Currently, the AC speed control system commonly referred to mainly refers to the frequency conversion speed control system for AC motors using electronic power converters. Due to its superior characteristics compared to DC drives, frequency conversion speed control systems are often the preferred transmission solution in many applications. Modern frequency conversion speed control systems generally use 16-bit or 32-bit microcontrollers as the control core, thus achieving fully digital control. The speed control performance is basically similar to DC speed control. However, the maintenance of frequency converters is more complex than that of DC drives. If a fault occurs, it is difficult for ordinary electrical personnel in the company to handle. This article analyzes the causes and solutions for common frequency converter faults.
I. Parameter setting related faults
In the use of common frequency converters, whether they can meet the requirements of the drive system depends heavily on the parameter settings of the frequency converter. If the parameters are not set correctly, the frequency converter will not work properly.
1. Parameter settings
Common frequency inverters typically have default values for each parameter set by the manufacturer at the time of shipment; these are called factory values. Under these parameter values, the inverter can operate normally via the control panel, but control panel operation does not meet the requirements of most drive systems. Therefore, before using the frequency inverter correctly, users should check the inverter parameters from the following aspects:
(1) Confirm the motor parameters. The inverter sets the motor's power, current, voltage, speed, and maximum frequency in the parameters. These parameters can be obtained directly from the motor nameplate.
(2) The control method adopted by the frequency converter, namely speed control, torque control, PID control or other methods. After adopting a control method, static or dynamic identification is generally required depending on the control accuracy.
(3) Set the start mode of the frequency converter. Generally, the frequency converter is set to start from the panel when it leaves the factory. Users can choose the start mode according to the actual situation. There are several options such as panel, external terminal, and communication.
(4) Selection of the given signal: Generally, the frequency of a frequency converter can be given in several ways, such as panel setting, external setting, external voltage or current setting, and communication setting. Of course, the frequency of the frequency converter can also be one or a combination of these methods. After correctly setting the above parameters, the frequency converter can basically work normally. If you want to obtain a better control effect, you can only modify the relevant parameters according to the actual situation.
2. Handling parameter setting-related faults
Once a parameter setting fault occurs, the frequency converter will not operate normally. Generally, the parameters can be modified according to the instruction manual. If this doesn't work, it's best to restore all parameters to factory settings and then reset them following the steps above. However, the parameter restoration method may vary depending on the company's frequency converter manufacturer.
II. Overvoltage-related faults
Overvoltage in frequency converters is primarily manifested in the DC bus voltage. Under normal circumstances, the DC voltage of the frequency converter is the average value after three-phase full-wave rectification. If calculated using a 380V line voltage, the average DC voltage Ud = 1.35Uline = 513V. When an overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage reaches approximately 760V, the frequency converter's overvoltage protection will activate. Therefore, frequency converters have a normal operating voltage range. Exceeding this range can potentially damage the frequency converter. There are two common types of overvoltages.
1. Input AC power overvoltage
This situation refers to an input voltage exceeding the normal range. It usually occurs during holidays when the load is light, causing the voltage to rise or fall and resulting in a circuit fault. In this case, it is best to disconnect the power supply and check and handle the problem.
2. Overvoltages in power generation
This situation is relatively common, mainly because the synchronous speed of the motor is higher than the actual speed, causing the motor to be in a generating state, and the frequency converter does not have a braking unit installed. There are two situations that can cause this fault.
(1) When the frequency converter drives a large inertia load, its deceleration time is set relatively short. During deceleration, the output speed of the frequency converter is relatively fast, while the load decelerates relatively slowly due to its own resistance. This causes the speed of the motor driven by the load to be higher than the speed corresponding to the frequency output of the frequency converter. The motor is in a generating state, but the frequency converter does not have an energy feedback unit. As a result, the voltage of the DC circuit of the frequency converter rises, exceeds the protection value, and a fault occurs. This often occurs in the drying section of paper machines. To deal with this fault, a regenerative braking unit can be added, or the frequency converter parameters can be modified to set the deceleration time of the frequency converter to be longer. The functions of adding a regenerative braking unit include energy consumption type, parallel DC bus absorption type, and energy feedback type. The energy consumption type connects a braking resistor in parallel in the DC circuit of the frequency converter and controls the on and off of the power tube by detecting the DC bus voltage. The parallel DC bus absorption type is used in multi-motor drive systems. In such systems, one or more motors often work in a generating state, generating regenerative energy. This energy is absorbed by the motors in the motoring state through the parallel bus. The grid-side converter of the energy feedback type frequency converter is reversible. When regenerative energy is generated, the reversible converter feeds the regenerative energy back to the grid.
(2) This fault can also occur when multiple motors drive the same load, mainly due to a lack of load distribution. For example, if two motors drive a single load, and the actual speed of one motor is greater than the synchronous speed of the other, the motor with the higher speed acts as the prime mover, while the one with the lower speed is in generator mode, causing the fault. This often occurs in the press and wire sections of paper machines, requiring load distribution control during troubleshooting. The characteristics of the frequency converters located in the paper machine's transmission speed chain branches can be softened.
III. Overcurrent Fault
Overcurrent faults can be categorized into acceleration, deceleration, and constant speed overcurrent. They may be caused by factors such as excessively short acceleration/deceleration times of the inverter, sudden load changes, uneven load distribution, or output short circuits. Solutions generally include extending acceleration/deceleration times, reducing sudden load changes, adding energy-consuming braking components, improving load distribution design, and inspecting the circuit. If the inverter still experiences an overcurrent fault after disconnecting the load, it indicates a loop in the inverter circuit, requiring replacement of the inverter.
IV. Overload Failure
Overload faults include inverter overload and motor overload. These may be caused by factors such as excessively short acceleration time, excessive DC braking, low mains voltage, or excessive load. They can generally be resolved by extending the acceleration time, extending the braking time, and checking the mains voltage. Excessive load may be caused by the selected motor and inverter being unable to drive the load, or it may be due to poor mechanical lubrication. In the former case, a higher-power motor and inverter must be replaced; in the latter case, the production machinery must be overhauled.
