Analysis of common faults of medium frequency furnace (I)

May 16, 2022

1) Fault phenomenon: the equipment cannot be started. When starting, only the DC ammeter has indication, and there is no indication of DC voltage and intermediate frequency voltage.


a. The inverter trigger pulse is missing;

b. Inverter thyristor breakdown;

c. Capacitor breakdown;

d. Short circuit and grounding of load;

e. If signal sampling circuit has open circuit or short circuit.

2) Fault phenomenon: it is difficult to start. After starting, the intermediate frequency voltage is more than twice the DC voltage, and the DC current is too large.


a. One thyristor of the inverter circuit is damaged;

b. One of the inverter thyristors does not conduct, that is, the "three legs" work;

c. If signal sampling circuit has open circuit or wrong polarity;

d. The inverter leading forward angle phase-shifting circuit fails;

3) Fault phenomenon: it is difficult to start, the DC voltage after starting is difficult to reach full load or close to full load, and the reactor vibrates greatly and the sound is dull.


e. Open circuit, breakdown, soft breakdown or electrical parameter performance degradation of rectifier thyristor

f. A set of rectifier pulses is missing

g. Rectifier thyristor gate open circuit or short circuit

4) Fault phenomenon: it can be started, but shut down immediately after startup, and the equipment is in the state of repeated startup.


h. The leading front angle is too small;

i. The load oscillation frequency is at the edge of its excitation frequency

5) Fault phenomenon: after the equipment is started, when the power rises to a certain value, it is prone to overcurrent protection. Sometimes the original thyristor is burned out before it is restarted. The phenomenon remains the same


j. If overcurrent occurs at low voltage just after startup, the front angle of the inverter is too small to turn off the thyristor reliably

k. The cooling effect of inverter thyristor water-cooling jacket decreases

l. Poor contact of groove connecting wire

6) Fault phenomenon: there is no response when the equipment is started, and the light on the control board is missing


Rapid melting and burning out

7) Fault phenomenon: when the equipment is running, the DC current has reached the rated value, but the DC voltage and intermediate frequency voltage are low.


This phenomenon is not caused by the failure of intermediate frequency power supply, but due to the low load impedance

a. The series capacitor is damaged

b. The inductor has inter turn short circuit

8) Fault phenomenon: when the equipment is running, the DC voltage and intermediate frequency voltage have reached the rated value, but the DC current is small and the power is low.


This phenomenon is just opposite to the fault phenomenon in 7) and is caused by high load impedance

a. Insufficient compensation of load compensation capacitor

b. The contact resistance of the groove connecting node is too large, and the ash shall be cleaned

9) Fault phenomenon: the equipment operates normally, and the DC current indication is on the high side. If the current is set at the rated value, the voltage is too low, and the multiplication between the indication of power meter and that of ammeter and voltmeter is inconsistent


This phenomenon is usually caused by the increased contact resistance caused by the dirt and oxide layer between the shunt and the wiring and the increased voltage generated on the shunt

10) Fault phenomenon: the equipment operates normally, but there is no response or protection after stopping and starting. instructions.


a. If start switch damaged

b. Protection circuit failure is usually caused by the problem of swept frequency circuit integrated block NE556 on the circuit board

c. In a given circuit, the given signal is interrupted

11) Fault phenomenon: frequently burn out the thyristor original, and then burn out after replacement


Refer to fault E) for details:

a. When the thyristor is turned off in reverse, the instantaneous burr voltage bearing the reverse voltage is too high. Check the resistance capacitance absorption

b. The insulation of the load to the ground is reduced, the ground is ignited, or a high voltage is formed at both ends of the thyristor

c. Pulse trigger circuit failure, sudden loss of trigger pulse, resulting in open circuit of thyristor

d. Load open circuit during equipment operation

e. Load short circuit during equipment operation

f. Protection system failure (protection failure)

g. Thyristor cooling water system failure

h. Reactor failure causes intermittent current at the inverter side, and the thyristor is burnt out due to magnetic saturation and loss of current limiting effect of the reactor

i. The commutation inductance is too large, or the insulation is reduced, resulting in current instability

12) Fault phenomenon: when starting the equipment, when the intermediate frequency start switch is turned on, the main circuit switch protection trips or overcurrent protection


a. The power adjustment knob is at the highest position, and the instantaneous current impact is too large

b. The current regulator fails, especially the current transformer is damaged or the wiring is open, there is no current feedback suppression at startup, and the current impact is too large

13) Fault phenomenon: the intermediate frequency transformer is burnt out, and the starting equipment is still burnt out after replacement


This phenomenon generally occurs on the step-up load equipment, which is mainly caused by the virtual connection and open circuit of the discharge inductance. The voltage of the two groups of capacitors in the step-up mode is inconsistent. When discharging, the high voltage discharges slowly. If you start charging again before discharging, you will accumulate DC charge on the capacitor. If you discharge through the discharge inductance and open circuit, you will be released through the medium frequency transformer. Because the capacity of the medium frequency transformer is small, it will be burned out

14) Fault phenomenon: the discharge inductance in the boost circuit is heated or burnt out


a. Small discharge inductance

b. Inverter pulse asymmetry

c. When one of the inverter thyristors is burnt out, the waveform of the intermediate frequency output voltage changes, causing a large current flowing through the discharge inductor, causing heating or burning out

15) Fault phenomenon: the equipment can be started successfully, the frequency is much higher than the original, and sometimes it is difficult to start


a. There is a short circuit between the turns of the load coil

b. There is an open circuit in the load capacitor column

16) Fault phenomenon: it is easy to start, but the voltage is easy to overvoltage when boosting, and sometimes overvoltage and overcurrent occur at the same time


a. The front angle of the inverter is too large, resulting in too high inverter burr voltage

b. The main circuit inside the power cabinet has virtual connection, reduced insulation and ignition

c. The load coil or capacitor has false connection, reduced insulation and ignition

d. There is a problem with the trigger of the inverter thyristor, the connection is loose or the gate is open

17) Fault phenomenon: the equipment can be started, but the voltage rise is not high, the reactor is loud and dull, and the voltage rise is unstable. Sometimes the over-current voltage protection and sometimes the thyristor are burned out, but the rectification is good


a. The reactor has large inductance and magnetic saturation, which can not play the role of filtering

b. Poor insulation of reactor