A brief analysis of the reasons for overcurrent and overvoltage protection at the inverter end of high-frequency induction annealing furnace

I believe everyone knows that high-frequency induction annealing furnaces will set a rated current. It is not allowed to exceed the rated current, otherwise it will burn out the equipment. Therefore, a current protection module needs to be built. When the current exceeds the set current, the equipment will automatically power off. , to protect the equipment and the IGBT mainboard from being burned out. This is called overcurrent protection. Overvoltage protection refers to a protection method that disconnects the power supply or reduces the voltage of the controlled equipment when the voltage of the protected line exceeds a predetermined maximum value.

The reasons for overcurrent at the inverter end are generally:

(1) Load fluctuations during operation cause overcurrent. The load of induction furnaces fluctuates greatly during the smelting process, especially in the early stages of smelting. Parameters change more drastically, often causing overcurrent.

(2) During operation of the bridge inverter, the commutation errors of the two pairs of bridge arm thyristors cause the inverter to fail, resulting in short-circuit current.

(3) During operation, the bridge inverter triggers a pulse, and the sudden interruption causes a diagonal thyristor diagonal short circuit in the bridge arm. The short-circuit current caused by this, as well as the overcurrent caused by various other reasons, are very harmful to the inverter side. Using fast-acting fuses to protect overcurrent will not be an economical and reliable method.

The reasons for overvoltage at the inverter end are generally:

(l) The intermediate frequency voltage Ue=1.1Ud/cosα. Since the lead angle α is too large, when the rectified voltage Ud is constant, the intermediate frequency voltage Ue is too high.

(2) The inverter trigger pulse is determined by the intersection of the Uc and Ie signals. If the Ue signal from the self-voltage transformer is suddenly interrupted, the intersection will be determined by the Ie signal, and α will increase rapidly, causing the intermediate frequency voltage. Ue is too high.

(3) The furnace induction coil suddenly opened, causing overvoltage.

(4) The peak overvoltage generated by the thyristor when it is turned on and off.

After fully understanding the reasons for overcurrent and overvoltage protection at the inverter end of the high-frequency induction annealing furnace, our company's induction heat treatment engineers suggested that it is better to use the pulse blocking method for protection, and the effect is more obvious.