The English translation of frequency converter is VFD (Variable-frequency Drive). This may be one of the few examples of modern technology reversely translated from Chinese to English. (However, VFD can also be interpreted as Vacuum fluorescent display, so this translation is not commonly used). Frequency converter uses frequency conversion technology and microelectronics technology to control the power transmission components of AC motors by changing the frequency and amplitude of the motor’s operating power supply. Frequency converters were once called VVVF (Variable Voltage Variable Frequency Inverter) in Asian regions such as China and South Korea due to the influence of Japanese manufacturers.
1: VVVF is the abbreviation of Variable Voltage and Variable Frequency, which means changing voltage and changing frequency, which is what people call variable voltage and frequency.
2: CVCF is the abbreviation of Constant Voltage and Constant Frequency, which means constant voltage and constant frequency, which is what people call constant voltage and constant frequency.
The power supply we use is divided into AC power supply and DC power supply. The general DC power supply is mostly obtained by transforming the AC power supply through a transformer and rectifying and filtering it. AC power accounts for about 95% of the total power used by people.
Whether used in homes or factories, the voltage and frequency of single-phase AC power supply and three-phase AC power supply have certain standards according to the regulations of various countries. For example, mainland my country stipulates that direct user single-phase AC power is 220V, and three-phase AC power is 220V. The wire voltage is 380V and the frequency is 50Hz. The power supply voltage and frequency in other countries may be different from those in our country, such as single-phase 100V/60Hz, three-phase 200V/60Hz, etc., standard voltage and frequency AC power supply. It’s called power frequency alternating current.
Generally, a device that converts power frequency alternating current with fixed voltage and frequency into alternating current with variable voltage or frequency is called a “frequency converter”. In order to produce variable voltage and frequency, the device first converts the alternating current from the power supply into direct current (DC), a process called rectification. The scientific term for a device that converts direct current (DC) into alternating current (AC) is “inverter”.
Generally, an inverter converts DC power into an inverter power supply with a certain fixed frequency and voltage. The inverter with adjustable frequency and adjustable voltage is called a frequency converter.
The waveform output by the frequency converter is a simulated sine wave, which is mainly used for speed regulation of three-phase asynchronous motors, also called a variable frequency speed regulator. For variable frequency inverters that are mainly used in instrumentation and testing equipment and have higher waveform requirements, the waveforms need to be sorted and can output standard sine waves, which are called variable frequency power supplies. Generally, variable frequency power supply is 15-20 times more expensive than the inverter. Since the main device in the inverter equipment that produces changing voltage or frequency is called “inverter”, the product itself is named “inverter”, that is: inverter.
Frequency converters can also be used in home appliances. Among the home appliances that use inverters, there are not only motors (such as air conditioners, etc.), but also products such as fluorescent lamps.
Frequency converters used for motor control can change both voltage and frequency. But inverters for fluorescent lamps are mainly used to adjust the frequency of power supply. Devices used in automobiles that generate alternating current from batteries (direct current) are also sold under the name “inverter.” The working principle of frequency converter is widely used in various fields. For example, computer power supply. In this application, the frequency converter is used to suppress reverse voltage, frequency fluctuations and instantaneous power outages.
1. What is a frequency converter?
Frequency converter is a power control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency. It is mainly composed of two parts of circuits, one is the main circuit (rectifier module, electrolytic capacitor and inverter module), and the other is the control circuit (switching power supply board, control circuit board). The CPU is installed on the control circuit board, and the operating software of the inverter is burned on the CPU. The software of the inverter of the same model is fixed. The only exception is the Sanjing inverter. The software can be changed according to the use needs.
2. What is the difference between PWM and PAM?
PWM is the abbreviation of Pulse Width Modulation in English. It is a value adjustment method that changes the pulse width of the pulse train according to certain rules to adjust the output volume and waveform.
PAM is the abbreviation of Pulse Amplitude Modulation in English. It is a modulation method that changes the pulse amplitude of the pulse train according to certain rules to adjust the output value and waveform.
3. What is the difference between voltage type and current type?
The main circuit of the frequency converter can be roughly divided into two categories: the voltage type is a frequency converter that converts the DC of the voltage source into AC, and the filter of the DC circuit is a capacitor; the current type is a frequency converter that converts the DC of the current source into AC. The DC link filter is an inductor.
4. Why does the voltage of the inverter change in proportion to the frequency?
The torque of an asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. At the rated frequency, if the voltage is constant and the frequency is only reduced, the magnetic flux will be too large and the magnetic circuit will be saturated. In severe cases, Will burn the motor. Therefore, the frequency and voltage should be changed in proportion, that is, while changing the frequency, the output voltage of the inverter should be controlled to keep the magnetic flux of the motor constant and avoid the occurrence of field weakening and magnetic saturation. This control method is mostly used in energy-saving frequency converters for fans and pumps.
5. When the motor is driven by industrial frequency power supply, the current increases when the voltage drops. For inverter drive, if the voltage also drops when the frequency drops, does the current increase?
When the frequency decreases (low speed), if the same power is output, the current will increase; but under the condition of constant torque, the current will almost remain unchanged.
6. When running with a frequency converter, what are the starting current and starting torque of the motor?
Using a frequency converter to operate, the frequency and voltage increase as the motor accelerates, and the starting current is limited to less than 150% of the rated current (125% to 200% depending on the model). When starting directly with a commercial power supply, the starting current is 6 to 7 times the rated current, so mechanical and electrical shocks will occur. Using frequency converter drive can start smoothly (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70%~120% of the rated torque; for inverters with automatic torque enhancement function, the starting torque is more than 100% and can be started with full load.
7. What does V/f mode mean?
When the frequency decreases, the voltage V also decreases proportionally. This question has been explained in answer 4. The proportional relationship between V and f is predetermined taking into account the characteristics of the motor. There are usually several characteristics stored in the memory device (ROM) of the controller, which can be selected with a switch or dial.
8. How does the torque of the motor change when V and f are changed proportionally?
When the frequency decreases, the voltage is completely reduced in proportion. Then, since the AC impedance becomes smaller and the DC resistance remains unchanged, the torque generated at low speed will tend to decrease. Therefore, when V/f is given at low frequency, the output voltage should be increased in order to obtain a certain starting torque. This compensation is called enhanced starting. Various methods can be used to achieve this, including automatic methods, selecting V/f mode or adjusting potentiometers, etc.
9. The instruction manual says that the variable speed range is 60~6Hz, that is, 10:1, so there is no output power below 6Hz?
Power can still be output below 6Hz, but depending on conditions such as motor temperature rise and starting torque, the minimum operating frequency is around 6Hz. At this time, the motor can output rated torque without causing serious heating problems. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz depending on the model.
10. For general motor combinations that require a certain torque above 60Hz, is this okay?
Normally this is not possible. Above 60Hz (there is also a mode above 50Hz), the voltage does not change and it is generally a constant power characteristic. When the same torque is required at high speed, attention must be paid to the selection of motor and inverter capacity.
11. What does open loop mean?
If a speed detector (PG) is provided for the motor device used and the actual speed is fed back to the control device for control, it is called “closed loop”. If it operates without PG, it is called “open loop”. Most general-purpose inverters are open-loop, and some models can use options to provide PG feedback. The speed sensorless closed-loop control method is to calculate the actual speed of the motor based on the magnetic flux based on the established mathematical model, which is equivalent to using a virtual speed sensor. Form closed loop control.
12. What should I do if the actual speed deviates from the given speed?
In open loop, even if the inverter outputs a given frequency, when the motor is running with load, the motor’s speed will fluctuate within the rated slip range (1%~5%). For occasions that require relatively high speed regulation accuracy and require operation close to a given speed even if the load changes, an inverter with PG feedback function (optional) can be used.
13. If a motor with PG is used, can the speed accuracy be improved after feedback?
The frequency converter with PG feedback function has improved accuracy. However, the value of speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter output frequency.
14. What does the stall prevention function mean?
If the given acceleration time is too short and the change in the output frequency of the inverter far exceeds the change in the rotational speed (electrical angular frequency), the inverter will trip due to the flow of overcurrent and stop running, which is called stalling. In order to prevent stalling and allow the motor to continue running, it is necessary to detect the magnitude of the current and perform frequency control. When the acceleration current is too large, slow down the acceleration rate appropriately. The same goes for slowing down. The combination of the two is the stall function.
15. There are models where the acceleration time and deceleration time can be given separately, and there are models where the acceleration and deceleration time can be given together. What is the significance of this?
Models whose acceleration and deceleration can be specified separately are suitable for short-time acceleration and slow deceleration situations, or for small machine tools that require strict production cycle times. However, for fan transmission and other situations, the acceleration and deceleration times are longer. Acceleration time and deceleration time can be given jointly.
16. What is regenerative braking?
If the command frequency of the motor is reduced during operation, the motor will become an asynchronous generator and work as a brake. This is called regenerative (electrical) braking.
17. Can I get greater braking force?
The energy regenerated from the motor is stored in the filter capacitor of the frequency converter. Due to the relationship between the capacity and withstand voltage of the capacitor, the regenerative braking force of a general frequency converter is approximately 10% to 20% of the rated torque. If the optional braking unit is used, it can reach 50%~100%.
18. Please explain the protection function of the frequency converter?
Protection functions can be divided into the following two categories:
(1) Automatically perform corrective actions after detecting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) After detecting the abnormality, block the PWM control signal of the power semiconductor device to cause the motor to automatically stop. Such as over-current cut-off, regenerative over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power outage protection, etc.
19. Why does the protection function of the inverter activate when the clutch is used for continuous load?
When a clutch is used to connect a load, at the moment of connection, the motor changes sharply from the no-load state to an area with a large slip, and the large current flowing causes the inverter to trip over current and become unable to operate.
20. In the same factory, as soon as a large motor is started, the inverter stops during operation. Why is this?
When the motor starts, a starting current corresponding to the capacity will flow through, and the transformer on the stator side of the motor will produce a voltage drop. When the motor capacity is large, the voltage drop will also have a greater impact. The inverter connected to the same transformer will cause undervoltage or instantaneous shutdown. Therefore, sometimes the protection function (IPE) operates, causing the operation to stop.
21. What is frequency conversion resolution? has no meaning?
For digitally controlled inverters, 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 usually takes a value of 0.015~0.5Hz. For example, if the resolution is 0.5Hz, then the upper part of 23Hz can be changed to 23.5 or 24.0 Hz, so the movement of the motor is also followed in steps. This causes problems for applications such as continuous coil control. In this case, if the resolution is about 0.015Hz, it can be fully adapted to a 4-stage motor with a step difference of less than 1 r/min. In addition, the given resolution and output resolution of some models are different.
22. Are there any restrictions on the installation direction when installing the inverter?
The structure inside and on the back of the inverter considers the cooling effect, and the up-down relationship is also important for ventilation. Therefore, the unit type should be installed in the longitudinal position in the panel or hung on the wall, and should be installed as vertically as possible.
23. Is it possible to directly put the motor into a fixed frequency inverter without using soft start?
It is possible at a very low frequency, but if the given frequency is high, it will be similar to the conditions for direct starting of the power frequency power supply. A large starting current (6 to 7 times the rated current) will flow, and the motor cannot start because the inverter cuts off the overcurrent.
24. What should we pay attention to when the motor runs above 60Hz?
When operating above 60Hz, please pay attention to the following matters:
(1) The machinery and equipment must be fully capable of operating at this speed (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain operation (the output power of shafts such as fans and pumps increases in proportion to the cube of the speed, so pay attention when the speed increases slightly).
(3) The issue of bearing life must be fully considered.
(4) For motors with medium capacity or above, especially 2-pole motors, please discuss carefully with the manufacturer when operating above 60Hz.
25. Can the inverter drive a gear motor?
Depending on the structure and lubrication method of the reducer, several issues need to be paid attention to. In terms of gear structure, 70~80Hz can usually be considered as the maximum limit. When oil lubrication is used, continuous operation at low speed may cause damage to the gear.
26. Can the frequency converter be used to drive a single-phase motor? Can single phase power be used?
Basically unusable. For single-phase motors started by the speed regulator switch, the auxiliary winding will be burned in the speed regulation range below the operating point; for capacitor starting or capacitor operation, the capacitor will explode. The power supply of the inverter is usually three-phase, but for small capacity, there are also models that operate on single-phase power.
27. How much power does the inverter itself consume?
It is related to the inverter model, operating status, frequency of use, etc., but it is difficult to answer. However, the efficiency of the inverter below 60Hz is about 94% to 96%, and the loss can be calculated based on this. However, with the built-in regenerative braking (FR-K) inverter, if the loss during braking is also taken into account, the power consumption will become larger, so attention must be paid to the design of the operating panel.
28. Why can’t it operate continuously in the whole range of 6~60Hz?
Post time: Feb-28-2024