground point is made to flow as a ground detection current by the direct current power supply 6, and if the detected current exceeds the set value of the meter relay 7, the alarm contact will cause the inverter 3 to stop operating. The signal Sf / Sa for protection / warning operation is formed.

[Problems to be Solved by the Invention]

However, in the ground protection device using the meter relay according to the conventional method as described above, it is difficult to protect the inverter main circuit part, especially its semiconductor element from overvoltage even if it takes time to detect grounding and is effective in preventing the progress of insulation deterioration of the heating coil. There are cases. In addition, if the main circuit including the heating coil is incompletely grounded, the ground detection current will be a broken arc, which is so small that the protection circuit does not operate until the level exceeds the specified level. It sometimes caused the destruction of. In view of this, it is an object of the present invention to provide a ground protection device that accurately and promptly detects the ground state in the main circuit.

[Means for Solving the Problems]

In order to achieve the above object, an inverter ground protection device of the present invention is an inverter ground protection device having a load circuit composed of an inductive load and a capacitor, wherein the negative side input bus voltage of the inverter to ground voltage. Driven by the output of this voltage detection circuit, a voltage detection circuit for detecting the positive or negative polarity output voltage of the detection circuit, and a voltage comparison circuit for comparing the absolute value with the overvoltage setting value and storing the comparison result. A relay circuit is provided, and the output signal of the voltage comparison circuit and the relay circuit is used as a signal for stopping the operation of the inverter or an alarm signal.

[Action]

The heating coil of the induction heating device having a resistance component and an inductance component is grounded via a stray capacitance to ground. In addition, the output voltage of the inverter that supplies power by using the series connection of the heating coil and the resonance capacitor as a load includes a harmonic component generated in the process of creating a required AC voltage from the input DC voltage.

Therefore, when grounding occurs in the main circuit of the inverter and the load circuit consisting of the heating coil and the resonance capacitor connected in series, the earth-to-ground stray capacitance is connected in parallel to the heating coil via the ground point. Therefore, at this time, if the resonance frequency in the total impedance seen from the output terminal side of the inverter and the harmonic component contained in the output voltage of the inverter match, harmonic resonance occurs at that frequency, and as a result, If an intermittent arc is generated at the grounding point, a high voltage is generated due to the intermittent current of the arc, and the ground potential of the main circuit may rise abnormally. Therefore, the ground potential on each element connected to the main circuit is a risk that the main circuit ground potential is superimposed on the normal operating voltage of each of these elements and the potential rises rapidly.

In the earth protection device of the present invention, when the earth occurs in the main circuit as described above, the absolute value of the earth voltage of the main circuit that fluctuates positively and negatively with respect to the earth becomes larger than the allowable set value. The state is instantly detected and stored, and a protection command such as the operation stop of the inverter and an alarm or an operation condition command are issued.

〔Example〕

An embodiment of the present invention will be described below with reference to the circuit diagram shown in FIG. Incidentally, in FIG. 1, the same reference numerals are attached to the constituent elements having the same functions as in the case of the embodiment of the prior art shown in FIG.

FIG. 1 is a circuit diagram of the choke coil shown in FIG.
The same device 10 replaces the grounding detection and protection device consisting of series connection of CH, current limiting resistance Rs, DC power supply 6 and meter relay 7. In FIG. 1, 11 is a voltage dividing circuit for the grounding overvoltage composed of high resistance, 12 is a differential voltage increasing circuit composed of an operational amplifier OA and a resistor, and 13 is a voltage dividing circuit varying in both positive and negative polarities. Full-wave rectifier circuit that aligns the output voltage polarity of the circuit 11 to a single polarity, CP is a voltage comparator, and outputs when the passing voltage of the rectifier circuit 13 is larger than the voltage set by the overvoltage level setter 14. Is a voltage comparator that emits. Further, FF is a flip-flop circuit, which holds the output signal of the voltage comparator CP which changes with time until a reset signal Sr is applied and outputs it as a signal Sf.
Sf is a protection command such as the operation stop of the inverter. 15
Is an output relay circuit, which is composed of a relay Ry and its driving transistor TR, and is connected to the signal S by a negative logic element NOT.
The signal Sa is output as an operating condition command to the inverter or the like when f is not generated.

〔The invention's effect〕

According to the present invention, by detecting and storing the voltage between the ground of the inverter bus and the ground, it becomes possible to promptly and surely detect the ground fault state which varies with time, and the heating coil connected to the inverter and the bus. It is possible to reduce the size and cost by optimizing the ground insulation of the device components including the load such as the above, and further improve the reliability of the operation of the device.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the prior art. 1 ... Insulation transformer, 2 ... Rectifier, 3 ... Inverter, 4 ... Resonant capacitor, 5 ... Heating coil, 6 ...
DC power supply, 7 …… meter relay, 10 …… ground protection device,
11 …… Voltage divider circuit, 12 …… Differential amplification circuit, 13 …… Full wave rectifier circuit, 14 …… Overvoltage level setting device, 15 …… Output relay circuit, Cf …… Smoothing capacitor, CH …… Choke coil,
CP ... voltage comparator, FF ... flip-flop circuit, OA
…… Computation amplifier, Rs …… Current limiting resistance, NOT …… Negative logic element, Ry …… Relay, TR …… Transistor.

Claims (1)

[Claims] 1. A ground protection device for an inverter having a load circuit composed of an inductive load and a capacitor, a voltage detection circuit for detecting a ground-to-ground voltage of an input bus voltage of a negative side of the inverter, and a positive circuit of the detection circuit. Alternatively, the negative voltage output voltage is compared with its overvoltage set value in its absolute value, a voltage comparison circuit for storing the comparison result, and a relay circuit driven by the output of this comparison circuit, and the voltage comparison circuit, A ground protection device for an inverter, characterized in that an output signal from the relay circuit is used as a signal for stopping the operation of the inverter or a signal for an alarm.