RU353U1 - Induction heating electric stove - Google Patents

Abstract

An induction heating electric stove containing a power supply connected in series, a high-frequency thyristor converter and an inductor, as well as a control unit connected to the control input of a high-frequency thyristor converter, characterized in that a diode, an additional control unit and a power source are introduced, the voltage of which is an order of magnitude lower than the main voltage a power source connected through according to it an included diode to the same outputs of the main power source, and the outputs are additional itelnogo control unit connected to the control inputs of the main power source.

Description

Induction heating electric stove

The device relates to a conversion technique and can be used in induction electric stoves for domestic and industrial use.

Known induction furnace according to US patent and 4556770, class MKI H05B 6/06, publ. 12/03/85, t. 1061 "1. The furnace has an AC power source, a rectifier and a smoothing filter, supplying a high-frequency transistor converter, the load of which is an induction coil (inductor) with a cookware made of the corresponding material located on it. The converter generates in the inductor a periodic sequence of packs of high-frequency current pulses with an adjustable duty cycle of the packs, while the electromagnetic field arising in the inductor induces eddy currents in the bottom of the cookware that heat it.

The disadvantages of this device include significant power loss in the inductor of the low-pass power filter.

Electric cookers CS-140 and CS-150 from Mitsubishi Electric, IC-60 from Sagno, KY-1600T from Panasonic (see the catalogs of the listed companies), the high-frequency converters of which are also built on transistors, are also known. These devices are reliable and provide smooth control of the heating power.

However, these devices do not allow operation from an alternating current network of 220 V, since high-frequency data converters of electric stoves are supplied with directly rectified voltage of the supply network, and the voltage at the transistor of the converter reaches its maximum permissible values even at a supply voltage of 127 V.

Closest to the technical nature of the proposed device is a control circuit and current limits for the induction line according to US patent No. 4564733, class MKI H05B 6/06, publ. 01/14/86, t. 1062 No. 2, which is taken as the closest analogue. The device contains two electrically excited heating sections, connected to the mains supply through a bridge rectifier and a low-pass filter, consisting of inductors with

class MKI N05V 6/06

capacitors, the oscillations in which are excited with the help of thyristors of a high-frequency converter. The device also contains a control circuit, which is constructed in such a way that eliminates the simultaneous operation of the thyristor control circuits, and, therefore, limits the surges of the current consumed from the mains.

Among the shortcomings of the closest analogue, significant power losses in the inductor of the low-frequency power filter and the damping circuit resistor of the thyristor of the high-frequency converter can be noted, which reduces the overall efficiency (efficiency) of the device and increases the unwanted heat generation inside the device, the largest at the rated (maximum) heating power.

When creating the device, it was decided to increase the efficiency of the device and, accordingly, reduce heat dissipation on the elements of the device by eliminating power losses in the low-pass filter of the power supply circuit and reducing sweat, power loss in the resistor of the damping circuit of the thyristor of the high-frequency converter. The problem is solved by changing the power circuit of the high-frequency thyristor converter, which in the proposed device does not contain a low-frequency power filter. This eliminates power losses in the filter inductor (ohmic losses in the winding, core losses). However, the supply voltage of the thyristor converter will not be a constant rectified voltage, but a sequence of half-sine waves of the same polarity, which can lead to the overturning of the converter and short circuit of the device rectifier.

Indeed, when the instantaneous voltage at the input decreases

less than a certain level, the thyristor current of the converter decreases and approaches the holding current of the thyristor. Accordingly, the resistance of the anode - cathode in the open state increases and the condition for overcharging the discharge capacity of the converter is violated: RBH + Rnp 2,

- 2 EG

Mosffve

where: RBH is the resistance of the inductor, taking into account the insertion resistance of the cookware;

Rnp is the direct resistance of the anode - cathode of the open thyristor;

Lp, Cp - respectively, the inductance and capacity of the discharge circuit of the Converter (see figure 2, elements 3 and 17).

This phenomenon leads to a decrease in the time provided by the circuit to the thyristor for restoring its electrical strength, causing it to re-unlock and, as a consequence, to overturn the converter. To prevent overturning, an additional low-power power source with a voltage an order of magnitude lower than the rectifier amplitude voltage is introduced into the device circuit. This source is connected through an additional diode to the power input of the converter. This provides alternating power to the converter from two sources: from the rectifier, when its output voltage exceeds the output voltage of the additional source, and from the additional source at other time intervals. The value of the output voltage of the additional source is selected from the condition of providing the necessary time for turning off the thyristor.

The regulation of the heating power with this form of supply voltage converter should be carried out by changing the duty cycle of the half-periods of the output voltage of the rectifier at a constant current frequency in the inductor.

When regulating the heating power from zero to maximum, the duty cycle of the half-periods of the rectified network voltage at the output of the rectifier varies from infinity to unity (in the case of a half-wave rectifier). The applied converter power system allows you to reduce the power released in the thyristor damping circuit resistor.

- Z Compare the power loss in the resistor of the damping circuit of the device - the closest analogue and the proposed device in maximum power mode, which is assumed to be the same for both devices.

Let the voltage utilization of the thyristors be the same in both devices, i.e. the maximum values of direct voltages at the thyristors are Um. We take equal the parameters R and C of the damping circuits, as well as the frequency at which the heating is carried out. We also take into account that the frequency of the supply voltage is much lower than the output frequency of the converter.

Under these conditions, the power dissipated in the damping circuit resistor for the device - the closest analogue will be equal to:

where Ra is the power released in the damping resistor

chains for the device - an analogue;

Ue is the effective value of the voltage at the input of the converter equal to Um (see Fig. 3a); fu is the output frequency of the converter.

The power dissipated in the damper of the proposed device is determined by:

Pn 0. 0.,

i.e., the power loss in the damping circuit resistor of the proposed device is two times less than the corresponding losses of the closest analogue, because U "for the proposed device is equal to:

(see Fig. 36).

Thanks to the adopted technical solutions, the proposed device has a higher efficiency due to the reduction of heat loss. At the same time, the device provides a lighter thermal regime of electrical elements, which increases the reliability of its operation.

- L Re. With Ue fu,

Ue

In addition, the use of a supply voltage converter in the form of a series of half-sinusoids of the same polarity allows this voltage to be switched (when regulating power, when turning on and off the plate) at its minimum instantaneous values, which reduces overvoltage on the thyristor of the converter, ensuring its reliable operation.

In FIG. 1 is a structural diagram of an induction heating electric stove; FIG. 2 gives a specific embodiment of the structural design of the plate. In FIG. 3 shows graphs explaining the operation of the device, wherein: in FIG. For and 36 - shows the voltage at the input of the thyristor

the converter, respectively, the device of the closest analogue and the proposed device; in FIG. Sv - shows the pulses at the output of the multivibrator

with adjustable duty cycle

control unit 6; in FIG. Zg - shows the pulses at the output of the shaper

pulses of the additional control unit 6; in FIG. Zd - shows control pulses at the output of the block

management 6; in FIG. Ze - shows the voltage at the output of the additional

power source 7; in FIG. Zzh - shows the voltage at the source output

power supply 1; in FIG. Зз - the resulting supply voltage is shown,

lead to the Converter 2; in FIG. Zi - shows the control pulses at the output of the block

management 4; in FIG. Зк - current pulses in the inductor 3 are shown.

In figures 4 and 5, specific embodiments of control units 4 and 6, respectively, are given.

The proposed device contains a serially connected power source 1, a high-frequency thyristor converter 2 and an inductor 3, as well as a main control unit 4, the output of which is connected to a control input (control thyristor electrode) of a high-frequency thyristor converter 2. The device also contains an additional control unit 6, additional - / power supply point 7, one of the outputs of which, according to it, the switched-on diode 5 is connected to the same output of the main power source 1. The second outputs and power sources are interconnected. The inputs of both power supplies are connected to the mains.

The additional power source 7 contains a step-down transformer 19, diodes 20 and 21, a filtering capacitor 22. In this case, the outputs of the additional control unit 6 are connected to the control inputs of the main power source 1, which is made according to the bridge circuit on thyristors 8, 9, and diodes 10, 11 (see figure 2). The control inputs of the power source 1 are the control electrodes of the thyristors 8, 9. The high-frequency thyristor converter 2 contains a series-connected high-frequency inductor 12 and a thyristor 13 with a reverse diode 14. Parallel to the thyristor 13, a damping circuit is connected from the resistor 15 and the capacitor 16. Between one of the outputs of the converter 2 and a capacitor 17 is turned on and a thyristor 13. An inductor 3 is connected to the outputs of the converter 2 with the cookware 18 installed on it.

The control unit 4 contains (see figure 4) a self-oscillating multivibrator and amplifier. The multivibrator is made according to the standard scheme on three AND-NOT elements. The duration of the short pulse generated by the multivibrator to control the thyristor of the converter is determined by the charge time of the capacitor C1 through the internal resistance of the diode VD1. The repetition period of the control pulses is determined by the discharge time of the capacitor C1 through the resistor R2. The resistor R1 is used to limit the input current of the element DD1. Element DD1.4 is used as a buffer. The output of the multivibrator is connected to the input of the amplifier. The amplifier is made on transistor VT1 and has a transformer output. Resistor R3 limits the base current of transistor VT1. The output of the control unit is the secondary winding of the amplifier transformer T1, the control signal from which is fed to the control electrode of the thyristor 13 (see figure 2) of the high-frequency converter. The shape of the control pulses is shown in FIG. Zee.

A specific embodiment of the control unit 6 is shown in FIG. 5. The control unit 6 contains a pulse shaper, a multivibrator with adjustable duty cycle, circuit (j

- / mu matches and amplifier. The pulse shaper is made on a Schmitt trigger DD1 and an inverter DD2.1. It forms short pulses at the moments when the voltage of the supply network passes through zero (see Fig. Zg). The resistor R1 determines the input impedance of the pulse shaper, the capacitor C1 - separation. The resistive divider R3, R4 determines the bias voltage for the DD1 chip. The duration of the pulses generated by the shaper is determined by the differentiating chain C2, R6, R7 and VD3. For a half-wave rectifier, the pulse repetition rate at the output of the shaper is equal to twice the frequency of the supply network. The multivibrator with adjustable duty cycle is made according to the standard scheme on three AND-NOT elements (DD2.2, DD2.3 and DD2.4). The period of pulses generated by the multivibrator is determined by the parameters of capacitor C6 and resistor R2, the duty cycle of the pulses (see Fig. 3S) is regulated by a variable resistor R2. The outputs of the pulse shaper and multivibrator with adjustable duty cycle are connected to the inputs of the matching circuit. The match scheme is made on the element AND NOT DD3.1. At the output of the coincidence circuit, packs of short unipolar pulses are formed (see Fig. Zd), the beginning of which coincides with the moment the voltage of the supply network passes through zero, and the pulse repetition rate is equal to twice the frequency of the supply network.

Duty rate and pulse train repetition period are determined by a multivibrator. The amplifier is made on transistor VT1 and has a transformer output. Resistor R8 limits the base current of transistor VT1. The outputs of the additional control unit are the secondary windings of the amplifier transformer T1, the control signals from which (see Fig. Zd) are fed to the control electrodes of thyristors 8 and 9 (see Fig. 2) of the bridge rectifier.

The proposed device operates as follows.

When the device is connected to AC power, a voltage appears at the output of the additional power source 7, the heating power Regulator (potentiometer R2, figure 5) of the additional control unit 6 is in the minimum position, so the control pulses from the outputs of the additional control unit 6 to the thyristors 8 and 9 the power source 1 is not supplied, and its output voltage is zero, and the high-frequency converter 2 is powered by an additional source 7. The capacitor 17 of the converter 2 is charged from an additional the source through the diode 5 and the inductor 12. At the same time, the supply voltage is supplied to the control units 4 and b, and the control pulses 4 begin to flow from the output of the control unit 4 (see FIG. the thyristor 13 electrode is unlocked and the capacitor 17 starts to recharge through the inductor 3 and the open thyristor 13. The recharge of the capacitor 17 is oscillatory in nature, therefore, the positive half-wave of the recharge current passes through the thyri side 13, and negative through diode 14, creating a reverse voltage for thyristor 13, so that it restores its blocking properties, and the current in the valve group (thyristor 13 and diode 14) ceases. Two half-waves of current passing through the inductor 3 induce eddy currents in the bottom of the cookware, which heat it. Then, the capacitor 17 is again charged from the additional power source 7, and when the next control pulse comes from the control unit 4, the processes in the device are repeated. The shape of the current in the inductor 3 is shown in FIG.

The regulation of the heating power is carried out by changing the average voltage value supplying the converter 2. This change is made by adjusting the duty cycle of the half-periods of the rectified network voltage applied to the converter. When the heating power controller (variable resistor R2, Fig. 5) is installed in the required position, control pulses are supplied from the output of the additional control unit 6 to the control electrodes of the thyristors 8 and 9 of the power source 1 (Fig. Zd). The beginning of each control pulse corresponds to the moment the line voltage passes through zero, and the period between pulses in the packet is equal to the half-period of the line voltage. The number of control pulses in the packet is determined by the pulse duration of the multivibrator with adjustable duty cycle (Fig. Sv), which is included in the additional control unit 6, the period of the sequence of pulses is also determined by this multivibrator. Each of the 8 and 9 control pulses arriving at the control electrodes of the thyristors opens in turn one of these thyristors, the anode voltage (gzvs- e

which is positive, while a rectified voltage appears at the output of power supply 1, the number of half-sinusoids of which (Fig. 3g) corresponds to the number of control pulses received, and the amplitude voltage exceeds the voltage of additional power supply 7 (Fig. 3), so the diode 5 is locked to the time during which the voltage of the power source 1 exceeds the output voltage of the additional power source 7 by the reverse voltage of the power source 1 and the power of the converter 2 is provided from power source 1. After passing the last half-sine wave of the rectified voltage to the input of the converter 2, the diode 5 is unlocked and the converter is powered from the source 7 to the start of the first pulse of the next packet of control pulses. The resulting voltage supplied to the converter has the form shown in FIG. The operation of the thyristor converter in this case does not differ from that described above.

SZo & fV

LF Formula

Claims (1)

An induction heating electric stove containing a power supply connected in series, a high-frequency thyristor converter and an inductor, as well as a control unit connected to the control input of a high-frequency thyristor converter, characterized in that a diode, an additional control unit and a power source are introduced, the voltage of which is an order of magnitude lower than the main voltage a power source connected through according to it an included diode to the same outputs of the main power source, and the outputs are additional itelnogo control unit connected to the control inputs of the main power source.