VFD Drive Working Principle

What is VFD Drive

VFD Drive is historical, applications requiring precise speed control, such as paper reels, expensive DC motors, or hydraulic couplings, have been used to control a machine’s speed. In contrast, in other applications, processes have controlled by opening and closing dampers and valves or changing speed performance with gears, pulleys, and similar devices. At the same time, the engine runs at a constant speed.

Variable Frequency Converter (VFD) is a type of motor controller that controls the electric motor by changing the frequency and voltage supplied to the electric motor. Other names for the VFD include variable speed drive, variable speed drive, variable frequency drive, AC drive, microdrive, and inverter.

VFD Drive

VFD is a device based on power electronics, which converts the central fixed frequency. The power of a sinusoidal signal with a fixed voltage (linear power supply) into an alternating output voltage with a variable frequency used to control an induction motor’s speed.

It controls the speed of a three-phase asynchronous motor by controlling the frequency and voltage of the power supplied to the motor. The speed of an AC motor is regulated in two ways – by controlling the voltage or frequency. Frequency control provides better control due to constant flux density than voltage control.

This is a power conversion device that converts a fixed voltage, a fixed frequency of the input power into alternating voltage, an alternating output frequency for controlling asynchronous AC motors.

VFD for AC Motors

VFD for AC Motors was an innovation that brought the use of AC motors back to prominence. An AC induction motor can change its speed by changing the frequency of the voltage used to power it. This means that if the voltage supplied to the AC motor is 50 Hz, the motor runs at rated speed.

If the frequency exceeds 50 Hz, the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than 50 Hz, the motor will run slower than its rated speed, according to the principle of operation of the frequency converter. It is an electronic controller specially designed to change the frequency of the voltage supplied to the induction motor.

Variable Frequency Drive Working Principle

A frequency converter is also known as a VFD, variable speed drive, variable speed drive, electronic motor controller, or inverter. The two main characteristics of the frequency converter are adjustable speeds and soft start/stop capabilities. Each VFD is unique in its component characteristics, so each VFD’s operation depends on the components within the VFD.

These two features make VFD a powerful controller for controlling AC motors. VFD consists mainly of four sections; This is a rectifier, an intermediate DC circuit, an inverter, and a control circuit.

Working of VFD Drive

Variable frequency drive controllers are solid-state electronic energy conversion devices. A conventional design first converts the input AC power to an intermediate DC power using a rectifier or converter bridge. The rectifier is usually a three-phase, half-wave diode bridge. The intermediate DC power is then converted to quasi-sinusoidal AC power using an inverter switching circuit.

The inverter circuit is probably an essential part of the VFD, changing DC energy into three channels of AC energy that can be used by an AC motor. The two main characteristics of the frequency converter are adjustable speeds and soft start/stop capabilities. These two features make VFD a powerful controller for controlling AC motors.

Although the converter consists of six diodes, which are similar to non-return valves used in plumbing systems, they allow current to flow in only one direction; the arrow on the diode symbol indicates the direction. For example, whenever the voltage of the A phase (voltage is similar to the pressure in water supply systems) is more positive than the voltage of phase B or C, this diode opens and transmits current.

When the B phase becomes more positive than the A phase, the B phase diode opens, and the A-phase diode closes. The same is true for three diodes on the negative side of the bus. Thus, we get six current pulses when each diode opens and closes. This is called a “six-pulse VFD,” which is the standard configuration for current frequency converters.

While the two main characteristics of the frequency converter are adjustable speeds and soft start/stop capabilities. These two features make VFD a powerful controller for controlling AC motors. VFD consists mainly of four sections. This is a rectifier, an intermediate DC circuit, an inverter, and a control circuit.

1: Rectifier

The Rectifier is the first stage of a variable frequency drive. It converts AC power from the mains into DC power. This section can be unidirectional or bidirectional depending on the application, such as a quadrant motor’s operation. It uses diodes, SCRs, transistors, and other electronic switching devices.

When the VFD input voltage is 480 volts AC, the DC voltage output from the rectifier section will be approximately 670 volts DC. The pulsed DC voltage supplied to the DC bus in this system. The DC bus identified by the + DC and -DC wires runs along the entire length of the VFD circuit.

Rectifier, The rectifier’s principle of operation is to change the input alternating current (AC) to direct current (DC). Although various designs are available, which are selected depending on the required performance of the frequency converter. The rectifier’s design will affect the degree to which electric harmonics induced on the incoming power source. It can also control the direction of power flow.

2: DC Bus

DC power from the rectifier section is supplied to the DC line. This section consists of capacitors and inductors to smooth out ripples and conserve DC power. The main function of the DC link is to receive, store, and deliver DC energy.

Typical DC applications in which one rectifier module provides power to many inverter units via a universal DC bus. These inverter units can be connected to many motors.

Use of a DC bus as a discharge medium in case of faster braking of the load. A resistor connected to the DC bus can act as a braking resistor that dissipates the pulsed power generated by the fast decelerating load. This protects other parts of VFD.

3: Invertor

The Inverter’s basic principle is to turn on and off the direct current so fast that the motor receives a ripple voltage similar to alternating current. The switching speed is adjusted to change the simulated AC frequency that supplied to the motor.

This section consists of electronic switches, such as transistors, thyristors, IGBT, etc. It receives direct current energy from the direct current line and converts it into alternating current, which is supplied to the motor. It uses modulation methods, such as pulse width modulation, to change the output frequency to control the speed of an induction motor.

4: Control Unit circuit

The control circuit controls the frequency converter; It monitors and controls the rectifier, intermediate circuit, and inverter to ensure the correct output in response to an external control signal.

It consists of a microprocessor device and performs various functions. Such as control, it was setting drive parameters, fault status, and communication protocols. It receives a feedback signal from the motor as a setpoint value of the current speed and accordingly adjusts the voltage to frequency ratio to control the motor’s speed.

Its function is to control the output voltage, that is, the inverter’s voltage vector supplied to the motor. And maintain a constant voltage to frequency ratio (V / Hz). It consists of an electronic circuit that receives feedback from the drive motor and adjusts the output voltage or frequency to the required values. The control scheme can be based on SPWM (PWM sine wave), SVPWM (PWM) with spatial vector modulation), or some algorithm based on soft calculations.

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