Six Methods of Adjusting Power for Commercial Induction Cookers

1、 Transformer primary tap switching, frequency modulation, and voltage regulation methods
By switching the primary tap of the transformer, high, medium, and small power levels are obtained. Due to the use of pulse power output transformers, the power output stage is well matched with the load, and the power stage can work in the optimal state. At the high, medium, and small power levels, frequency tracking is used to make the power transistor work in zero voltage switch and zero current switch states. From small to medium The power is continuously adjusted by fine-tuning the excitation pulse frequency within the power adjustment range from the middle gear to the large gear. At this time, the power transistor operates in a zero voltage switch and zero current on, approximately zero current off state. Using frequency modulation within the small gear to zero power range will disrupt the zero voltage switch state of the power transistor, so the power is adjusted by adjusting the output rectifier voltage
This power regulation method is relatively complex and costly. Its outstanding advantages include a large power regulation range, good matching of power output stage and load, and a commercial induction cooker with a radiator temperature rise of no more than 6 ℃ at a power of 8 kW (tested at an ambient temperature of 25 ℃)
2、 Frequency modulation method
At maximum power, the circuit operates in a near resonant state, increasing the excitation pulse frequency, and the circuit operates in a detuned state, resulting in a decrease in power. The advantage of this method is that the circuit is simple, but if power adjustment occurs when the output power of the electromagnetic furnace is high, the current phase lags behind the voltage phase, and the voltage phase is large. In a state of high current, the power tube consumption is high. In this way, even if the temperature rise on the heat sink does not significantly increase, It is also possible that the tube core has overheated and damaged the ICBT power module, so this method should not be used for power regulation above 8kW
3、 Change rectification voltage method
The three-phase rectifier module is replaced by a controllable rectifier module, which uses a control voltage of 0-10V to change the DC output voltage of the rectifier module after rectification to change the output power of the electromagnetic furnace. At the rated input voltage, the rectifier module is fully conductive to obtain the rated maximum power. At non rated maximum power, due to the decrease in DC voltage of the rectifier output, the power shows a decrease in the square relationship with the voltage. The advantage of this power regulation method is that the circuit is simple and can be continuously adjusted, Frequency tracking can be realized in the whole power regulation range. The disadvantage of this circuit is that the controllable rectifier module is not fully connected during power regulation, and there is a chopping gap, which has a great impact on electromagnetic compatibility indicators. To make electromagnetic compatibility indicators pass, there are high requirements for power filter and rectifier filter circuit. There are relatively many components in this part, and the overall volume and cost increase
4、 Phase shift pulse width modulation method
In the circuit shown in the figure, if the transformer does not have a tap, it only serves to isolate the induction coil from the main power supply and match the load. T1 and T2 are the left bridge arms, and T3 and T4 are the right bridge arms. The excitation pulses of T1 and T2 are inverted and have sufficient dead time to ensure that T1 and T2 do not generate common state conduction. Similarly, the excitation pulses of T3 and T4 are inverted and also have sufficient dead time. When the phase difference of the excitation pulses of the left and right bridge arms changes from 180 ° to 0 °, The power of commercial induction cookers varies continuously and smoothly from maximum to minimum, and the power transistor on half of the bridge arm achieves zero voltage switching; In addition, the power transistor on half of the bridge arm achieves zero current switching. When using the phase-shifting pulse width modulation method in practice, the compensation inductance is omitted, and when the power of the commercial electromagnetic furnace is low, the zero voltage switch of the leading bridge arm will lose accuracy. Therefore, it is advisable to use gap heating method to adjust the power when the power is low
The disadvantage of phase-shifting pulse width modulation method is that there is a large amount of current passing through the freewheeling diode in the IGBT tube, and the working state of the power tube is not as good as the transformer tap method mentioned above. Its advantage is that it can continuously adjust the power, eliminating the need for three AC contactors
5、 Primary tap switching method for pulse power output transformers
The normally open contacts J1-1, J2-1, and J3-1 of three AC contactors can be connected through the control circuit to connect the primary tap of the pulse power output transformer, in order to obtain high, medium, small, and third gear power (as shown in the attached figure). This power regulation method has a larger volume of the movement due to the presence of contactors, but in large, medium, and When the power is in the third gear, frequency tracking can be used to make the power transistor work in zero voltage switch and zero current switch states (see the soft switch frequency tracking circuit in the "Electronic News", Issue 14, 2005). Among all power regulation methods, the temperature rise of the power transistor in this method is the lowest, and the thermal efficiency of the induction furnace is over 95%
But be careful! The connection and release of the contactor must first turn off the excitation pulse to switch between the power output stage and the current free state
6、 Gap heating method
Apply excitation pulses to the gap to intermittently heat the induction cooker, and control the intermittent heating time interval to adjust the power of the induction cooker. This method has a simple circuit, but there may be electromagnetic noise during on/off and current impact on the power supply. When using this power adjustment method, it should be noted that the excitation pulse must be turned off at the time of current crossing zero, otherwise the electromagnetic noise at the bottom of the pot will be large during shutdown

The above content is organized by Superb heater Company, super heater. Mainly professional production: industrial heating elements, temperature measuring elements, production equipment, accessories, and raw materials such as tubular heaters, tubular heaters, belt heaters, ceramic heaters, silicone rubber heaters, thermocouples, etc.

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