Now, frequency converters have perfect protection measures. Overvoltage and overcurrent are the two most important protection in all protection, which directly affect the safety and reliability of converter. Overcurrent protection mainly refers to the character of mutation, and the peak of current exceeds the allowable value of the frequency converter. Over current protection has the characteristic of anti time limit. Therefore, how to suppress overcurrent in the application of frequency converter is a key problem. The following are the reasons for the analysis of the current and how to prevent it.
1. Causes of over current generation
There are many reasons for overcurrent, such as soft fault and hard fault.
1. reasons for soft fault
When the inverter parameters in the acceleration or deceleration time is too short, the motor power is heavy, it means that in acceleration, the working frequency of the inverter is rising too fast motor synchronous speed N0 increases rapidly, and the motor speed is n due to load inertia and to keep up, causing the rotor speed of cutting magnetic lines too fast (the equivalent slip is too large), resulting in excessive current, caused by the current transducer.
2. reasons for hard failure
(1) the transmission mechanism is blocked, the operation is not flexible, the motor load is too heavy, which causes the electric current of the motor to increase.
(2) the short circuit of the output terminal of the inverter or the unbalanced three-phase voltage causes the unbalanced three-phase current and causes overcurrent.
(3) the inverter itself is damaged, such as the aging of the inverter, the misoperation of the current transformer and so on.
When the cable wire between the inverter and the motor is too long (general inverter manufacturers recommend the output wire is less than 50m) will appear the phenomenon of insufficient output, in order to meet the load requirement needs to increase the output voltage of the inverter current; in addition to the high frequency state, the cable can be equivalent to a capacitor, the capacitance between lines, on the ground the capacitance is increased due to cable lengthening, if the output carrier frequency converter is very high, then the output attenuation is large, in order to meet the load requirements, it is necessary to increase the current, it may lead to excessive current.
Two, the solution of overcurrent
1., in order to meet the requirements of production equipment and process, accelerate or decelerate time as much as possible, so as to avoid overcurrent in acceleration or deceleration process.
2. check whether the matching of frequency converter, motor, production equipment is good, the transmission part is flexible, or whether the material has the phenomenon of card death.
3. whether the frequency converter is in good condition. Whether the three-phase voltage balance is in line with the requirements, if it does not meet the requirements, check the drive waveform of the frequency converter is normal. In addition, some of the inverter's current detection links fail to produce overcurrent display. Some brand converters do not show overcurrent indication even if there are problems in current detection section, which should be noticed in use. If the inverter main circuit device has problems, the phenomenon of overcurrent will appear.
4. when the output cable of the inverter is lengthened, the high frequency loss is added, which makes the frequency converter out of force. The following two methods should be used to deal with this problem.
(1) make some modifications on the parameters of the frequency converter. When the condition is allowed, the output carrier frequency of the inverter can be modified, the output frequency can be reduced and the high frequency loss can be reduced. In addition, the output torque can be improved to reduce the effect of high frequency loss.
(2) the AC reactor can be added at the output end of the inverter, which can restrain the sudden change of current and prevent over current. The selection of the reactor can be connected with the inverter manufacturer to choose the reactor with the power matching. In addition, the relevant materials can also be consulted for their own design.
In a word, there are many reasons for the over current produced by the frequency converter, which should be analyzed according to the specific conditions. In case of frequent overcurrent in the frequency converter and confirmation that overload is caused, a high grade inverter can be selected to overcome overcurrent caused by overload.
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