JPS6222075Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a high frequency induction heating device using an inverter, and more particularly to a high frequency induction heating device that aims to reduce high frequency leakage current sent out from the inverter.

Conventionally, high-frequency induction heating equipment has been used for quenching, melting, etc., which drives an induction heating work coil with a high-frequency voltage of several 10KHz to several 100KHz to inductively heat the workpiece (object to be heated). ing.

FIG. 1 is a diagram showing the main parts of an example of a conventional device of this type. In FIG. 1, a three-phase commercial power source whose S phase is grounded is applied to a forward converter 3 via a no-fuse breaker 1 and an earth leakage detector 2.
This earth leakage detector 2 has been commonly used in the past, and the sum of the vectors of currents (three phases) passing through the zero-phase transformer and flowing into the electrical circuit becomes zero when there is no earth leakage; The magnetic flux generated in the current transformer cancels each other and no voltage is generated in the secondary coil.
When there is an earth leakage, an imbalance occurs in the current flowing through the zero-phase current transformer, and a voltage is generated in the secondary coil.Using this, the voltage in the secondary coil is amplified to operate the breaker. It is something that makes you Further, the forward converter 3 is composed of, for example, a silicon controlled rectifier (SCR), and can rectify the three-phase commercial power supply by adjusting the gate pulse applied to its gate. The rectified output voltage of the forward converter 3 is smoothed by a smoothing circuit 4 including a choke coil L, and is applied to an inverter 5. Inverter 5
is composed of, for example, a static induction transistor (SIT), and is configured so that AFC can be applied according to the load condition by a control circuit (not shown). This inverter 5
The voltage (several
10 KHz to several 100 KHz) is sent to the work coil 6 connected as a load. When a high frequency voltage is applied to the work coil 6, the object to be heated is heated due to eddy current loss due to the alternating magnetic field of the work coil 6. Reference numeral 7 denotes a housing that houses these components, and the housing 7 is grounded.

By the way, according to such a configuration, since the commercial power supply is directly converted to DC and has a higher frequency,
The power circuit is in a floating state.
If we focus on the impedance between the power circuit and the casing (earth), at commercial frequencies the value is sufficiently high so no leakage current occurs, but in the high frequency range even a small amount of capacitance can cause high frequency leakage current. It will flow. For example, capacitance is 0.01
In terms of μF, at 3-phase full wave 300Hz, it is approximately 40KΩ, but at 100KHz, it is approximately 150Ω, resulting in a difference of 267 times. In the case of the device shown in FIG. 1, a high frequency leakage current flows through the stray capacitance Cs between the winding of the chiyoke coil L and the core, and when the output exceeds a certain level, the leakage detector 2 may operate.

However, such operation of the earth leakage detector 2 is due to the influence of stray capacitance Cs, and is essentially different from a failure of the high-frequency induction heating device itself. Therefore, it is necessary to reduce the influence of the high-frequency earth leakage current as much as possible. This is desirable.

The present invention has been made in view of these points, and its purpose is to provide a high-frequency induction heating device that substantially reduces high-frequency leakage current.

The present invention achieves this objective by rectifying and smoothing a commercial power supply, converting it into a high-frequency voltage using an inverter, and supplying the converted high-frequency voltage to a work coil. A capacitor with sufficiently low impedance compared to the impedance of the ground wire is connected between the ground wire and the high frequency earth leakage current sent out from the inverter is returned to the commercial power line through the capacitor. .

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram of main parts showing an embodiment of the present invention. Components that are the same as those in FIG. 1 are given the same reference numerals, and their explanations will be omitted. In FIG. 2, C _E is a capacitor having an impedance sufficiently lower than the impedance Z _E of the ground line, and is connected between the commercial power line and the ground line. The capacitance value of the capacitor C _E is determined by the value of the stray capacitance Cs, the value of the impedance Z _E of the ground wire, the value of the impedance Zl of the commercial power line, the magnitude of the high frequency voltage, the oscillation frequency, and the magnitude of the high frequency leakage current. By quantitatively determining factors such as the maximum allowable value, the optimum value can be set.

With this configuration, the inverter 5
Most of the high-frequency earth leakage current flowing from the ground line through the stray capacitance Cs to the ground line is returned to the commercial power line through the capacitor C _E whose impedance is sufficiently lower than the impedance Z _E of the ground line, and is returned to the commercial power line via the earth leakage detector 2. It can flow to the fuse breaker 1 side. Due to this return current, the earth leakage detector 2
Most of the unbalance of the current flowing through the circuit is canceled out, and the situation where the earth leakage detector is operated by high frequency earth leakage current is avoided.

This operation will be explained more specifically. In Figure 2, the 3-phase 200V S phase is grounded and used in Japan. As in the case of FIG. 1, a stray capacitance Cs exists between the winding of the chiyoke coil L and the core. Since the current issue is the operation of the earth leakage detector 2 in the high frequency range, the equivalent circuit in the high frequency range of the circuit shown in FIG. 2 is shown in FIG. 3. Incidentally, in order to simplify the explanation of the operation, the inductance of the line, the stray capacitance, etc. are omitted. In Figure 3, Z _E is the impedance of the ground wire as in Figures 1 and 2, and e _HF is the high frequency voltage (the frequency is
Hz), I _HF is the current flowing through the stray capacitance Cs, Iz is the current flowing through the impedance Z _E , and Is is the current flowing through the capacitor C _E. Conventionally, since there is no capacitor C _E , all of the current I _HF (value of Iz when C _E = 0)
flows through the impedance _ZE , and this is the leakage current that activates the earth leakage detector 2. However, in the present invention, Is of the current _IHF is returned to the commercial power line via the capacitor _CE , and the earth leakage detector 2 operates. Since it passes through
As a result, the leakage current to the earth leakage detector 2 is only the current Iz. That is, high frequency leakage current can be substantially reduced. The circumstances surrounding this will be explained below using formulas.

The following equation holds from the above equivalent circuit.

I _HF = e _HF / {(1/jωCs) +Z _E / (1+jωC _E )} …(1) IzZ _E = Is/jωC _E …(2) I _HF = Iz+Is …(3) Above (1) to (3) ) By eliminating the formula I _HF and Is and rearranging for Iz, the following formula is obtained.

Iz=e _HF / [(1+jωC _E Z _E ) {(1/jωCs)+Z _E /(1+jωC _E )}]
...(4) This is the leakage current at which the leakage detector 2 in the circuit of the present invention operates.

Here, for comparison, the high-frequency leakage current _Iz0 in the conventional circuit is calculated. This _Iz0 is calculated by C _E =
0, and is given by the following equation:

Iz ₀ = e _HF / {(1/jωCs) + Z _E } ...(5) From equations (4) and (5) above, Iz < Iz ₀ , so the high frequency leakage current Iz can be reduced by using the capacitor C _E can. According to this embodiment, the capacitor C _E is 0.1μF to 0.4μF, 550V _AC WV in the device shown in FIG.
When a commercially available capacitor was connected, it was confirmed that the magnitude of the high frequency leakage current in the leakage detector 2 could be reduced to about 1/3. Although such capacitors are relatively inexpensive and account for a very small proportion of the overall cost of the device, they are extremely effective in reducing high-frequency leakage current.

In order to reduce the high frequency leakage current without using the capacitor C _E as in the present invention, it is conceivable to electrically insulate the core of the chiyoke coil L from the casing with an insulator such as a baking plate. However, the chiyoke coil L used in such a device is quite large, with an outer diameter of 500 mm x 500 mm x 300 mm and a weight of several hundred kg, and requires considerable mechanical strength as an insulator. , resulting in high costs. Another method would be to reduce the stray capacitance between the winding of the chiyork coil L and the core, but this would inevitably require the chiyork coil L itself to become even larger.
The cost will be high. It is also conceivable to remove the ground wire that grounds the casing, but this is inappropriate from both legal and handling standpoints.

As explained above, according to the present invention, in a high-frequency induction heating device using an inverter, the high-frequency leakage current sent out from the inverter can be reduced with a simple configuration, and the practical effect is great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a main part of a conventional device, FIG. 2 is a block diagram of a main part of an embodiment of the present invention, and FIG. 3 is an equivalent circuit diagram of the circuit shown in FIG. 2 in a high frequency region. 1... No fuse breaker, 2... Leakage detector, 3... Forward converter, 4... Smoothing circuit, 5... Inverter, 6... Work coil, 7... Housing, C
_E ...Capacitor, Z _E ...Earth wire impedance, Zl...Commercial power line impedance.

Claims (1)

[Scope of utility model registration request] In a high-frequency induction heating device configured to rectify and smooth commercial power, convert it into a high-frequency voltage using an inverter, and supply the converted high-frequency voltage to a work coil, a ground wire is connected between the commercial power line and the ground wire. A high-frequency induction heating device characterized in that a capacitor having an impedance sufficiently low relative to the impedance is connected, and a high-frequency earth leakage current sent out from an inverter is returned to a commercial power line through the capacitor.