This document provides an overview of a training course on variable frequency drive (VFD) maintenance. It introduces VFDs and discusses their main components, including the rectifier, intermediate DC circuit, inverter, and control/regulation sections. It describes the functions of these components in converting incoming AC power to a variable frequency AC output to control motor speed. The document also outlines maintenance tasks like inspecting components and connections and discusses using a bypass system to run a motor at full speed if the VFD fails.

1. AWI 106 Att. 8 Rev. 3 02 Jan. 2011
Date :
VFD Panel
Maintenance
FM-EL-VFD-L2-01
QC Department

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Preface
 This course introduces to Variable Freqency
Drives.
 It will enable the technicians to obtain basic
knowledge required to perform Operational and
Maintenance tasks, classified as L1 in Electrical
task book, for VFD Maintenance.

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Background
 This course is intended to technicians who have
basic knowledge and qualification in the field of
Electrical.
 Technicians experience in similar field is
advantageous.

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Topics / Tasks
The Following Topics will be covered during this course
I. What is VFD ?
II. Main components of VFD?
IV. Clean the room and panel, visual inspection of all
components and cable termination. (check for any
loose connections, chances for heating & burning )
(FM-EL-TB# 024)
V. Check tightness of terminals of connections
including earth connections (Check for any loose
connections, chances for heating & burning ) (FM-EL-
TB# 024)
VI. Question & Answers

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What is VFD ?
The simplest and least expensive way to control the
speed of a process or piece of equipment is to
operate all the equipment at full speed.
Many applications require the speed of a process or
piece of equipment to be varied.
A variable-frequency drive (VFD) is a system to
control the rotational speed of an AC motor by
controlling the frequency of the electrical power
supplied to the motor.

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What is VFD ?
 AC Adjustable Speed Drives can be thought of as
electrical control devices that change the
operating speed of a motor. ASD’s are able to
vary the operating speed of the motor by
changing the electrical frequency input to the
motor. The speed an AC induction motor
operates is given by the following equation:
Speed = 120 f
P
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Variable Frequency Drive
 Where:
f =Electrical frequency of the power supply in Hz.
P= Number of electrical poles in the motor stator.
Motors speed can be changed by altering the
electrical frequency, the # of poles, or both.
Motor speed can be changed by altering the # of
poles in a motor from 4 to 2:
4 pole motor operating on 60 hertz = 1800 rpm.
2 pole motor operating on 60 hertz = 3600 rpm.
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Variable Frequency Drive
4 pole motor operating on 50 hertz = 1500 rpm.
4 pole motor operating on 40 hertz = 1200 rpm.
By varying frequency, we can adjust the speed over a
wide range or vary the speed precisely using precise
changes in the electrical frequency input to the motor.
Many ASD systems make use of a bypass starter in
parallel with the drive which allows the drive to be
easily bypassed when critical application require a
back-up control means.
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Variable Frequency Drive
The ASD can be bypassed in a matter of seconds
where other types of speed control may be down
for hours or even weeks, while it is being repaired
if there is not a spare control
Some ASD's can control multiple motors from the
same drive which can be an advantage over other
types of variable speed control devices.
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Parts of a Variable Frequency Drive (VFD)

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Parts of a Drive
Parts of a Drive are
1.Rectifier
2 .Soft Charge Circuit
3 .Intermediate Circuit (DC Link)
4. Inverter
5. Control & Regulation Section
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Parts of a Drive
The Drive is just one part of a system. In the
diagram above, notice the disconnect switch,
fuses, bypass switch, thermal overloads, BAS,
etc. all play an important part in making an
application work correctly.
Inside the VFD there are 4 major sections:
rectifier, intermediate circuit (DC Link), inverter
and control/regulation. This fourth section, control
and regulation, interfaces with the other 3
sections.
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Rectifier Section
 Power first goes into the rectifier, where the 3-
phase AC is converted into a rippling DC voltage.
The intermediate circuits then smoothes and holds
the DC Voltage at a constant level or energy
source for the inverter. The last section, the
inverter, uses the DC voltage to pulse the motor
with varying levels of voltage and current
depending upon the control circuit. The pattern of
the pulses going to the motor makes it appear
similar to an AC sinusoidal waveform.
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Rectifier Section
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Rectifier Section
Its function is to change 3-phase AC into DC

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Rectifier Section
 The 3-phase AC voltage goes into the rectifier section
which is made up of a group of gated diodes (silicon
rectifiers or SCRs). In most VFDs, these diodes are in a
group of 6 as diagramed above.
 In this type of circuit, the DC voltage is 1.35 times the
AC line voltage . If 240 Vac is coming in, 324 Vdc is
generated.
 Because of line (power coming in) and load (power to
the motor) changes, the DC Voltage level is constantly
moving above and below this expected value.
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Rectifier Section
 This rectification
 can generate a
considerable amount
of heat, so the SCRs
are mounted onto
 a gold-colored
heatsink. fans
 mounted across the
top of this VFD pull the
air across the
heatsink.
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The Rectifier section contains
terminals for incoming
power, silicon rectifiers (SCR) and
heatsinks

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Soft Charge Circuit
With this circuit, when
power is applied, the
inrush of current is
restricted going to the
large capacitors in the DC
Link, so that they may
charge up slowly (within a
couple of seconds). If this
circuit was absent, line
fuses would be blown
every time the VFD was
started.
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Soft Charge Circuit
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Intermediate Circuit (DC Link)
Using a large bank of
capacitors and DC
reactors the rippling
DC voltage becomes
more stable.
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Intermediate Circuit (DC Link)
The rippled DC voltage coming in has now been
filtered to a relatively constant voltage. Remember
that this DC Link Voltage is 1.35 times the input
voltage. The value of the DC Link voltage can be
read from the display on the front of the drive.
When ever working around the drive always be
careful and give it a healthy respect.
Capacitors banks (black on newer units) area
major part of the DC Link and store a great deal of
energy.
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Inverter Section
The Inverters take
the voltage from the
DC Bus and using
Pulse Width
Modulation (PWM)
sends a signal which
appears to the motor
as an AC signal
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Inverter Section
 Inverter section takes the DC
voltage from the intermediate
section and, with the help of the
control section, fires each set of
IGBT (Insulated Gate Bipolar
Transistors) to the U,V and W
terminals of the motor. This
firing of the IGBTs is known as
Pulse Width Modulation (PWM)
IGBTs
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IGBTs

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Control & Regulation
The control section
coordinates and
regulates signals
inside the drive.Here
numerous calculations
are completed to
properly switch the
IGBTs.This control
sectionseparates the
torque producing
current from the
magnetizing current.
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Control & Regulation
The control section uses
the frequency (f), voltage
(V) and phase angle
(theta) to control the
inverter. The function of
all the parts of the drive,
Rectifier, DCLink, Inverter
and control/Regulation is
to make a clean
waveform to the motor.
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Bypass System for VFD.
 In Case any VFD failure comes , A Star Delta
starter has been Provided to Start the motor.
It will bypass the variable drive to run the motor
in rated full speed.
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PPM – Documents
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Question & Answers
V. QUESTIONS????

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 However, these course are clear in order to
perform maintenance of Variable Frequency Drive
for technicians in the field of Electrical.
Conclusion ‫׃‬ -