Chemical Engineering September 2015 - 42

Facts At Your Fingertips
Variable Frequency Drives
Department Editor: Scott Jenkins
Variable
E
lectric alternating current (a.c.)
motors power all types of equipment
in the chemical process
industries (CPI), including pumps, fans,
blowers, compressors and other types
of process equipment. Electric motor
systems offer opportunities for reductions
in power consumption because
while some applications require a constant
motor speed, half of all electric-motor
applications have a speed demand
that varies with different conditions. In
those situations, it is desirable to employ
a variable frequency drive (VFD),
also known as an adjustable speed
drive, to achieve flexible process speed
and torque, and thereby increase system
efficiency. This one-page reference
provides information on VFD operation
and potential benefits.
VFD principles
A VFD is a type of motor controller that
drives a.c. electric motors by varying
the frequency and voltage supplied to
the electric motor. Electrical frequency
(Hertz) is directly related to the motor's
speed (revolutions per minute; rpm),
so higher frequencies translate into
higher motor rpm. VFDs convert an
incoming electrical supply with fixed
frequency and voltage into a variable
frequency and variable voltage output
for the electric motor (Figure 1).
This allows motor speed to be varied
from 0 rpm to typically 100-120% of
its full rated speed, while up to 150%
of rated torque can be achieved at reduced
speed.
VFDs exhibit high versatility and are
available in a range of capacity sizes,
from 0.2 kW to several megawatts.
They are usually available as standalone
devices and are connected to
the motor's electrical supply. However,
some smaller motor designs may
have VFDs available as an integrated
motor-drive product.
VFD components
Major components of a VFD include
the following:
Rectifier. This converts incoming a.c.
supply to direct current (d.c.).
Intermediate circuit. The rectified
d.c supply is then conditioned in the
intermediate circuit, most commonly
42
Rectifier
Time
by a combination of inductors and
capacitors. Most currently available
VFDs use a fixed-voltage d.c. link.
Inverter. The inverter converts the rectified
and conditioned d.c. back into
an a.c supply of variable frequency
and voltage. This is normally achieved
by generating a high-frequency pulsewidth-modulated
(PWM) signal for
frequency and effective voltage. PWM
is a modulation technique where
the average value of voltage fed to
the motor is controlled by switching
power supply on and off at fast rates.
The ratio between on and off periods
determines the total power supplied
to the motor, with longer off periods
associated with lower motor speeds.
The high on-off frequency allows the
motor to perceive a smooth power
supply. Semiconductor switches are
used to create the output.
Control unit. This controls the operation
of the VFD by monitoring and
controlling the rectifier, the intermediate
circuit and the inverter to deliver
the correct output in response to an
external control signal.
VFDs are typically 92-98% efficient
with 2-8% losses due to additional
heat dissipation caused by the highfrequency
electrical switching and
the additional power required by the
electronic components.
Advantages of using a VFD
In many applications, variable speed
control can lead to substantial reductions
in energy cost. The use of VFDs
is particularly effective in fan and pump
applications, where they can replace
traditional power-regulation methods.
Here, an exponential relationship exists
between the machine speed (and
output) and the energy used.
VFDs can enable energy savings
and process efficiency due to improvements
in the following areas:
* Speed control
* Flow control
* Pressure control
* Temperature control
Inverter
FIGURE 1. VFDs work by converting
an incoming electrical supply from
alternating current to direct current,
then creating a variable-frequency
alternating current that can be used
to control motor speed
* Tension control
* Torque control
* Monitoring quality
* Acceleration/deceleration control
In addition to energy savings, many
a.c.-motor applications can benefit
from the use of VFDs because they
can also reduce operating costs by
increasing system reliability and lowering
maintenance requirements by
reducing overall wear and tear
Evaluating VFD usefulness
The following actions can help determine
whether installing a VFD makes
sense for a particular application:
* Understand how the motor system
meets the requirements of the
process. Determine whether the
demand is variable and to what
extent it can be varied or reduced.
Monitor the load profile to help establish
when motor speed can be
reduced and by how much
* Determine the load type, including
whether its torque requirements
vary and to what extent. Establish
whether VFD control can be implemented
on the system or if an alternative
solution would be more
appropriate or cost-effective
* Explore opportunities to maximize
the existing system efficiency
through low-cost measures, because
there is little benefit in fitting
a VFD to a system that already suffers
from poor efficiency. Improve
efficiency by other low-cost means
along with the VFD
* Monitor the system's current energy
consumption to estimate the
energy-saving potential
n
Further reading
1. Prachyl, Stephen, Understanding Variable Frequency
Drives, Plant Engineering, April 2011.
2. Shukla, D.K. and Chaware, D., Variable frequency drives:
An Algorithm for Selecting VFDs for Centrifugal Pumps.
Chem. Eng., February 2010, pp. 38-43.
3. Ramey, Joseph, Variable Frequency Drives for Centrifugal
Pumps, Chem. Eng., November 2012, pp. 31-42.
4. GoHz.com, Variable Frequency Drive, www.variablefrequencydrive.org,
accessed July 2015.
ChemiCal engineering www.Chemengonline.Com september 2015
Voltage
http://www.GoHz.com http://www.variablefre http://www.quencydrive.org http://www.Chemengonline.Com
Chemical Engineering September 2015

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