General-Purpose VFD Basics: Structure & Working Principle
Variable Frequency Drives (VFDs) play a vital role in modern industrial automation by controlling the speed of motors, improving energy efficiency, and providing flexible operation. Among various types of VFDs, general-purpose VFDs stand out due to their versatility and ability to work with a wide range of small and medium-sized motors. In this post, we will delve into the basic structure and working principle of a general-purpose VFD, helping you better understand how it functions and its components.
1. Basic Structure of a General-purpose VFD
A general-purpose VFD is a versatile device that can be used with almost all small and medium-sized AC asynchronous motors. While there are many specialized VFDs, most of them are based on general-purpose VFDs with additional features. Understanding how general-purpose VFDs work makes it easier to handle other types of VFDs, as their installation, operation, and maintenance are very similar.
A general-purpose VFD consists of two main parts: the power circuit and the control circuit. Small and medium-sized VFDs typically use an AC-DC-AC configuration, as shown in the diagram (Figure 1).
Figure 1. Block Diagram of an AC-DC-AC VFD
(1) Power Circuit
The power circuit of a general-purpose VFD is shown in Figure 2. It includes power electronic components that handle power conversion, such as rectifier, filtering, current-limiting, inverter, freewheeling, and braking circuits.
Figure 2. AC-DC VFD Main circuit
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Rectifier Circuit: This converts AC power into DC. A three-phase bridge rectifier is commonly used in small and medium VFDs, with diodes (VD1 to VD6) or rectifier modules. The input power comes from terminals R, S, T (L1, L2, L3, or A, B, C).
Filtering Circuit: The filtering circuit smooths the DC power by using capacitors. Capacitors CF1 and CF2 are connected to increase the overall capacity, while resistors RC1 and RC2 balance the voltage. -
Current-Limiting Circuit: This circuit limits the current to prevent overload. It includes a resistor (RS) and a switch (S), with a thyristor or relay used in smaller VFDs.
Inverter Circuit: The inverter, made of power electronic devices (VT1 to VT6), converts the DC into three-phase AC. This AC is then sent to the motor. - Freewheeling Circuit: This circuit (VD7 to VD12) helps return reactive current from the motor back to the DC circuit. When the motor slows down, this circuit allows the motor's regenerated energy to flow back into the DC circuit.
- Braking Circuit: When the motor decelerates, the braking circuit absorbs the regenerative energy, converting it into heat. This helps the motor stop quickly and efficiently. The main component (RB) absorbs the energy, and a power device (VTB) switches the braking circuit on or off.
(2) Control Circuit
The control circuit of a VFD is shown in Figure 3.
Figure 3. Block Diagram of the VFD Control Circuit
The control circuit includes the power supply board, main control board, keypad, and input/output terminals.
- Power Supply Board: This provides the necessary power for the main control board and drives the power circuit. It also supplies DC power to external control circuits.
- Main Control Board: This is the "brain" of the VFD. It processes input from the keypad, external control circuits, and internal signals (like voltage, current, and temperature). It then generates control signals (SPWM) for the inverter. The main control board also manages the display signals, protection commands, and sends signals about the VFD's status (e.g., normal operation, frequency reached, fault detection).
- Keypad: The keypad allows the user to input commands or settings into the main control board.
- Input/Output Terminals: These terminals connect the VFD to external devices like display units, adjustment controls (e.g., buttons, switches, knobs), and communication interfaces. The communication interface allows the VFD to interact with other equipment, such as PLCs, and exchange information.
2. Working Principle of a Variable Frequency Drive (VFD)
The working principle of an AC-DC-AC type VFD is explained with reference to the block diagram in Figure 1.
The three-phase AC power is first converted into pulsating DC by the rectifier circuit. This DC is then filtered by the intermediate circuit to ensure that both the inverter and control circuits receive a stable DC power supply. The filtered DC is sent to the inverter circuit, while the control system generates drive pulses. These pulses are amplified by the drive circuit and sent to the inverter circuit. Under the control of the drive pulses, the inverter converts the DC into variable-frequency AC, which is then supplied to the motor to drive it. By adjusting the frequency of the AC output from the inverter, the motor speed changes accordingly.
Since the main circuit operates under high-voltage and high-current conditions, protection is essential. To safeguard the circuit, VFDs are equipped with voltage and current detection circuits for the main power circuit. When the voltage is too high or too low, the voltage detection circuit informs the control circuit. Upon receiving this information, the control circuit takes appropriate action according to the set program, such as stopping the main circuit and issuing an alarm.
Similarly, if the output current exceeds safe limits (e.g., when the motor is overloaded), the current sensing elements or circuits generate an overcurrent signal. After the current detection circuit processes this signal, it is sent to the control circuit. The control circuit then takes the necessary actions according to the preset program.
In conclusion, understanding the structure and working principle of a general-purpose VFD is key to maximizing its efficiency and ensuring its longevity. The combination of the power circuit and control circuit enables the VFD to efficiently convert AC power into variable-frequency AC for motor speed control, while built-in protection features ensure safe operation. Whether you're looking to optimize motor performance or reduce energy consumption, a general-purpose VFD is a flexible and reliable choice for your industrial needs.