Classification methods of frequency converters

Ⅰ. Classification of Frequency Converters by Conversion Stages

      Based on the different stages of electrical energy conversion, frequency converters can be systematically divided into two main types: direct AC-AC converters and indirect AC-DC-AC converters. These two types exhibit significant differences in working principles and technical characteristics.

1. AC-AC Converters

      Also known as direct frequency converters, AC-AC converters are characterized by their ability to directly convert the frequency and voltage of power-frequency alternating current (AC). Through special power electronic devices, these converters transform fixed-frequency and fixed-voltage power-frequency AC into adjustable-frequency and adjustable-voltage AC output while maintaining the same number of phases. This "one-step" conversion method avoids energy loss in intermediate DC stages, giving it unique advantages in high-power, low-speed drive applications. For example, it is widely used in rolling mill drive systems in the metallurgical industry.

2. AC-DC-AC Converters

      Referred to as indirect frequency converters, AC-DC-AC converters feature a "two-step" energy conversion process. First, a rectifier converts the input power-frequency AC into direct current (DC), completing the initial transformation from AC to DC. Subsequently, an inverter converts the DC back into adjustable-frequency and adjustable-voltage AC. Compared with AC-AC converters, AC-DC-AC converters offer a wider frequency adjustment range and higher output waveform quality. As a result, they are widely used in scenarios requiring high-speed regulation performance, such as industrial automation production lines, elevator drive systems, and air conditioning equipment.

Ⅱ. Classification of Frequency Converters by DC Power Supply Characteristics

      Based on the differences in DC power supply characteristics, frequency converters can be divided into current-source inverters and voltage-source inverters . These two types of equipment have distinct advantages in energy storage methods, operational characteristics, and application scenarios.

1. Current-Source Inverters

      The core feature of current-source inverters lies in their use of a large inductor as an energy storage component in the intermediate DC link. By leveraging the inductor’s ability to suppress current transients, CSIs buffer reactive power in the system and make the output voltage waveform approach a sine wave. Due to the high internal resistance of this DC link, the current output remains relatively stable, hence the name "current-source inverter." This unique design endows CSIs with strong load current regulation capabilities, enabling them to effectively handle frequent and drastic fluctuations in load current. As a result, they are widely used in industrial scenarios with stringent dynamic response requirements and severe load current variations, such as steel rolling mills and hoisting machines.

2. Voltage-Source Inverters

      Voltage-source inverters utilize large capacitors as energy storage components in the intermediate DC link, relying on the capacitors’ voltage-stabilizing properties to buffer reactive power. The DC link features low internal resistance and provides a stable output voltage, approximating an ideal voltage source—this is the origin of the name "voltage-source inverter." Compared with current-source inverters, VSIs excel at handling significant fluctuations in load voltage. They perform exceptionally well in driving equipment with high requirements for output voltage stability, such as fans, water pumps, and air conditioners, delivering stable and efficient power support to loads.

Ⅲ. Classification of Frequency Converters by Voltage Modulation Methods

Description of Classification of Frequency Converters by Voltage Modulation Methods

      Based on different voltage modulation techniques, frequency converters can be classified into Pulse Width Modulation (PWM) frequency converters and Pulse Amplitude Modulation (PAM) frequency converters. These two types of converters meet the requirements of different application scenarios through differentiated voltage regulation mechanisms and play a crucial role in the fields of industrial automation and electric drive.

1. Sinusoidal Pulse Width Modulation (SPWM) Frequency Converters

      Pulse Width Modulation (PWM), especially Sinusoidal Pulse Width Modulation (SPWM), is the mainstream modulation method in the current frequency converter field. This type of frequency converter controls the magnitude of the output voltage by precisely adjusting the duty cycle of the pulse signal, which changes the equivalent average value of the output voltage. This modulation method can effectively reduce the harmonic content of the output voltage, improve the power quality, and also has high control accuracy and dynamic response capabilities. Due to its excellent performance and relatively simple circuit design, almost all general-purpose frequency converters with medium and small capacities adopt SPWM technology and are widely used in scenarios such as fans, water pumps, and machine tool drives.

2. Pulse Amplitude Modulation (PAM) Frequency Converters

      Pulse Amplitude Modulation (PAM) frequency converters indirectly adjust the magnitude of the output AC voltage by adjusting the amplitude of the DC-side voltage. This modulation technology first converts the input AC power into DC power through the rectification process, then changes the amplitude of the DC voltage using a controlled rectifier or chopper, and finally outputs AC power with the corresponding amplitude through an inverter. Compared with PWM technology, the control logic of PAM frequency converters focuses on the direct adjustment of voltage amplitude. However, due to its high dependence on the DC power supply, the system structure is relatively complex, and its harmonic suppression ability is weak. Currently, it is mainly applied to special industrial equipment with extremely high requirements for dynamic response speed, such as high-speed motor drives and other specific scenarios.

Ⅳ.  Classification by the Number of Input Power Phases

Based on the difference in the number of input power phases, frequency converters can be divided into two categories:

1. Three-Phase Input and Three-Phase Output Frequency Converters

• Technical Features: Both the input and output sides are three-phase alternating current (such as a 380V three-phase power supply), and energy conversion is achieved through a three-phase bridge circuit.
• Application Scenarios: This is the mainstream configuration in the industrial field, suitable for driving three-phase motors (such as fans, pumps, and machine tool spindles), accounting for more than 80% of the market share.
• Advantages: Balanced three-phase power supply can reduce harmonic distortion and support high-power loads (usually ≥7.5kW). It is commonly used in scenarios such as production lines, compressors, and metallurgical equipment.

2. Single-Phase Input and Three-Phase Output Frequency Converters

• Technical Features: The input side is single-phase alternating current (such as a 220V domestic power supply), and the output side is three-phase variable-frequency power (such as 220V/380V three-phase).
• Application Scenarios: Used in low-power civilian and light industrial scenarios. Typical applications include:

• Household Appliances: Driving the external units of variable-frequency air conditioners and drum washing machines.
• Small Equipment: Laboratory stirrers, commercial kitchen equipment, and small servo motors in automated production lines.
  • Limitations: Limited by the single-phase input power (usually ≤3kW), it is not suitable for high-load industrial environments.

Ⅴ. Classification by Supply Voltage Levels

      According to the rated input voltage range, frequency converters can be divided into three levels:

Ⅵ. Classification by Functional Characteristics

Based on load characteristics and industry requirements, frequency converters can be divided into five categories:

1. Constant Power Frequency Converters

• Characteristics: The output power remains constant (P = constant), and the rotational speed is inversely proportional to the torque.
• Applications: Used in scenarios that require constant power speed regulation, such as machine tool spindle drives and coil winders.

2. Square Torque Frequency Converters

• Characteristics: The torque is proportional to the square of the rotational speed (T∝n²), with significant energy-saving effects.
• Applications: Used in fluid machinery such as fans and pumps (accounting for more than 40% of industrial energy consumption).

3. Simple Frequency Converters

• Characteristics: Simplified functions (only basic speed regulation is supported), with high cost-effectiveness.
• Applications: Used in scenarios with low requirements for control accuracy, such as small conveyor belts and agricultural irrigation equipment.

4. General-Purpose Frequency Converters

• Characteristics: Integrated with multiple control modes (U/f, vector control), highly compatible.
• Applications: Used in general industrial automation production lines, textile machinery, and other common scenarios.

5. Elevator-Specific Frequency Converters

• Characteristics: Equipped with built-in safety functions such as brake control and zero-speed hovering, with a dynamic response accuracy ≤±0.5%.
• Applications: Used in the drive systems of passenger and freight elevators, complying with safety standards such as EN 81-1/2.

Ⅶ. Classification by Control Methods

Based on differences in motor control algorithms, the core control methods include:

1. U/f Control Method (Voltage-Frequency Ratio Control)

• Principle: Maintaining a constant ratio of voltage to frequency (U/f = constant) to achieve approximately constant torque speed regulation.
• Advantages: Simple algorithm, low hardware cost.
• Limitations: Significant torque decay at low speeds, suitable for scenarios with low requirements for dynamic performance.

2. Slip Frequency Control Method

• Principle: Adjusting the output frequency of the frequency converter by detecting the slip rate to achieve closed-loop control.
• Advantages: Higher speed regulation accuracy than the U/f control (error ≤1%), supports sudden load compensation.
• Applications: Used in scenarios that require stable speeds, such as printing machinery and packaging equipment.

3. Vector Control Method (Field-Oriented Control)

• Principle: Decomposing three-phase current into exciting current and torque current to simulate the control of a DC motor.
• Advantages: Fast dynamic response speed (speed regulation range 1:1000), torque control accuracy ≤±5%.
• Applications: Used in high-end fields such as high-precision machine tools, servo robots, and wind power converters.

Ⅷ. Classification by Main Switch Devices

According to the types of core power devices, frequency converters can be divided into:

1. IGBT (Insulated Gate Bipolar Transistor)

• Characteristics: Fast switching speed (10 - 20kHz), low on-state loss, withstand voltage up to 6500V.
• Applications: Adopted by more than 90% of medium and low-voltage frequency converters, such as Infineon FF300R12KT4 and other models.

2. GTO (Gate Turn-Off Thyristor)

• Characteristics: High current capacity (up to 6000A per tube), but low switching frequency (<1kHz).
• Applications: Used in early high-voltage frequency converters (above 6kV), gradually replaced by IGBTs.

3. BJT (Bipolar Junction Transistor)

• Characteristics: Low saturation voltage drop, good thermal stability, but complex drive circuits.
• Applications: Used in niche scenarios (such as some welding machine-specific frequency converters), accounting for less than 5% of the market share.

Ⅸ. Classification by Appearance Structure

According to physical forms and power levels, frequency converters can be divided into:

1. Plastic-Cased Frequency Converters (Low Power)

• Power Range: 0.75 - 15kW.
• Features: Integrated injection-molded shell, built-in fan for heat dissipation, supports wall-mounted installation.
• Examples: Mitsubishi FR-D700 series (0.4 - 7.5kW).

2. Metal-Cased Frequency Converters (Medium Power)

• Power Range: 18.5 - 250kW.
• Features: Metal shell with independent air duct design, supports installation in cabinets or wall mounting.
• Examples: Siemens G120C series (1.1 - 250kW).

3. Cabinet-Mounted Frequency Converters (High Power)

• Power Range: ≥280kW.
• Features: Split cabinet structure, integrated with cooling systems (water or air cooling) and filtering devices.
• Examples: ABB ACS880-07 series (355 - 5000kW), used in scenarios such as mine hoists.

Note: In practical applications, frequency converters may possess multiple classification attributes (such as "three-phase input and output + high voltage + vector control + cabinet-mounted"). When selecting a frequency converter, a comprehensive evaluation should be conducted considering load characteristics, grid conditions, and control accuracy requirements.