About the control circuit of the inverter and common fault analysis

Li Fuling
(Liaocheng Wanhe Industrial Manufacturing Co., Ltd., Liaocheng, Shandong 251022)
Abstract: This paper introduces the wide range of applications of frequency converters, understands the structure of frequency converters, and briefly introduces the application of frequency converters from electrical characteristics and some common parameters and faults.
Key words: frequency converter; control circuit; interference measure; fault display
With the increasing use of frequency converters in production, understanding the structure of the frequency converter, the electrical characteristics of the main components and the role of some common parameters and their common faults are becoming more and more important for practical work.
1. Inverter control circuit
    The main circuit for supplying power to the asynchronous motor (voltage and frequency adjustable) provides a control signal, which is called a control loop. The control circuit consists of a frequency, a voltage operation circuit, a voltage of the main circuit, a current detection circuit, and a speed detection circuit of the motor. A drive circuit for amplifying the control signal of the arithmetic circuit, and a protection circuit for the inverter and the motor. The speed-free detection circuit is open-loop control; the speed detection circuit is added to the control circuit, that is, the speed command is increased, and the speed of the asynchronous motor can be more accurately closed-loop controlled.
(1) The arithmetic circuit compares the external speed and torque commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) The voltage and current detection circuit is isolated from the main circuit potential to detect voltage, current, and the like.
(3) The driving circuit is a circuit that drives the main circuit device, which is isolated from the control circuit to control the turning on and off of the main circuit device.
(4) The I/O circuit makes the frequency conversion better human-computer interaction, and it has the input of multiple signals (such as running multi-speed operation, etc.), and the input of various internal parameters (such as current, frequency, protection action drive, etc.). .

(5) The speed detecting circuit sets the signal of the speed detector (TG, PLG, etc.) mounted on the asynchronous motor shaft as a speed signal, and sends it to the arithmetic circuit to operate the motor at the command speed according to the command and calculation.
(6) The protection circuit detects the voltage, current, etc. of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent damage to the inverter and the asynchronous motor, the inverter is stopped or the voltage and current are suppressed.
2. Protection of asynchronous motor 1 Overload protection, overload detection device is shared with inverter protection, but when considering overheating at low speed, embed temperature detector in asynchronous motor or use electronic heat installed in inverter Protect to detect overheating. When the frequency is too high, consider reducing the motor load and increasing the capacity of the motor and inverter.
2 Overspeed protection, when the output frequency of the inverter or the speed of the asynchronous motor exceeds the specified value, the inverter operation is stopped.
3 Prevent stall overcurrent. When the asynchronous motor is slow to track during acceleration, the overcurrent protection circuit operates and the operation cannot continue (stall). Therefore, control should be performed before the load current is reduced to suppress the frequency rise or decrease the frequency. The same control is sometimes performed for overcurrent in constant speed operation.
4 Preventing the stall from regenerating the overvoltage, and the regenerative energy generated during deceleration increases the DC voltage of the main circuit. In order to prevent the regenerative overvoltage circuit from protecting, the DC voltage is controlled before the DC voltage is lowered to suppress the frequency drop and prevent the inability to operate (stall).
3. Anti-interference measures of inverter control loop
    Due to the nonlinearity of the main circuit (switching action), the inverter itself is a source of harmonic interference, while the peripheral control loop is a small energy, weak signal loop, which is highly susceptible to interference from other devices, causing the inverter itself and its surroundings. The device is not working properly. Therefore, when the inverter is installed and used, it must take anti-interference measures to the control loop.
(1) Basic control loop of the inverter Generally speaking, the basic return route for signal exchange with the outside has two kinds of analog and digital:
4 ~ 20MA current signal loop (analog); 1 ~ 5V / 0 ~ 5V voltage signal loop (analog).
2 switch signal loop, inverter start and stop command, forward and reverse command, etc. (digital).
External control, the command signal is introduced into the inverter through the above basic circuit, and the interference source also generates an interference potential on the circuit, and the control cable is used as a medium to invade the inverter.
(2) Basic types of interference and anti-interference measures 1 Electrostatic coupling interference refers to the potential generated by the static coupling of the control cable and the surrounding electrical circuit in the cable. When the distance from the interference source cable is increased and the conductor diameter is more than 40 times, the degree of interference will be less obvious. It is also possible to provide a screen conductor between the two cables and then ground the shield conductor.
2 Electrostatic induction interference refers to the potential induced by the magnetic flux generated by the surrounding electrical circuit in the cable. Its strength depends on the amount of magnetic flux generated by the interference source cable, the closed loop area formed by the control cable, and the relative angle between the interference source cable and the control cable.
3 Radio wave interference means that the control cable becomes an antenna, and an external incoming wave generates an electric potential in the cable. The anti-interference measures are the same as 12, if necessary, put the inverter into the iron box for electric wave shielding, and the iron box for shielding must be grounded.
4 Poor contact interference means that the contact between the electrical contact and the relay contact of the inverter control cable is poor, and the resistance changes in the cable. For this, the parallel contact or the level of the electric device is used to solve the problem. The cable connection points should be tightened and reinforced regularly.
5 Grounding interference refers to the grounding of the body or the grounding of the signal. For weak voltage, current loop, any unreasonable grounding can induce various unexpected interferences. For example, if more than two grounding points are set, the potential difference will be generated at the grounding, and interference will occur. . The control cable with a given speed can be grounded at one point. The grounding wire is not used as a signal path. The grounding of the cable is performed on the inverter side. The dedicated grounding terminal is used and is not shared with other grounding terminals.
4, the common fault analysis of the inverter (1) Inverter charging start circuit failure, general-purpose inverter is generally used with pressure type inverter, using AC-DC-AC working mode. When the inverter is just powered up, the charging current is very large due to the very large capacity of the smoothing capacitor on the DC side. A starting resistor is usually used to limit the charging current. After charging is complete, the control circuit shorts the resistance through the contacts of the relay. The start circuit fault generally shows that the start resistor is burned out, and the inverter alarm shows the DC line voltage fault. Generally, when designing the inverter, in order to reduce the volume of the inverter, choose a small starting resistor, the value is mostly 10-50Ω, the power is 10-50W; when the AC input power of the inverter is frequently turned on, or bypass When the contacts of the contactor are in poor contact, the starting resistance will burn out. Therefore, when replacing the resistor, it is necessary to find out the cause. If the fault is caused by the input side power frequency, the inverter must be eliminated to put the inverter into use. If the fault is caused only by the bypass contact component, these must be replaced. Device.
(2) The inverter has no fault display, but it can't run at high speed. After checking the inverter parameters are set correctly, the speed input signal is normal. After the power-on operation test, the inverter DC bus voltage is only about 450V (normal should be 580V-600V) , then test the input side and found that there is a missing phase. The cause of the malfunction is caused by a poor contact of one of the air switches on the input side. The inverter input phase loss alarm does not alarm, still can work in the low frequency band, because the lower limit of the bus voltage of most inverters is 400V, and the inverter only reports the fault when the bus voltage drops below 400V. And when the two-phase input, the DC bus voltage is 380V × 1.2 = 452V > 400V. When the inverter is not running, the DC voltage can also reach the normal value due to the action of the smoothing capacitor. The new inverter adopts PWM control technology. The operation of voltage regulation and frequency modulation is completed in the inverter bridge, so the input phase is missing in the low frequency band. It can still work normally, but due to the input voltage, the output voltage is low, causing the asynchronous motor to rotate at a low frequency.
(3) The inverter displays overcurrent. When this display occurs, first check if the acceleration time parameter is too short, if the torque boost parameter is too large, and then check if the load is too heavy. If these phenomena are not available, the current transformer on the output side and the Hall current detection point on the DC side can be disconnected and operated after reset to see if overcurrent occurs. If yes, it is likely that the IPM module is faulty because the IPM module contains overvoltage, overvoltage, overload, overheat, phase loss, short circuit, etc., and these fault signals are output via the module control pin. The pin is transferred to the controller. After receiving the fault information, the microcontroller blocks the pulse output on the one hand and displays the fault information on the panel on the other hand. The IPM module should be replaced.
(4) The inverter shows an overvoltage fault, and the inverter has an overvoltage fault, which is generally a thunderstorm. Because the lightning is connected to the power supply of the inverter, the voltage detector on the DC side of the inverter is tripped. In this case, usually Simply disconnect the power supply of the inverter for about 1 minute and then power it on. In another case, the inverter drives a large inertia load and an overvoltage phenomenon occurs. In this case, first, increase the deceleration time parameter or increase the braking resistor (brake unit); second, set the stop mode of the inverter to the free stop mode.
(5) The motor is hot, and the inverter shows overload. For the inverter that has been put into operation, the load condition must be checked. For the newly installed inverter, such a fault may occur, the V/F curve is improperly set or the motor parameter setting is faulty. At this time, various parameters must be correctly set. In addition, the heat dissipation performance of the motor is degraded during low-frequency operation, and this may occur. At this time, a heat sink is required.
The frequency converter is used as the asynchronous motor driver. Although its reliability is very high, if it is used improperly or accidentally, it will cause damage to the inverter. In order to use the inverter in the production process, familiarize yourself with the structural principle of the inverter. Understanding common faults is especially important for technicians.

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