1. What should be noted when using one frequency converter to power multiple motors?
(1) It can be used to move multiple devices simultaneously, but it must be used in translation mechanisms.
(2) The control mode must be v/f, and vector control cannot be used.
(3) The inverter capacity should be greater than or equal to the motor capacity. The specific capacity depends on the load characteristics. Each motor should have heating protection.
2. Why is an output reactor added to the output end of a frequency converter, and what is its function?
Adding an output reactor to the output terminal of the frequency converter increases the distance between the frequency converter and the motor. The output reactor can effectively suppress the instantaneous high voltage generated when the frequency converter's IGBT switches, reducing the adverse effects of this voltage on cable insulation and the motor.
The main function of a reactor is to limit the capacitive charging current of the motor connection cable and to limit the voltage rise rate on the motor winding to within 540V/μs. It is also used to passivate the steepness of the inverter output voltage (switching frequency) and reduce the disturbance and impact on power components (such as IGBTs) in the inverter.
3. Why can't a frequency converter be used as a variable frequency power supply?
The entire circuit of a frequency converter power supply consists of AC-DC, AC, and filtering components, resulting in pure sine waves for both its output voltage and current, closely resembling an ideal AC power supply. It can output the grid voltage and frequency of any country in the world.
A frequency converter is composed of circuits such as AC-DC-AC (modulated wave). The standard name for a frequency converter should be variable frequency speed controller. Its output voltage waveform is a pulse square wave with many harmonic components. The voltage and frequency change proportionally simultaneously and cannot be adjusted separately, which does not meet the requirements of AC power supply. In principle, it cannot be used as a power supply and is generally only used for speed control of three-phase asynchronous motors.
4. What is the effect of carrier frequency on frequency converters and motors?
(1) The higher the operating frequency, the larger the duty cycle of the voltage wave and the smaller the higher harmonic components of the current. That is, the higher the carrier frequency, the better the smoothness of the current waveform.
(2) The higher the carrier frequency, the smaller the current that the inverter can output;
(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor, and the greater the leakage current caused by the high-frequency pulse.
The higher the carrier frequency, the less vibration, the lower the operating noise, and the less heat the motor generates. However, the higher the carrier frequency, the higher the frequency of harmonic currents, the more severe the skin effect of the motor stator, the greater the motor losses, and the lower the output power.
5. Can an AC servo motor be controlled by a frequency converter?
Because frequency converters and servo drives differ in performance and function, and their applications are also quite different, it is not possible to use them together.
(1) In applications where speed and torque control requirements are not very high, frequency converters are generally used. There are also frequency converters used for position control by adding position feedback signals to the host computer to form a closed loop. However, the accuracy and response are not high. Some frequency converters now accept pulse sequence signals to control speed, but they seem unable to directly control position.
(2) In situations where there are strict position control requirements, only servo can be used. Also, the response speed of servo is much greater than that of frequency converter. In some situations where the speed accuracy and response requirements are high, servo control is also used. In almost all situations where frequency converter can be used for motion control, servo can be used instead.
6. What is the frequency adjustment range of the frequency converter?
The maximum output frequency of a general-purpose variable frequency drive (VFD) is generally no higher than 400Hz; the minimum output frequency is no lower than 0.1Hz. The frequency adjustment range varies among different VFDs.
For general industrial electric motors, the maximum operating frequency should not exceed 100Hz.
7. What are the different starting methods for a frequency converter?
Most frequency converters can be set to three starting modes:
(1) Linear method, i.e., ordinary starting method
(2) S-shaped method
(3) Half-S-shaped method
8. Under what circumstances is an external braking unit required?
When the working machinery requires rapid braking, and the braking resistor connected inside the frequency converter cannot consume the regenerated electrical energy within the required time, resulting in the DC portion being depleted, a braking unit needs to be added to accelerate the consumption of regenerated electrical energy.
9. How to determine the resistance value of the external braking resistor?
Generally, you can refer to the data provided in the instruction manual to make the selection. If you need to enhance the braking effect or shorten the braking time, you can also determine it yourself through testing.
During the experiment, the following principles should generally be followed:
(1) The braking current IB shall not exceed the rated current IN of the frequency converter. During initial selection, the braking resistor value should be determined according to IB≤(1/3~1/2)IN;
RB≤UDmax/(1/3~1/2)IN厖厖厖(3-1)
In the formula, UDmax is the peak value of the DC voltage that may occur when regenerative braking begins, within the allowable fluctuation range of the power supply voltage. When the power supply voltage is 380V, UDmax can be calculated as 695V.
(2) Under the premise that the braking effect is satisfied, the value of RB should be selected as large as possible.
10. Why is it necessary to set multiple speeds?
In the program control of machinery, different program segments often require different speeds. Therefore, frequency converters can be pre-set with multiple operating frequencies to meet user needs.
When setting up the frequency converter, users can pre-set several operating frequencies according to the requirements of the machine for selection during program control.
11. Under what circumstances might a frequency converter trigger voltage protection?
There are two main situations:
(1) Overvoltage of power supply: The inverter generally allows the power supply voltage to fluctuate upward within a range of +10%. If this range is exceeded, protection should be implemented.
(2) If the deceleration time is set too short, the braking resistor will not have enough time to release the energy during the regenerative braking process, resulting in excessively high DC circuit voltage and forming excessively high voltage.
(3) Circuit board bus fault detection
12. What are the possible consequences of interference signals?
When the capacity of a variable frequency speed control system is large enough, the high-frequency signals it generates will be sufficient to interfere with the operation of various surrounding electronic devices. The main consequences are:
(1) It affects the normal reception of radio equipment.
(2) It affects the normal operation of surrounding machines, causing them to malfunction due to receiving incorrect signals, or causing judgment errors due to affecting the detection accuracy of sensing circuits.
13. Why can't a capacitor be connected to the output of a frequency converter to improve the current waveform flowing into the motor?
Because the output voltage of the frequency converter is a rectangular pulse sequence, it contains many high-order harmonic components. Since the capacitive reactance of the capacitor is small under high-order harmonics, the high-order harmonic current is large. This increases the burden on the inverter transistors, and the capacitors themselves are also prone to damage due to overheating.
14. What are the environmental requirements for frequency converters?
The ideal operating environment should have appropriate temperature and humidity, and the installation environment should be free of conductive particles and dust.
Temperature and humidity: The operating ambient temperature should be between -10℃ and 40℃. If the temperature exceeds 40℃, the inverter must be derated. The maximum operating temperature should not exceed 50℃. For every 1℃ increase in ambient temperature above 40℃, the derated value should be 4%. The relative humidity of the air should be ≤90%, with no condensation. Also, avoid placing the inverter in direct sunlight.
Altitude: When the frequency converter is installed at an altitude below 1000M, it can operate at its rated power. When the altitude exceeds 1000M, the power of the frequency converter needs to be derated.
To improve the reliability of frequency converter operation, the frequency converter should be installed in a well-ventilated place. When used in a closed enclosure, a cooling fan or air conditioner should be installed to keep the ambient temperature below 40 degrees Celsius.
What problems might occur if a frequency converter (A) is left unused for an extended period?
If a frequency converter has been idle for more than six months, the leakage current of the electrolytic capacitors will increase. It is best to run it under no-load for at least half an hour before use!
B. What issues should be considered when selecting a frequency converter?
Inverters do not work properly under all conditions, so users need to have a better understanding of the load, environmental requirements, and inverters.
(1) Selection of load type and frequency converter:
Different loads driven by electric motors require different frequency converters.
A: Fans and water pumps are the most common loads: the requirements for frequency converters are the simplest, as long as the capacity of the frequency converter is equal to the capacity of the motor (air compressors, deep water pumps, sludge pumps, and rapidly changing musical fountains require larger capacities).
B: Crane-type loads: These loads are characterized by a large impact during startup, thus requiring the frequency converter to have a certain margin. Additionally, there is energy feedback during the lowering of the heavy load, necessitating the use of a braking unit or a shared bus configuration.
C: Uneven load: Some loads are sometimes light and sometimes heavy. In this case, the inverter capacity should be selected according to the heavy load situation, such as rolling mill machinery, crushing machinery, mixers, etc.
D: High-inertia loads: such as centrifuges, punch presses, and rotary kilns in cement plants. These loads have large inertia, so they may oscillate during startup, and the motor will regenerate energy during deceleration. A higher-capacity frequency converter should be used to speed up startup and avoid oscillation. A braking unit should be used to eliminate the regenerative energy.
(2) Long-term low-speed operation results in high heat generation of the motor and reduced cooling capacity of the fan. Therefore, it is necessary to increase the reduction ratio or use a 6-pole motor to make the motor operate at a higher frequency.
(3) The installation location of the frequency converter must meet the requirements of the standard environment, otherwise it may cause failure or shorten the service life; the distance between the frequency converter and the drive motor is generally no more than 50 meters. If a longer distance is required, the carrier frequency must be reduced or the output reactor option must be added to ensure normal operation.
15. What kind of motor is an AC variable frequency motor?
Simply put, it means that frequency conversion technology is used in the control of AC motors. An AC variable frequency motor is actually a type of motor that adjusts its speed by regulating the frequency of the AC power supply. Adjusting the AC frequency requires a frequency converter; the motor itself does not change frequency. In many applications where the requirements are not high, a regular motor is used as an AC variable frequency motor by adding a frequency converter for speed regulation.
16. What is regenerative braking?
If the command frequency is reduced while the electric motor is running, the motor will operate as an asynchronous generator and act as a brake. This is called regenerative braking.
17. What is variable frequency resolution? What is its significance?
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in steps. The smallest unit of this step difference is called the frequency conversion resolution. Frequency conversion resolution is typically set to 0.015~0.5Hz. For example, with a resolution of 0.5Hz, 23Hz can be changed to 23.5Hz or 24.0Hz, thus the motor's operation also follows in steps. This causes problems for applications like continuous winding control. In such cases, a resolution of around 0.015Hz is sufficient for a 4-pole motor, where one step difference is less than 1 r/min. Additionally, some models have a different given resolution than their output resolution.
18. What issues should be considered when a motor is running at a frequency exceeding 60Hz?
The following precautions should be taken when operating above 60Hz.
(1) The machinery and equipment must be able to operate at this speed with sufficient capacity (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain operation.
(3) The issue of bearing life should be fully considered.
(4) Pay attention to the problem of motor overheating.
19. Why do frequency inverters need to have upper and lower frequency limits?
After the upper and lower frequency limits are set, the operating frequency of the frequency converter is limited to the upper and lower frequency limits, which can prevent the motor speed from exceeding the application range due to operational errors, thus preventing accidents or losses.
20. Why should frequency converters be installed vertically?
For better heat dissipation and ventilation. Absolutely do not install it upside down! Installing it horizontally may cause the inverter to overheat!
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