Gingoog Electric Trading (G.E.T.) ™

March 6, 2009

What is a VFD? Part 5

Filed under: GET to know VFD's — Ralf @ 1:58 AM

What is a VFD and how does it work? Part 5

As promised we introduce now the last part of this series but as promised too, this one will go again a little deeper into the matter, ready for some shop talk?

We hope that you liked all 5 parts and that we were able to paint a little picture for us to see what kind of technology the VFD’s actually use, how it works and what it can do, probably a few basics in order to understand much better how the products can be used in our applications, the main topic actually here.

After this we will go more into particular examples in more specific categories through our category GET Applications.

I am sure you will like it.

For now the much awaited Part 5:


A Variable Frequency Drive or VFD is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of Adjustable Speed Drive. Variable Frequency Drives are also known as Adjustable Frequency Drives (AFD), Variable Speed Drives (VSD), AC Drives, Micro Drives or Inverter Drives, just to mention a few other common names. Since the voltage is varied along with frequency, these are sometimes also called VVVF Variable Voltage Variable Frequency Drives.

Variable Frequency Drives are widely used. For example in ventilations systems for large buildings, Variable Frequency motors on fans save energy by allowing the volume of air moved to match the system demand. Variable Frequency Drives are also used on pumps and machine tool Drives.


Operating principle

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 X f / p

where

RPM = Revolutions per minute

f = AC power frequency (Hertz/Hz)

p = Number of poles (an even number)

The constant, 120, is 60 cycles per second multiplied by 2 poles per pole pair. Sometimes 60 is used as the constant and p is stated as pole pairs rather than poles. By varying the frequency of the voltage applied to the motor, its speed can be changed.

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 X 60 / 4 = 1,800RPM

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

When the motor is connected to a speed controller that provides power at 50 Hz, the synchronous speed would be 1500 RPM:

120 X 50 / 4 = 1,500RPM


VFD types

All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off. Attempting to use a linear device such as transistor in its linear mode would be impractical, since power dissipated in the output devices would be about as much as power delivered to the load.

Drives can be classified as:

Constant voltage

Constant current

Cyclo converter

In a constant voltage converter, the intermediate DC link voltage remains approximately constant during each output cycle. In constant current drives, a large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant. A cyclo converter has no input rectifier or DC link and instead connects each output terminal to the appropriate input phase.

The most common type of packaged VF drive is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.


VFD system description

A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.


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.


VFD controller

Variable frequency drive controllers are 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. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be rerated when using single phase input as only part of the rectifier bridge is carrying the connected load.

As new types of semiconductor switches have been introduced, these have promptly been applied to inverter circuits at all voltage and current ratings for which suitable devices are available. Introduced in the 1980’s, the Insulated Gate Bipolar Transistor (IGBT) became the device used in most VFD inverter circuits in the first decade of the 21’st century.

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. 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 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary, but nominally a constant volt per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor.

The usual method used for adjusting the motor voltage is 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.

Operation at above synchronous speed is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called “field weakening” and, for AC motors, is operating at less than rated volts/hertz and above synchronous speed. Example, a 100 HP, 460 VAV, 60 Hz, 1775 RPM (4 pole) motor supplied with 460 VAC, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power.

An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements.

At 460 Volts, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies (CSF) of 2.5 KHz. The lower CSF can produce audible noise at the motors. The 2.5 KHz and 5 KHz CSFs cause less motor bearing problems than caused by CSF’s at 20 KHz. Shorter cables are recommended at the higher CSF of 20 KHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 KHz.


VFD operator interface

The operator interface, also commonly known as an HMI (Human Machine 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 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 controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most 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.


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 a VFD 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 50% 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 50% 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 50% current.

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.


Applications considerations

The output voltage of a PWM VFD consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between the Drive and motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. If the cable is long enough, the resulting voltages can produce up to twice the rated line voltage, putting high stress on the cable and eventual insulation failure. Because of the standard ratings of cables, this phenomenon is of little concern for 230 volt motors, may be a consideration for long runs and 480 volt motors, and frequently a concern for 600 v motors.


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 0.2 KW or ¼ Horse Power (HP) up to at least 750 KW or 1000 HP. Medium voltage VFD controllers are designed to operate at 2400/4160 Volts (60 Hz), 3000 volts (50 Hz) or up to 10 KV. In some applications a step up transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375 KW or 500 HP and above. Medium Voltage Drives rated above 7 KV and 5000 or 10000 Hp should probably be considered to be one-of-a-kind (one-off) designs.


Brushless DC motor Drives

Much of the same logic contained in large, powerful VFDs is also embedded in small brushless DC motors such as those commonly used in computer fans. 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.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

Advertisements

What is a VFD? Part 4

Filed under: GET to know VFD's — Ralf @ 1:43 AM

What is a VFD and how does it work? Part 4

The following article is a bit more compressed, just a quick overview, made with just a few short words to gain an initial idea what VFD’s are all about.

This is our chance to take a little breather because right after this we will go to the last part of this series and dig one more time a little deeper into this issue.


Variable Frequency Drives or VFD’s are electronic devices used to control the speed of an Alternating Current Motor (AC Motor).  Variable Frequency Drives are also commonly known as Adjustable Frequency Drives, Adjustable Speed Drives, AC Drives, Frequency Inverters, Frequency Converters often just only Drives or else simply VLT.

Variable Frequency Drives have a wide range of application use that include, but are not limited to:

  • Variable Air Volume Systems
  • Circulating Pumps for Hot Water Heating Systems
  • Chilled Water Circulating Pumps
  • Geothermal Heat Pump Systems
  • Injection-molding Equipment
  • Air Compressors
  • Conveyors
  • Chillers
  • Cooling Towers

Variable Frequency Drives operate as load controls within these applications that may accomplish up to a 50% reduction in energy costs.  In general, an electric motor will turn at a rate proportional to the frequency of the alternating current (AC) applied to it.  The majority of Variable Frequency Drives in the market today contain electronic circuitry that converts a 60Hz Line power into direct current.  The VFD converts this line power into a pulsed output voltage that duplicates varying alternating current to a desired frequency.


Advances in technology over the past decade have allowed for Variable Frequency Drives to become a very cost efficient way to reduce energy costs and increase system efficiencies.  More and more companies within a wide range of industries are finding more ways to apply VFD’s to their applications.

An AC Variable Frequency Drive (VFD) is commonly referred to as an “Inverter”. This is because of the way a VFD works.


The following details the inner workings of a VFD:

1.) Alternating Current (AC) power is applied to the input of the VFD and feeds a bridge rectifier.

2.) The rectifier converts the Alternating Current (AC) voltage into Direct Current (DC) voltage.

3.) The Direct Current (DC) voltage then feeds the Direct Current (DC) bus capacitors on the VFD where it is stored for use by a transistor or Insulated-Gate Bipolar Transistor (IGBT).

4.) Direct Current (DC) from the capacitors feed the input of the transistor(s).

5.) The transistor(s) then continuously turns on and off at the appropriate frequency to build a new sine wave for use by the motor connected to the output of the VFD.


The process above is often referred to as inversion because it changes from one form to another then back again.

The voltage frequency, as distributed in the Philippines, is 60 cycles per second and the unit of measurement is Hertz (Hz). The output frequency and voltage of an AC Variable Frequency Drive is variable and controlled by the speed at which the output transistor is continuously turned on and off.

The variable speed is controlled digitally in modern VFD’s and changed by the operator through programming, an operator interface, or by changing an analog input to the VFD that is programmed as “speed reference input”.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

March 5, 2009

What is a VFD? Part 3

Filed under: GET to know VFD's — Ralf @ 8:16 AM

What is a VFD and how does it work? Part 3


In order to provide our readers here the best possible information we are using a variety of resources such as manufacturer’s support materials, our extensive experience in this field but also online researches.

When we want to go more into applications in order to achieve the ultimate goal of enjoying all the benefits that come along with a Variable Frequency Drive we should also know what is a VFD and how does it work?

And to understand the VFD’s much better we found for you also the following very interesting material for your further review.

It explains in a very comprehensive way what a VFD actually is and how it works, it confirms what we have posted earlier in our category GET Savings that we can really safe a lot, it gives us an initial idea how we select a Drive and very important also how to size it up. We learn about the relevance of torque and that at times we do have to oversize the VFD for one step in order to reach the required torque. It touches also special applications where we have to watch out or should have at least a closer eye on and it tackles some important corresponding considerations.

It answers a lot of our questions and I would rate this article as another MUST READ, it is not the usual shop talk you might read on other blogs, we make it all easy digestible.

GET it easy, GET is easy, GET more today, with GET you GET it, GET it here and GET it now:

However, this is a larger entry which doesn’t upload in one piece, so we will do it in a few parts:

This is Part 3:


Three basic types of Variable Frequency Drives offer certain advantages as well as disadvantages depending on your motor application. The new flux vector Drive is also discussed.

While all Variable Frequency Drives (VFD’s) control the speed of an AC induction motor by varying the motor’s supplied voltage and frequency of power, they all do not use the same designs in doing so. There are three major VFD designs commonly used today: pulse width modulation (PWM), current source inverter (CSI), and voltage source inverter (VSI). Recently, the flux vector drive also has become popular.


Let’s compare these technologies.

PWM design:

The PWM Drive has become the most commonly used Drive controller because it works well with motors ranging in size from about 1/2 HP to 500 HP. A significant reason for its popularity is that it’s highly reliable, affordable and reflects the least amount of harmonics back into its power source. Most units are rated either 230VAC or 460VAC, 3-phase, and provide output frequencies from about 2 Hz to 400 Hz. Nearly 100 manufacturers market the PWM controller.

Let’s take an example wherein an AC line supply voltage is brought into the input section. From here, the AC voltage passes into a converter section that uses a diode bridge converter and large DC capacitors to create and maintain a stable, fixed DC bus voltage. The DC voltage passes into the inverter section usually furnished with Insulated Gate Bipolar Transistors (IGBTs), which regulate both voltage and frequency to the motor to produce a near sine wave like output.

The term “pulse width modulation” explains how each transition of the alternating voltage output is actually a series of short pulses of varying widths. By varying the width of the pulses in each half cycle, the average power produced has a sine-like output. The number of transitions from positive to negative per second determines the actual frequency to the motor.

Switching speeds of the IGBT’s in a PWM drive can range from 2 KHz to 15 KHz. Today’s newer PWM designs use power IGBT’s, which operate at these higher frequencies. By having more pulses in every half cycle, the motor whine associated with VFD applications is reduced because the motor windings are now oscillating at a frequency beyond the spectrum of human hearing. Also, the current wave shape to the motor is smoothed out as current spikes are removed.


PWM’s have the following advantages:

* Excellent input power factor due to fixed DC bus voltage.

* No motor cogging normally found with six-step inverters.

* Highest efficiencies: 92% to 96%.

* Compatibility with multi motor applications.

* Ability to ride through a 3 to 5 Hz power loss.

* Lower initial cost.


The following are disadvantages, however, that you should also consider:

* Motor heating and insulation breakdown in some applications due to high frequency switching of transistors.

* Non-regenerative operation.

* Line-side power harmonics (depending on the application and size of the Drive).


CSI design:

Let’s take another example wherein the incoming power source to the CSI design is converted to DC voltage in an SCR converter section, which regulates the incoming power and produces a variable DC bus voltage. This voltage is regulated by the firing of the SCR’s as needed to maintain the proper volt/hertz ratio. SCR’s are also used in the inverter section to produce the variable frequency output to the motor. CSI drives are inherently current regulating and require a large internal inductor to operate, as well as a motor load.


CSI’s have the following advantages:

* Reliability due to inherent current limiting operation.

* Regenerative power capability.

* Simple circuitry.


The following are disadvantages, however, in the use of CSI technology:

* Large power harmonic generation back into power source.

* Cogging below 6 Hz due to square wave output.

* Use of large and costly inductor.

* HV spikes to motor windings.

* Load dependent; poor for multi motor applications.

* Poor input power factor due to SCR converter section.


VSI design:

The VSI drive is very similar to a CSI drive in that it also uses an SCR converter section to regulate DC bus voltage. Its inverter section produces a six-step output, but is not a current regulator like the CSI Drive. This Drive is considered a voltage regulator and uses transistors, SCRs or gate turn off thyristors (GTO’s) to generate an adjustable frequency output to the motor.


VSI’s have the following advantages:

TERMS TO KNOW:

Cogging:

Pulsating symptom of a motor while operating at a very low frequency, usually 2 to 6 Hz. Shaft of motor jerks in a rotational manner. The term “cogging” comes from gear cogs.


Non-regenerative:

The inability of a drive to regenerate, or reverse, the power flow back from the motor through the Drive.

* Basic simplicity in design.

* Applicable to multi motor operations.

* Operation not load dependent.


As with the other types of drives, there are disadvantages:

* Large power harmonic generation back into the power source.

* Poor input power factor due to SCR converter section.

* Cogging below 6 Hz due to square wave output.

* Non-regenerative operation.


Flux Vector PWM Drives:

PWM Drive technology is still considered new and is continuously being refined with new power switching devices and smart 32-bit microprocessors. AC Drives have always been limited to “normal torque” applications while high torque, low RPM applications have been the domain of DC Drives. This has changed recently with the introduction of a new breed of PWM drive, the Flux Vector Drive.

Flux Vector Drives use a method of controlling torque similar to that of DC Drive systems, including wide speed control range with quick response. Flux Vector Drives have the same power section as all PWM drives, but use a sophisticated closed loop control from the motor to the Drive’s microprocessor. The motor’s rotor position and speed is monitored in real time via a resolver or digital encoder to determine and control the motor’s actual speed, torque, and power produced.

By controlling the inverter section in response to actual load conditions at the motor in a real time mode, superior torque control can be obtained. The personality of the motor must be programmed into or learned by the Drive in order for it to run the vector control algorithms. In most cases, special motors are required due to the torque demands expected of the motor.


The following are advantages of this new drive technology:

* Excellent control of motor speed, torque, and power.

* Quick response to changes in load, speed, and torque commands.

* Ability to provide 100% rated torque at 0 speed.

* Lower maintenance cost as compared to DC motors and Drives.


As usual, there are disadvantages:

* Higher initial cost as compared to standard PWM Drives.

* Requires special motor in most cases.

* Drive setup parameters are complex.

While flux vector technology offers superior performance for certain special applications, it would be considered “over-kill” for most applications well served by standard PWM Drives.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

What is a VFD? Part 2

Filed under: GET to know VFD's — Ralf @ 8:13 AM

What is a VFD and how does it work? Part 2


In order to provide our readers here the best possible information we are using a variety of resources such as manufacturer’s support materials, our extensive experience in this field but also online researches.

When we want to go more into applications in order to achieve the ultimate goal of enjoying all the benefits that come along with a Variable Frequency Drive we should also know what is a VFD and how does it work?

And to understand the VFD’s much better we found for you also the following very interesting material for your further review.

It explains in a very comprehensive way what a VFD actually is and how it works, it confirms what we have posted earlier in our category GET Savings that we can really safe a lot, it gives us an initial idea how we select a Drive and very important also how to size it up. We learn about the relevance of torque and that at times we do have to oversize the VFD for one step in order to reach the required torque. It touches also special applications where we have to watch out or should have at least a closer eye on and it tackles some important corresponding considerations.

It answers a lot of our questions and I would rate this article as another MUST READ, it is not the usual shop talk you might read on other blogs, we make it all easy digestible.

GET it easy, GET is easy, GET more today, with GET you GET it, GET it here and GET it now:

However, this is a larger entry which doesn’t upload in one piece, so we will do it in a few parts.

This is Part 2:


Key VFD specifications:

While there are many specifications associated with drives, the following are the most important.

Continuous run current rating:

This is the maximum RMS current the VFD can safely handle under all operating conditions at a fixed ambient temperature, usually 40 degrees Celsius. Motor ball load sine wave currents must be equal to or less than this rating.


Overload current rating:

This is an inverse time/current rating that is the maximum current the VFD can produce for a given time frame. Typical ratings are 110% to 150% overcurrent for 1 min., depending on the manufacturer. Higher current ratings can be obtained by oversizing the VFD. This rating is very important when sizing the VFD for the currents needed by the motor for break-away torque.


Line voltage:

As with any motor controller, an operating voltage must be specified. VFD’s are designed to operate at some nominal voltage such as 240VAC or 480VAC, with an allowable voltage variation of plus or minus 10%. Most motor starters will operate beyond this 10% variation, but VFDs will not and will go into a protective trip. A recorded voltage reading of line power deviations is highly recommended for each application.


Applications to watch out for:

If you answer any of the following questions with YES, be extra careful in your VFD selection and setup parameters of the VFD.


Will the VFD operate more than one motor?

The total peak currents of all motor loads under worst operating conditions must be calculated. The VFD must be sized based on this maximum current requirement. Additionally, individual motor protection must be provided here for each motor.


Will the load be spinning or coasting when the VFD is started?

This is very often the case with fan applications. When a VFD is first started, it begins to operate at a low frequency and voltage and gradually ramps up to a preset speed. If the load is already in motion, it will be out of sync with the VFD. The VFD will attempt to pull the motor down to the lower frequency, which may require high current levels, usually causing an overcurrent trip. Because of this, VFD manufacturers offer drives with an option for synchronization with a spinning load; this VFD ramps at a different frequency.


Will the power supply source be switched while the VFD is running?

This occurs in many buildings, such as hospitals, where loads are switched to standby generators in the event of a power outage. Some drives will ride through a brief power outage while others may not. If your application is of this type, it must be reviewed with the Drive manufacturer for a final determination of Drive capability.


Is the load considered hard to start?

These are the motors that dim the lights in the building when you hit the start button. Remember, the VFD is limited in the amount of overcurrent it can produce for a given period of time. These applications may require oversizing of the VFD for higher current demands.


Are starting or stopping times critical?

Some applications may require quick starting or emergency stopping of the load. In either case, high currents will be required of the Drive. Again, oversizing of the VFD may be required.


Are external motor disconnects required between the motor and the VFD?

Service disconnects at motor loads are very often used for maintenance purposes. Normally, removing a load from a VFD while operating does not pose a problem for the VFD. On the other hand, introducing a load to a VFD by closing a motor disconnect while the VFD is operational can be fatal to the VFD. When a motor is started at full voltage, as would happen in this case, high currents are generated, usually about six times the full load amps of the motor current. The VFD would see these high currents as being well beyond its capabilities and would go into a protective trip or fail altogether. A simple solution for this condition is to interlock the VFD run permissive circuit with the service disconnects via an auxiliary contact at the service disconnect. When the disconnecting contact is closed, a permissive run signal restarts the VFD at low voltage and frequency.


Are there power factor correction capacitors being switched or existing on the intended motor loads?

Switching of power factor capacitors usually generates power disturbances in the distribution system. Many VFD’s can and will be affected by this. Isolation transformers or line reactors may be required for these applications.

Power factor correction at VFD-powered motor loads is not necessary as the VFD itself does this by using DC internally and then inverting it into an AC output to the motor. All VFD manufacturers warn against installing capacitors at the VFD output.


Application considerations:

* Starting torque currents

* Running torque currents

* Peak loading currents

* Duty cycle

* Load type

* Speed precision required

* Performance (response)

* Line voltages (deviations)

* Altitude

* Ambient temperature

* Environment

* Motoring/regenerating load

* Stopping requirements

* Motor nameplate data

* Input signals required

* Output signals required

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

What is a VFD? Part 1

Filed under: GET to know VFD's — Ralf @ 8:09 AM

What is a VFD and how does it work? Part 1


In order to provide our readers here the best possible information we are using a variety of resources such as manufacturer’s support materials, our extensive experience in this field but also various online researches, etc, etc, etc.

When we want to go more into applications in order to achieve the ultimate goal of enjoying all the benefits that come along with a Variable Frequency Drive we should also know what is a VFD and how does it work?

And to understand the VFD’s much better we found for you also the following very interesting material for your further review.

It explains in a very comprehensive way what a VFD actually is and how it works, it confirms what we have posted earlier in our category GET Savings that we can really safe a lot, it gives us an initial idea how we select a Drive and very important also how to size it up. We learn about the relevance of torque and that at times we do have to oversize the VFD for one step in order to reach the required torque. It touches also special applications where we have to watch out or should have at least a closer eye on and it tackles some important corresponding considerations.

It answers a lot of our questions and I would rate this article as another MUST READ, it is not the usual shop talk you might read on other blogs, we make it all easy digestible.

GET it easy, GET is easy, GET more today, with GET you GET it, GET it here and GET it now:

However, this is a larger entry which doesn’t upload in one piece, so we will do it in a few parts.

Here is Part 1:


Understand VFD’s:

A thorough understanding of how to match the VFD to the driven load is the key to a successful application.

When applied properly, the Variable Frequency Drive (VFD) is the most effective motor controller in the industry today. Modern VFD’s are affordable and reliable, have flexibility of control, and offer significant electrical energy savings through greatly reduced electric bills.

They are used in a wide variety of applications for various reasons. For example, they are the most effective energy savers in pump and fan applications; they enhance process operations, particularly where flow control is involved. VFD’s provide soft-start capabilities, which decrease electrical stresses and line voltage sags associated with full voltage motor start-ups, especially when driving high-inertia loads.

To obtain a clear understanding of the proper and most effective application of VFD’s, you first should gain a working knowledge of VFD basic theory as well as a strong familiarity with practical know-how.


Basic VFD theory:

Applying a VFD to a specific application is no mystery when you understand the requirements of the load. Simply put, the VFD must have ample current capability for the motor so that the motor can produce the required torque for the load. You must remember that machine torque is independent of motor speed and that load horsepower increases linearly with RPM.


VFD applications can be divided into the following individual load types.

Constant torque loads:

These loads represent 90% of all general industrial machines (other than pumps and fans). Examples of these load types include general machinery, hoists, conveyors, printing presses, positive displacement pumps, some mixers and extruders, reciprocating compressors, as well as rotary compressors.


Constant horsepower loads:

These loads are most often found in the machine-tool industry and center driven winder applications. Examples of constant horsepower loads include winders, core-driven reels, wheel grinders, large driller machines, lathes, planers, boring machines, and core extruders.

Traditionally, these loads were considered DC Drive applications only. Having available high-performance flux vector VFD’s, many DC Drive applications of this type can be now handled by VFD’s.


Variable torque loads:

Variable torque loads are most often found in variable flow applications, such as fans and pumps. Examples of applications include fans, centrifugal blowers, centrifugal pumps, propeller pumps, turbine pumps, agitators, and axial compressors. VFD’s offer the greatest opportunity for energy savings when driving these loads because horse power varies as the cube of speed and torque varies as square of speed for these loads. For example, if the motor speed is reduced 20%, motor horsepower is reduced by a cubic relationship (.8 X .8 X .8), or 51%. As such, utilities often offer subsidies to customers investing in VFD technology for their applications. Many VFD manufactures have free software programs available for customers to calculate and document potential energy savings by using VFDs.


Sizing VFD’s for the load:

How do you size a VFD Drive for an application and feel confident it’s going to work? First, you must understand the requirements of the load. It helps also if you understand the difference between horse power and torque. As electrical people, we tend to think of loads in horse power ratings instead of torque ratings. When was the last time you sized something based on torque? Thus, both torque and horse power must be carefully examined.


Torque:

Torque is an applied force that tends to produce rotation and is measured in lb-ft or lb-in or in the metric system nowadays also with NM (Newton Meter). All loads have a torque requirement that must be met by the motor. The purpose of the motor is to develop enough torque to meet the requirements of the load.

Actually, torque can be thought of as “OOUMPH”. The motor has to develop enough “OOUMPH” to get the load moving and keep it moving under all the conditions that may apply.


Horse power:

Horse power (HP) is the time rate at which work is being done. One HP is the force required to lift 33,000 lbs 1 ft in 1 min. If you want to get the work done in less time, get yourself more horses!

Here are some basic equations that will help you understand the relationship between HP, torque, and speed.

HP = (Torque x Speed)/5250 (eq. 1)

Torque = (HP x 5250)/Speed (eq. 2)

As an example, a 1HP motor operating at 1800 rpm will develop 2.92 lb-ft of torque.

Know your load torque requirements every load has distinct torque requirements that vary with the load’s operation; This torques must be supplied by the motor via the VFD.


You should have a good understanding of this torques:

* Break-away torque:

Torque required to start a load in motion (typically greater than the torque required to maintain motion).

* Accelerating torque:

Torque required to bring the load to operating speed within a given time.

* Running torque:

Torque required to keep the load moving at all speeds.

* Peak torque:

Occasional peak torque required by the load, such as a load being dropped on a conveyor.

* Holding torque:

Torque required by the motor when operating as a brake, such as downhill loads and high inertia machines.


Practical knowhow guidelines

The following guidelines will help ensure a correct match of VFD and motor.

1. Define the operating profile of the load to which the VFD is to be applied.

Include any or all of the “torques” discussed above. Using a recording true RMS Ampere meter to record the motor’s current draw under all operating conditions will help in doing this. Obtain the highest “peak” current readings under the worst conditions. Also, see if the motor has been working in an overloaded condition by checking the motor full-load amps (FLA). An overloaded motor operating at reduced speeds may not survive the increased temperatures as a result of the reduced cooling effects of the motor at these lower speeds.


2. Determine why the load operation needs to be changed.

Very often VFD’s have been applied to applications where all that was required was a “soft start” reduced voltage controller. The need for the VFD should be based on the ability to change the load’s speed as required. In those applications where only one speed change is required, a VFD may not be necessary or practical.


3. Size the VFD to the motor based on the maximum current requirements under peak torque demands.

Do not size the VFD based on horse power ratings. Many applications have failed because of this. Remember, the maximum demands placed on the motor by the load must also be met by the VFD.


4. Evaluate the possibility of required oversizing of the VFD.

Be aware that motor performance (break-away torque, for example) is based upon the capability of the VFD used and the amount of current it can produce. Depending on the type of load and duty cycle expected, oversizing of the VFD may be required.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

Blog at WordPress.com.

%d bloggers like this: