A brief analysis of the types and applications of frequency converters

A variable frequency drive (VFD or VVVF) is an electrical device that uses an inverter circuit to convert an industrial frequency power supply into a variable frequency power supply with variable frequency and voltage, and then regulates the speed of the motor.

1. Types and characteristics of frequency converters

There are many types of frequency converters, and they can be classified in various ways. They can be classified according to purpose, conversion method, power supply properties, voltage regulation method, frequency conversion control, etc. according to needs.

1. Classification by purpose

Frequency converters can be divided into two categories according to their uses: general-purpose frequency converters and special-purpose frequency converters.

General frequency converters are frequency converters with strong versatility in many aspects. This type of frequency converter simplifies the system functions and is mainly used for energy saving. Most of them are small and medium-capacity frequency converters, and are generally used in water pumps, fans, blowers and other occasions that do not require high system speed regulation performance.

Special frequency converters are frequency converters designed and developed specifically for a certain aspect or field. They are highly targeted, have unique functions and advantages, can better play the role of frequency conversion and speed regulation, and have poor versatility.

At present, the more common special frequency converters mainly include fan special frequency converters, constant pressure water supply (water pump) special frequency converters, machine tool special frequency converters, heavy load special frequency converters, injection molding machine special frequency converters, textile special frequency converters, elevator special frequency converters, etc.

2. Classification by transformation method

Inverters are mainly divided into AC-DC-AC inverters and AC-AC inverters according to the conversion method, as shown in Figure 2.

Figure 2 Classification of frequency converters by conversion method

3. Classification according to power source properties

Inverters can be divided into voltage type inverters and current type inverters according to the nature of the power supply, as shown in Figure 3.

Figure 3 Classification of inverters by power supply properties

The characteristic of voltage-type inverters is that the intermediate circuit uses capacitors as energy storage elements to buffer the reactive power of the load, and the DC voltage is relatively stable, which is often used in situations where the load changes greatly. The characteristic of current-type inverters is that the intermediate circuit uses inductors as energy storage elements to buffer the reactive power of the load, that is, to suppress the change of current, which is often used in situations where the load current changes greatly, and is suitable for production machinery that requires feedback braking and frequent forward/reverse rotation.

Voltage-type inverters and current-type inverters are not only different in circuit structure, but also in performance and application range. Figure 4 shows a comparison of the two types of inverters.

Figure 4 Comparison between voltage-type inverter and current-type inverter

In addition to the above classification methods, inverters can be divided into voltage/frequency (U/f) control inverters, slip frequency control inverters, vector control inverters, direct torque control inverters, etc. according to frequency conversion control. Inverters can be divided into PAM inverters and PWM inverters according to the voltage regulation method. PAM (Pulse Amplitude Modulation) inverters can modulate the pulse amplitude of the pulse train according to a certain rule. The

pulse amplitude is controlled by a microprocessor. PWM (Pulse Width Modulation) inverters can modulate the pulse width of the pulse train according to a certain rule. The pulse width is controlled by a microprocessor. Inverters are divided into three-input three-output and single-input three-output inverters according to the number of phases of the input current. The input and output sides of the three-input three-output inverter are both powered by three-phase AC. The input side of the single-input three-output inverter is powered by single-phase AC, and the output side is powered by three-phase AC.

2. Functional application of inverter

Figure 5 shows the functional principle of the frequency converter. As can be seen from the figure, the frequency converter can convert the constant frequency AC power supply into the variable frequency AC power supply to control the motor speed.

Figure 5 Functional principle of the frequency converter

1. Function of the inverter

The frequency converter is a control device that integrates start/stop control, frequency conversion speed regulation, display and button setting functions, and protection functions. It is mainly used in equipment that requires speed adjustment.

(1) The inverter has a start/stop control function

After receiving the start and stop instructions, the inverter can control the start and stop of the motor according to the pre-set start and stop methods. The main control functions include soft start control, acceleration/deceleration control, shutdown control and braking control.

① The inverter has a soft start function, which can realize the starting current of the controlled motor from zero, and the maximum value does not exceed 150% of the rated current, reducing the impact on the power grid and the requirements for power supply capacity, as shown in Figure 6.

② When using a frequency converter to control a motor, the frequency and voltage output by the frequency converter can be accelerated from low frequency and low voltage to rated frequency and rated voltage, or decelerated from rated frequency and rated voltage to low frequency and low voltage, as shown in Figure 7.

③There are two commonly used braking methods for frequency converters, namely DC braking and external braking resistor and braking unit, to meet the needs of different users.

a. DC braking.

The DC braking of the inverter is that when the operating frequency of the motor drops to a certain value, the inverter sends a DC voltage to the motor winding to stop the motor quickly. In DC braking, the user needs to set the parameters of the inverter such as DC braking voltage, DC braking time and DC braking starting frequency.

b. External braking resistor and braking unit.

When the output frequency of the inverter drops too quickly, the motor will generate feedback braking current, causing the DC voltage to rise and damaging the inverter. At this time, adding a braking resistor and a braking unit to the feedback circuit can consume the feedback braking current, thereby protecting the inverter and achieving braking.

Figure 6 Soft start function of the inverter

Figure 7: Acceleration/deceleration control function of the inverter

(2) The inverter has speed control function

In the inverter control circuit, the inverter can convert the industrial frequency power supply into a variable frequency power supply voltage through a series of conversions to adjust the speed of the motor. At present, the speed control of the inverter mainly includes voltage/frequency (U/f) control mode, slip frequency control mode, vector control mode and direct torque control mode, as shown in Figure 8.

Figure 8 Speed ​​control function of the inverter

(3) The inverter has display and key setting functions

The user can set various parameters through the display screen and operation buttons on the front panel of the inverter, and view the set values, operating status and other information through the display screen.

(4) The inverter has a safety protection function

The inverter is equipped with a protection circuit inside, which can realize various abnormal protection functions for itself and the motor, including overload protection and anti-stall protection, as shown in Figure 9.

Figure 9 Safety protection function of the frequency converter

In order to facilitate communication and human-computer interaction, the frequency converter is usually equipped with different communication interfaces, which can be used to communicate with PLC and remote operation panel, communication module, computer, etc.

As a new type of motor control device, the frequency converter has the characteristics of high operation accuracy and controllable power factor in addition to the above-mentioned functions and features.

2. Application of frequency converter

The frequency converter is a new type of intelligent drive and control device developed based on frequency conversion technology. It is widely used in various fields. Simply put, as long as an AC motor is used, a frequency converter must be used.

Figure 10 Application of frequency converters in improving product quality or production efficiency

Figure 11 Application of frequency converter in energy saving

Figure 12 Application of frequency converter in civil environment improvement