(1) When the inverter is connected to the large-capacity power transformer (more than 500kva) power grid, perhaps the thyristor converter converter is connected to the same power transformer without using the converter reactor, or when there is a switching capacitor bank for power improvement in the same power grid, AC reactor or DC reactor should be equipped, which also has the effect of improving the power factor of the power supply side of the inverter and reducing the input higher harmonic current.
(2) Low-voltage circuit breakers and AC contactors with short-circuit and overload protection should be installed between the inverter and the power supply to avoid accident expansion when the inverter has shortcomings. The emergency stop control of the electronic control system should make the AC contactor on the power side of the inverter be interrupted, completely block the power supply of the inverter, and ensure the safety of equipment and personnel.
(3) The input terminal R, S, T and the output terminal U, V and W of the inverter cannot be connected incorrectly. The input terminals R, S and T of the inverter are connected to the input of the three-phase rectifier bridge, while the output terminals U, V and W are inverter circuits connected to the three-phase asynchronous motor. If the two are connected wrongly, the speed regulation of the frequency conversion cannot be realized, the motor will not work, and the frequency converter will be burned.
(4) In the case of starting and intermittent repeatedly, do not use the on/off of the main circuit power supply to control the start and interruption of the inverter, and use the run/stop button or SF/SR control terminal on the inverter control panel. Since the inverter is initiated, the large-capacity electrolytic capacitor of the DC circuit must be charged first, assuming that the inverter is repeatedly initiated, it will inevitably constitute a severe heating of the current-limiting resistor for capacitor charging, and at the same time, it also shortens the service life of the large-capacity electrolytic capacitor.
(5) The terminal "n" of the inverter is the low-level end of the central DC circuit, and it is forbidden to connect with the neutral line or ground in the three-phase four-wire power supply line, otherwise it will constitute a three-phase rectifier bridge and damage the inverter due to a short circuit in the power supply.
(6) The output side of the inverter generally cannot be equipped with an electromagnetic contactor, if there is a necessary equipment, the following conditions must be satisfied: if the inverter is in operation, stop switching the electromagnetic contactor on the output side; To switch the contactor, it is necessary to compare the intermittent output of the inverter. Assuming that switching the contactor on the output side during the normal output of the inverter will generate a high overvoltage at the moment when the contactor contact is disconnected, and it is very easy to damage the power electronics in the inverter. Therefore, in order to switch the contactor on the output side of the inverter, it is necessary to switch the power tube of the inverter when it is closed.
(7) In case of inverter with built-in braking unit and need to add braking resistor, be sure to pay attention to the correct wiring of braking resistor. The braking resistor should be connected between P and DB, not between P and N, otherwise the inverter that will constitute the inverter will operate at full load when the three-phase rectifier bridge is not in operation, and the inverter will not work normally, and the braking resistance may also burn out.
(8) The multi-speed motor with frequency conversion speed regulation cannot change the number of pole pairs during operation. Assuming that in the frequency conversion speed regulation system, it is necessary to select a multi-speed motor in order to expand the scale of speed regulation, because the multi-speed motor is to change the stator winding wiring method, change the number of pole pairs, and realize the speed regulation intention. It is assumed that changing the winding wiring of the motor during the inverter operation will cause a large impulse current, which will constitute a serious accident of overload tripping and even burning of the inverter. Therefore, in order to safely switch the windings of a multi-speed motor, it is necessary to compare the intermittent output of the inverter.
(9) The correct application of mechanical brakes in the frequency conversion speed regulation system. In a pulse-width modulated (PWM) inverter, the ratio of the output frequency to the output voltage is a constant, i.e., v/f=c. When the output frequency is low, the output voltage is also low, assuming that the electromagnetic brake coil of the mechanical brake is connected to the U, V, and W ends, then the mechanical brake is always in a tight state at the low speed of the inverter, and the inverter will trip due to overload, so the electromagnetic coil of the mechanical brake can only be connected to the input end of the inverter R, S, and T ends.
(10) Characteristics and countermeasures of inverter during low-speed operation. The conventional design of the self-ventilated asynchronous motor will not exceed the rated temperature rise under the rated working conditions and within the regular ambient temperature scale, but in the frequency conversion speed regulation system, the situation is different. When the self-ventilated asynchronous motor is operating below 20Hz, the cooling capacity of the rotor blades decreases, and then assuming that the long-term operation under the condition of constant torque load will inevitably constitute an increase in the temperature rise of the motor, which will make the characteristics of the speed regulation system deteriorate. Therefore, when the self-ventilated asynchronous motor is operating at low frequency and dragging the constant torque load, it is necessary to adopt forced cooling measures to improve the heat dissipation capacity of the motor and ensure the stability of the frequency conversion speed regulation system.
(11) When the wiring between the inverter and the motor is too long, with the increase of the length of the output cable of the inverter, its dispersion capacitance increases significantly, and then the capacitive spike current that constitutes the inverter output of the inverter is too large and causes the inverter trip protection, so it is necessary to use the output reactor or DU/DT filter or sine wave filter and other equipment to limit the capacitive spike current.
The output filter reactor is used to compensate for the line capacitor charging current caused by the long-distance laying of the motor cable, and can also suppress harmonics. When multiple motors are driven in groups, an output filter reactor can be connected, and the total cable length is the sum of the cable lengths of each motor. Theoretically speaking, the length of the motor cable allowed to be laid by the inverter of different power levels is different, and the length of the motor cable allowed to be laid by the inverter of different manufacturers is also different. Therefore, the motor cable laid by the inverter should be equipped with an output filter reactor when the distance exceeds the long distance, and the operation manual provided by each inverter manufacturer should also be consulted.
(12) Do not install power capacitors, surge suppressors, and radio noise filters on the output side of the inverter, as this will lead to defects in the inverter or damage to the capacitors and surge suppressors.
(13) The leakage current of the inverter and its countermeasures. Due to the existence of dispersion capacitance in the input and output wiring of the inverter and the motor winding, and most of the current inverters use the PWM modulation method, there will be leakage current flowing through them, and its value is proportional to the dispersion capacitance and the carrier frequency of the inverter. Therefore, to reduce the leakage current of the inverter, one is to shorten the wiring length between the inverter and the motor as much as possible, and the other is to reduce the carrier frequency of the inverter as much as possible.
In order to protect equipment and personal safety, leakage circuit breakers can be installed on the incoming side of the inverter. When the leakage circuit breaker for the frequency converter is selected, the rated sensitivity current is as follows:
i△n≥10×(ig1+ig2+ign+igm)
When a general leakage circuit breaker is selected, its rated sensitivity current:
i△n≥10×【ig1+ign+3×(ig2+igm）】
Where: IG1 - the leakage current of the input circuit of the inverter during the operation of the power frequency power supply;
IG2—the leakage current of the output circuit of the inverter during the operation of the power frequency power supply;
IGN—the leakage current of the noise filter on the input side of the inverter;
igm—the leakage current of the motor during the operation of the power frequency power supply.
Assuming that the basic data of leakage current in the above equation is not easy to admit, you can select from the following experience. The rated sensitivity current of the special leakage circuit breaker for the inverter is budgeted at 20mA per inverter, and the rated sensitivity current of the general leakage circuit breaker is budgeted at 50mA per inverter.
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