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.)

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