Four common faults and incentives of the inverter

As a very popular control device in the field of electronic control, the frequency converter almost penetrates into every corner of industrial production activities. However, for some newcomers or newcomers who are just beginning to contact the inverter, they are still relatively unfamiliar and mysterious, especially if the inverter fails, it is easy to get started. Today, based on years of experience in using and overhauling inverters, I have summarized the four major faults common to inverters and given the relevant incentives for causing them.

First, over current (short circuit)
Overcurrent faults are most common among various faults in the frequency converter. This fault is mostly caused by the following problems: First, as long as the inverter speeds up, it reports a fault, indicating that the overcurrent is very serious, mostly the load short circuit, mechanical parts are stuck, the inverter module is soft breakdown damage and the acceleration time is over. Shortly caused; secondly, the inverter reports a fault when it is powered, and it cannot be “reset” to be eliminated. Most of it is caused by damage to the internal drive circuit of the inverter and damage to the current detection circuit. The last phenomenon is the most extreme. When the power is turned on instantaneously or after a short delay, it directly causes the upper air to trip and the internal fuselage is blown out or sparks. The inverter rectifier unit and the power inverter component directly cause breakdown failure. Second, overvoltage
This type of fault phenomenon is also often found in inverter faults. In addition to eliminating the supply voltage is too high, there is also a case of such a fault – when the inverter is stopped. The main reason for this may be caused by a slow deceleration time or a problem with the braking resistor and the brake unit.
Third, under voltage
The above overvoltage fault corresponds to the undervoltage fault of the inverter. Generally, the power supply voltage is too low. It may also be caused by the following reasons: the power supply phase loss, the open circuit fault of one bridge arm of the rectifier circuit, and the main circuit If the capacity of the filter electrolytic capacitor becomes small or a problem occurs in the voltage detection circuit, the inverter may experience an undervoltage fault. In addition, if the internal snubber resistance of the inverter is not cut off by short circuit, an undervoltage fault will occur as long as the inverter is loaded.
Fourth, over temperature
Over-temperature is also a common fault of the inverter. This fault is mostly caused by the temperature of the inverter working environment is too high, the cooling holes are blocked, the cooling fan is damaged, the temperature sensor and the temperature detection circuit are damaged.
Through the above analysis of the four common faults of the inverter (of course, faults such as overload and output imbalance), it is not difficult for the peers to see that the inverter needs to work harder in daily maintenance, and more needs to be done in daily maintenance. We will use it flexibly according to the situation on the spot, and try our best to prevent it before it happens!

Causes and treatment methods of inverter tripping fault

One. Power failure handling
If the power supply is temporarily powered off or the voltage is low, the “undervoltage” display, or the “overvoltage” display of this transient overvoltage may cause the inverter to trip and stop. Generally, the power can be restarted after the power supply returns to normal.
2. External fault handling
If the input signal is open, the output line is open circuit, phase failure, short circuit, grounding or insulation resistance is very low, the motor is faulty or overloaded, etc., the inverter will display “external” fault and trip and stop. After the fault is eliminated, it can be re-enabled.
three. Internal fault handling
If the internal fan is open or overheated, the fuse is open, the device is overheated, the memory is faulty, the CPU is faulty, etc., it can be switched to the power frequency operation without affecting the production. After the internal fault is eliminated, the inverter operation can be resumed.

In the event of an internal fault in the inverter unit, if it is within the warranty period, the manufacturer or the manufacturer’s agent should be notified as soon as possible to be responsible for the warranty. Perform the following checks based on the category and data displayed by the fault:

1. After opening the chassis, first check whether there are any broken wires, solder joints, burnt smells or deterioration of the components inside, if necessary, do the corresponding treatment in time.

2. Use a multimeter to check the resistance of the resistor and the on-off resistance of the diode, switch tube and module to determine whether to break or break down. If yes, replace it with the original nominal value and the withstand voltage value, or replace it with the same type.

3. Use the dual trace oscilloscope to detect the waveform of each working point, and use the step-by-step elimination method to judge the fault location and components.

Issues to be aware of during maintenance:

1. Strictly prevent false soldering, virtual connection, or wrong soldering, continuous soldering, or miswired, especially do not mistakenly connect the power cable to the output.

2. Power-on static check indicator, digital tube and display are normal, and the preset data is appropriate.

3. If there are conditions, a small motor can be used for the simulation dynamic test.

4. With load test.

four. Improper handling of function parameters
When the parameters are preset, the no-load test is normal. If an “overcurrent” trip occurs after loading, the starting torque setting may be insufficient or the acceleration time may be insufficient. Some may also decrease the moment of inertia after running for a period of time, resulting in “overvoltage” during deceleration. “Trip, generally modify the function parameters and then increase the acceleration time to solve.

Causes of inverter overvoltage

(1) The overvoltage of the breaking transformer is based on the theory of intercepting overvoltage. When the transformer is disconnected, the current in the transformer inductance cannot be abrupt, and the magnetic field energy stored therein oscillates between the transformer magnetizing inductance and the ground capacitance. An overvoltage has occurred.

(2) Overvoltage generated by transformer with load closing In the actual test, the combined no-load transformer has detected several times the overvoltage of the power supply voltage. The physical principle is that the no-load transformer can still be equivalent to a magnetizing inductance and Parallel connection of the equivalent capacitance of the transformer itself. If the neutral point of the transformer is not grounded, the switch is aperiodic closing (one phase or two phase first), due to the inductance of the feeder, the capacitance of the transformer to the ground, the longitudinal capacitance and the inductance of the transformer. As a result, a higher overvoltage is generated, especially in the transformer neutral point overvoltage. Although the transformer is basically loaded with a load, the transformer will also generate an overvoltage when it is loaded with a load, but it is smaller when it is relatively empty. There is a relatively large capacitance in the real load, because the storage of the capacitor does not suddenly increase, and the transmission cable has a distributed capacitance to the ground when transmitting a high frequency oscillating voltage, and these capacitors have an absorption effect on the overvoltage. The combination of the two causes the overvoltage of the transformer during the closing process to be suppressed, but sometimes the value is still high and may even be higher than the withstand voltage of the component, which is very dangerous.

(3) The commutation overvoltage rectifying element of the rectifying element is high in steering, and therefore the steering overvoltage is also high. This not only damages components, but also creates electromagnetic interference.

Frequency converter overvoltage processing method

(1) For the breaking overvoltage of the phase shifting transformer of the frequency converter, the overvoltage absorption circuit is formed by the RC absorption network and the zinc oxide arrester, and good results are obtained.

(2) For the overvoltage generated by the transformer with load closing, a switch with good cycle performance can be selected (the switch will have different periods after long-term operation); a good resistance-capacitance absorption circuit or active suppressor technical solution is adopted; The shielded transformer can also effectively suppress the closing overvoltage. However, the difficulty of making a high-power transformer in the formation of an electrostatic shielding layer will be considerable.

(3) For the overvoltage generated by the commutation of the rectifying element, the point of attention is: the reverse withstand voltage of the rectifying element is sufficient, and the second is that the absorption circuit and the freewheeling circuit must be properly controlled. Otherwise, the rectifier device may be broken down by an overvoltage. (4) Since the overvoltage during the operation of the inverter is basically generated when the transformer is opened, it is necessary to start from the transformer to find a way to suppress the overvoltage of the inverter. Can be used:

1 Increase the magnetizing inductance of the transformer and the capacitance to the ground, increase the excitation inductance to reduce the no-load current, which will increase the cost of the transformer.

2 increase the capacitance of the transformer to the ground: in principle, it is easy to analyze, but in fact, due to the structure and material limitations of the transformer itself, it is impossible to make a transformer with any insulation method or high insulation level, so it is necessary to increase it greatly. The capacitance to ground C of the transformer is also quite difficult.

Solutions to Overcurrent /Overvoltage/Undervoltage of VFD

Overcurrent
Overcurrent is the most frequent phenomenon of inverter alarm.
1.1 phenomenon
(1) When restarting, it will trip at a raise speed. This is a very serious phenomenon of overcurrent. The main reasons are: load short circuit, mechanical parts stuck; inverter module damage; motor torque is too small and so on.
(2) Power-on jump, this phenomenon can not be reset generally, mainly due to: bad module, bad drive circuit, bad current detection circuit.
(3) When restarting, it does not immediately trip, but when accelerating, the main reasons are: too short acceleration time setting, too low current limit setting, and high torque compensation (V/F) setting.
1.2 Examples
(1) An LG-IS3-4 3.7kW inverter jumps “OC” as soon as it starts
Analysis and maintenance: Open the cover did not find any signs of burnt, online measurement IGBT (7MBR25NF-120) basic judgment is no problem, to determine the problem, after the removal of the IGBT measured 7 units of high-power transistors turn on and off both well. When measuring the drive circuit of the upper half bridge, it was found that there was a clear difference between the other two circuits. After careful inspection, it was found that the output pin of the optocoupler A3120 was short-circuited to the negative terminal of the power source. After the replacement, the three paths were basically the same. The module is powered on and everything is running fine.
(2) A BELTRO-VERT 2.2kW inverter energizes to jump “OC” and cannot be reset.
Analysis and maintenance: First check the inverter module found no problems. Secondly, there is no abnormality in checking the drive circuit. It is estimated that the problem is not in this block. It may be in the part of the over-current signal processing. After removing the circuit sensor and powering it on, it shows that everything is normal. Therefore, it is considered that the sensor is broken and a new one is found. After loading the experiment, everything is normal. The
2 Overvoltage
Overvoltage alarms usually occur at the time of shutdown. The main reason is that the deceleration time is too short or there is a problem with the braking resistor and the brake unit.
(1) Examples
A Taian N2 series 3.7kW inverter jumps “OU” when it stops.
Analysis and maintenance: Before repairing this machine, we must first understand what causes the “OU” alarm. This is because when the frequency converter is decelerating, the rotor rotor winding cuts the rotating magnetic field faster, and the rotor’s electromotive force and current increase. Make the motor in the power generation state, the energy returned by the diode in the inverter circuit in parallel with the high-power switch tube flows to the DC link, so that the DC bus voltage is increased, so we should focus on checking the brake circuit, measuring the discharge resistance is no problem, When the brake pipe (ET191) was measured, it was found to have broken down. After the replacement, it was powered on, and there was no problem with the quick stop.
3 Undervoltage
Underpressure is also a problem we often encounter in use. Mainly because the main circuit voltage is too low (220V series is lower than 200V, 380V series is lower than 400V), the main reason: a damaged rectifier bridge or SCR three-way work may lead to undervoltage fault Appeared, followed by the main circuit contactor damage, resulting in DC bus voltage loss above the charging resistor may cause undervoltage. There is a voltage detection circuit failure and undervoltage problems.
3.1 Examples
(1) Jump on “Uu” on a CT 18.5kW inverter.
● Analysis and maintenance: After checking that the charging resistance of the rectifier bridge of this inverter is good, but no contactor action is heard after power-on, because the charging circuit of this inverter is not using SCR but by the contactor. The pull-in process completes the charging process, so it is considered that the fault may be in the contactor or the control circuit and the power supply section, and the removal of the contactor alone plus the 24V DC contactor is working properly. Then check the 24V DC power supply. After careful inspection, the voltage is output after being stabilized by the LM7824 regulator. Measure the voltage regulator tube is damaged. After finding a new one, the power is normal after the replacement.
(2) For a DANFOSS VLT5004 inverter, the power-on display is normal, but jump “DC LINK UNDERVOLT” after load.
● Analysis and maintenance: This inverter is special in terms of phenomena, but if you carefully analyze the problem is not so complicated, the inverter also through the charging circuit, contactor to complete the charging process, there is no power Any abnormal phenomenon is estimated to be caused by the voltage drop of the DC circuit when the load is applied, and the voltage of the DC circuit is rectified by the full bridge of the rectifier bridge and then provided by the capacitor after the flat wave. Therefore, the rectifier bridge should be checked and measured. The rectifier bridge was found to have an open bridge and the problem was solved after replacing the new one.

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