JPS6344214A - Ac power control method by thyristor for resistance electric furnace - Google Patents

Abstract

PURPOSE:To increase response speed, and to improve control accuracy, by obtaining a pulse output by a voltage-frequency converter corresponding to a supplied analog control signal, and turning on a thyristor for a pair of positive and negative cycle periods. CONSTITUTION:The power source circuit of a resistance electric furnace 10 consists of a matching transformer 20, an inversely parallel connected thyristor 30, a control circuit 40, a temperature adjuster 60, a temperature setting apparatus 70, and an effective value conversion circuit 80. And the energizing control of the thyristor 30 is performed by turning on the thyristor 30 for a pair of positive and negative cycle periods. At this time, the analog control signal A by which electric energy of 0-100% supplied to the electric furnace 10 can be obtained, is sent to the control circuit 40, and corresponding to the above, the pulse output of 0-50Hz (or 0-60Hz) can be obtained, and the pulse is distributed in an ON/OFF state having equal intervals. And control is performed so that a state can be moved to the ON/OFF state having a new equal interval immediately from an arbitrary time.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an AC power control method using Cyrisk, which is applied when operating a resistance electric furnace equipped with a transformer for supplying electric power according to the specifications of the resistance heating element in the furnace. It is about the method.

Conventional power control using thyristors employs (1) a phase control method that changes the firing phase angle, and (2) a cycle control method that switches at the zero-crossing point of the power supply, with a control cycle of around 3 seconds. .

The former phase control method has drawbacks such as waveform distortion that adversely affects the AC power supply system and deterioration of the equivalent power factor.

In addition, the latter cycle control method has the following drawbacks: (1) In order to improve output accuracy, it is necessary to increase the control cycle; The fluctuation of the follow-up temperature becomes large with respect to the set temperature.

In practice, a relatively long control cycle of around 3 seconds is required.

(2) When the control period is around 3 seconds, the control amount cannot be changed within the control period, so the response speed to the analog control signal becomes slow.

(3) Since vibrations in the length of the control period are removed when detecting the effective values of voltage, current, and power, detection accuracy and detection speed cannot be improved.

(4) In order to improve the accuracy of control B, it is possible to determine the on/off delimiter in half-wave units, or to control the start timing of the control 275 cycle while ignoring the polarity of the power supply, resulting in an accurate pair of positive and negative sine waves. Since it is not obtained, if there is a transformer in the load,
The last excitation direction of the transformer in the previous control cycle is the same as the current excitation direction, and the magnetic flux of the iron core is saturated, which tends to cause an accident in which overcurrent occurs.

It is known that there is.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the phase control method and cycle control method related to the prior art, and to provide economical control of AC power with the accuracy necessary for the operation of a resistance electric furnace. The goal is to provide a method to do so.

This object is achieved by an AC power control method having the structure described in the claims.

In this method, no specific control period is provided as in conventional cycle control methods. Therefore, corresponding to a given analog control signal 0-100%, the voltage-to-frequency converter converts 0-50Hz (or 0-60Hz
), and by turning on a pair of positive and negative thyristors with a fixed initial direction immediately after the output pulse for one cycle period, the on/off states are distributed at approximately equal intervals, and the analog control signal changes. Accordingly, AC power control is achieved that immediately transitions to a new evenly spaced on-off state from any point in time.

Note that if you carry out this method as is, the output frequency will be 1
Since lighting may flicker around 0 Hz, the output in that frequency range is removed due to the characteristics of the voltage-frequency converter.

The invention will now be described in detail with reference to the accompanying drawings, which illustrate examples.

FIG. 1 shows output waveforms for analog control signals in the AC power control method according to the present invention.

The power frequency is 50 hertz.

FIG. 2 is an output waveform for an analog control signal in a conventional cycle control method. The horizontal axis is the time axis, and the vertical axis shows the output waveform corresponding to the magnitude of the analog control signal A, and shows the case where the power supply frequency is 50 Hz and the control period is 1 second.

In this case, the average power within the control period can be controlled, but there are drawbacks as described above.

In the control method according to the present invention shown in FIG. 1, the output waveform corresponding to the magnitude of the analog control signal A on the vertical axis is shown, and the state of frifka frequency band removal is not shown. Compared to FIG. 2, the distribution of the output waveform on the time axis is much more averaged, so the temperature inside the electric furnace is stabilized.

In addition to the method according to the present invention, there is a method for obtaining the output waveform as shown in FIG. 1, for example, by storing the output pattern shown in FIG. 1 in a memory, and then depending on the value of the analog control signal. This is also possible by switching the sirisk by selecting the corresponding output pattern. However, in this case, a large capacity memory is required to improve output accuracy, and accurate control is theoretically difficult when the analog control signal changes and transitions to a new pattern. That is, it is difficult to grasp the current situation at the time of pattern transition, and a judgment function must be provided for this purpose, and it is undeniable that the cost is higher than that of the voltage-frequency conversion described above.

FIG. 3 shows an example of the configuration of a control circuit according to the present invention and a power supply circuit for operating a resistance electric furnace. It is equipped with a thermocouple 12 that detects the temperature of.

The load matching transformer 20 is a transformer for supplying power to the resistance electric furnace 10, generates an appropriate voltage determined from the required power and resistance value of the resistance heating element 11, and has a required current capacity. Depending on the specifications of the resistance heating element, it is also possible to directly supply the power supply voltage controlled by the anti-parallel connected thyristor switches 30 to the resistance heating element without using a load matching transformer. However, in high-temperature electric furnaces, the electrical insulation strength is extremely reduced, so from a safety standpoint as well, it is desirable to use a load matching transformer as an isolation transformer.

The anti-parallel connected thyristor 30 is a semiconductor switch that adjusts the power supplied to the electric furnace 10 in order to control the temperature inside the furnace to a desired value, and loads the voltage and current switched by receiving a commercial AC power supply. Supplied to matching transformer 20.

The switching mode is determined by the control circuit 40 as described later.
By the control signal generated by.

The temperature controller 60 uses the signal SV from the temperature setting device 70 as a set value and performs PID calculation so that the furnace temperature θ of the resistance electric furnace follows the set value SV.

The resulting control amount 4H is outputted to the control circuit 40 in the form of an analog control signal A for obtaining the amount of power supplied to the electric furnace from 0 to 100%.

The control circuit 40 outputs a thyristor drive signal C (G+, Gz) to the thyristor switches 30 connected in antiparallel in order to supply power corresponding to the analog control signal A from the temperature regulator 60 to the load. At this time, the voltage supplied to the load is a sine wave with a positive and negative pair or zero voltage, and the sine wave part and zero voltage part are as finely divided as possible and evenly distributed on the time axis. .

Also, according to the connection shown in Fig. 3, the anti-parallel connected thyristor 3
0 and the load matching transformer 20, an effective value conversion circuit 80 is connected to the load matching transformer 20.
is connected. As will be described later, since these devices are operated at a frequency proportional to the desired output that is lower than the power supply frequency, the pointer of the indicating instruments often vibrates, making it difficult to obtain stable readings. Therefore, it is necessary to convert the voltage and current supplied to the transformer 20 into smoothed effective values and handle them. This effective value conversion circuit 80 performs this function.

FIG. 4 shows a schematic block diagram of the control circuit 40, and only the main functions are shown compared to the detailed block diagram of FIG. 5, which will be described later. In the figure, the output calculation circuit 41 is
The voltage-frequency conversion circuit 42 inputs the analog control signal A and calculates the output after correcting the non-linearity, boundary conditions, etc. of the control system. A periodic pulse of 60 Hz) is generated. The flip-flop 43 as a storage circuit is a circuit that stores the output B of the voltage-frequency conversion circuit 42 up to the zero-crossing point of the power supply in the positive direction that occurs immediately after. Mono staple multivibrator M/M 4
4 is one cycle of both positive and negative cyrisk drive signals C (
The signal is held for the time required to generate cl+cZ). An AND circuit 45 located between the flip-flop 43 and the monostaple multivibrator M/M 44 receives a power synchronization pulse generation circuit 46 at one input terminal to obtain synchronization of the zero-cross point.

FIG. 5 is a detailed block diagram of the control circuit 40. Fourth
The same parts as in the figures are given the same reference numerals. The output arithmetic circuit 41 multiplies the analog control signal A, which is an input signal, by the upper limit power setting value [α%], and transmits the output A1 to the fringe frequency band removal circuit 49. The function of this circuit is to keep the power supplied to the electric furnace within a limited range (0.0
~α%) is used when it is desired to control.

At this time, the analog control signal O~100% corresponds to the output voltage O~α%. Flicker frequency band removal circuit 49
is a circuit that converts input A, to output A2, and for most input values A1, A I = A z, but input A
, is between the removal bandwidth (γ) centered on the removal target frequency set value (β), it is fixed to Az=β ±T. Also, when A1 is between the rejection bandwidths centered on (50 Hz - β), Ax = 50 H
It is fixed at z−β±T. In addition, as shown in FIG. 5, a delay action relay 50 for starting is arranged, and by delaying the quick signal transmission of each part formed by electronic components by a predetermined time at the time of starting, the safe operation of the entire device is ensured. This is what I am trying to do.

FIG. 6 shows the flicker frequency band removal circuit 4 in FIG.
This is an enlarged version of 9. This function is necessary when the power consumption of the electric furnace is too large to be ignored relative to the power receiving capacity of general power consumers. In other words, when the voltage fluctuation of the power receiving transformer appears as the voltage fluctuation of the electricity consumer's in-house lighting power supply, 10)! This is intended to prevent voltage fluctuations around z from particularly sensitively affecting human vision.

Therefore, β is usually chosen to have a value of A2 (therefore A,) corresponding to 10 Hz, and γ is chosen to be around 5 Hz. However, in a case where the voltage fluctuation of the lighting power source can be ignored, or when a plurality of electric furnaces are operated at the same time even if there is a voltage fluctuation, γ=0 can be set practically. The capacity of a resistance electric furnace alone is usually 500kVA.
Since it does not cause voltage fluctuations in the electric power company's distribution system, if the power receiving transformer for lighting power supplies is separated from the power receiving transformer for electric furnaces, r = o. .

The voltage-frequency conversion circuit 42 converts 0 to 10 of the input signal A2.
A narrow pulse B1 of 0 to 50H2 or 0 to 60Hz is generated corresponding to 0%. The time from outputting one pulse to outputting the next pulse is determined by the integral value of the input signal, so if the input signal is fluctuating,
It can be considered that the output frequency is determined by the average value of the input signal during that time.

In the case of an average electric furnace, the heat capacity is large, and there is a time delay in the thermocouple for temperature detection and the amplifier circuit for thermocouple output, so within about 0.1 seconds there is almost no effect on the furnace temperature θ. do not have. Therefore, the input A2 of the voltage-frequency conversion circuit 42 also hardly changes between two adjacent output pulses. As a result, it can be understood that the function of the flicker frequency removal circuit 49 does not directly control the output frequency, but as a result, it prohibits the generation of output in the desired frequency band. Details will be described later with reference to FIGS. 7 and 8.

Note that the voltage-frequency conversion circuit 42 and the flip-flop 4
A contact R)F is connected between 0 and 3, which is the a contact of the starting delay action relay 50 shown in FIG. 5, and is closed in normal operating conditions.

The flip-flop 43 as a memory circuit is composed of 7 D-type flip-flops, and is turned on at the rising edge of the output signal B of the voltage-frequency conversion circuit 42 and turned off at the rising edge of the output signal C of the monostaple multivibrator 44. .

The AND circuit 45 for synchronization with the power supply waveform outputs the logical product B3 of the output B of the flip-flop 43 and the positive half-wave output signal Sl of the output of the power synchronization pulse generation circuit 46. Once the output B of the AND circuit 45 is turned on, the monostable multivibrator 44 generates an output C that continues the ON state. This time of 15 m5 may be any other value as long as it is less than one cycle time of 60 Hz, about 16.7+ss, and greater than about 10 m5, a half cycle time of 50 Hz.

The power synchronization pulse generation circuit 46 generates a signal S2 that is turned on during a half-wave period in the negative direction of the output SI, which is turned on during a half-wave period in the positive direction of the AC power source.

The lowest frequency determination circuit 51 in FIG. 5 compares the output At of the flicker frequency band removal circuit 49 with the value of the lowest frequency setting value δ, and generates a signal B0 that becomes true when A>δ. The purpose of this circuit is that since the voltage-frequency conversion circuit 42 has an integration function, even when the input value is very close to zero, it will eventually reach a level equivalent to the output of one cycle with the passage of time. , 0OIHz, which is actually meaningless, will be generated at an extremely low frequency, so such output is prohibited.

The AND circuit 47a receives a signal S1 indicating a positive half wave of the power supply, an output C of the monostable multivibrator 44,
It is a circuit that calculates and outputs the logical product CI of three people with the output B0 of the lowest frequency determination circuit 51. This output C1 becomes a signal for turning on the thyristor TH connected in the positive direction.

The AND circuit 47b is similar to the AND circuit 47a except that it corresponds to the signal S2 indicating the negative half-wave of the power supply. In this case, the output C2 is the thyristor TH connected in the negative direction.
, is the signal to turn on.

When the output C1 of the AND circuit 47a reaches the value of 1 odor (ON), the monostable multivibrator 48a detects ?
+++s Generates an output C1 that continues the on state. This output C1 passes through the contact Ry of the delay action relay 50, which is normally on, and becomes an input to the signal isolator 52a. This signal insulator 52a is for insulating a control circuit with a low breakdown voltage from a main circuit including a thyristor, and can be constructed from a photocoupler or the like. That is, it generates an output G having the same waveform as the human power C3, and drives the thyristor TH connected in the positive direction.

Further, the monostable multivibrator 48b and the signal isolator 52b correspond to negative half waves, and have the same functions as the monostable multivibrator 48a and the signal isolator 52a, respectively. Note that the output pulse width of the monostable multivibrators 48a and 48b, 7S, is larger than the 50Hz 1/4 cycle time, and the 60Hz 1/2 cycle time 8
, 3 ms. this is,
Ensure time for the turn-on current of Cyrisk to be established with respect to the delay (1/4 cycle) of the excitation current of the transformer,
This is to prevent the thyristor drive from being delayed and shifting to the next half wave. Even if the Cyrisk drive pulse is delayed and shifts to the next half wave, if the Cyrisk is connected normally, it will not fire because the phase is reversed, but the third
If the polarity of the power supply circuit shown in the figure is incorrectly connected when the power is turned on after the construction is completed, the transformer will be half-wave excited, resulting in an overcurrent flow, which is dangerous.

The delay-action relay 50 for starting is a delay-action relay having mechanical contacts as described above, and is excited by the control circuit power supply and turned on after a certain period of time. This relay 50
The contact points are: (1) between the voltage-frequency conversion circuit 42 and the flip-flop circuit 43; (2) between the monostable multivibrator 48a and the signal isolator 52a; (2) between the monostable multivibrator 48b and the signal isolator 52b.
It is used in three locations between. By using such a contact point, it is possible to prevent a situation in which a thyristor drive signal is output during the establishment of the control power supply and the load matching transformer 20 is half-wave excited and a large current flows.

FIG. 7 is a block diagram showing the principle of the voltage-frequency conversion circuit 42, and FIG. 8 is a time chart showing the relationship between the signals of the seventh response section. The input signal At is input to the integrating circuit 4
2a and becomes the signal AH. The signal Az1 is
It is compared with the frequency setting signal ε in the comparator 42b, and ε
When the signal A becomes larger, the signal A becomes true (on).

When the signal Azz becomes true (on), the monostable multivibrator 42c generates a pulse B with a pulse width of 20 us.
Generates 1. This pulse B causes the integration circuit 42a to
The integral value of is returned to 0. Therefore, signals Azl and A are reset to false (off) while pulse B is positive. With the above features, the power frequency is 50Hz and 60Hz.
By changing ε according to Hz, signal A1 becomes 10
It is possible to output an output of 50 Hz or 60 Hz at 0%. It can also be understood that the pulse generation point of output B1 is completely unrelated to the zero-crossing point of the power supply, and that the interval between the pulses of two adjacent outputs B is exactly inversely proportional to the integral value of A2 between them. Will.

FIG. 9 is a time chart of signals at various parts within the circuit device. Analog control signal A is displayed in a condensed manner due to space constraints. In reality, things change over a longer period of time than shown. Here, each symbol corresponds as follows.

El: Power supply voltage, S, St: Positive and negative outputs of the power synchronization pulse generation circuit 46, A: Analog control signal, Bo:
Output of lowest frequency determination circuit 51, B1: Output of voltage-frequency conversion circuit 42, B2: Flip-flop circuit 4
3 output, B3: output of AND circuit 45, C: output of monostable multivibrator 44, C+: AN
Output of D circuit 47a, C2: Output of A N D circuit 47b, C3 (GI): Output of monostable multivibrator 48a, C4 (Gg): Output of monostable multivibrator 48b, E2: Input waveform of transformer By applying the embodiment circuit device shown here, the control method according to the present invention can be advantageously implemented.

[Brief explanation of drawings]

FIG. 1 is a control output waveform of the AC power control method according to the present invention. FIG. 2 shows the control output waveform of the conventional AC power control method. FIG. 3 is a block diagram showing an example of the configuration of a control circuit for implementing the control method according to the present invention and a power supply circuit for operating the resistance electric furnace. FIG. 4 is a schematic block diagram of the control circuit, and FIG. 5 is a detailed block diagram of the control circuit. FIG. 6 is a block diagram of the frifka frequency removal circuit. FIG. 7 is a block diagram of the voltage-frequency conversion circuit. FIG. 8 is a time chart showing signal waveforms of each part, and FIG. 9 is a time chart showing signal waveforms of each part. The correspondence of the main reference symbols in the figure is as follows: 10: Resistance electric furnace 20: Load matching transformer 30: Anti-parallel connection sirisk switch 40: Control circuit 41: Output calculation circuit 42: Voltage-frequency conversion circuit 43 : Fritz flop (memory) circuit 44; Monostable multivibrator 45: AND circuit 46: Power synchronization pulse generation circuit 47a, 47b: AND circuit asa, 43b: Monostable multivibrator 49: Fritz frequency band removal circuit 50: Delayed action relay 51: Minimum frequency determination circuit 52a, 52b: Signal isolator 60: Temperature regulator 70: Temperature setter 80: Effective value conversion circuit

Claims (1)

[Claims] (1) In an AC power control method that controls the power supplied to a resistance electric furnace using antiparallel thyristors, an analog control signal is input to a voltage-frequency conversion circuit, and the power waveform is changed in response to one output pulse. A thyristor drive signal is generated for one cycle from a zero-crossing point in a certain direction (positive direction or negative direction) to the next zero-crossing point in the same direction, and this thyristor drive signal turns on the voltage supplied to the load. The period is a sinusoidal waveform that is the same as the power supply waveform, and the on and off periods occur alternately in the smallest detail. Furthermore, if the analog control signal is constant, the on and off periods are almost evenly distributed. An alternating current power control method characterized in that the AC power is controlled so as to immediately shift to a new distribution state in response to a change in an analog control signal.