The object of over-current protection in frequency converter mainly refers to those with sudden change property, the peak value of current exceeds the over-current detection value (about 200% of rated current), and the frequency converter shows OC as over-current. Because the overload capacity of inverter is poor, the over-current protection of frequency converter is an important part.

1、 Cause analysis of converter over current fault

Over current fault can be divided into acceleration, deceleration and constant speed over current. It may be caused by the short acceleration and deceleration time, sudden load change, uneven load distribution, short-circuit output and other reasons. At this time, it can be solved by prolonging the acceleration and deceleration time, reducing the sudden change of load, adding energy consumption braking elements, designing load distribution, checking the line, etc. If the load converter is disconnected or over-current fault, the inverter circuit of the converter is broken, and the converter needs to be replaced. According to the display of frequency converter, the reasons can be found from the following aspects:

(1) Over current in operation, that is, over current occurs in the working process of the driving system. The reasons are as follows:

First, the motor encounters impact load or "stuck" phenomenon of the structure of the driving machine, which causes the sudden increase of the motor current;

Second, there is a short circuit at the output side of the frequency converter, such as the short circuit between the connecting lines between the output end and the motor, or the short circuit inside the motor, etc., grounding (contact and grounding caused by the motor burning, insulation deterioration, cable damage, etc.)

Third, the inverter works abnormally, for example, the two inverters of the same arm in the inverter bridge are abnormal in the continuous alternate working process. For example, the environment temperature is too high, or the components of the inverter are aging, which causes the parameters of the inverter to change, so that in the alternating process, one component has been turned on, while the other component has not yet been turned off, causing the "direct on" of the upper and lower components of the same bridge arm, and making the positive and negative pole of the DC voltage in a short circuit state.

(2) Over current during speed up and speed down: when the inertia of load is large and the speed up time or speed down time is set too short, over current will also be caused. In the process of speed increase, the working frequency of the frequency converter rises too fast, the synchronous speed of the motor rises rapidly, and the speed of the motor rotor cannot keep up with the load inertia, so the speed increase current is too large; in the process of speed decrease, the speed decrease time is too short, and the synchronous speed drops rapidly, However, the rotor of the motor still maintains a high speed because of the large inertia of the load. At this time, the speed of cutting the magnetic line of force of the rotor winding is also too high, resulting in overcurrent.

2、 Handling method of converter over current fault

(1) When starting, it will trip as soon as the speed is increased, which is a very serious phenomenon of overcurrent. The main inspection is as follows:

A. Whether the working machinery is stuck;

B. Check whether there is short circuit on the load side, use a megger to check whether there is short circuit to the ground and whether the converter power module is damaged;

C. The starting torque of the motor is too small, so the driving system can't rotate.

(2) When starting, it does not trip immediately, but trips during operation. The main inspection is as follows:

Speed up time is set too short, speed up time and speed down time are set too short, speed down time is set too long;

The setting of torque compensation (U / F ratio) is too large, which causes the current overload in low frequency space-time;

The setting of electronic thermal relay is improper, and the action current is set too small, which causes the inverter misoperation.

Finally, it is reminded that if the above methods can not solve the problem, that is, the converter hardware circuit is broken, generally there is a problem in the drive circuit or module, replacing the optocoupler or IGBT can solve most problems.