5 Key Classification Methods of VFDs and Their Features
Variable Frequency Drives (VFDs) are essential components in motor control systems, widely used to regulate the speed, torque, and operation of electric motors. Understanding the classification of VFDs is important for selecting the right system based on performance, efficiency, and application requirements. In this blog post, we'll break down the different types of VFDs and their unique characteristics, helping you make an informed decision when choosing the best solution for your motor control needs.
1. Classification of VFDs by Frequency Conversion Method
(1) AC-DC-AC VFD
The AC-DC-AC VFD (also known as an indirect VFD) operates by converting the AC input from the grid into DC through a rectifier. This DC voltage is then converted back into AC through an inverter, offering control over both frequency and voltage. This method is widely used due to its versatility in managing a broad range of motor speeds and voltage levels. The basic principle is shown in Figure 1-1.
Figure 1-1 AC-DC-AC VFD
(2) AC-AC VFD
The AC-AC VFD (direct VFD) is more efficient in certain situations, as it directly converts AC power to AC output with variable frequency and voltage. The basic principle of the AC-AC VFD is shown in Figure 1-2. The system consists of two sets of thyristor rectifiers connected in reverse parallel. The positive and reverse sets switch periodically, providing an alternating output voltage uo across the load.
Figure 1-2 AC-AC VFD
Table 1-1 Comparison of Main Features Between AC-AC VFD and AC-DC-AC VFD
2. Classification of VFDs by Main Circuit Design
(1) Voltage-type VFD
The main circuit of a voltage-type VFD is shown in Figure 1-3. In this type of VFD, the rectifier circuit generates the DC voltage required for inversion. After filtering by large capacitors in the intermediate DC link, the output is obtained. Due to the large capacitor filter, the DC voltage waveform is relatively flat, and in an ideal case, it can be treated as a voltage source with zero internal resistance. The output AC voltage waveform of a voltage-type VFD is typically a square or staircase wave, commonly used in general VFDs that do not require reverse operation or rapid acceleration/deceleration.
Figure 1-3 Main Circuit of a Voltage-type VFD
(2) Current-type VFD
The main circuit of a current-type VFD is shown in Figure 1-4. Its feature is the use of large inductors in the intermediate DC link for filtering. The DC current waveform is relatively flat due to the inductor. Therefore, the internal impedance of the DC source is high and can be approximated as a current source. The output AC current waveform is typically a square or staircase wave. The primary advantage of current-type VFDs is that they allow for four-quadrant operation, enabling energy feedback to the power source. They are also easier to handle in cases of load short circuits, making them suitable for frequently reversible or large-capacity VFDs.
Figure 1-4 Main Circuit of a Current-type VFD
A comparison of the main features between current-type and voltage-type VFDs is shown in Table 1-2.
3. Classification of VFDs by Voltage Adjustment Method
(1) PAM VFD (Pulse Amplitude Modulation)
Pulse Amplitude Modulation (PAM) is a method that adjusts the output by varying the amplitude of the voltage Ud or current source Id . In this method, the inverter only adjusts the output frequency, while the rectifier part controls the output voltage or current. When voltage adjustment is applied, the output voltage waveform of the VFD is shown in Figure 1-5.
The main circuit of a PAM-controlled VFD is shown in Figure 1-5a.
PAM is used in VFDs with a variable intermediate circuit voltage. When controlling frequency, the output voltage's frequency is adjusted by changing the working cycle of the inverter. During each working cycle, the power switching devices are turned on and off multiple times. Since the circuit implementation of PAM is relatively complex, requiring simultaneous control of both the rectifier and the inverter, and because the average DC voltage after thyristor rectification does not have a linear relationship with the phase-shifting angle, coordinating the rectification and inversion becomes quite difficult. For these reasons, this modulation method is generally not adopted.
(2) PWM and SPWM VFD (Pulse Width Modulation and Sine Pulse Width Modulation)
Pulse Width Modulation (PWM) generates multiple pulses within one cycle of the output waveform, approximating a sinusoidal waveform. This results in smoother output with fewer harmonics. The main circuit of a PWM-controlled VFD is shown in Figure 1-5b. PWM can be further classified into fixed pulse width PWM and sine wave PWM (SPWM).
Figure 1-5 PAM and PWM Control Main Circuit with Output Voltage Waveform
a) PAM Control Main Circuit b) PWM Control Main Circuit
4. VFDs Classified by Control Method
When an induction motor is adjusted with a VFD, the VFD can control the supply voltage, current, and frequency according to the motor's characteristics. The performance, characteristics, and application of VFDs vary according to the control method used. Therefore, VFDs can also be classified based on the control method.
(1) U/f Control VFD (VVVF Control)
The U/f control method, also known as VVVF (Variable Voltage Variable Frequency) control, involves controlling both the voltage and frequency output of the VFD to maintain a constant U/f ratio. This method allows for constant torque below the base frequency and constant power above the base frequency. The control circuit cost of U/f control VFDs is relatively low, making them suitable for general-purpose VFDs with less stringent accuracy requirements.
(2) Slip Frequency Control VFD
Slip frequency control is an improvement over U/f control. In VFDs using this method, the converter forms a speed feedback closed-loop control system with the motor and a speed sensor. The output frequency of the converter is automatically set by the motor's actual speed and the slip frequency, allowing the output torque to be controlled while regulating speed. This control method is a closed-loop control, so compared to the U/f control method, it can maintain higher speed accuracy and better torque characteristics even when there are significant load changes. However, because this control method requires a speed sensor to be installed on the motor and the slip needs to be adjusted based on the motor's characteristics, its versatility is relatively low.
(3) Vector Control VFD
The principle of vector control is to decompose the stator current of an AC induction motor into two components: the magnetizing current (flux-producing) and the torque-producing current. Both components are controlled separately. Vector control significantly improves dynamic performance and allows for more precise torque control, making it comparable to DC motor speed control systems. Vector control VFDs are widely used in precision control applications.
5. Classification of VFDs by Application
(1) General-purpose VFD
General-purpose VFDs can adjust the speed of standard induction motors. They can be classified into two types: low-cost simple general-purpose VFDs and high-performance multi-functional general-purpose VFDs.
Simple VFDs are designed for energy-saving purposes and are mainly used in applications like pumps and fans, where speed control performance requirements are not high. They offer advantages such as compact size and lower cost.
High-performance multi-functional VFDs are designed to meet various needs, with more advanced hardware and software features. They are suitable for applications like conveyors, hoists, machine tools, and electric vehicles, where higher performance and functionality are required.
(2) High-performance Dedicated VFD
High-performance dedicated VFDs adopt vector control to deliver performance that rivals or exceeds that of DC motor speed control systems. These VFDs are typically designed for specific industries, such as metallurgy, CNC machines, and elevators, where performance and cost-efficiency need to be optimized.
(3) High-frequency VFD
High-frequency VFDs are used to control high-speed motors in applications requiring ultra-precision, such as in high-end machining.
(4) Single-phase and Three-phase VFD
Finally, VFDs are available in both single-phase and three-phase versions, depending on the type of motor they are designed to control. The main difference lies in the circuit design, with three-phase VFDs offering greater efficiency for high-capacity applications.
For the circuit structure and working principle of VFDs, please refer to General-Purpose VFD Basics: Structure & Working Principle.
Conclusion:
Understanding the different classifications and characteristics of VFDs is crucial for selecting the right motor control solution. Whether you're looking for a cost-effective VFD for general use or a high-performance drive for precision control, there is a wide range of VFDs designed to meet specific needs. By considering factors like frequency conversion methods, voltage adjustment techniques, control methods, and application requirements, you can make an informed choice that ensures optimal performance and energy efficiency for your motor systems.