JPS6342829B2 - - Google Patents

Description

Detailed Description of the Invention The present invention provides a power supply circuit for an induction heating device that inductively heats an object to be heated, which is transported at a predetermined speed by a transport device, to a predetermined temperature by the electromagnetic induction action of an induction heating coil. Regarding. In particular, this invention
In this type of induction heating device, in order to improve the load power factor of the induction heating coil, a power supply circuit is designed to reduce the rush current to the phase advance capacitor when the phase advance capacitor is inserted into the power supply circuit. This is what we are trying to provide.

Fig. 1 is a circuit diagram showing the power supply circuit of a conventional induction heating device of this kind, in which 1 is a stepless output power supply device that can output continuously in a stepless manner, 2 is an induction heating coil, and 3 is this induction heating device. A phase advance capacitor for improving the power factor of the coil 2; 4 a switch for supplying a variable amount of the phase advance capacitor 3; 5 a reactor for controlling the rush current to the phase advance capacitor 3; , 6 is a current transformer for detecting the load circuit current ``'', 7 is an instrument transformer for detecting the output voltage ``'' of the stepless output power supply device 1, 8 is the current transformer 6, and Instrument transformer 7
The current detected by ``'', and the voltage ``''
This is a reactive power detection device for detecting the reactive power "Var" of a load.

Since the power supply circuit of a conventional induction heating device is configured as described above, the power supply voltage is normally kept constant at a predetermined value, and the induction heating coil 2 is excited by this power supply voltage to reduce the load. Although the object to be heated is heated by induction, the impedance of the load changes depending on the temperature, so the load side (induction heating coil side) as seen from load end a-b.
The load power changes as the power factor changes. Then, this fluctuation is detected by the current transformer 6, and the power supply voltage is detected by the instrument transformer 7, and the reactive power detection device 8 is operated, and the output signal of this is used to detect the switch 4. is used to turn on the phase advance capacitor 3 for improving the load power factor. Note that the peak value of the rush current flowing through the switch 4 at this time is expressed by the following equation.

Here, the above IP is the capacitor inrush current peak value [Apeak], is the power supply voltage [ ], and L is the line reactance from the terminal of the switch 4 to the phase advance capacitor 3, and the reactance of the inrush current suppression reactor 5. The total reactance [H] and C are the capacitance [F] of one variable stage of the phase advance capacitor 3.

Further, the steady-state current c after the phase advance capacitor 3 is turned on is expressed by the following equation.

IC=wCV ......[1] Here, IC is the unit of [Arms], w is the angular frequency [rad], and has a relationship with the power supply frequency f [HZ] as w=2πf.

However, when the phase advance capacitor 3 is turned on using the conventional constant voltage control method, the rush current IP to the phase advance capacitor 3 described above is several times or more the steady current IC of the phase advance capacitor 3. Therefore, when selecting the switch 4, one with a large switching capacity must be used, and the inrush current suppressing reactor 5 must also have a large current capacity. It has the disadvantage of being small in size and expensive. In addition, 1 of the switch 4
If the amount of input capacitors per unit is reduced, the required number of switches 4 must be increased, resulting in a significant increase in the amount of control circuit equipment and wiring, and the need for a larger installation space, among other disadvantages. have.

This invention has been made with attention to this point, and is made by sending the output signal of the reactive power detection device 8 to the stepless output power supply device 1, and lowering the output voltage to an arbitrary set voltage using a voltage controller. Therefore, it is an object of the present invention to provide a power supply circuit for an induction heating device that suppresses inrush current to the phase advancing capacitor 3.

That is, although FIG. 2 shows one embodiment of the present invention, the entire circuit of the power supply circuit of the induction heating apparatus is the same as the conventional one (FIG. 1) described above, and therefore will not be described here.

Reference numeral 9 denotes an output contact of the reactive power detection device 8, and reference numeral 10 denotes a plurality of voltage controllers built into the stepless output power supply device 1 and connected in series, each of which uses, for example, a variable resistor.

The power supply circuit of the induction heating device of the present invention is configured as described above, and the operation of turning on the phase advance capacitor 3 to improve the power factor is the same as that of the conventional one described above. By changing the input value by changing the resistance value between c and d of the voltage controller 10 of the output voltage control circuit of the stepless output power supply 1 using the output contact 9 of the reactive power detection device 8 immediately before inputting the capacitor for Since the output voltage can be automatically lowered to the set value, for example, if the power supply voltage is lowered to half of the original voltage, the inrush current IP to the phase advance capacitor 3 will also be reduced by half according to the above formula [1]. Therefore, not only the current carrying capacity of the switch 4 can be reduced, but also the capacity of the inrush current suppressing reactor can necessarily be reduced.

Therefore, according to the present invention, the electromagnetic mechanical force due to inrush current and the amount of heat generated by the reactor can be significantly reduced, so that the switch 4, the inrush current suppression reactor 5, and the phase advance capacitor 3 are Not only can the lifespan be significantly extended, but also the cost and size of the power supply device can be reduced. In addition, according to the present invention, since the inrush current becomes a nearly steady current within several Hz, the voltage is lowered from just before the switch 4 is closed to immediately returns to the original circuit voltage after the switch is closed. It also has the effect that it takes only a short time and has almost no negative effect on the load operating rate.

In the embodiment described above, the voltage is lowered by the stepless output power supply device 1, but as another means, for example, a stepwise output power supply device having a tap changer at the time of load may be used. It goes without saying that similar effects can be obtained. Furthermore, it goes without saying that the power supply device of the present invention can be used not only for induction heating devices but also for supplying power factor improving capacitors for other inductive loads.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a power supply circuit of a conventional induction heating device, and FIG. 2 is a circuit diagram of a main part showing an embodiment of the present invention. In the drawing, 1 is a stepless output power supply device, 2 is an induction heating coil, 3 is a phase advancing capacitor for power factor improvement,
4 is a switch, 5 is a reactor for suppressing rush current, 6
7 is a current transformer, 7 is an instrument transformer, 8 is a reactive power detection device, 9 is an output contact of the reactive power detection device, and 10 is a voltage controller. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A plurality of series circuits in which a capacitor for power factor improvement and a switch are connected in series are connected in parallel with an induction heating coil, and a voltage controller is installed in a power supply device that provides an output voltage to the induction heating coil. What is claimed is: 1. A power supply circuit for an induction heating device, characterized in that the resistance value of the voltage controller is changed by an output contact of a built-in reactive power detection device to lower the output voltage of the power supply device. 2. The power supply circuit for an induction heating apparatus according to claim 1, wherein a stepless output power supply device that outputs a stepless continuous output voltage is used as the power supply device. 3. The power supply circuit for an induction heating apparatus according to claim 1, wherein a variable resistor is used as the voltage controller.