1 The failure phenomenon of intermediate frequency furnace

In General, the failure phenomenon of intermediate frequency power supply can
be divided into two categories, First, Can not start to at all. Second, cannot work
normally after starting. These fuults are genrally caused by phase loss,
Rectifier thyristor and fast thyristor burnout, capacitor bank break down,
water-cooled cable break, reactor failure, coil inductor failure and control board
failure.
For the analysis and repair of general faults and repair solutions, the following
is introduced for reference from Smithermal.

2. The Intermediate frequency power can not start properly.

2.1 Can not start successfully
Fault phenomenon is that press the start button and rotate the power
potentionmeter and the sweep process is started but the startup can not be
sucessful.
Trouble shooting: Judging that there maybe a problem with Intermediate
furnace feedback transformer, you can try to exchange the connect 2-6 and 2-7
on the Intermediate furnace transformer.

2.2 Cannot work normally after starting.

(1) The sweep frequency is not correct. Use an oscilloscope to measure the
output frequency and set the start frequency correctly.
(2) Check the current feedback signal line: whether the current transformer,
voltage transformer is damaged or not, and whether the connection line is open;
(3) The control circuit board is faulty and the circuit board is replaced.

2.3 Control board Low voltage indicator light (“LV” in MPU-2FK. “QY” in MPU-
6FK)

(1) Use a multimeter to measure whether the quick-blow fuse at the incoming
line is disconnected. If there is damage, replace the fuse with a new one;
(2) The board is broken and the board is replaced.

2.4 DC current big and DC voltage low.

The fault phenomenon is to press the start button, rotate the power
potentiometer, the DC current is large, the DC voltage is low, and the overvoltage
and overcurrent indication is accompanied. (“OV”,”OC” In MPU-2FK, “GY”,”GL”
in MPU-6FK)
Analysis processing and trouble shootting from Smithermal.
(1) Check the forward and reverse resistance of the fast thyristor of the inverter
circuit with a multimeter. If the forward and reverse resistance is 0, the
breakdown of the thyristor is broken and the thyristor is replaced.
(2) Check if the GK pole resistance of the inverter thyristor is 10~20Ω. If it is too
large, the thyristor may not be triggered, and the three-legged arm will run.
(3) Check whether the capacitor is broken down, separate the capacitor terminal
from the copper strip, and measure the resistance of the two poles with a
multimeter. If it is 0, it means that the capacitor is broken down and the
damaged capacitor must be replaced.
(4) Check if the induction coil of the intermediate frequency furnace has sparks
between the turns and the spark between the yoke and the place where the
ignition is handled. If it cannot be handled, the induction coil needs to be
replaced.
(5) Check the circuit for ignition.

2.5 fast fuse burned out

The fault phenomenon is to press the start button, rotate the power
potentiometer, no DC voltage, and the fast fuse burns out.
Analysis and processing: Check the forward and reverse resistance of the rectifier
thyristor of the rectifier circuit with a multimeter. If the forward and reverse
resistance is 0, the breakdown of the rectifier thyristor is damaged, and the
rectifier thyristor is replaced.

3 The boot device can be started, but the working status is abnormal.

3.1 Intermediate Furnace automatic stop
The fault phenomenon is that the intermediate frequency automatically stops
after the start of running for a period of time, and with overcurrent protection
(“OC” in MPU-2FK, “GL” in MPU-6FK)
Analysis processing and trouble shotting from Smithermal: Whether the inverter thyristor is
installed and fastened to ensure that the thyristor and the heat sink are closely
attached.

3.2 A fixed frequency occurs in the Itermediate Furnace automatic stop
The fault phenomenon is that the intermediate frequency automatic stop
phenomenon occurs every fixed time of melting, and the component inspection
has no fault and regularity occurs.
Analysis processing and trouble shooting: related to the external power
compensation circuit, stop connecting with the power compensation device or
adjust the power compensation device parameters to prevent interference with
the electric furnace control panel.

3.3 Frequent overvoltage (OV in MPU-2FK and GL in MPU-6FK) and overcurrent
protection (OC in MPU-2FK, GL in MPU-6FK)
The fault phenomenon is that overvoltage and overcurrent protection occurs
during operation, the components are routinely checked without failure, and the
frequency of occurrence is getting higher and higher.

Analysis processing and trouble shotting from Smithermal.
(1) The smoothing reactor is short-circuited between the turns;
(2) After the voltage rises, the capacitor withstand voltage is not enough, and the
internal ignition;
(3) The electric furnace induction coil is ignited between turns or between the
coil and the yoke;
(4) The copper bar screws are loose.

3.4 DC voltage can not rise
(1) Check if the GK pole resistance of the rectifier thyristor is about 10-20Ω;
(2) Observe the rectifying touch pulse with an oscilloscope, if there is any loss or
the pulse width is too narrow, and the trigger power is not enough;
(3) Check if the rectifier thyristor control pole is open.
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3.5 is operating normally, but the current cannot rise after some value
The board is not adjusted, in the current limit state, adjust the board 1w1 and
2w1 potentiometer.

3.6 Normal operation, the intermediate frequency voltage cannot be adjusted to
the design value
The circuit board intermediate frequency voltage setting is not adjusted, and the
circuit board 3w5 potentiometer is adjusted.

3.7 Intermediate Furnace voltage and DC voltage ratio is wrong
The board is not adjusted, and the 3w3 and 3w4 potentiometers on the board
are adjusted.

4 Analysis and processing of other faults

4.1 Water-cooled cable heating
(1) Check if the water circuit of the water-cooled cable is unobstructed.
(2) Whether there is an open circuit in the same group of water-cooled cables,
resulting in a single cable with increased current.
4.2 cable bracket heating
(1) Whether the cables are cross-mounted at the brackets in the same direction
in a diagonal manner;
(2) The cable is broken.
4.3 Thyristor temperature is too high
(1) Whether the cooling waterway is unblocked;
(2) Whether the thyristor is fastened to the heat sink.

5 Conclusion
In summary, in order to use the correct method for fault repair of intermediate
frequency power supply, it is necessary to be familiar with the characteristics
and causes of common faults of intermediate frequency power supply, in order to
save time, eliminate faults as soon as possible, and restore normal operation of
intermediate frequency power supply. In order to ensure the smooth progress of
production.

6. Buy the Spare Parts from Smithermal.:)