We often hear about inverter air conditioners, but what exactly is an inverter?
Inverter principle
Variable frequency drives (VFDs) are devices developed in the field of modern power electronics technology, commonly used to convert direct current (DC) to alternating current (AC). They can also change the frequency of AC power to control AC motors. A VFD mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit.
Inverters adjust the voltage and frequency of the output power supply by switching the internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, inverters have many protection functions, such as overcurrent, overvoltage, and overload protection.
The diagram above shows a circuit for converting AC frequency. P and N are inverter circuits that can convert AC power into DC power and apply it to the load Z. When the pulse signal supplied to P is sinusoidal, the converted DC power also exhibits a sinusoidal pattern, forming the upper half of a sine curve. The period of this DC power is an integer multiple of the period of the sinusoidal pulse signal. Through the coordination of P and N, a periodic sinusoidal current is formed on the load Z, and the frequency can be adjusted according to the pulse signal period. This is the working principle of a frequency converter.
Applications of frequency converters
Inverter air conditioners are equipped with this type of inverter, which allows the air conditioner to operate at different frequencies. It automatically adjusts the operating frequency based on a comparison between the set temperature and the room temperature. A larger temperature difference results in a higher operating frequency and faster cooling or heating, while a smaller temperature difference results in a lower frequency. When the room temperature is close to the set temperature, it maintains the lowest operating frequency (the lower the operating frequency, the lower the power consumption), avoiding frequent starts of the outdoor compressor and thus saving energy.
In addition, frequency converters, when used in electric motors, can achieve stepless speed regulation, which is widely used in automobiles, locomotives and other fields, and realizes continuous speed regulation.
Inverter main circuit structure
Generally speaking, based on the structure and principle of the main circuit, the circuit can be divided into voltage-type structure and current-type control structure; in terms of working mode, the main function of the frequency converter is to realize the conversion of AC to AC power, so the working mode of this circuit is AC-AC conversion or AC-DC-AC conversion.
These two types of conversion circuits are fundamentally different in function, each with its own characteristics. For AC-AC converters, the DC intermediate stage is eliminated, but the number of switching transistors is not reduced; often, the number of transistors required for a single bridge arm doubles. This circuit structure is commonly found in ultra-high power, low-speed regulation circuits. Its biggest drawback is that the output power frequency must be less than 1/3 or 1/2 of the grid frequency; otherwise, the output voltage waveform distortion will be significant. Therefore, it is suitable for low-speed motor applications. In recent research, matrix-type current structures have received increasing attention. However, the biggest problem with this circuit structure lies in the complexity of control, often requiring complex modulation strategies.
Another more versatile circuit structure is the AC-DC-AC main circuit structure, which can be further divided into voltage-type and current-type structures based on its operating mode. The former has a wider range of applications.
Its features include: an electrolytic capacitor in the middle to store and provide the bus voltage; uncontrolled diode rectification in the front stage, which is simple and reliable; and three-phase PWM modulation for inverter operation (currently, the modulation algorithm is space vector). Because of the use of a certain capacity electrolytic capacitor, the DC bus voltage is stable. Therefore, by controlling the switching sequence (output phase sequence, frequency) and duty cycle (output voltage magnitude) of the inverter IGBTs, excellent control characteristics can be obtained.
The voltage-source AC-DC-AC converter, a type of rectifier-frequency converter, boasts advantages such as simple structure, low harmonic content, and adjustable stator and rotor power factors. It can significantly improve the operating status and output power quality of doubly-fed generators. Furthermore, this structure completely separates the grid side and rotor side through the DC bus-side capacitor. The stator field-oriented vector control system of the doubly-fed generator using the voltage-source AC-DC-AC converter achieves decoupled control of the generator's active and reactive power based on the wind turbine's maximum power point tracking, representing a significant direction in variable-speed constant-frequency wind power generation.
To adapt to different power grid operating conditions, more requirements are placed on frequency converters. To accommodate different power grid voltage requirements, some frequency converters incorporate a DC boosting component in their circuit structure to increase the voltage according to the motor's operating conditions, such as by adding a boost circuit. In situations with high power grid noise, a front-end filtering circuit is added to ensure normal circuit operation.
How to select a frequency converter
Industrial frequency converters are frequently used in industrial control, and selecting the right frequency converter for these moving motors is a major concern. This selection should be based on the on-site working environment, the controlled object, the required speed range, steady-state speed accuracy, torque requirements, and the existing wiring conditions. A balance must be struck between production processes and economic efficiency.
The selection of frequency converters is based on the principle of their operating current characteristic curve, including the load current curve. Qualitatively speaking, selection is based on voltage matching, current matching, and torque matching. This requires a comprehensive understanding of the on-site power conditions, including the voltage level and waveform quality, to ensure the frequency converter operates normally. Secondly, it is necessary to have a certain understanding of the load; the load's performance curve determines the application method of the frequency converter. For ordinary centrifugal pumps, the rated current of the frequency converter matches the rated current of the motor, while deep-water pumps require a larger current.
Engineering experience suggests that the power of the inverter should match the power of the motor used; generally, a slightly larger inverter is used to allow for a margin of safety. If the motor requires frequent starting and braking, a braking resistor must be installed, and its size should be selected based on the power rating. If the factory environment is harsh, with high dust levels and difficult heat dissipation, a water-cooled inverter can be chosen to effectively prevent module explosion and reduce noise. For equipment requiring aging tests, a four-quadrant inverter can be considered to effectively reduce power loss. If there is a separate DC power supply on site, a pure inverter module can be used to save on investment. Furthermore, a reactor must be added at the input of high-power inverters to improve the power quality of the input equipment and increase its power factor.
Furthermore, depending on the function of the frequency converter, different precision motors can be controlled. Typical industrial motors are AC induction motors, which can use constant voltage or constant current control. The equipment used varies depending on the type of motor. The function of the frequency converter also varies depending on the purpose of motor control.
When using a frequency converter to drive a high-speed motor, the increased high-order harmonics due to the motor's low reactance lead to a higher output current. Therefore, the capacity of the frequency converter selected for high-speed motors should be slightly larger than that for ordinary motors. If the frequency converter is to operate with a long cable, measures must be taken to suppress the influence of the cable's ground coupling capacitance to avoid insufficient output. In such cases, the frequency converter capacity should be increased by one level, or an output reactor should be installed at the output terminal. For some special applications, such as high temperature or high altitude, the frequency converter's capacity may be reduced, requiring a capacity increase of one level.
In applications requiring high precision, a series of tests must be conducted on the motor before selection, and even specific requirements may be placed on the motor's rotary encoder. In short, adapting to local conditions and selecting different motors and frequency converters based on varying site conditions remains a constant principle.
Twelve Questions about Frequency Converters
1. What are the differences between voltage-source and current-source voltage ...
The main circuit of a frequency converter can be broadly divided into two categories: voltage-source frequency converters convert DC voltage to AC voltage, and the DC circuit filter is a capacitor; current-source frequency converters convert DC current to AC current, and the DC circuit filter is an inductor.
2. When a motor is driven by a mains frequency power supply, a decrease in voltage will increase the current; for a frequency converter drive, if a decrease in frequency also causes a decrease in voltage, will the current increase?
When the frequency decreases (low speed), the current increases if the output power is the same, but the current remains almost unchanged under the condition of constant torque.
3. What are the starting current and starting torque of the motor when operating with a frequency converter?
When operating with a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%~200% depending on the model). When starting directly with a mains frequency power supply, the starting current is 6 to 7 times the rated current, thus causing mechanical and electrical shocks.
Using a frequency converter drive allows for smooth starting (though starting time is longer). The starting current is 1.2 to 1.5 times the rated current, and the starting torque is 70% to 120% of the rated torque; for frequency converters with automatic torque boosting function, the starting torque is over 100%, allowing for full-load starting.
4. The instruction manual states the speed range is 60~6Hz, or 10:1. Does this mean there is no output power below 6Hz?
While power can still be output below 6Hz, the minimum operating frequency is around 6Hz, considering factors such as motor temperature rise and starting torque. At this frequency, the motor can output rated torque without causing serious overheating problems. The actual output frequency (starting frequency) of the inverter ranges from 0.5Hz to 3Hz, depending on the model.
5. Is it possible to maintain a constant torque for general motor combinations even at frequencies above 60Hz?
Normally, it is not possible.
6. What does "open-loop" mean?
A closed-loop control system uses a speed detector (PG) to feed back the actual speed to the control unit. A system operating without a PG is called an open-loop control system. Most general-purpose frequency converters are open-loop, although some models offer PG feedback as an option. Sensorless closed-loop control calculates the actual motor speed based on a mathematical model and magnetic flux, essentially using a virtual speed sensor to create a closed-loop control.
7. What should be done if the actual rotational speed deviates from the given speed?
In open-loop operation, even if the inverter outputs a given frequency, the motor speed will vary within the rated slip range (1%~5%) when the motor is under load. For applications requiring high speed control accuracy and operation close to the given speed even with load variations, an inverter with PG feedback function (optional) can be used.
8. What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation; this is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of these two measures constitutes the stall function.
9. What is the significance of models where acceleration and deceleration times can be set separately, and models where both acceleration and deceleration times can be set together?
For machine types where acceleration and deceleration can be set separately, this is suitable for short-time acceleration and slow deceleration applications, or for small machine tools where strict production cycle time needs to be set. However, for applications such as fan drives, where acceleration and deceleration times are relatively long, acceleration and deceleration times can be set together.
10. Why does the inverter's protection function activate when a clutch is used to connect the load?
When a clutch is used to connect a load, at the moment of connection, the motor changes drastically from an unloaded state to a region with high slip. The large current flowing through it causes the inverter to trip due to overcurrent and become unable to operate.
11. 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 stages. The smallest unit of this stage difference is called the frequency resolution.
The frequency conversion resolution is typically set between 0.015 and 0.5 Hz. For example, with a resolution of 0.5 Hz, 23 Hz can be changed to 23.5 or 24.0 Hz, resulting in stepped motor operation. This poses a problem for applications like continuous winding control.
In this case, if the resolution is around 0.015Hz, it is sufficient for a 4-pole motor with a speed difference of less than 1 r/min per stage. However, some models have a different given resolution than their output resolution.
12. Why can't a frequency converter be used as a variable frequency power supply?
The entire circuit of the frequency converter consists of AC-DC-AC-filtering components, so its output voltage and current waveforms are pure sine waves, which are very close to an ideal AC power supply.
It can output the grid voltage and frequency of any country in the world. A frequency converter, on the other hand, is composed of AC/DC/AC (modulated wave) circuits, and its standard name should be variable frequency speed controller.
Its output voltage waveform is a pulsed 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 regulation of three-phase asynchronous motors.
