A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the motor's power supply.
A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit. The frequency converter adjusts the output voltage and frequency by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
The main circuit is the power conversion section that provides voltage and frequency adjustable power to the asynchronous motor. The main circuit of the frequency converter can be broadly divided into two categories.
Voltage-source inverters convert DC to AC from a voltage source, and their DC circuit filtering uses capacitors. Current-source inverters convert DC to AC from a current source, and their DC circuit filtering uses inductors. They consist of three parts: a rectifier that converts the mains frequency power supply into DC power, and a smoothing circuit that absorbs voltage ripples generated in the converter and inverter.
rectifier
Diode-based converters are widely used to convert mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate in regenerative mode because its power direction is reversible.
Smoothing circuit
The DC voltage after rectification contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. A three-phase AC output is obtained by switching six switching devices on and off at predetermined times. The switching time and voltage waveforms are shown as an example using a voltage-source PWM inverter.
The control circuit is a loop that provides control signals to the main circuit that supplies power (voltage and frequency are adjustable) to the asynchronous motor. It consists of a frequency and voltage "operation circuit", a main circuit "voltage and current detection circuit", a motor "speed detection circuit", a "drive circuit" that amplifies the control signals from the operation circuit, and an inverter and motor "protection circuit".
(1) Operational circuit: It compares and calculates the external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit: isolated from the main circuit potential to detect voltage, current, etc.
(3) Drive circuit: The circuit that drives the main circuit devices. It is isolated from the control circuit to turn the main circuit devices on and off.
(4) Speed detection circuit: The speed signal is sent to the operation circuit by the speed detector (tg, plg, etc.) installed on the shaft of the asynchronous motor. According to the instructions and operations, the motor can run at the commanded speed.
(5) Protection circuit: Detects the voltage and current of the main circuit, and protects the inverter and asynchronous motor from damage in case of overload or overvoltage.
Functions and uses
Variable frequency energy saving
The energy-saving benefits of frequency converters are mainly seen in the application of fans and pumps. To ensure production reliability, various production machines are designed with a certain margin of safety in their power drives. When a motor cannot operate at full load, the excess torque beyond meeting the power drive requirements increases active power consumption, resulting in energy waste. Traditional speed control methods for equipment such as fans and pumps involve adjusting the opening of inlet or outlet baffles or valves to regulate airflow and water flow. This method has high input power, and a significant amount of energy is consumed in the flow throttling process of the baffles and valves. When using frequency converter speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan.
The purpose of using a frequency converter for an electric motor is to regulate its speed and reduce starting current. To generate variable voltage and frequency, the device first converts the AC power supply to DC power; this process is called rectification. The scientific term for the device that converts DC to AC power is "inverter." Generally, an inverter converts DC power to a power supply with a fixed frequency and voltage. Inverters that convert DC to AC with adjustable frequency and voltage are called frequency converters. The output waveform of a frequency converter is a simulated sine wave, mainly used for speed control of three-phase asynchronous motors; it is also called a variable frequency speed controller. For variable frequency inverters used in instrumentation and testing equipment with higher waveform requirements, the waveform needs to be shaped to output a standard sine wave; this is called a variable frequency power supply. Generally, a variable frequency power supply costs 15-20 times more than a frequency converter. Because the main device in a frequency converter that generates changing voltage or frequency is called an "inverter," the product itself is named "inverter," i.e., a frequency converter.
Inverter technology doesn't always save electricity; in many situations, it doesn't necessarily save power. As an electronic circuit, the inverter itself consumes power (approximately 3-5% of its rated power). A 1.5 horsepower air conditioner consumes about 20-30W, equivalent to a light bulb left on continuously. It's true that inverters operate at the mains frequency and have energy-saving capabilities. However, this is conditional:
First, high power and fan/pump type load;
Second, the device itself has a power-saving function (software supported);
These are the three conditions that demonstrate energy-saving effects. Beyond these, whether or not an inverter saves energy is meaningless. Claiming that an inverter saves energy when operating at power frequency without considering the underlying conditions is exaggeration or commercial hype. Knowing the facts will allow you to skillfully utilize it to your advantage. It is crucial to pay attention to the application environment and conditions to ensure correct application; otherwise, you risk being blindly followed, easily misled, and "deceived."
Power factor compensation energy saving
Reactive power not only increases line losses and equipment heating, but more importantly, the reduction in power factor leads to a decrease in the active power of the power grid. A large amount of reactive power is consumed in the lines, resulting in low equipment efficiency and serious waste. After using a variable frequency drive (VFD), the reactive power loss is reduced and the active power of the power grid is increased due to the effect of the internal filter capacitor of the VFD.
Soft start energy saving
1. Hard starting of a motor causes severe impact on the power grid and places excessive demands on grid capacity. The large current and vibration generated during startup cause significant damage to baffles and valves, severely shortening the service life of equipment and pipelines. However, using a variable frequency drive (VFD) energy-saving device utilizes the VFD's soft-start function to ensure the starting current starts from zero, with a maximum value not exceeding the rated current. This reduces the impact on the power grid and the demand on power supply capacity, extending the service life of equipment and valves and saving on equipment maintenance costs.
2. Theoretically, frequency converters can be used in all mechanical equipment with electric motors. When a motor starts, the current is 5-6 times higher than the rated current, which not only affects the motor's lifespan but also consumes more electricity. While the system design includes a certain margin in motor selection, and the motor speed is fixed, in actual use, sometimes it needs to operate at lower or higher speeds. Therefore, frequency conversion is essential. Frequency converters can achieve soft starting of motors and compensate for power factor distortion.
