Inverter air conditioners we often hear. So what exactly is the inverter?
Principle of frequency converterFrequency conversion is a development of modern power electronics technology, and it is a common conversion device between DC and AC. It can also change the frequency of our AC to control the power control equipment of the AC motor. The frequency converter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, and detection unit micro processing unit.
The inverter adjusts the voltage and frequency of the output power supply by the internal IGBT breaking, and supplies the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the frequency converter has many protection functions. Such as overcurrent, overvoltage, overload protection and so on.
The figure above shows the circuit with variable AC frequency. P and N are converter circuits. The AC can be set to DC and loaded to the load Z. When the pulse signal to P is sinusoidal, the DC is set to be sinusoidal. , which is the upper half of the sinusoid, the period is the sinusoidal regular period of the pulse signal, which is an integer multiple of the alternating current period. Through the cooperation of P and N, a periodic sinusoidal current on the load Z is formed, and the frequency can be adjusted according to the pulse signal period, which is the working principle of the frequency converter.
Application of frequency converterThe inverter air conditioner is equipped with this kind of frequency converter, which can make the air conditioner run at different frequencies. It can automatically adjust the running frequency according to the temperature you set and the room temperature. The temperature difference is high, the running frequency is high, and the cooling or heating is fast. On the contrary, it is small. When the room temperature is close to the set temperature, keep the lowest frequency operation (the lower the running frequency, the smaller the power, the lower the power consumption), avoiding the frequent start of the outdoor compressor, thus saving energy.
In addition, the frequency converter is applied to the motor, which can realize stepless speed regulation, which is widely used in the fields of automobiles and locomotives, and realizes the continuity of speed regulation.
Inverter main circuit structureIt is generally believed that the circuit can be divided into a voltage type structure and a current type control structure from the structure and principle of the main circuit; from the working mode, the main function of the frequency converter is to realize the electric energy conversion from alternating current to alternating current. Therefore, the circuit works. The way is the intersection transformation or the form of the intersection transformation.
These two types of conversion circuits belong to different circuit structures in their essential functions, and each has its own characteristics. For the AC to the inverter, the intermediate part of the DC is omitted, but the number of the switching tubes is not reduced, and the number of switching tubes required by one bridge arm is often doubled. This circuit structure is common in the ultra-high-power low-speed regulating circuit. The biggest disadvantage is that the output power frequency must be less than 1/3 or 1/2 of the grid frequency. Otherwise, the output voltage waveform is very distorted, so it is suitable for low speed motors. In the latest research, the matrix current structure has received more and more attention. However, the biggest problem of this circuit structure is the complexity of control, which often requires complex modulation strategies.
Another versatile circuit structure is a cross-connected main circuit structure, which can be divided into a voltage type and a current type structure from the working mode, and the former has a wide range of uses.
Its characteristics are: the bus voltage is provided for electrolytic capacitor storage in the middle, the front stage adopts diode uncontrolled rectification, simple and reliable, and the inverter adopts three-phase PWM modulation (current modulation algorithm is space voltage vector). Since the electrolytic capacitor of a certain capacity is used, the DC bus voltage is stable. At this time, as long as the switching sequence (output phase sequence, frequency) and duty ratio (output voltage magnitude) of the inverter IGBT are controlled, excellent control can be obtained. characteristic.
The voltage-type AC-DC converter has the advantages of simple structure, less harmonic content, adjustable stator and rotor power factor, etc., which can obviously improve the operating state and output power quality of the doubly-fed generator, and the structure The separation of the mesh side and the rotor side is completely achieved by the DC bus side capacitor. The stator-field oriented vector control system of the doubly-fed generator of the voltage-type AC-DC converter realizes the decoupling control of the active and reactive power of the generator based on the maximum power point tracking of the wind turbine. It is a representative direction of the current variable-speed constant-frequency wind power generation.
In order to adapt to different grid working conditions, more requirements are put on the frequency converter. In order to meet the requirements of different grid voltages, some inverters will add a DC-carrying part to the circuit structure, and the voltage will be in accordance with the working conditions of the motor. Boost, such as adding a boost circuit. In the case of relatively large grid noise, in order to ensure the normal use of the circuit, the filter circuit of the previous stage is added.
Industrial frequency converters are often used in industrial control. How to choose the right frequency converter for these motion motors is a problem that everyone is concerned about. This problem should be based on the site's working environment, the object being controlled, the range of speed requirements, steady-state speed accuracy, torque requirements, and field wiring conditions. A balance is made between production process and production economy.
The selection of the inverter is based on the principle that the operating current characteristic curve of the inverter includes the load current curve. Qualitatively, the selection is made from voltage matching, current matching, and torque matching. This requires a comprehensive and thorough understanding of the power consumption situation of our team. We have a comprehensive understanding of the voltage level of the voltage and the waveform quality of the voltage to ensure that the inverter can work normally. Secondly, it must have a certain load. Cognition, the performance curve of the load determines the application method of the frequency converter. The rated current of the ordinary centrifugal pump frequency converter is consistent with the rated current of the motor, and the deep water pump needs more current.
The engineering experience is that the power of the motor used is how much power the inverter is used. Generally, in order to leave a margin, a slightly larger inverter will be used. If the on-site motor requires frequent starting and braking, a braking resistor must be installed. The size of the braking resistor should be selected according to the power. If the environment of the factory is harsh, the dust is large, and the heat dissipation is difficult, the water-cooled inverter can be selected, which can effectively prevent the module from bursting and effectively reduce the noise. If you need to do the aging test equipment, you can consider using four-quadrant products to effectively reduce power loss. If there is a separate DC power at the site, you can save money by using only purely inverter modules. In addition, in the high-power inverter input end, a reactor must be added, which can improve the power quality of the input device and improve the power factor of the device.
In addition, according to the function of the inverter, it is possible to select different precision control motors. The general industrial motor is an AC induction motor, which can use constant voltage control or constant current control. The equipment used is not exactly the same depending on the type of motor. The functions of the inverters that can be selected according to the control purpose of the motor are also different.
When a high-speed motor is driven by an inverter, the output current value increases due to the small reactance of the high-speed motor and the increase in higher harmonics. Therefore, the selection of the inverter for high-speed motors is slightly larger than that of ordinary motors. If the inverter is to run with a long cable, measures should be taken to suppress the influence of the long cable to the ground coupling capacitor to avoid the insufficient output of the inverter. Therefore, in this case, the inverter capacity should be amplified by one gear or at the output of the inverter. Install the output reactor. For some special applications, such as high temperature, high altitude, this will cause the frequency converter to derate, the inverter capacity should be amplified by one gear.
In some cases where the precision is critical, it is necessary to carry out a series of tests on the motor and then select the type, and even put forward certain requirements for the rotary encoder of the motor. All in all, depending on the local conditions, choosing different motors and inverters according to different site conditions is a constant criterion.
Inverter twelve questions1. What is the difference between voltage type and current type?
The main circuit of the inverter can be roughly divided into two types: the voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, the filtering of the direct current circuit is a capacitor, and the current type is a frequency converter that converts the direct current of the current source into an alternating current. Its DC loop filtering is an inductor.
2. When the motor is driven by the commercial frequency power supply, the current increases when the voltage drops. For the inverter drive, if the voltage drops when the frequency decreases, does the current increase?
When the frequency drops (low speed), if the same power is output, the current increases, but under a certain torque, the current hardly changes.
3. What is the starting current and starting torque of the motor when the inverter is running?
The inverter is operated, and the frequency and voltage are increased correspondingly with the acceleration of the motor. The starting current is limited to 150% of the rated current (125% to 200% depending on the model). When starting directly with the commercial frequency power supply, the starting current is 6 to 7 times the rated current, so a mechanical and electrical impact will occur.
It can be started smoothly with the inverter drive (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 rated torque. For the inverter with automatic torque boosting function, the starting torque is 100% or more, and it can start with full load.
4. In the manual, the shift range is 60~6Hz, which is 10:1. Is there no output power below 6Hz?
The power can still be output below 6 Hz, but according to the conditions of the motor temperature rise and the starting torque, the minimum use frequency is about 6 Hz. At this time, the motor can output the rated torque without causing serious heat generation. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the model. .
5. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?
Usually not allowed.
6. What does it mean to open a ring?
A speed detector (PG) is provided for the motor device to be used, and the actual speed is fed back to the control device for control, which is called "closed loop". If the PG is not operated, it is called "open loop". The general-purpose inverter is mostly open-loop mode, and some models can use the option to perform PG feedback. The speed sensor-free closed-loop control mode is based on the established mathematical model to calculate the actual speed of the motor according to the magnetic flux, which is equivalent to using a virtual speed sensor. Form a closed loop control.
7. What should I do if the actual speed is different for a given speed?
When the ring is open, even if the inverter outputs a given frequency, the motor's speed will fluctuate within the range of rated slip (1%~5%) when the motor is running with load. For the requirement that the speed regulation accuracy is relatively high, even if the load is changed, it is required to operate at a speed close to a given speed, and an inverter (optional) having a PG feedback function can be used.
8. What does the stall prevention function mean?
If the given acceleration time is too short, the output frequency of the inverter changes far more than the change of the speed (electrical angle frequency), the inverter will trip due to the overcurrent, and the operation stops. This is called stall. In order to prevent the stall from running, the magnitude of the current is detected for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. The same is true when decelerating. The combination of the two is the stall function.
9. What are the meanings of the models that can be given separately for the acceleration time and deceleration time, and the acceleration and deceleration time?
Acceleration and deceleration can be given separately. It is suitable for short-time acceleration, slow deceleration, or for small machine tools where the production tact time is strictly required. However, for fan drive and other occasions, the acceleration/deceleration time is longer. The acceleration time and deceleration time can be given together.
10. Why does the protection function of the inverter operate when the load is connected by a clutch?
When the load is connected by the clutch, at the moment of connection, the motor changes sharply from the no-load state to the region where the slip ratio is large, and the large current flowing causes the inverter to over-current trip and cannot operate.
11. What is the frequency conversion resolution? What is the point?
For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is given in a given order. The smallest unit of this level difference is called the frequency conversion resolution.
The frequency conversion resolution usually takes a value of 0.015~0.5Hz. For example, the resolution is 0.5Hz, then the upper of 23Hz can be changed to 23.5, 24.0 Hz, so the motor's action is also stepped. This creates problems for applications like continuous take-up control.
In this case, if the resolution is about 0.015 Hz, and the level difference of the four-stage motor is 1 r/min or less, it can be sufficiently adapted. In addition, some models have different resolutions and output resolutions.
12. Why can't the inverter be used as a variable frequency power supply?
The whole circuit of the variable frequency power supply is composed of alternating current, alternating current, and filtering. Therefore, the output voltage and current waveforms are pure sine waves, which are very close to the ideal AC power supply.
It can output grid voltage and frequency in any country in the world. The inverter is composed of alternating current, alternating current (modulated wave) and other circuits. The standard of the inverter should be the inverter.
The waveform of the output voltage is a pulse square wave, and the harmonic components are many. The voltage and the frequency are proportionally changed at the same time, and cannot be separately adjusted, and does not meet the requirements of the AC power source. In principle, the use of power supply cannot be used, and it is generally only used for the speed regulation of three-phase asynchronous motors.
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