A frequency converter (also called a variable frequency drive, VFD) is a power control device that converts a fixed‑frequency AC supply into an AC output with adjustable frequency and voltage. Most converters in use today adopt the AC‑DC‑AC conversion method (VVVF or vector control). They first convert the mains AC power into DC power through a rectifier, and then convert that DC power into an AC supply with controllable frequency and voltage to feed the motor.
The circuit of a frequency converter generally consists of four parts: rectifier, DC link, inverter, and control. The rectifier section is usually a three‑phase bridge uncontrolled rectifier, and the inverter section is an IGBT three‑phase bridge inverter with a PWM output.
2.1 Function and Role
The rectifier circuit converts the mains AC (typically 50 Hz or 60 Hz) into DC power. In general‑purpose converters, the rectifier is a three‑phase bridge rectifier. Its function is to rectify the mains power, which is then smoothed by the DC link to provide the required DC supply for both the inverter and the control circuits.
2.2 Configuration and Types
Common rectifier circuits include uncontrolled rectifiers (e.g., diode rectifiers) and controlled rectifiers (e.g., thyristor rectifiers).
Uncontrolled rectifier: simple structure, low cost, but output voltage is not adjustable.
Controlled rectifier: output voltage can be adjusted by controlling the firing angle.
A three‑phase AC supply is generally introduced to the input of the rectifier bridge through a network of absorption capacitors and varistors. This network absorbs high‑frequency harmonics and surge overvoltages from the AC grid, thereby protecting the converter from damage.
2.3 Key Parameters
When the supply voltage is 380 V three‑phase, the maximum reverse voltage of the rectifier devices is typically 1200–1600 V, and the maximum rectified current is twice the rated current of the converter.
3.1 Function and Role
The DC link filters and stores energy from the rectified DC output. It smoothes the pulsating DC voltage and supplies a stable or smooth DC voltage to the inverter. The DC link acts as an energy storage device – the motor draws energy from the intermediate circuit through the inverter. During deceleration or braking, it can also store regenerative energy fed back from the motor.
3.2 Configuration and Types
The DC link typically consists of filter capacitors and inductors. The filter capacitors smooth the DC voltage ripple, while the inductors limit the rate of change of current and improve the stability of the DC circuit.
There are three types of intermediate circuits:
Convert rectified voltage into DC current.
Smooth the pulsating DC voltage for the inverter.
Transform a fixed DC voltage into a variable DC voltage.
3.3 Current‑Source vs. Voltage‑Source Types
Depending on the energy storage element in the DC link, converters are classified as:
Current‑source type: uses a large inductor as the energy storage element. Reactive power is buffered by this inductor, and regenerative energy can be fed back directly to the grid.
Voltage‑source type: uses a large capacitor as the energy storage element. Reactive power of the load is buffered by the capacitor, but reactive energy cannot be easily returned to the AC grid.
Function and Role
The inverter circuit, under the control of the control section, converts the DC voltage from the DC link into an AC supply with arbitrarily adjustable frequency and voltage. The output of the inverter is the output of the converter, making it one of the core circuits of the frequency converter.
Configuration and Working Principle
The inverter circuit is typically composed of several power switching devices, such as IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs. By controlling the turn‑on and turn‑off of these switches, DC power can be inverted into AC power of different frequencies and voltages.
The most common inverter topology is a three‑phase bridge inverter using six power switching devices (GTR, IGBT, GTO, etc.). By regularly controlling the switching sequence, a three‑phase AC output of any desired frequency can be obtained.
The control circuit generates corresponding Pulse Width Modulation (PWM) signals based on the given frequency and voltage commands. By adjusting the duty cycle and frequency of the PWM signals, the output voltage and frequency of the AC supply are regulated.
Auxiliary Circuits
The inverter circuit is always equipped with a freewheeling circuit, which provides a path for regenerative energy from the asynchronous motor to flow back to the DC circuit when the frequency decreases. In addition, because simultaneous conduction of two switches on the same bridge arm would cause a short circuit and destroy the devices, general‑purpose converters also include snubber circuits and other auxiliary protection circuits to ensure normal operation and to protect the switching devices under fault conditions.
For small‑ and medium‑capacity converters, the main circuit components are often integrated into integrated modules or intelligent power modules (IPMs), which internally integrate the rectifier, inverter, sensors, protection circuits, and gate drive circuits.
5.1 Function and Role
The control circuit is the core part of the frequency converter. It sends signals to the rectifier, DC link, and inverter, and simultaneously receives feedback signals from these sections. Based on external control inputs (e.g., set frequency, set voltage) and feedback signals (e.g., motor speed, current), the control circuit generates switching signals for the inverter devices according to a certain control algorithm, thereby achieving precise control of the output frequency and voltage.
The main tasks of the control circuit include inverter switching control, rectifier voltage regulation, and various protection functions.
5.2 Configuration and Control Methods
The control circuit typically comprises an arithmetic unit, detection circuits, control signal input/output circuits, and gate drive circuits. Its control methods can be analogue or digital. Nowadays, many converters adopt fully digital control using microcomputers, where hardware is kept as simple as possible and various functions are implemented through software.
Common control methods include:
V/F control: simultaneously adjusts both frequency and voltage.
Slip frequency control: an improved version of V/F control.
Vector control: decomposes the stator current of an AC motor into a field‑generating component and a torque‑generating component, controlling each separately, so that the AC motor achieves speed control performance similar to that of a DC motor.
Direct torque control (DTC): takes torque as the direct controlled variable – a new AC variable‑speed control technology developed after vector control.
The complete operating sequence of a frequency converter can be summarised as follows:
Rectification: mains AC is fed into the converter; the rectifier converts it into pulsating DC.
DC link processing: filter capacitors smooth the pulsating DC into a relatively stable DC voltage, while also storing energy.
Inversion: the control circuit generates PWM signals to control the switching of the inverter devices, converting the DC back into AC with adjustable frequency and voltage.
Motor speed control: by changing the output AC frequency, the motor speed is precisely controlled (motor speed *n* = 60*f*/*p*, where *f* is the supply frequency and *p* is the number of pole pairs of the motor).
These four sections work together to realise the core function of the frequency converter: transforming the fixed‑frequency mains supply into an AC source with both frequency and voltage adjustable.