Variable Frequency Drives (VFD) basics

The Variable Frequency Drives (VFD) are using in applications ranging from small appliances to large motors, pumps and industrial purpose. About 30% of the world's electrical energy is consumed by electric motors in industrial applications, which can be more efficient when using Variable Frequency Drives (VFD) in centrifugal load service.
A variable frequency drive is a device used in a drive system that includes the following three main subsystems.
1, AC motor
2, Main drive controller assembly
3, and drive/operator interface.

Variable Frequency Drives controller


Variable Frequency Drives (VFD) basics
Variable Frequency Drives


The VFD controller is a solid-state power electronics conversion system consisting of three distinct subsystems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage Source Inverter (VSI) drives are the most common types of drives so far. Most drives are AC-AC drives in which they convert the AC line input into AC inverter output. However, in some DC bus or applications such as solar applications, drives are configured as DC-AC drives. The most basic correctional converter for VSI drive is configured as a three-phase, six-pulse, and full-wave diode.

In a VSI drive, a DC link has a capacitor which facilitates the DC output ripple of the converter and provides a hard input to the inverter. This filtered DC voltage is converted to quasi-sinusoidal (sin wave) AC voltage output using the inverter's active switching elements. The VSI drives provide higher power factor and less harmonic distortion compared to a phase controlled current source inverter (CSI) and load-cumulated inverter. The drive controller can also be configured as a phase converter, which has single-phase converter input and three-phase inverter output.

The controller advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid-state power devices over the past six decades. Introduced in the Insulated Gate Bipolar Transistor (IGBT) has in the past two decades come to dominate Variable Frequency Drives (VFD) as an inverter switching device.

VFD in an AC Motor

The Variable Frequency Drives (VFD) system is usually Ac electric motor used in induction Motor. Some types of single phase motors or synchronous motor advantageous in some situations can be used, but three-phase induction motors are generally preferred as the most economical motor choice. Motors that are designed for fixed-speed operation are often used. Elevated-voltage stresses imposed on induction motors that are supplied by Variable Frequency Drives required.

Performance of the Variable Frequency Drives

AC drives are used to bring about the process and quality improvements in industrial and commercial applications' acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque.
Fixed-speed loads the motor under a high initial torque and current surge, which is up to eight times the full load current. AC drives gradually ramps the motor at a dynamic pace to reduce mechanical and electrical stress, reduce maintenance and repair costs, and extend the life of the motor and powered equipment.

Variable speed drives can also run a motor in special patterns to reduce mechanical and electrical stress. Variable Frequency Drives (VFD) can be applied to a conveyor application for smoother deceleration and acceleration control, which reduces the backlash that can occur when a conveyor is accelerating or decelerating.

Operator interface

The operator interface provides an operator the means for starting the motor and stopping and adjusting operating speed. The operator interface often includes alphanumeric displays or indicator lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit are often provided on the front of the Variable Frequency Drive controller. Most Variable Frequency Drives (VFD) are provided with input and output terminals for connecting push buttons, switches, and other operator interface devices or control signals. A serial communication port is also often available to allow the VFD to be configured, adjusted, monitored, and controlled using a computer.

Calculate Synchronous speed

Synchronous speed = 120F/P

The speed at which the magnetic field rotates depends on the number of poles or coils. The poles or coils distributed around the stator and the frequency of the supply current. This is called synchronous speed.

Energy savings

Fixed speed motors load applications that are supplied directly from AC line power can save energy when they are operated at variable speed by means of Variable Frequency Drives (VFD). Such energy cost savings are especially evident in a convertible- torque centrifugal fans and pump applications, where the torque and power of the load vary with class and cube, respectively. This change gives the reduction of large power compared to fixed speed operation for relatively less reduction in speed. E.g. At 75% speed, a motor load consumes only 30% of its full speed power. This reduction is in accordance with affinity laws that define the relationship between various centrifugal load variables.

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