Analysis of the causes of overcurrent in the operation of frequency converters

In automated production, more and more frequency converters are being used for transmission control, so frequency converters have been widely used in various equipment.

Nowadays, various brands of frequency converters have good protection measures. During the protection process, overvoltage and overcurrent are two important types of protection, which directly affect the stability of the frequency converter. Overcurrent protection mainly refers to the situation where the peak current exceeds the allowable value of the frequency converter due to sudden changes. Overcurrent protection has inverse time characteristics. Therefore, how to suppress overcurrent in the application process of frequency converters is a crucial issue. Below is an analysis of the causes of overcurrent and how to prevent it.

1、 The causes of overcurrent

There are many reasons for overcurrent, including soft and hard faults.

1. Cause of Soft Fault

When the acceleration or deceleration time in the parameters of the frequency converter is set too short and the motor power is heavy, it means that during acceleration, the working frequency of the frequency converter increases too quickly, and the synchronous speed n0 of the motor increases rapidly. However, the motor speed n cannot keep up due to the large inertia of the load, causing the rotor to cut the magnetic field lines too quickly (equivalent to excessive slip), resulting in excessive current and causing the frequency converter to overcurrent.

2. Hard fault cause

(1) The transmission mechanism is blocked, the operation is not flexible, and the motor load is too heavy, which in turn causes an increase in the current of the motor.

(2) Short circuit or three-phase voltage imbalance at the output end of the frequency converter can cause three-phase current imbalance, leading to overcurrent.

(3) The frequency converter itself is damaged, such as aging of inverter components, incorrect operation of current transformers, etc.

When the cable lead between the frequency converter and the motor is too long (usually the recommended output wire by the frequency converter manufacturer is within 50m), insufficient output will occur. To meet the load requirements, it is necessary to increase the current; In addition, the output voltage of the frequency converter is in a high-frequency state, and the cable lead can be equivalent to a capacitor. At this time, the line to line capacitance and ground capacitance increase due to the elongation of the cable. If the output carrier frequency of the frequency converter is high at this time, the output attenuation will be large. In order to meet the requirements of the load, it is necessary to increase the current, which may cause overcurrent.

2、 Solution to overcurrent

Different measures will be taken to address the aforementioned issues to avoid the occurrence of overcurrent.

1. On the premise of meeting the requirements of production equipment and technology, the acceleration or deceleration time should be increased as much as possible to avoid overcurrent during the acceleration or deceleration process.

2. Check whether the matching of the frequency converter, motor, and production equipment is good, whether the transmission part is flexible, and whether the materials are stuck.

3. Is the frequency converter itself intact. Whether the three-phase voltage balance meets the requirements. If it does not meet the requirements, check whether the drive waveform of the frequency converter is normal. In addition, some frequency converters, such as Danfoss's products, may also display overcurrent if there is a fault in the current detection process. Some brands of frequency converters, even if there are problems with the current detection process, do not display overcurrent, which should be noted during use. If there is a problem with the inverter main circuit components of the frequency converter, overcurrent may also occur.

4. When the output cable of the frequency converter is extended, it increases high-frequency loss, resulting in insufficient output of the frequency converter. The following two methods should be used to solve this problem.

(1) Make some modifications to the parameters of the frequency converter. If conditions permit, the output carrier frequency of the frequency converter can be modified to lower the output frequency and reduce high-frequency losses. In addition, the output torque can be increased to reduce the impact of high-frequency losses.

(2) An AC reactor can be added at the output end of the frequency converter to suppress sudden changes in current and prevent overcurrent. The selection of reactors can be contacted with the frequency converter manufacturer to select reactors that match the power. Additionally, you can refer to relevant materials and design your own.

In short, there are many reasons for the overcurrent generated by the frequency converter, and it should be analyzed and dealt with according to the specific situation. If the frequency converter frequently experiences overcurrent and it is confirmed that it is caused by excessive load, a higher-level frequency converter can be selected to overcome the overcurrent caused by overload.