Overcurrent and overload are the most common faults of frequency converters. To distinguish between overcurrent tripping and overload tripping of a frequency converter, one must first make clear the difference between them. Generally speaking, overload must also be overcurrent, but why does the inverter have to separate the overcurrent from the overload?

(1) the protected objects are different

Overcurrent is mainly used to protect the inverter, and overload is mainly used to protect the motor. Because the capacity of the inverter sometimes need to increase the capacity of the motor one or two gears, in this case, the motor overload, the inverter may not be over-current. The overload protection is carried out by the electronic thermal protection function inside the frequency converter. When the electronic thermal protection function is preset, the"Current access ratio" should be preset accurately, that is, the motor rated current and frequency converter rated current ratio of the percentage:

IM% = IMN * 100% i/IM type, 

IM% ー current access ratio; 

IMN ー motor rated current, A;

IN ー inverter rated current, A.

(2) the change rate of current is different

Overload protection occurs during the working process of production machinery, the current change rate di/DT is usually small; in addition to the overload other over-current, often with a burst, the current change rate di/dt is often larger.

(3) the overload protection has the inverse time characteristic. 

The overload protection is to prevent the motor from overheating, so it has the inverse time characteristic similar to the thermal relay. That is, if it is not much more than the rated current, it can be allowed to run for a longer time, but if it is much more than that, it will be allowed to run for a shorter time. In addition, the motor's heat dissipation becomes worse as the frequency drops. Therefore, the lower the frequency, the shorter the run time allowed under the same 50% overload.

Overcurrent trip of frequency converter

The over-current trip of frequency converter can be divided into short-circuit fault, trip in operation and trip in speed-up and speed-down.

1.Short-circuit Fault: 

(1) fault characteristics

(a) the first trip may occur in the course of operation, but if reset and then start, often a speed trip trip. 

(b) has a high impulse current, but the zero converter is already capable of protective tripping without damage. Because the protection tripping is very fast, it is difficult to observe the magnitude of its current.

(2) judgment and treatment

The first step is to determine whether the preferred short circuit. In order to be easy to judge, after reset before starting again, can be connected to the input side of a voltmeter, restart, potentiometer from zero slowly rotating, at the same time, pay attention to the voltmeter. If the frequency of the inverter output just rose immediately trip, and the voltmeter pointer has an instant back to"0" signs, indicating that the output of the inverter has been short-circuited or grounded.

The second step, to determine whether the inverter is internal or external short-circuit. At this time, should be the output of the inverter wiring off, and then rotating the potentiometer, so that the frequency rise, if still tripping, indicating that the converter internal short-circuit; if not tripping, indicating that the converter external short-circuit, check the line from the inverter to the motor, as well as the motor itself.

2.Light load overcurrent load is very light, but overcurrent trip:

This is the unique phenomenon of frequency conversion speed regulation. In the V/F control mode, there is a very prominent problem: that is, in the process of operation, the motor magnetic circuit system instability. The basic reason for this is that in order to drive a heavy load at low frequencies, it is often necessary to compensate for the torque (that is, to increase the U/F ratio, also known as torque boost) . The saturation of the motor magnetic circuit varies with the weight of the load. This kind of over-current trip caused by the saturation of the motor magnetic circuit mainly occurs in the case of low frequency and light load. Solution: adjust the U/F ratio repeatedly.

3.Overload overcurrent:

(1) fault phenomenon some production machinery in the process of operation load suddenly increased, or even“Stuck”, the speed of the motor because of the belt does not move and drop significantly, the current increases rapidly, overload protection too late to act, cause overcurrent trip. 

(2) solution 

(a) first find out if there is a fault in the machine itself, and if there is a fault, repair the machine. 

(b) if such overloading is a frequent occurrence in the production process, the possibility of increasing the transmission ratio between the motor and the load should be considered first? Appropriately increasing the transmission ratio, can reduce the resistance torque on the motor shaft, to avoid the case of belt immobility. If you can not increase the transmission ratio, then only consider increasing the capacity of the motor and inverter.

4.Overcurrent in speed up or down:

This is due to speed up or speed down caused by the following measures can be taken: 

(1) to extend the speed up (speed down) time first know according to the requirements of the production process whether it is allowed to extend the speed up or speed down time, if so, can extend the rise (fall) speed time. 

(2) the accurate pre-position device for the increase (decrease) speed self-processing (anti-stall) function the frequency converter has the self-processing (anti-stall) function for the over-current in the process of the increase and decrease of speed. When the up (down) current exceeds the preset upper limit current, the up (down) speed will be suspended until the current drops below the set value, then continue the up (down) speed.

Overloading trip of frequency converter

The motor can rotate, but the running current exceeds the rated value, called overload. The basic reflection of overload is: although the current exceeds the rated value, but the range is not large, generally do not form a larger impact current

1. The main cause of overload (1) mechanical overload, the main feature of overload is the motor heating, and can be read from the display running current to find. (2) the unbalance of three-phase voltage leads to the over-current of a certain phase, which leads to overload trip, reading the running current from the display is not necessarily detectable (because the display only shows one phase of current) . (3) misoperation, the internal current detection part of the inverter fault, the detected current signal is too large, leading to tripping. 

2, check method (1) check whether the motor is hot, if the temperature of the motor is not high, first of all, check whether the electronic heat protection function of the frequency converter is reasonable, if there is room for the frequency converter, should be relaxed electronic thermal protection function of the pre-value. If the temperature rise of the motor is too high, and the overload that appears belongs to normal overload, it means that the load of the motor is too heavy. At this point, the first should be appropriate to increase the transmission ratio to reduce the load on the motor shaft. If can increase, then increase the transmission ratio. If the transmission ratio can not be increased, then should increase the capacity of the motor. (2) check whether the three-phase voltage on the motor side is balanced, if the three-phase voltage on the motor side is not balanced, then check whether the three-phase voltage at the output end of the inverter is balanced, if it is not balanced, then the problem is in the inverter. If the voltage balance at the output end of the inverter, the problem in the line from the inverter to the motor, should check all the wiring screws have been tightened, if there is a contactor or other electrical appliance between the inverter and the motor, it should also check whether the terminals of the relevant electrical appliance have been tightened, and whether the contact condition is good, etc. .

If the three-phase voltage on the motor side is balanced, the operating frequency of tripping should be known: if the operating frequency is low and vector control is not used (or not used) , then the U/F ratio should be reduced first, if the lower U/F ratio can still drive the load, it shows that the original prearranged U/F ratio is too high, the peak value of the excitation current is on the high side, the current can be reduced by reducing the U/F ratio; if the lower U/F ratio can not drive the load, if the inverter has the function of vector control, the vector control mode should be adopted.

Source: internet (for study only, intrusion and deletion)