JP2797663B2 - Series resonant inverter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an induction heating device or a device utilizing series resonance, in which a high-frequency inverter is used to connect a heating coil to a high voltage (for example, several thousand volts) with respect to the ground. The present invention relates to an inverter circuit for preventing an inverter main circuit from vibrating at a high voltage and a high frequency with respect to the ground when vibrating at a high frequency.

FIG. 2 is a circuit diagram showing a conventional example of a series resonance type inverter.

In the figure, 1 is a power receiving transformer (for example, three-phase, 6600V
/ 220V, 60Hz), 2 is rectifier, 3 is smoothing capacitor, 4
Is an inverter unit for converting DC into high-frequency AC by the switching action of a power semiconductor including a transistor; 5 is a matching transformer (insulating transformer) for matching the output of the inverter unit 4 with the input of the heating coil 7; Reference numeral 6 denotes a resonance capacitor for improving the power factor of the heating coil 7.
The heating coil 7 is water-cooled to absorb Joule heat due to electric current and radiant heat from the object to be heated. Therefore, both ends of the heating coil 7 are electrically grounded by the water resistance 8, and vibrate at a high voltage and a high frequency with respect to the ground. However, in this example, since the matching transformer 5 is provided, reference numerals 1 to 4
The inverter main circuit does not vibrate at a high voltage and a high frequency with respect to the ground.

By the way, this matching transformer 5 is unnecessary when the output current of the inverter and the current required by the heating coil 7 are matched to achieve matching, and it is quite expensive.
There are many cases where it is desired to use this in an omitted form as shown in FIG.

[Problems to be solved by the invention]

However, in the circuit configuration as shown in FIG. 3, the entire main circuit other than the heating coil vibrates at high frequency and high voltage. Therefore, it is necessary to strengthen the insulation of the entire main circuit, and the amount of the insulating material used increases. Further, when the grounded object comes into contact with the heating coil 7, the main circuit may vibrate at the full voltage of the heating coil depending on the contact location, and further strong insulation is required, and the apparatus is enlarged. If the entire main circuit vibrates at a high frequency and a high voltage with respect to the ground, this may cause a large noise for a control device and the like, which may cause a malfunction. Moreover,
There is also a problem that radiated electric wave noise becomes very large.

[Means for solving the problem]

A resonance capacitor is connected to both ends of a heating coil that heats the object to be heated via the inverter, and a capacitor is connected between each phase of the input-side commercial frequency three-phase power supply and the ground.

[Action]

If the capacitor placed between each phase of the power supply on the input side and the ground is selected to have high impedance for the commercial frequency and low impedance for the high frequency on the load side, both ends of the heating coil However, even if the main circuit vibrates at a high frequency and a high voltage with respect to the ground, the main circuit except for the heating coil and the resonance capacitor is grounded at a high frequency and does not vibrate with respect to the ground. In addition, if the resonance capacitor is connected to both ends of the heating coil in a balanced manner, the inverter main circuit will apply only half of the total voltage to the ground even if the heating coil is grounded. Can be alleviated.

〔Example〕

 FIG. 1 is a circuit diagram showing an embodiment of the present invention.

This embodiment differs from that shown in FIG. 3 in that the three phases on the input side of the commercial frequency are connected to the ground via a capacitor 9 and that a resonance capacitor 8 is connected to both ends of the heating coil 7. However, the rest is the same as FIG.

Assuming that the input side capacitor 9 operates by resonating at a commercial frequency of 60 Hz and an inverter output at a frequency of 50 KHz,
The capacitance is, for example, 0.1 μF. Note that the capacitor 9
Also serves to absorb input surges. By doing so, the impedance Z of the capacitor 9 as viewed from the commercial frequency power supply is , But at 50KHz, The impedance becomes low as shown in FIG.

Therefore, even if the heating coil oscillates between the ground and the commercial frequency, the main circuit is grounded with low impedance at high frequency, and can be regarded as substantially ground potential. As a result, the portion surrounded by the dotted line in FIG. 1 requires only low-voltage insulation. Further, since the capacitors 6 are connected to both ends of the heating coil 7 so as to be balanced with each other (there is an advantage that the circuit configuration is symmetrical and the design is simplified). Only a voltage of 1/2 or less is applied to the main circuit, and the withstand voltage in such a case can be reduced. In this way, the inverter main circuit does not generate a high voltage with respect to the ground unless an abnormality occurs.Conversely, this fact is actively used to monitor the main circuit and the ground to reduce the high voltage. If it occurs, it is possible to detect that an abnormality including the grounding accident of the heating coil 7 has occurred and to detect it.

〔The invention's effect〕

According to the present invention, the insulation of the inverter main circuit does not need to be strengthened more than necessary, and it is possible to reduce the insulating material and reduce the size of the device. Since the main circuit does not vibrate between the ground, the reliability of each part in the device can be improved, and radiated radio noise can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional example of a series resonance type inverter, and FIG. 3 is a circuit diagram showing another conventional example of a series resonance type inverter. 1 ... power receiving transformer, 2 ... rectifier, 3 ... smoothing capacitor, 4 ... inverter section, 5 ... insulating transformer, 6 ...
... Resonant capacitor, 7 ... Heating coil, 8 ... Water resistance,
9 Input capacitor.

Claims (1)

(57) [Claims] 1. A resonance coil is connected to both ends of a heating coil for heating an object to be heated via an inverter, and capacitors are connected between each phase of a commercial frequency three-phase power supply on the input side of the inverter and ground. And a series resonance type inverter circuit.