The principle and selection of the inverter and the main circuit structure

We often hear about inverter air conditioners, etc. So what exactly is an inverter?

Inverter principle

Frequency conversion is a modern development in the field of power electronics and is a conversion device between DC and AC power that we commonly use. It can also change the frequency of our AC power to control the AC motor power control equipment. The inverter is mainly composed of a rectifier (AC to DC), filter, inverter (DC to AC), brake unit, drive unit, detection unit, microprocessor unit, and so on.

In addition, the inverter has many protection functions, such as over-current, over-voltage, overload protection, and so on.

The diagram above shows the circuit of variable AC frequency. P and N are the converter circuit, which can rectify the AC power into DC power and load it to the load Z. When the pulse signal to P becomes sinusoidal, the rectified DC is sinusoidal, which is the upper part of the sinusoidal curve, and the period is the sinusoidal period of the pulse signal, which is an integer multiple of the AC power period. Through the cooperation of P and N, it forms the periodic sinusoidal current on load Z, and the frequency can be adjusted according to the pulse signal period, which is the working principle of the inverter.

Application of inverter

The inverter air conditioner is loaded with this kind of inverter, which can make the air conditioner run at different frequencies and automatically adjust the running frequency according to your set temperature and room temperature comparison. The temperature difference is high running frequency, cooling or heating is fast, and vice versa 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) to avoid the frequent start of the outdoor compressor, thus saving power.

In addition, the inverter can realize stepless speed regulation when applied to the motor, widely used in automobiles and locomotives, and realize the continuity of speed regulation.

Inverter main circuit structure

It is generally considered that the circuit can be divided into voltage type structure and current type control structure from the structure and principle of the main circuit; from the working mode, the main function of the inverter is to realize the AC to AC electrical energy conversion. Therefore, this circuit works in the form of AC to AC conversion or AC to DC conversion.

These two conversion circuits belong to different circuit structures in terms of their essential functions, and both have different characteristics. For the AC-DC converter, the intermediate link of DC is eliminated. Still, the number of switching tubes is not reduced, and often the number of switching tubes required for a bridge arm is doubled, and this circuit structure is commonly used for 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 is very large, so it is suitable for low-speed motor applications. In the latest research, the current matrix 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 more versatile circuit structure is the AC-DC main circuit structure, which can be further divided into voltage-type and current-type structures in operating mode. The former is more widely used.

Its characteristics are:

• The middle provides bus voltage for electrolytic capacitor storage.

• The front stage uses uncontrolled diode rectification, which is simple and reliable.

• The inverter uses three-phase PWM modulation (the current modulation algorithm is a space voltage vector).

Since a certain capacity of the electrolytic capacitor is used, the DC bus voltage is stable. As long as the switching sequence (output phase sequence and frequency) and duty cycle (output voltage size) of the inverter IGBT are controlled, superior control characteristics can be obtained.

This rectifier inverter with a simple structure, low harmonic content, and adjustable stator-rotor power factor can significantly improve the doubly-fed generator's operation and output power quality, and the structure completely separates the grid side and rotor side through the DC bus side capacitor. The double-fed generator stator magnetic field directional vector control system with voltage type AC/DC inverter realizes the decoupling control of generator active and reactive power based on the maximum power point tracking of the wind turbine, which is a representative director of the current variable speed and constant frequency wind power generation.

To adapt to different grid working conditions, more requirements are put forward for inverters. To adapt to grid voltage requirements, some inverters will add a DC wave-occupying part to the circuit structure to boost the voltage according to the requirements of motor working conditions, such as adding a boost circuit. In the case of high grid noise, a front-end filter circuit will be added to ensure the normal use of the circuit.

How to select the inverter

Industrial frequency converters are often used in industrial control, and how to choose the right inverter for these motion motors is an issue that is of more concern. This issue should be based on the working environment of the site, the object to be controlled, the range of speed requirements, steady-state speed accuracy, torque requirements, and the site wiring to make the requirements. A balance is made between the production process and the production economy.

The inverter selection is based on the principle that the operating current characteristic curve includes the load current curve. Qualitatively, the selection is made regarding voltage matching, current matching, and torque matching. This requires us to have a comprehensive understanding of the electricity situation at the site, the level of the voltage, and the quality of the voltage waveform, to ensure that the inverter can normally work, secondly, to have a certain knowledge of the load, the performance curve of the load determines the application method of the inverter, the rated current of the inverter of an ordinary centrifugal pump matches the rated current of the motor. In contrast, the deep water pump requires a larger current.

Engineering experience is that the use of the motor power is how much power the use of inverters, generally to leave a margin, will use slightly larger specifications of the inverter. If the site of the motor needs frequent starting and braking, it is necessary to install a braking resistor. The size of the braking resistor should be selected according to the size of the power. Suppose the factory environment is harsh, dusty, and difficult to dissipate heat. In that case, you can choose a water-cooled inverter, which can effectively prevent the module from blowing up and reduce the noise. If the equipment needs to do the aging test, consider using four-quadrant products to reduce the power loss effectively. If the site has a separate DC power, you can use only pure inverter modules to save input. In addition, at the inlet end of the high-power inverter, a reactor must be added, which can improve the input equipment's power quality and the equipment's power factor.

In addition, according to the function of the inverter can choose different precision control motors. You can use constant voltage or current control in the general industrial motor for AC induction motors. According to the type of motor, the use of equipment is not the same. Depending on the purpose of the motor, control can be selected for different inverter functions are also different.

When using an inverter to drive a high-speed motor, the output current value increases due to the high harmonics due to the small reactance of the high-speed motor. Therefore the selection of inverter for high-speed motor capacity is slightly larger than the selection of ordinary motor. If the inverter to long cable operation, at this time, take measures to suppress the impact of a long cable to ground coupling capacitance to avoid the inverter output power shortage, so in such cases, the inverter's capacity to enlarge a gear or the output reactor installed at the output of the inverter. For some special applications, such as high temperature and high altitude, this will cause the inverter to reduce the capacity, the inverter capacity to enlarge a gear.

In some occasions with strict accuracy requirements, the motor needs to be selected after a series of tests, and even the rotary encoder of the motor also puts forward certain requirements. All in all, according to the situation, local situation, according different site conditions to choose different motors and inverters is a constant guideline.

Inverter twelve questions

1.What is the difference between voltage type and current type?

The main circuit of the inverter can be broadly divided into two categories: voltage type is the inverter that converts the DC of the voltage source into AC, and the DC circuit filter is capacitor; current type is the inverter that converts the DC of current source into AC, and the DC circuit filter is an inductor.

2.When the motor is driven by an industrial frequency power supply, the voltage drops, and the current increases; for inverter drive, if the voltage drops when the frequency drops, does the current increase?

When the frequency drops (low speed), if the output power is the same, the current will increase, but under the condition of certain torque, the current is almost unchanged.

3.What is the starting current and starting torque of the motor when using an inverter?

When using an inverter, the starting current is limited to 150% of the rated current (125%~200% depending on the machine type) as the motor accelerates, and the frequency and voltage are increased accordingly. When starting directly with an industrial frequency power supply, the starting current is 6~7 times rated current. Therefore, it will produce a mechanical and electrical shock.

Using inverter drive can start smoothly (starting time becomes longer). The starting current is 1.2~1.5 times of rated current, and the starting torque is 70%~120% of the rated torque; for the inverter with automatic torque enhancement function, the starting torque is more than 100%, and it can start with full load.

4.It is written in the manual that the variable speed range is 60~6Hz, i.e. 10:1, so there is no output power below 6Hz?

But according to the temperature rise of the motor and the size of starting torque, the minimum frequency is about 6Hz, at which time the motor can output the rated torque without causing serious heat problems. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the machine type.

5.For the general motor combination is above 60Hz, it also requires a certain torque, is it possible?

Usually, it is not possible. Above 60Hz (there are also models above 50Hz), the voltage remains unchanged, the general constant power characteristics, the same torque required at high speed

6.What is the meaning of the so-called open loop?

To the motor device used to set the speed detector (PG), the actual speed feedback to the control device for control, known as "closed loop", without PG operation is called "open loop". General-purpose inverters are mostly open-loop, but some models can use options for PG feedback. The closed-loop control method without speed sensor is based on the established mathematical model to project the actual speed of the motor according to the magnetic flux, equivalent to a virtual speed sensor to form a closed-loop control.

7.What should I do if the actual speed deviates from the given speed?

In open-loop, even if the inverter outputs a given frequency, the motor speed varies within the range of the rated speed difference (1%~5%) when the motor is running with a load. For the requirements of speed regulation accuracy is relatively high, even if the load changes also requires the operation of the occasion in close to the given speed, can be used with PG feedback function of the inverter (optional parts).

8.What is the meaning of stall prevention function?

If the given acceleration time is too short, the output frequency change of the inverter far exceeds the speed (electric angle frequency) change, the inverter will trip due to over-current flow and the operation stops, which is called stall. In order to prevent stall so that the motor continues to run, we must detect the size of the current for frequency control. When the acceleration current is too large, the acceleration rate is slowed down appropriately. The same is true when decelerating. The combination of the two is the stall function.

9.What is the significance of the models that have acceleration and deceleration times that can be given separately and those that have acceleration and deceleration times that can be given together?

The acceleration and deceleration times can be set separately, which is suitable for short acceleration and slow deceleration, or for small machine tools where the production beat time needs to be strictly set, but for fan drives and other applications where the acceleration and deceleration times are long, the acceleration and deceleration times can be set together.

10.Why does the protection function of the inverter act when the load is connected with the clutch?

When the load is connected with a clutch, at the moment of connection, the motor changes sharply from the no-load state to the area with a large turndown rate, and the large current flowing causes the inverter to trip over the current and cannot operate.

11.What is inverter resolution? What is the significance?

For digitally controlled inverter, even if the frequency command is an analog signal, the output frequency is given in steps. The smallest unit of this level difference is called the frequency resolution.

For example, if the resolution is 0.5 Hz, then 23 Hz can be changed to 23.5 or 24.0 Hz, so the motor action is also followed in steps. This causes problems for applications like continuous winding control.

In this case, if the resolution is about 0.015 Hz, it can be sufficiently adapted to a 4-stage motor with 1 stage difference of 1 r/min or less. In addition, some models are not the same as the given resolution and output resolution.

12.Why the inverter can not be used as a variable frequency power supply?

The whole circuit of the inverter power supply is composed of AC constant current, AC filtering and other parts, so its output voltage and current waveform are pure sine wave, very close to the ideal AC power supply.

It can output the voltage and frequency of the power grid of any country in the world. The inverter is composed of AC constant current and AC (modulating wave) circuit, and the standard name of the inverter should be inverter speed regulator.

Its output voltage waveform is pulse square wave, and more harmonic components, voltage and frequency change proportionally at the same time, can not be adjusted separately, does not meet the requirements of the AC power supply. In principle, it cannot be used as a power supply, and is generally used only for speed regulation of three-phase asynchronous motors.