Frequent fault analysis of frequency converter

1) Inverter charging start circuit is faulty

The general-purpose inverter is generally a voltage-type inverter, and the AC-DC-AC operation method is adopted, that is, the input is a communication power source, and the communication voltage is converted into a DC voltage by the three-phase rectifier bridge, and then the DC voltage is passed through the three-phase bridge inverter circuit. The three-phase communication output that is converted to voltage regulation and frequency modulation is output to the load. When the inverter is just powered on, because the capacity of the smoothing capacitor on the DC side is very large, the charging current is very large. Generally, a starting resistor is used to limit the charging current. The more common frequency conversion start circuit is shown in Figure 1. After the charging is completed, the control circuit short-circuits the resistor through the contact or thyristor of the relay. The fault of the starting circuit is generally characterized by the burning of the starting resistor. The alarm of the inverter appears as a DC bus voltage problem. When the planner plans the starting circuit of the inverter, In order to reduce the size of the inverter, select the starting resistor, and select a smaller one. The resistance value is 10 to 50 Ω, and the power is 10 to 50 W.

When the communication input power of the inverter is frequently turned on, or the contact of the bypass contactor is poor, and the conduction resistance of the bypass thyristor becomes large, the starting resistance will be burnt out. In this case, it is possible to purchase a resistor of the same specification, and it is necessary to find out the cause of the burnout of the lead-out resistor. If the fault is caused by the input side power frequency opening and closing, it is necessary to eliminate the scene to put the inverter into operation; if the fault is caused by the bypass relay contact or the bypass thyristor, it is necessary to replace the equipment.

2) The inverter has no symptoms, but it cannot run at high speed.

The condition of one inverter in our factory is normal, but it can't be adjusted to high speed. After checking, the inverter has no faults, the parameters are set accurately, the speed input signal is normal, and the test shows that the DC bus voltage of the inverter is only about 450V when the power is running. The normal value is 580 ~ 600V, and then the input side is measured. It is found that one phase is missing. The cause of the fault is that one phase of the air switch on the input side is poorly connected. Why does the inverter input phase loss alarm still work in the low frequency band? In fact, when the inverter lacks one-phase input, it can work. Most of the inverter's bus voltage lower limit is 400V. When the DC bus voltage drops below 400V, the inverter only states that the DC bus low voltage is faulty. When two-phase input, the DC bus voltage is 380*1.2=452V>400V. When the inverter is not running, the DC voltage can reach the normal value because of the effect of the smoothing capacitor. The new inverters use PWM control skills. The operation of voltage regulation and frequency modulation is completed at the inverter bridge, so the input in the low frequency band is missing. The phase can still work normally, but because the input voltage is low and the output voltage is low, the asynchronous motor is low in torque and does not go up.

3) The inverter shows overcurrent

When this problem appears, the primary check is whether the acceleration time parameter is too short, the torque increase parameter is not too large, and then check if the load is too heavy. If there is no such scene, the current transformer on the output side and the Hall current check point on the DC side can be disconnected and operated after resetting to see if the overcurrent scene is present. If it is presented, it is likely that the 1PM module is faulty because 1PM The module contains maintenance functions such as overvoltage and overcurrent, undervoltage, overload, overheat, phase loss, short circuit, etc., and these fault signals are transmitted to the microcontroller through the output Fn pin of the module control pin, and the microcontroller receives After the fault information, on the one hand, the pulse output is closed, and on the other hand, the fault information is displayed on the panel, and the 1PM module is generally replaced.

4) The inverter shows over-pressure

The inverter presents over-voltage problems, which are generally thunderstorms. Because the lightning is connected to the power supply of the inverter, the voltage checker on the DC side of the inverter is tripped. In this case, generally only the inverter power supply must be disconnected for 1 min. Left and right, and then close the power supply, you can reset; in another case, the inverter drives the large inertia load, it presents an overvoltage scene, because in this case, the deceleration of the inverter is aborted due to regenerative braking, during the suspension process, The output frequency of the inverter decreases linearly, and the frequency of the load motor is higher than the output frequency of the inverter. The load motor is in the power generation condition, the mechanical energy is converted into electric energy, and is absorbed by the smoothing capacitor on the DC side of the inverter. When this energy is sufficient When it is large, the so-called “pumping scene” will occur, and the voltage on the DC side of the inverter will trip beyond the maximum voltage of the DC bus. For this problem, firstly, set the deceleration time parameter to be longer or increase the braking resistor. Or increase the brake unit; the second is to set the suspension method of the inverter to be free parking.

5) The motor is hot, the inverter shows overload

Regarding the inverter that is now in operation, it is necessary to check the load if there is such a problem; if the inverter of the new device presents such a problem, it is likely that the V/F curve is not set properly or the motor parameter setting is in doubt. For example, if a newly installed inverter is driven by a variable frequency motor, the additional parameters of the motor are 220V/50Hz, and the inverter is set to 380V/50Hz when it leaves the factory, because the device personnel do not accurately set the V/F parameters of the inverter. When the motor is running for a period of time, the rotor is magnetically full, causing the motor speed to drop, heating and overloading. Therefore, in the new inverter application, it is necessary to set this parameter. When using the speed sensorless vector control method of the inverter, the parameters such as extra voltage, current and capacity of the load motor are not accurately set, which will also cause the motor to be hot. Overloading, there is also a phenomenon that when the set carrier frequency of the inverter is too high, it will also cause the motor to be overloaded. The final situation is that the electric planner plans that the inverter often operates in the low frequency band without considering the operation in the low frequency band. The problem of poor heat dissipation of the motor causes the motor to heat up and overload after a period of time. In this case, a heat sink is required.

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