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Encyclopedia > Variable Frequency Drive
Small Variable Frequency Drive (VFD)

Variable frequency drives operate under the principle that the synchronous speed of an AC motor is determined by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

$RPM = {{{120 times f}over{p}}}$

where

RPM = Revolutions per minute

f = AC power frequency (Hertz)

p = Number of poles (an even number)

Synchronous motors operate at the synchronous speed determined by the above equation. The speed of an induction motor is slightly less than the synchronous speed.

### Example

A 4-pole motor that is connected directly to 60 Hz utility (mains) power would have a synchronous speed of 1800 RPM:

${{{120 times 60}over{4}}} = 1800 RPM$

If the motor is an induction motor, the operating speed at full load will be about 1750 RPM.

If the motor is connected to a speed controller that provides power at 40 Hz, the synchronous speed would be 1200 RPM:

${{{120times 40}over{4}}} = 1200 RPM$

## VFD system description

VFD system

### VFD Motor

The motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed mains voltage operation are often used, but certain enhancements to the standard motor designs offer higher reliability and better VFD performance. Three phase systems have 3 waveforms (usually carrying power) that are 2/3π radians (120 degrees,1/3 of a cycle) offset in time. ... The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. These three conductors are commonly housed in a single conduit (e. ...

### VFD Controller

Variable frequency drive controllers solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Low power models are often configured for use with single-phase input power. In physics, the solid state is one of the three phases of matter (solid, liquid, and gas). ... Look up Electronic in Wiktionary, the free dictionary Electronic can refer to many things: Objects related to electronics The band Electronic. ... It has been suggested that this article or section be merged with rectification (electricity). ...

PWM VFD Diagram

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). For optimum performance, some further voltage adjustment may be necessary, but nominally constant volts per hertz is the general rule. The latest method used for adjusting the motor voltage is called pulse width modulation PWM. With PWM voltage control, the inverter switches are used to divide the quasi-sinusoidal output waveform into a series of narrow voltage pulses and modulate the width of the pulses.

PWM VFD Output Voltage Waveform

### VFD Operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the AFD controller. Most AFD controllers are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer. Process control is an engineering discipline that deals with architectures, mechanisms, and algorithms for controlling the output of a specific process. ... Generally speaking, the term alphanumeric refers to anything that consists of only letters and numbers. ... Input/output, or I/O, is the collection of interfaces that different functional units (sub-systems) of an information processing system use to communicate with each other, or to the signals (information) sent through those interfaces. ... In telecommunications and computer science, serial communications refers to any data transmission scheme in which data is sent one symbol at one time, sequentially over a communications channel. ... In computing, a port (derived from seaport) is usually a connection through which data is sent and received. ...

## VFD Operation

When a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. When an AFD starts, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while drawing only 150% of its rated current. When a motor is simply switched on at full voltage, it initially draws at least 300% of its rated current while producing less than 150% of its rated torque. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed while drawing only 150% current. Jump to: navigation, search Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the gradient of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of... The concept of torque in physics, also called moment or couple, originated with the work of Archimedes on levers. ...

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit to dissipate the braking energy or return it to the power source. Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the gradient of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ...

## Available VFD power ratings

Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 110 volts to 690 volts are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2kW or 1/4 horsepower (Hp) up to at least 750kW or 1000Hp. Medium voltage VFD controllers are designed to operate at 1000 to 10,000 volts. Medium voltage units are typically designed for use with motors rated to deliver 375kW or 500Hp and above. Medium voltage drives rated above 7000 volts and 5000 or 10,000Hp should probably be considered to be one-of-a-kind (one-off) designs. Jump to: navigation, search The horsepower (hp) is the name of several non-metric units of power. ...

## Brushless DC motor drives

Much of the same logic contained in large, powerful VFDs is also embedded in small brushless DC motors. In this case, the chopper usually converts a low DC voltage (such as 12 volts) to the three-phase current used to drive the electromagnets that turn the permanent magnet rotor. A brushless DC motor is an electric motor that operates like a DC motor, but with the roles of the rotor and stator reversed. ... An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. ... Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ...

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