Variable Frequency Drives (VfD)
Introduction
Variable Frequency Drive
(VfD) is a type of motor controller that drives an electric
motor by varying the frequency and voltage supplied to the electric motor.
Other names for a VfD are variable speed drive,
adjustable speed drive, adjustable frequency drive, AC drive, micro-drive, and inverter.
Variable-frequency drive also termed adjustable-frequency drive,
“variable-voltage/variable-frequency (VVVF) drive”, variable speed drive
) is a type of adjustable-speed drive used in electro-mechanical drive
systems to control AC motor speed and torque by
varying motor input frequency and voltage. VfDs
are used in applications ranging from small appliances to large compressors.
About 25% of the world's electrical energy is consumed by electric motors in
industrial applications, which can be more efficient when using VfDs in
centrifugal load service; however, VfDs' global market penetration for all
applications is relatively small. Over the last four decades, power
electronics technology has reduced VfD cost and size and has
improved performance through advances in semiconductor switching devices, drive
topologies, simulation and control techniques, and control hardware and
software. VfDs are made in a number of different low- and medium-voltage AC-AC and
DC-AC topologies.
Variable-frequency
types
There are three common
types of VfDs. Current source inversion (CSI) has been successfully used in
signal processing and industrial power applications. CSI VfDs are the only type
that has regenerative power capability. In other words, they can absorb power
flow back from the motor into the power supply. CSI VfDs give a very clean
current waveform but require large, expensive inductors in their construction
and cause cogging (pulsating movement during rotation) below 6 Hz.
Voltage source inversion
(VSI) drives have poor power factor, can cause motor cogging below 6 Hz, and
are non-regenerative. Consequently, CSI and VSI drives have not been widely
used.
Pulse-width modulation
(PWM) VFDs are most commonly used in industry because of excellent input power
factor due to fixed DC bus voltage, no motor cogging, higher efficiencies, and
lower cost. A PWM VFD uses a series of voltage pulses of different lengths to
simulate a sinusoidal wave . Ideally,
the pulses are timed so that the time average integral of the drive yields a
perfect sinusoid. The current method of choice to produce this waveform runs a
triangle wave and sine wave through a comparator, and outputs a voltage pulse
whenever the sine wave's value is greater than the triangle wave. The current
electric component of choice to generate the voltage pulse is the insulated
gate bipolar transistor (IGBT), although silicon-controlled rectifiers (SCRs)
can work as well. In the near future, injection-enhanced gate transistors
(IEGTs) will be used to perform this task. Much more long term, memristors will
probably become the component of choice for this task. Memristors are the
fourth passive circuit element, linking electric charge and magnetic flux Regardless of the component used to form the
sine wave, the switching action causes problems.
System Description
Frequency (or hertz) is directly related to
the motor’s speed (RPMs). In other words, the faster the frequency, the faster
the RPMs go. If an application does not require an electric motor to run at
full speed, the VfD can be used to ramp down the frequency and voltage to meet
the requirements of the electric motor’s load. As the application’s motor speed
requirements change, the VfD can
simply turn up or down the motor speed to meet the speed requirement.
A
variable-frequency drive is a device used in a drive system consisting of the
following three main sub-systems: AC motor, main drive, controller assembly, and drive/operator
interface
The first stage of a Variable Frequency AC
Drive, or VfD, is the Converter. The converter is comprised of six diodes,
which are similar to check valves used in plumbing systems. They allow current
to flow in only one direction; the direction shown by the arrow in the diode
symbol. For example, whenever A-phase voltage (voltage is similar to pressure
in plumbing systems) is more positive than B or C phase voltages, then that
diode will open and allow current to flow. When B-phase becomes more positive
than A-phase, then the B-phase diode will open and the A-phase diode will
close. The same is true for the 3 diodes on the negative side of the bus. Thus,
we get six current “pulses” as each diode opens and closes. This is called a
“six-pulse VFD”, which is the standard configuration for current Variable
Frequency Drives.
AC motor
The
AC electric motor used in a VFD system is usually three-phase induction motor.
Some types of single-phase motors or synchronous motors
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 VfDs require that such motors be
designed for definite-purpose inverter-fed duty in accordance with such
requirements as Part 31 of NEMA Standard MG-1.
Controller
The VFD controller is
a solid-state power electronics conversion system consisting of
three distinct sub-systems: a rectifier bridge
converter, a direct current (DC) link, and an inverter. Voltage-source
inverter (VSI) drives Most drives are AC-AC drives
in that they convert AC line input to AC inverter output. However, in some
applications such as common DC bus or solar applications,
drives are configured as DC-AC drives. The most basic rectifier converter for
the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge.
In a VSI drive, the DC link consists of a capacitor which
smooth’s out the converter's DC output ripple and provides a stiff input to the inverter. This
filtered DC voltage is converted to quasi-sinusoidal AC
voltage output using the inverter's active switching elements. VSI drives
provide higher power factor and lower harmonic distortion than phase-controlled current-source inverter
(CSI) and load-commutated inverter (LCI) drives The
drive controller can also be configured as a phase converter having
single-phase converter input and three-phase inverter outputController 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 1983, the insulated-gate bipolar transistor (IGBT)
has in the past two decades come to dominate VFDs as an inverter switching
device.
In variable-torque applications
suited for Volts-per-Hertz (V/Hz) drive control, AC motor characteristics
require that the voltage magnitude of the inverter's output to the motor be
adjusted to match the required load torque in a linear V/Hz
relationship. For example, for 460 V, 60 \ Hz motors, this linear V/Hz
relationship is 460/60 = 7.67 V/Hz. While suitable in wide-ranging
applications, V/Hz control is sub-optimal in high-performance applications
involving low speed or demanding, dynamic speed regulation, positioning, and
reversing load requirements. Some V/Hz control drives can also operate in quadratic V/Hz mode or can even be programmed to suit
special multi-point V/Hz paths.
The two
other drive control platforms, vector control and direct torque control (DTC), adjust the motor voltage
magnitude, angle from reference, and frequency[14] so as to precisely control the motor's magnetic flux and
mechanical torque.
Although
space vector pulse-width modulation (SVPWM)
is becoming increasingly popular, sinusoidal PWM (SPWM) is the most
straightforward method used to vary drives' motor voltage (or current) and
frequency. With SPWM control quasi-sinusoidal, variable-pulse-width output
is constructed from intersections of a saw-toothed carrier signal with a modulating sinusoidal signal which is variable in
operating frequency as well as in voltage (or current).
Operation of the
motors above rated nameplate speed (base speed) is possible, but is limited to
conditions that do not require more power than the nameplate rating of the
motor. This is sometimes called "field weakening" and, for AC motors,
means operating at less than rated V/Hz and above rated nameplate speed. Permanent magnet synchronous
motors have quite limited field-weakening speed range due to the constant
magnet flux linkage. Wound-rotor synchronous motors and induction
motors have much wider speed range. For example, a 100 HP, 460 V,
60 Hz, 1775 RPM (4-pole) induction motor supplied with 460 V,
75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125%
speed (2218.75 RPM) = 100% power. At higher speeds, the induction motor torque
has to be limited further due to the lowering of the breakaway torque of the
motor. Thus, rated power can be typically produced only up to 130-150% of the
rated nameplate speed. Wound-rotor synchronous motors can be run at even higher
speeds. In rolling mill drives, often 200-300% of the base speed is used. The
mechanical strength of the rotor limits the maximum speed of the motor.
An embedded microprocessor governs
the overall operation of the VFD controller. Basic programming of the microprocessor is provided as
user-inaccessible firmware. User programming of display,
variable, and function block parameters is provided to control, protect, and
monitor the VFD, motor, and driven equipment.
The basic drive
controller can be configured to selectively include such optional power
components and accessories as follows:
·
Connected upstream of converter -- circuit breaker or
fuses, isolation contactor, EMC filter, line reactor,
passive filter
·
Connected downstream of inverter—output reactor,
sine wave filter, dV/dt filter.
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
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 push buttons, 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
Sizing
VFD
sizing should always be based on motor current and voltage, not hp. Motors can be rated for multiple input
voltages, but it is extremely important to know the input voltage for the
application because VfDs are not rated for multiple voltages and only operate
on the voltage for which they are designed, such as 100 to 120 V, 200 to 240 V,
380 to 48 0V or 525 to 600/690 V. A VfD should never be sized solely on the hp
rating of the motor. It is critical that other points be taken into account to
ensure proper sizing of the VFD. Let us start with FLAs.
Full
load amps FLAs are the amps on which the motor will draw when operating at full
load and full speed. The actual measured FLA may be lower than the nameplate
based on the actual loading of the motor (this may be because of a reduced load
or a slower speed). It is critical when sizing the VfD to use the motor FLA
plus any additional overload that the application may require. This additional
overload rating may actually push the VfD selection to the next size, so it is
important to use care in sizing the VFD for this additional overload.
Service
factor amps. In some pumping applications, the pump manufacturer may size the
motor to operate into its service factor to get the pump to maintain flow or
head conditions as required. When operating in this condition, it is important
to verify the continuous amps required to operate the motor in its new normal
conditions. These overload motor amps are used to size the VfD. Sometimes the
motor nameplate details the service factor amps of the motor, otherwise it is
critical to multiply the running amps by the service factor to ensure proper
VFD sizing. The overload rating of the VfD will typically have the VfD running
into its intermittent output current rating (which the VfD can run into for one
minute out of every 10 minutes, depending on manufacturer), so the VFD should
be sized so it is possible to see this overload operation as its continuous amp
rating.
For
high overload usage of the VFD, it is important to understand how the motor
will be used so it can be sized to accommodate the motor overload protection
required for more difficult applications such as conveyers, cranes, positive
displacement pumps and blowers. These constant torque applications may require
150 to 160 percent overload of motor current when in an overload condition.
This overload condition may be required to provide the necessary torque to
overcome the starting inertia or during heavy operating conditions in some
loads like rock crushers, positive displacement pumps, belt presses, cranes or
other applications.
Again,
it is critical when sizing the VFD to know as much about the operating
conditions as possible. Different manufacturers use different terms when
describing the overload operation like high overload, constant torque or
automation/industrial VFD to alert the user to the VFD’s overload sizing. For
applications such as centrifugal pumps, fans, blowers or other variable torque
loads, these applications may only require 110 to 120 percent overload current
for one minute. This overload may be required to provide the necessary torque
to overcome the starting inertia or operation during heavy conditions in some
loads like pumping solids or other viscous materials. Most pumping and fan VFDs
today are sized with variable torque as their primary sizing.
Other Factors
It
is also important to know the type of motor in the installation. For example,
be careful when operating a submersible motor. Submersible motors are not National
Electrical Manufacturers Association
(NEMA) B-type motors and typically draw more current per hp than conventional
NEMA B-type motors. Low-rpm motors sometimes require more current to operate.
Equipment alignment will always be better if the correct nameplate motor current
is determined and the VfD is sized based on those facts.
Single-phase
input is another useful application of a VfD for phase conversion. When only
single-phase power is available, some VfDs can use single-phase input power to
provide three-phase output power and operate a three-phase motor. Some manufacturers
provide specific VfD units designed for this while others simply oversize the
VFD and derate for this feature. De-rating the VfD may not be fully UL listed.
Users should be cautious in their search for VFDs and purchase fully tested UL
VfDs certified on single-phase inputs with three-phase outputs.
High-altitude
installations also require special attention. As the air thins with higher
altitude, the ability of the air to transfer heat out of the VFD is reduced.
The higher the altitude, the less heat the VfD can exhaust, requiring a de-rating
or reduction in its ability to produce output current. This is different by
manufacturer and size, so for the best information, visit the VFD
manufacturer’s website or call the tech support hotline.
Higher
ambient temperature can also affect the VfD size since heat dissipation is the
big design issue with VfDs. The higher the ambient air temperature, the hotter
it is when running the VfD. The issue is that the VfD is unable to cool itself
as it generates heat while producing current to run the motor. With a high ambient
temperature, the VfD is unable to dissipate the generated heat it needs to
dissipate and keep itself cool. The sizing may require the need to de-rate for
higher ambient temperature or cool the VFD with cooler air. An air conditioner
is not always the solution. Sometimes an exhaust fan may change enough air to
allow the VfD to operate satisfactorily. The de-rated value varies by
manufacturer and by size, so no good rule applies other than to visit the
manufacturer’s website or call the tech support hotline.
Low
ambient temperature can also be a potential pitfall. VfDs need a certain amount
of heat to keep all components operating at their optimal designs. The low
thermal rating needs to be taken into account when mounting VfDs in
unconditioned or outside installations. Capacitors, insulated-gate bipolar
transistors and LCD displays are some of the components that do not operate
well in below-freezing temperatures or when frozen. If a VfD is unpowered, has
been in a below-freezing temperature and is frozen, it is crucial that the
drive be warmed up before powering because this extreme condition could cause
the capacitors to explode.
Heat,
power losses, and harmonics. The first
problem a VFD manufacturer needs to address is heat. Although VFDs are highly
efficient devices, manufacturers are unable to produce an ideal set of
components. The heat lost in the drive is governed by the following equation:
Hloss= Pt (1-η)
Where Hloss is the power
lost (W), Pt is the power through the drive (W), and η is the
efficiency of the drive. Usually, VfDs have an efficiency rating between 95%
and 98%. This means the amount of air that must be moved through the drive is
governed by the equation:
m = Hloss÷(CpΔT)
= Pt(1-η)÷(CpΔT)
Where m is the mass flow rate (kg/s), Cp
is the specific heat of air [kJ÷(kg×K)], and ΔT is the difference in
temperature between the incoming air and the outgoing air (K). This heat can
cause significant cooling costs to be added into the design, especially if the
drive is unable to be placed in an unclassified location (area free of
flammable gases or particles). If the drive must be placed in a classified
location, then the airflow going to the drive will need to be purged and
pressurized.
Heating is only one of the problems
with VFDs. The other major problem lies with system harmonics. A picture of the
PWM and the harmonics they cause The
irregularities in the sine wave are called harmonics. In an ideal power circuit
world, these harmonics should not exist. They do nothing but cause problems.
Fortunately, there are a number of ways to mitigate harmonics.
One of the simplest methods of dealing
with harmonics is to place a sine wave filter on either side of the VfD. On the
line side, these are typically called line reactors and have reactance values
anywhere between 1.5% and 5.0% impedance. Higher impedance not only stops more
harmonics, but it also limits the power going to the VfD.
Another tactic that can be used on the
line side of the VFD is to place capacitors at a common bus. Because the
impedance of a capacitor is inversely proportional to the frequency of a
signal, the harmonics see a short through the capacitor and travel through the
capacitor to ground, hopefully ignoring the other loads on the bus. VfDs may
also use an active front end to limit the harmonics that the line side sees. An
active front end has another IGBT switching at an inverse voltage as the main
IGBT, but it is placed through a high pass filter so that the fundamental power
signal goes to ground. The summation of the two harmonic signals ideally should
be zero. If an active front-end drive is not suitable for some reason, a
passive front-end VfD might be procured. Passive front-end VFDs use multiple
phase-shifting transformers and diode bridges to mitigate harmonics.
The more pulses a passive front-end
VFfD has, the fewer problems with harmonics exist. The trade-off is that the
line voltages must be well balanced, and with each additional phase shifting
transformer there is increased cost and a loss in efficiency. In extreme cases,
an isolation transformer might be procured. Although this is one of the most
effective ways to prevent harmonics from spreading, it's also one of the most
costly.
If harmonics are not sufficiently
mitigated on the line side of the VfD, crosstalk and overheating could become
issues. Overheating could either cause bus sizes to be de-rated or increase
cooling costs. Crosstalk is defined as the signal from one circuit interfering
with another circuit. Generally speaking, it is a larger issue than
overheating. An example of this is a radio just slightly out of tune. Although
it is possible to hear the music through the static, the static is annoying.
Crosstalk is an annoying thing in telecommunication circuits. In power
circuitry, crosstalk will cause overheating and frequency relay trips.
Just as harmonics left unchecked on the
line side can cause problems, they can create issues on the load side as well.
This is because of the nature of waves. For example, a small force exerted on a
Slinky at either end will cause a high amplitude sine wave. Electromagnetic
waves act in the same fashion, meaning a small amount of reactance can cause
large voltage spikes. Because this reactance is inductive in nature, most
output filters are capacitors connected in a delta configuration. Ideally, this
should make the reactance portion of the impedance go to zero. If the impedance
is matched properly, then this does not occur.
A note of caution: Capacitors connected
on the load side of the VFD can create a large number of problems, up to and
including destroying a drive. Therefore, it's wise to check with the drive
manufacturer before installing a sine wave filter on the load side of the VfD.
On rare occasions, an active filter may be used. Although these tend to work
well, they are rather expensive and usually have to be custom designed.
Benefits
Energy
savings. Many fixed-speed motor load applications that are
supplied direct from AC line power can save energy when they are operated at
variable speed by means of VfD. Such energy cost savings are especially
pronounced in variable-torque centrifugal fan and pump applications, where the
load's torque and power vary with the square and cube,
respectively, of the speed. This change gives a large power reduction compared
to fixed-speed operation for a relatively small reduction in speed. For
example, at 63% speed a motor load consumes only 25% of its full-speed power.
This reduction is in accordance with affinity laws that
define the relationship between various centrifugal load variables. AN actual
audit of a industry utilizing VfDs is
presented as follows:
|
Savings Textile Mill
|
|||
|
11-Sep
|
Jan-14
|
diff %
|
|
|
kWh
|
1489019
|
1515574
|
1.78
|
|
bags
|
6532
|
5932
|
-9.19
|
|
kWh/bag
|
227.9576
|
255.4912
|
12.08
|
|
Rs./month
|
22335285
|
22733610
|
26.75
|
|
savings /month
|
179849.74
|
||
|
savings/year
|
2.16
|
Rs. M
|
|
|
Inv
|
12.88
|
Rs. M
|
|
|
payback
|
5.97
|
Years
|
|
The above suggests that the investment in VfDs has a payback
period of less than 6 years , there was an element of subsidy involved and the
actual pay back period nay be less than 8 years or so,
If you have an application that does
not need to be run at full speed, then you can cut down energy costs by
controlling the motor with a variable frequency drive, which is one of the benefits of Variable Frequency
Drives. VfDs allow
you to match the speed of the motor-driven equipment to the load requirement.
There is no other method of AC electric motor control that allows you to
accomplish this.
Electric motor systems are
responsible for more than 65% of the power consumption in industry today.
Optimizing motor control systems by installing or upgrading to VfDs can reduce
energy consumption in your facility by as much as 70%. Additionally, the
utilization of VFDs improves product quality, and reduces production costs. , returns on investment for VFD installations
can be as little as 6 months. By operating
your motors at the most efficient speed for your application, fewer mistakes
will occur, and thus, production levels will increase, which earns your company
higher revenues.
Equipment Life- Your equipment will last longer
and will have less downtime due to maintenance when it’s controlled by VfDs
ensuring optimal motor application speed. Because of the VfDs optimal control
of the motor’s frequency and voltage, the VfD will offer better protection for
your motor from issues such as electro thermal overloads, phase protection,
under voltage, overvoltage, etc.. When you start a load with a VfD you will not
subject the motor or driven load to the “instant shock” of across the line
starting, but can start smoothly, thereby eliminating belt, gear and bearing
wear. It also is an excellent way to reduce and/or eliminate water hammer since
we can have smooth acceleration and deceleration. Electrically, VfDs run at a high
power factor. Any class of induction motors usually has a low power factor at
half and three-quarters load (0.75 to 0.85). This actually decreases the life
of the motor, because the unnecessary increase in current overheating the
winding insulation. VfDs bypass this problem by running the load at a frequency
below the fundamental
Speed Control- The most obvious reason to procure a VfD is speed control.
This is usually done for process, operation, and economic benefits. One
economic benefit comes from the reduction of maintenance when using a VfD,
especially not having to deal with the DC motor carbon brushes or mechanical
speed-control gearboxes (transmissions). The most obvious economic benefits of
VfDs occur with fans and pumps. The power that a pump or fan consumes is
directly proportional to the cube of the velocity. This means if an operator
can run a fan at 80% of full speed, it theoretically uses 51% of full load
power.
Starting current - VfDs also optimize motor starting characteristics. VfDs bring
motors up to full speed quickly and by drawing only 100% to 150% of full load
amps (FLAs). This ability to start at normal FLA is very important if the power
supply cannot withstand the normally six times FLA across-the-line starting
draw, or even the 350% FLA soft-start device current. VFDs do this by managing
the magnetic flux of an induction motor. Magnetic flux is directly proportional
to the voltage and inversely proportional to the frequency. By keeping the flux
constant, the inrush current does not exceed the FLA rating of the motor, and
full torque is maintained. This is a significant improvement on a soft-start,
which has significant voltage drop problems and cannot start under full load.
Another potentially
useful aspect of VFDs is the maintenance
of a constant torque, and constant horsepower. The
constant torque region is fairly self explanatory; the VfD is regulating the
flux so that the current is constant. Once the VfD surpasses the rated system
frequency, the voltage cannot increase due to the physical constraints of the
system. Because the voltage is static — and the frequency is increasing — the
flux is forced to decrease. When this occurs, the current and torque are forced
to decrease as well. This is called field weakening. Although not necessarily a
good thing, it can be useful if there is a need to power a partial torque load
above the rated speed. In addition to this capability, VfDs can also take any
form of input power whether it's single-phase AC, 3-phase AC, or DC. VfDs fed
from a DC source still power an AC load without an internal rectifier.
Power Grid- VfDs also have some
applications on the power grid. One classic example of this is a doubly fed
induction generator, in which the VfD can force a fixed frequency and voltage
signal out of a variable-speed (frequency) input. This is commonly seen in wind
turbines and other small hydroelectric generation projects that will be
connected to the power grid. Other renewable energy sources, such as
photovoltaic cells, can use VfDs to act as an inverter before connecting to the
power grid, although inverters with buck-boost technology are more common.
Conclusion
Whenever a load has either a variable torque
or a variable speed, a VfD should be considered. A VfD might be considered if a
large motor has a problem with voltage drop, torque, or inrush current during
start-up. Even though VfDs undoubtedly solve their fair amount of problems and
provide substantial energy savings, the heat they generate must be dissipated —
and the harmonics they produce must be mitigated.
