The carbon silicon heating element can be regarded as a conventional resistive load, and its electrical law is simple, in which V is the voltage (in volts; I is the ampere current intensity, W power is watts, and R is the Omar resistance).

Element connection

Elements can be connected in parallel or in series, but in most cases, parallel connection is considered preferred because in this connection, the change of resistance will balance the service life of the elements. When serially connecting elements, this does not happen and shortens the service life of the elements.

Since the resistance of elements increases relatively slowly, the effect of imbalance is still small, and as long as they have a consistent amount of resistance, up to four elements can be connected continuously. When the temperature of the electric burner exceeds 1400 ° C, it is recommended to limit the number of elements in series to two.

The combination of parallel and serial element connection is usually considered as an effective solution. In this case, it is recommended that the serial connection element group is connected in parallel. If the parallel connection group is connected in series, the failure of one element will lead to a large amount of congestion of the remaining elements in the group.

In a three-phase network, elements can be connected by star or increment. When using a star connection, it is recommended to use a 4-wire power supply to balance the phase voltage regardless of the phase resistance value. When using the same 3-wire connection to the star phase resistance value, it must be matched very clearly.

Resistance negotiation

It is recommended to connect the elements in series into the group and select different elements within ± 5% of the original resistance. When connected in parallel, the limit for changing the resistance of the original element is wider: ± 10%.

If the element fails or breaks after a short period of time, replace it with a new one, preferably with a higher resistance.

If the element has been running long enough, the whole element group must be replaced. Otherwise, the old element (in serial connection) or the new element (in parallel connection) will bear too much load, resulting in premature failure of the element.

It is better to divide the total number of elements in the furnace into relatively small control groups so that the resistance can be coordinated more easily in the future. For example, if 48 elements in the oven were split into 8 groups of 6 elements instead of 3 groups of 16 elements, it would be much easier to select resistance elements.

When replacing a group of components, make sure that the voltage is set to the required value before turning on the power output voltage, because even in a short time, component overload may lead to irreparable faults. The old element can be left for further use, but has resistance with its "peers". If possible, before disassembling the component, the voltage and current readings must be cleared and the increased resistance must be specified on the pin to facilitate resistance negotiation when using the resistance later. It must also be remembered that at room temperature, the resistance of the element does not match that at operating temperature, and the reading must be taken at a constant temperature above 1000 ° C.

Voltage reserve

Variable supply voltage is usually used to compensate for the inevitable increase in resistance during the operation of the element. The size of the required voltage reserve will depend on the "aging" rate and the estimated service life of the element, but it is usually about 50-100% of the voltage required to obtain full load with a new electric heating element in the initial state.

For example, if 125 V is required to provide full power for a new component, 125-250 V is required to produce 100% voltage reserve.

If the element must be operated at 1400 ° C or higher for a long time and at a high "aging" rate, 100% voltage reserve is required. Conversely, if the element temperature is very low and the use frequency of the electric furnace is not high, a voltage reserve of 50% or less is sufficient.

Power supply equipment

In order to operate the estimated power that must be maintained throughout the service life of the element, a voltage varying power supply will be used. The type of equipment used will affect the characteristics of components. It is very important to select the appropriate equipment to achieve the longest service life of components.

There are different types of power supplies:

1. Adjustable power transformer

Filter 2 Dash unit (varistor voltage regulator)

）a. Phase on (phase ignition adjustment) B

Pulse regulation

3. Combined system: converter

Thyristor 4 Direct insertion into the grid

In essence, transformers with stepped outputs only control on / off unless they are used with thyristors. Although time-consuming and insensitive to slight overload, in most cases, the transformer is considered a heavy, bulky and relatively expensive power supply. Thyristor control is the most compact solution, with the ability of round less power regulation and proportional differential accurate temperature control. However, titration control requires more power and may cause interference to the power supply line. Generally, in order to ensure more than 50% voltage reserve, not only thyristor control is used. If a large amount of voltage reserve and proportional phase bottom control are to be provided, the best solution is to combine the advantages of titrator control and transformer high voltage control.

An iris control is usually used for low-temperature furnaces, Laboratory Furnaces and other equipment, in which the change rate of element resistance must be low. For high temperature continuous industrial furnaces that require a large amount of voltage reserve and accurate temperature control, the additional cost of using combined transformer system usually proves its performance reasonable.

1. Adjustable power transformer

Power transformers with continuous regulation from scratch are used to power small laboratory cookers or heat experimental supports, but these transformers are considered very expensive for large cookers with more economical power transformers with stepped output. The maximum voltage difference between the steps of this transformer shall not exceed 7% of the initial voltage required to distribute full power (+), where w = installed capacity of the electric cooker and resistance of the R - circuit. It is calculated according to the rated resistance of the element and when calculating the transformer (= maximum value of secondary current). The resistance tolerance of the element must be considered. For example, if the furnace is designed to be 5 kW and equipped with elements with 2 OM circuit resistance (± 15%), the characteristics of the transformer can be calculated as follows:

Rated voltage and power=

, the difference between stages should not exceed 100 V = 7 v

Minimum circuit resistance = 2 000 - 15% = 1.7o

Minimum two

Secondary current = minimum required voltage=

It is assumed that the required voltage reserve is 100%. Then, the characteristics of the transformer are as follows:

Input: and power supply

Corresponding single-phase output: at 15 7V levels (= 4 large * 4 small shunts)

Power variation from 92v to 197v, rated data of power transformer: 5 KVA at 92v and above (maximum two

The secondary current is 54 A) 1) when using a power transformer with steps, it must be considered to reduce the power of the electric furnace. For example, if the voltage changes between 7% levels, the power during operation will decrease by about 12.5% before switching the voltage to the next level of the transformer.

2) If required, for lower power, multiple less than 92 v.3 can be provided

If the secondary current meter is installed, the reading of the secondary current meter must accurately reflect the power distribution in the primary circuit.

Dash unit (thyristor voltage regulator)

Thyristor is a semiconductor device used to adjust the average conduction power of electric heating element and immediately switch it from open state to closed state. Each thyristor conducts electricity in only one direction. In order to adjust the AC charge, the anti parallel connected thyristors are provided in pairs. The thyristor is enabled by a series of pulses from the corresponding power amplifier cascade or temperature controller. Thyristors are quite simple instruments, but they are usually equipped with closed circuits to compensate for fluctuations in voltage, load, etc. Typical loop operation modes include current control (I * I feedback), voltage control (V * V feedback) and active power control (VI feedback). Generally, only voltage control (V * V feedback) is suitable for silicon carbide heaters, although. There are some exceptions.

Current control may increase the power on the element with the increase of resistance, and when the resistance increases, both power and current control may cause serious damage to the element. If there is such a high load resistance, both current and power control may cause the maximum voltage to be supplied to the element. If the available voltage is higher than the allowable voltage, the element may be seriously damaged. Under voltage control, the power supply will be controlled according to the resistance and temperature characteristics of the element, starting from a relatively low power and gradually increasing with the heating of the element.

The output characteristics of thyristor voltage regulator depend on the activation (regulation) mode of vesicles. There are two methods:

2.1 phase on (phase ignition adjustment)

Each titrator is turned on once every half wheel of variable current for a period of time. The conductivity of the titrator ends at the end of the half cycle, that is, the current drops to zero. In this case, the power sine wave is decomposed to reduce the average quadrilateral voltage of the thyristor output, which means that only part of the voltage reaches the load. In order to adjust the power supply voltage of carbon silicon electric heating element, the ignition angle of vesicle must be changed, so it can be increased by increasing it. The "aging" of the element is compensated by increasing the power supply voltage of the heater.

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The dash unit with phase ignition device can be equipped with current limiter, which can protect the thyristor from accidental overload regardless of the voltage setting, so as to keep the current output below the set value. Please note that the current limiting system required to protect the dash cannot be used to adjust the power input to the element, because the power input (= I * I * r) will gradually increase with the increase of element resistance, resulting in overload of the heating element.

Because the thyristor regulators with phase ignition (start-up) are characterized by stable and non reflective change of power supply voltage heaters, they are very suitable for supplying power to carbon silicon electric heating elements. At the same time, they may cause radio interference and waveform distortion; Special attention must be paid to these factors when selecting the power supply system of the furnace. In many countries, the use of phase ignition (including) is undesirable or strictly limited to local regulations.

Due to the phase adjustment of vesicle ignition angle, especially at small ignition angle, there will be a large number of high harmonics, resulting in the release of large injection power in the circuit. Therefore, even under pure resistance load, it may lead to the adjustment problem of high-power device. In short, in order to reduce this effect, the starting voltage with new elements must be at least 60% of the supply voltage.

The power cable must be calculated at the medium clutch current through the element (= power depends on the input voltage, not on the power supply voltage). This means that the power supply power must be higher than the design power of the furnace, and the greater the voltage reserve, the higher the recalculation required.

In the three-phase setting, the occurrence of the third high harmonic will lead to the cumulative distortion of the power wave and may lead to excessive neutral current in the zero line, which is twice the linear current in the star connected 3-phase 4-wire load. Therefore, the neutral cable (neutral line) must be calculated correctly to withstand this excessive current. However, a 3-wire star connection must be used. In order to eliminate such problems, this may lead to imbalance between phases, especially when the elements are not very clear in terms of resistance.

In order to reduce power distortion in large devices, it is recommended to use six wired power supplies with open triangles (increments).

In this case, the total electrical installation cost may be significantly reduced, because the thyristor can only calculate the phase current, not the linear current required in the traditional triangular (closed incremental) load setting, and the phase can be controlled independently, so as to achieve more flexible control.

As with transformers, the element resistance tolerance must be considered when calculating the maximum current to calculate the voltage and current in the phase ignition regulator.

The power control is the phase angle.

Most types of voltmeters and ammeters do not provide accurate mean azimuth data on phase adjustable loads, so it is necessary to closely monitor that the electric heating elements will not be seriously overloaded. The available digital instruments will accurately indicate non sine waves, but in order to determine the correctness of the selected instrument, you need to consult its manufacturer. In order to obtain correct mean square digit data, it is recommended to use Hall sensors with an amplitude coefficient of 7 or higher.

2.2 pulse conditioning - fast cycle

Under pulse power regulation, the thyristor starts at the beginning of the main cycle, remains open for one or more complete cycles, and then locks one or more cycles. "This operation is repeated to limit the average power input to the element. Manual power limits are used to change the on / off ratio to compensate for the" aging "of the element.

Although the average power supplied to the element may be at the allowable operating limit, temperature and atmosphere, the instantaneous full voltage surge of the main circuit may cause the instantaneous load to exceed this value several times, thus accelerating the "aging" speed and causing premature (if not immediate) failure of the element. To do this, the elements must be connected like this. Make the instantaneous load of components not more than 15W / cm2 during voltage surge of main circuit.

In order to reduce the impact of "enable" surge, its main duration must be minimized, preferably less than 30 cycles at 50 Hz power frequency (i.e. 50% power = 15 "on" cycles)+ 15 cycles "off").

The vesicle opening time of slow cycle is usually a few seconds. This kind of thyristor is not suitable for directly adjusting the power of carbon silicon electric heating element. They can be used in the secondary circuit of stepped power transformer instead of traditional electromechanical contacts.

The best type of silicon carbide battery pulse regulator is the thyristor with single pulse regulation cycle, in which the average power input is always reached, exceeding the number of possible full cycles (i.e. 50% power = 1 ring "on" + 1 "off" cycle).

The required voltage value of the fault finder unit will be the same as or higher than the supply voltage, but the current limit of the unit must be determined by dividing the intermediate frequency supply voltage by the minimum circuit resistance (i.e. thyristor current = supply voltage / minimum resistance).

Therefore, the estimated value of this titrator will be much higher than that of this phase ignition vesicle unit.

In order to obtain the required thyristor output value, the power at the power supply voltage must be calculated to calculate the on / off relationship. In any case, a thyristor with a certain percentage limit will produce the required estimated power.

That is, if the power supply voltage is 200 V and the circuit resistance is 4 ram, the 200 V power supply is 10