First, overcurrent (OC)

Overcurrent is the most frequent phenomenon of inverter alarms.

1.1 Phenomenon

(1) When restarting, it will trip at a speed. This is a very serious overcurrent phenomenon. The main reasons are: load short circuit, mechanical parts are stuck; inverter module is damaged; motor torque is too small and so on.

(2) Jumping on power-on, this phenomenon can’t be reset generally. The main reasons are: bad module, bad drive circuit, and bad current detection circuit.

(3) When restarting, it does not trip immediately but during acceleration. The main reasons are: the acceleration time setting is too short, the current upper limit setting is too small, and the torque compensation (V/F) setting is high.

1.2 Examples

An LG-IS3-4 3.7kW inverter jumps “OC” as soon as it starts. Analysis and maintenance: No signs of burnout are found when opening the cover. There is no problem in the online measurement of IGBT (7MBR25NF-0) for further judgment. After the IGBT is removed, the high-power transistors that measure 7 cells are turned on and off very well. When measuring the driving circuit of the upper half bridge, it is found that there is a clear difference between the other two paths. After careful inspection, it is found that the output pin of one optocoupler A30 is short-circuited with the negative pole of the power supply, and the three paths are basically the same after replacement. The module is powered on and runs everything.

Second, overpressure (OU)

Overvoltage alarms generally occur when the machine is stopped. The main reason is that the deceleration time is too short or the braking resistor is damaged.

2.1 Examples

A Taian N2 series 3.7kW inverter jumps “OU” when it stops.

Analysis and maintenance: Before repairing this machine, we must first understand the reason of the “OU” alarm. This is because when the inverter is decelerating, the speed of the rotor winding of the motor is faster, and the electromotive force and current of the rotor increase. The motor is in the power generation state, and the feedback energy flows to the DC link through the diode connected in parallel with the high-power switch tube in the inverter link, so that the DC bus voltage is increased, so we should focus on checking the brake circuit and measuring the discharge resistance without problems. When the brake pipe (ET191) was measured, it was found to have broken down. After the replacement, the power was turned on and there was no problem with the quick stop.

Third, undervoltage (Uu)

Undervoltage 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 certain damage of the rectifier bridge or abnormal operation of the thyristor three-way may cause undervoltage faults. When it occurs, the secondary circuit contactor is damaged, which causes the DC bus voltage loss to cause undervoltage on the charging resistor. There is also a problem that the voltage detection circuit is malfunctioning and an undervoltage problem occurs.

3.1 Examples

A DANFOSS VLT5004 inverter, the power-on display is normal, but after loading, jump “DC LINK UNDERVOLT” (low DC link voltage).

Analysis and maintenance: This inverter is quite special in terms of phenomena, but if you analyze the problem carefully, it is not so complicated. The inverter also completes the charging process through the charging circuit and the contactor. It is not found when the power is turned on. Any abnormal phenomenon is estimated to be caused by the voltage drop of the DC loop when the load is applied, and the voltage of the DC loop is rectified by the full-wave rectification of the rectifier bridge, and then provided by the capacitor after the flat wave, so the rectifier bridge should be inspected and measured. The rectifier bridge has a bridge arm open circuit, and the problem is solved after replacing the new product.

Fourth, overheating (OH)

Overheating is also a common fault. The main reasons are: the surrounding temperature is too high, the fan is blocked, the temperature sensor performance is poor, and the motor is overheated.

4.1 Examples

An ABB ACS500 22kW inverter customer reported a “OH” jump for about half an hour.

Analysis and maintenance: Because it is faulty after running for a period of time, the temperature sensor is unlikely to be bad. The temperature of the inverter may be too high. After the power is turned on, the fan is slow to rotate, and the protective cover is covered with a lot of cotton wool. Because the frequency converter is used in the textile industry, the cleaning fan runs well after cleaning, and does not jump again after running for several hours.

Fifth, the output is unbalanced

The output imbalance is generally characterized by motor jitter and unstable speed. The main reasons are: bad module, bad drive circuit, and bad reactor.

5.1 Examples

A Fuji G9S 11KW inverter has an output voltage difference of about 100V.

Analysis and maintenance: Open the machine and initially check the inverter module (6MBI50N-0). No problems were found. The 6-channel drive circuit was not found to be faulty. The module was removed and the bridge was found to have a high-power transistor that could not be turned on and off normally. The module has been damaged. After confirming that the drive circuit is fault-free, everything is normal after replacing the new product.

Sixth, overload

Overload is also one of the more frequent faults of the inverter. When we see the overload phenomenon, we should first analyze whether the motor is overloaded or the inverter itself is overloaded. Generally speaking, the motor has strong overload capability, as long as the motor of the inverter parameter table When the parameters are set properly, there is generally no motor overload. The inverter itself is prone to overload alarms due to poor overload capability. We can detect the inverter output voltage.

Seven, the switching power supply is damaged

This is the most common fault of many inverters, usually caused by short-circuit of the load of the switching power supply. Danfoss inverter adopts the new pulse width integrated controller UC2844 to adjust the output of the switching power supply. At the same time, UC2844 also has current detection and voltage. Feedback and other functions, when there is no display, no voltage at the control terminal, DCV, 24V fan does not work, etc., we should first consider whether the switching power supply is damaged.

Eight, SC failure

The SC fault is a common fault of Yaskawa inverters. The IGBT module is damaged, which is one of the causes of the SC fault alarm. In addition, damage to the drive circuit can easily lead to SC failure alarms. Yaskawa in the design of the drive circuit, the upper bridge uses the drive optocoupler PC923, which is an optocoupler with an amplifier circuit dedicated to driving the IGBT module, Yaskawa’s lower bridge drive circuit uses the optocoupler PC929, which It is an optocoupler with an internal amplifier circuit and a detection circuit. In addition, motor jitter, three-phase current, voltage imbalance, frequency display but no voltage output, these phenomena may be IGBT module damage. There are many reasons for the damage of the IGBT module. The first is the failure of the external load, which causes damage to the IGBT module, such as short-circuit of the load, stalling, and the like. Secondly, the aging of the drive circuit may also cause the drive waveform to be distorted, or the drive voltage fluctuates too much and the IGBT is damaged, resulting in an SC fault alarm.

Nine, GF – ground fault

The ground fault is also a fault that will normally be encountered. In addition to the cause of the problem of grounding of the motor, the most likely part of the fault is the Hall sensor. The Hall sensor is easily affected by environmental factors such as temperature and humidity. A drift occurred, causing the GF to alarm.

Ten, current limit operation

In normal operation, we may encounter the inverter’s current limit. For a general inverter, when the current limit alarm occurs, the voltage (frequency) must first be lowered until the current drops to the allowable range. Once the current is lower than the allowable value, the voltage (frequency) will rise again. Causes instability of the system. Danfoss frequency converter uses internal slope control to find the working point without exceeding the predetermined current limit value, and control the motor to run smoothly at the working point, and feedback the warning signal to the customer. According to the warning information, we will check whether the load and motor are checked. has a problem.

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