Causes of overcurrent in intermediate frequency induction furnaces and solutions to overcurrent faults

A common problem in the operation of an intermediate frequency induction furnace is overcurrent protection. When the current exceeds a predetermined value, the protection device will be triggered, and a trip or alarm signal will occur. Many people ask what happened to the IF furnace trip? In fact, it is also related to the cause of the intermediate frequency furnace overcurrent. Let's take a look at the solutions to the overcurrent fault.

I. Reasons for overcurrent failure of intermediate frequency furnace

1, short circuit of induction coil

The induction coil has a short circuit to ground or a short circuit between turns. The induction coil is insulated from the ground. In the case of no water flow, the resistance value should be above 2M8. If water is passed, use a multimeter to measure it, it is above 5k8 (because water is connected to ground, but there is a certain resistance). The induction coil has a short-to-ground or inter-turn short-circuit, most of which is caused by leakage of molten iron from the furnace lining. After the molten iron is cooled, the induction coil communicates with the ground and inter-turn communication, resulting in short circuit.

2, capacitor short circuit

The capacitor has a short-to-phase or short-circuit to ground. The intermediate frequency capacitor is an electric heating capacitor with cooling water inside. When there are scales or foreign objects in the waterway, the water flow is greatly reduced or even cut off, which will cause the capacitor to heat up and burn out, causing short-circuits between phases. If the capacitor has a serious oil leakage, the capacitance will be greatly reduced, and it will also cause the capacitor to heat up and cause short-circuits between phases. The intermediate frequency capacitor casing is one pole and is directly placed on a bracket welded by angle steel. The capacitor bracket is separated from the ground of the electrical network by a bakelite seat. If the bakelite seat is greatly reduced due to moisture and oil pollution to the ground insulation, it is prone to short circuit to ground breakdown, and short circuit between phases or short circuit to ground will cause overcurrent.

3, inverter thyristor failure

The forward angle B is adjusted to be too small, so that the inverter thyristor commutation time tC is increased, and the corresponding reserve time tB is reduced. Therefore, the inverter commutation difficulty occurs, an overcurrent occurs, and the inverter fails. We know that tD = tC + tB (where tD is the pre-trigger trigger time, tC is the commutation time, and tB is the reserve time). Only when tB> tq (tq is the turn-off time), it can be ensured that the turned-off thyristor restores the forward blocking characteristic and makes the commutation successful. When tB decreases due to the increase of tC, the thyristor, which should be turned off during commutation, mis-conducts because it fails to restore the forward blocking characteristic, resulting in a shoot-through short circuit, which eventually leads to inverter failure.

4. Failure of inverter trigger circuit

What is the problem of the old burning of the inverter thyristor in the intermediate frequency induction furnace? Failure of the automatic frequency modulation circuit and the inverter trigger circuit causes the trigger pulse to be disordered and the waveform to be distorted. When the current is reversed, the tube that is turned on is not turned on, and the shutdown is turned off continuously. As a result, a short circuit is generated and an overcurrent is generated. Under normal circumstances, the diagonal pulses of the inverter bridge should overlap, and the waveform should be clear and neat, and the amplitude and width should meet the requirements. The phase difference between two adjacent pulses of the inverter bridge should be 180 °. If the voltage signal or current signal is distorted or disturbed due to a line failure or interference, the synthesized signal is bound to be abnormal. If the inverter trigger circuit fails, the trigger pulse will also be abnormal (in phase, amplitude, waveform). As a result, the inverter tube is short-circuited, causing overcurrent.

5. Change of inverter characteristics

There is a change in the characteristics of an inverter tube in the left arm or the right arm, but a forward turn occurs when it should not be turned on, causing a direct short circuit in the left arm or the right arm of the inverter circuit, causing overcurrent. This mainly means that the thermal characteristics of an inverter thyristor are not good. In the cold state, each characteristic parameter is normal, but once the power is transmitted, when the intermediate frequency voltage reaches a certain value (about 300V), a positive turn occurs, so that An overcurrent is formed due to the shoot-through.

Second, medium frequency furnace overcurrent solution

1. Run the device without load first and observe whether the voltage can rise to the rated value. If the voltage cannot rise to the rated value and over-current protection is performed near a certain value of the voltage multiple times, this may be caused by insufficient withstand voltage of the compensation capacitor or thyristor, but it is not ruled out that a part of the circuit is caused by fire.

2, the voltage can rise to the rated value, the equipment can be transferred to heavy load operation, and observe whether the current value can reach the rated value. If the current cannot rise to the rated value, and overcurrent protection is performed near a certain value of the current multiple times, this may be a large current interference. Pay special attention to the interference of the electromagnetic field of the medium frequency and large current on the control part and the signal line.

3. The thyristor with one arm of the inverter bridge may be short-circuited or open-circuited, causing the three-arm bridge of the inverter bridge to operate. The pulsator observes the thyristor voltage drop waveforms of the four bridge arms of the inverter bridge respectively. If the thyristor voltage drop waveform of one bridge arm is a line, the thyristor has passed through; if it is a sine wave, the thyristor is not conducting. Turn on, replace the thyristor, and find out why the thyristor is not conducting.

4. The reverse conversion flow angle is too small. Use an oscilloscope to watch the commutation angle of the inverter thyristor and adjust the commutation angle to an appropriate value.

5. The insulation resistance of the furnace is low or short-circuited. Use a megohmmeter to measure the resistance of the furnace to eliminate the short-circuit point of the furnace.

6. The resistance value of the relative induction coil of the charge steel is low. Use the megohmmeter to detect the resistance value of the relative induction coil of the charge. If the resistance value is low, rebuild the furnace.

The above is the introduction to the solution to the overcurrent fault. Frequent overcurrent protection will cause the impact of the current and electromagnetic force on the transformer, causing damage to the transformer or thyristor. Therefore, the daily maintenance of the intermediate frequency furnace should be carried out in a timely manner to ensure the normal operation.