Luoyang Judian Metal Hot Processing Equipment Co., LTD is mainly engaged in the manufacture of complete sets of equipment in the metal thermal processing industry and the integration of the entire production line.
As described in this chapter, a low frequency power supply unit is connected to a single-phase induction heating coil at the output and to a three-phase power supply system at the input. If the three-phase current imbalance exceeds the permissible range, additional equipment must be added to reduce the imbalance of the furnace load. The basic circuit of this power supply unit is shown in the article, using reactors and capacitors to balance the single-phase load to obtain a substantially balanced three-phase circuit.
The capacity of the reactors and capacitors can have fixed or variable values depending on the permissible balance. These additional reactive and capacitive currents introduced into the furnace inductor and the power factor compensation capacitor current must be carefully selected in conjunction with the electrical characteristics of the network to avoid series resonance during the furnace melting cycle. The input power is usually controlled by selecting the tap of the transformer secondary voltage to achieve the furnace voltage change.
Most units are designed with a "load-loading" transformer tap selector switch, when this switch is operated, the current is interrupted instantly. The high-frequency switching currents that occur when performing this operation not only increase the voltage drop on the power supply line, but also cause additional electrical stress on the insulation of the relevant equipment components.
Due to the frequent opening and closing of circuit breakers and switch contactors, the maintenance costs and the cost of replacing parts are also increased. Recently, power control has been able to be implemented using saturated reactor systems.
Experience has shown that such single-phase systems are only allowed if the furnace can be allowed to act as a distributor for single-phase loads in the network without using any phase balancing equipment. The use of these power control devices, which increase the complexity of the system, will cause more problems, because the harmonic currents that occur must be filtered to avoid the occurrence of undesirable harmonics that interfere with the power supply system.
In addition, the reactors and capacitors used as filtering circuits increase electrical losses, thereby reducing the efficiency of the entire equipment. In addition, the presence of reactors and capacitors will also increase the complexity of electrical stability problems in the power supply equipment, furnaces and power supply systems. Therefore, it is most undesirable to use magnetic saturation reactors as power controllers in places where three-phase balancing equipment is required; because the phase balancing device also has its own reactors and capacitors, it will of course further complicate the characteristics of the entire system.
The impulse current generated by the power circuit when the equipment is started can be reduced by installing a "balanced starting" system on the secondary winding of the convertible transformer. The power distribution cabinet can usually use the main circuit breaker to energize or de-energize the transformer, and then use the conversion contactor to cut off the connection between the secondary winding of the transformer and the furnace.
The operation process of the furnace equipment is often such that the main circuit breaker is closed only once per working day, and sometimes even less. For longer working cycles, for example, only once during the working life of the furnace lining. Therefore, the voltage drop and high-frequency transient voltage can be greatly reduced by starting with a resistor in series with the furnace at the beginning.
This starting resistor is then short-circuited within a very short time (depending on its heating rate) so that the full value of the selected secondary winding voltage is input into the inductor of the furnace coil. In places where the power system is weak (i.e. low short-circuit capacity) a two-stage or even a three-stage series resistor switch can be used to minimize the starting current and voltage drop.
Similarly, power factor compensation capacitors, especially in high-power furnace installations, can also be switched using a step-by-step resistor device. Well-designed power supply equipment includes good smooth start switching technology together with fully automatic phase balancing equipment. This system is fully applicable to feed systems with moderate voltage drops and phase imbalance values. In addition, experience has shown that the components of the power supply equipment composed under such equipment conditions also have good performance, and there are few failures caused by failure and damage to contactors, transformers and especially expensive power factor compensation capacitors.
