Solutions to Overcurrent /Overvoltage/Undervoltage of VFD

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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