(1) When the frequency converter is connected to a power grid with a large capacity power transformer (500kva or above), or when a thyristor converter is connected to the same power transformer but no converter reactor is used, or when there is a switching capacitor bank for power improvement on the same power grid, an AC reactor or DC reactor should be configured. They also have the effect of improving the power factor of the frequency converter power supply side and reducing the input high-order harmonic current.
(2) A low-voltage circuit breaker and an AC contactor with short-circuit and overload protection should be installed between the frequency converter and the power supply to prevent the accident from escalating when the frequency converter fails. The emergency stop control of the electrical control system should disconnect the AC contactor on the power supply side of the frequency converter to completely cut off the power supply to the frequency converter and ensure the safety of equipment and personnel.
(3) The input terminals r, s, t and the output terminals u, v, w of the frequency converter must not be connected incorrectly. The input terminals r, s, t of the frequency converter are connected to the input terminals of the three-phase rectifier bridge, while the output terminals u, v, w are connected to the inverter circuit of the three-phase asynchronous motor. If the two are connected incorrectly, at best the frequency conversion speed regulation cannot be achieved and the motor will not run, at worst the frequency converter will be burned out.
(4) In situations where frequent starts and stops occur, do not use the on/off state of the main circuit power supply to control the start and stop of the frequency converter. Instead, use the run/stop button or sf/sr control terminal on the frequency converter control panel. This is because when the frequency converter starts, it first charges the large-capacity electrolytic capacitor in the DC circuit. If the frequency converter is started frequently, the current-limiting resistor used for capacitor charging will inevitably overheat, and the service life of the large-capacity electrolytic capacitor will also be shortened.
(5) The terminal “n” of the inverter is the low-level terminal of the intermediate DC circuit. It is strictly forbidden to connect it to the neutral line or ground in the three-phase four-wire power supply line . Otherwise, the inverter will be damaged due to short circuit of the three-phase rectifier bridge.
(6) Electromagnetic contactors should generally not be installed on the output side of the frequency converter. If installation is necessary, the following conditions must be met: Switching the electromagnetic contactor on the output side is strictly prohibited while the frequency converter is running; switching the contactor must only be done after the frequency converter has stopped outputting. This is because switching the contactor on the output side while the frequency converter is outputting normally will generate a very high overvoltage at the moment the contactor contacts open, which can easily damage the power electronic components in the frequency converter. Therefore, switching the contactor on the output side of the frequency converter must only be done when the power transistors of the frequency converter are locked.
(7) For frequency converters with built-in braking units that require an external braking resistor, pay close attention to the correct wiring of the braking resistor. The braking resistor should be connected between p and db, not between p and n. Otherwise, the inverter's three-phase rectifier bridge will be fully loaded when the inverter is not running, causing the frequency converter to malfunction and the braking resistor to burn out.
(8) The number of pole pairs of a multi-speed motor with variable frequency speed control cannot be changed during operation. If a multi-speed motor must be selected in a variable frequency speed control system to expand the speed range, the multi-speed motor achieves speed control by changing the stator winding connection method to change the number of pole pairs. If the motor winding connection is changed while the frequency converter is running, it will cause a large inrush current, resulting in overload tripping of the frequency converter or even serious accidents such as burnout. Therefore, to safely switch the windings of a multi-speed motor, it is necessary to wait until the frequency converter stops outputting.
(9) Correct use of mechanical brakes in variable frequency speed control systems. In pulse width modulation (PWM) frequency converters, the ratio of output frequency to output voltage is a constant, i.e., v/f=c. When the output frequency is low, the output voltage is also low. If the electromagnetic brake coil of the mechanical brake is connected to the u, v, w terminals, the mechanical brake will always be in a tight state when the frequency converter is at low speed, and the frequency converter will trip due to overload. Therefore, the electromagnetic coil of the mechanical brake can only be connected to the r, s, t terminals of the frequency converter input.
(10) Characteristics and countermeasures of frequency converters operating at low speeds. Conventionally designed self-ventilated asynchronous motors will not exceed their rated temperature rise under rated operating conditions and within the specified ambient temperature range. However, the situation is different in a variable frequency speed control system. When a self-ventilated asynchronous motor operates below 20 Hz, the cooling capacity of the rotor fan decreases. Furthermore, if it operates under constant torque load conditions for an extended period, the motor temperature rise will inevitably increase, deteriorating the characteristics of the speed control system. Therefore, when a self-ventilated asynchronous motor operates at low frequencies and drives a constant torque load, forced cooling measures must be taken to improve the motor's heat dissipation capacity and ensure the stability of the variable frequency speed control system.
(11) When the wiring between the inverter and the motor is too long, the distributed capacitance increases significantly as the length of the inverter output cable increases, which causes the capacitive peak current of the inverter output to be too large, causing the inverter to trip. Therefore, it is necessary to use output reactors, du/dt filters or sine wave filters to limit this capacitive peak current.
Output filter reactors are used to compensate for the charging current of line capacitors caused by long-distance motor cable laying, and also to suppress harmonics. When multiple motors are driven in a group, one output filter reactor can be connected, and the total cable length is the sum of the lengths of the cables for each motor. Theoretically, the allowable length of motor cable varies depending on the power rating of the inverter, and also differs between inverters from different manufacturers. Therefore, regarding the maximum allowable distance for motor cables laid with inverters that requires the installation of an output filter reactor, it is essential to refer to the user manual provided by the respective inverter manufacturer.
(12) Do not install power capacitors, surge suppressors and radio noise filters on the output side of the inverter, as this will cause inverter failure or damage to capacitors and surge suppressors.
(13) Leakage current of frequency converters and its countermeasures. Due to the presence of distributed capacitance in the input and output wiring of the frequency converter and the motor windings, and since most current frequency converters use PWM modulation, leakage current will flow through them. The value of leakage current is proportional to the distributed capacitance and the carrier frequency of the frequency converter. Therefore, to reduce the leakage current of the frequency converter, one is to shorten the wiring length between the frequency converter and the motor as much as possible, and the other is to reduce the carrier frequency of the frequency converter as much as possible.
To protect equipment and personnel safety, a residual current circuit breaker (RCCB) can be installed on the incoming side of the frequency converter. When a dedicated RCCB for the frequency converter is selected, its rated sensitivity current is:
i△n≥10×(ig1+ig2+ign+igm)
When a general residual current circuit breaker is selected, its rated sensitivity current is:
i△n≥10×【ig1+ign+3×(ig2+igm)】
Where: ig1—leakage current of the inverter input circuit when the power frequency power supply is running;
ig2—Leakage current in the inverter output circuit during operation of the power frequency power supply;
IGN—Leakage current of the noise filter on the input side of the frequency converter;
igm—Leakage current of the motor when the power frequency power supply is running.
If the basic leakage current data in the above formula is difficult to determine, the following empirical guidelines can be used for selection. The rated sensitivity current of a dedicated leakage circuit breaker for frequency converters is estimated at 20mA per frequency converter, while the rated sensitivity current of a general leakage circuit breaker is estimated at 50mA per frequency converter.
