Global Journal of Advanced Engineering Technologies
ISSN (Online): 2277-6370 & ISSN (Print):2394-0921
Volume 5, Issue 1- 2016
PLC APPLICATION FOR SPEED CONTROL OF
INDUCTION MOTORS THROUGH VFD
Anshul Tiwari1, Abhay Pratap Singh2, Aditi Dixit3, Debojyoti Sen4
4th Year Student, EN Dept, IMS Engineering College, Ghaziabad, India.
3.
3rd Year Student, EN Dept, IMS Engineering College, Ghaziabad, India.
4
Assistant Professor, EN Dept, IMS Engineering College, Ghaziabad, India.
1,2.
Abstract: The paper is about controlling the speed of a
three phase induction motor using variable frequency
drive (Power Flex 4m) through programmable logic
controller(Compact Logix programmed using RSLogix
5000). Programmable logic controller is an industrial
controlling device and is used to automate machines
and factory assembly lines. A three phase induction
motor can also be controlled manually through
variable frequency drives. So, the main purpose of this
paper is to automate the three phase induction motor by
controlling the inputs to variable frequency drive
through PLC and therefore as a result the inputs to the
induction motor will be changed and thus the speed of
induction motor will change accordingly. VFD
employed in this experiment work on V/f method of
speed control in which flux remains constant. VFD
comprises of converter, filter and PWM inverter. The
output obtained from these three steps is fed to the
induction motor. Obtain speed control up to the range
of 0 to 1510 RPM.
Keywords: Programmable logic controller, Ladder
logic program, Variable frequency drive, V/f method.
I. INTRODUCTION
Automation has become the heart of industries.
Industries are reaching new heights only because of
advancement of automation technology. Plant
automation is the necessity for the manufacturing
industry to survive in the today’s globally competitive
market. Growth of Industries leads to the reduction of
manual controls day by day. One such contribution to
the growing era is made through this paper.
This paper is about controlling the speed of induction
motor, which is most economical motor, using variable
frequency drive (VFD) through programmable logic
controller (PLC). VFD has been chosen specifically
because they provide the advantages of energy savings,
low motor starting current, reduction of thermal and
mechanical stresses on motors and belts during starts,
simple installation, high power factor and lower KVA.
Variable frequency drives (Power Flex 4m) are
generally required because in many applications it is
not desired to run the motor at same speed all the time
www.gjaet.com
due to its surrounding circumstances. The revolution
per minute of the driven shaft need to be increased or
decreased depending on load changes, application
requirement or other circumstances. For example, a
pump delivering cooling liquid supply may require
peak load operation only for a requisite period of time
and may require only much less amount during
remaining time of the day. VFD will allow the speed of
the pump to run at a lower rate in such case thereby
enabling energy saving benefits [1].
The Compact Logix PLC is used which is interfaced
with the help of a software known as RSLogix 5000.
The PLC has been connected to control and monitor a
VFD which acts as a go-between the three phase
induction motor and the PLC. The PLC processes the
inputs according to the ladder logic programming and
initiates corresponding output to the VFD. The VFD in
turn once again processes the PLC input to it and
accordingly controls the speed of three phase induction
motor. Ladder logic programming is carried out in
RSLogix in the personal computer.
A PLC based control system was set up comprising of
an Allen Bradley Compact Logix PLC, an Allen
Bradley Power Flex 4M variable frequency drive, a
three phase induction motor in the Rockwell
automation laboratory GLA University Mathura U.P.
Speed control methods of three phase induction motors
·
·
Stator voltage control method used have many
disadvantages such as it offers control over
limited speed range, its use introduces
harmonic content and thus reduces the input
supply power factor and can be used for only
low power drives like fan, blowers, centrifugal
pumps etc[6].
Stator frequency control method used have
following disadvantages such as at low
frequencies reactances will be low leading to
high motor currents, more losses and reduced
efficiency, after saturation of magnetic circuit,
motor parameters will change leading to
inaccurate speed-torque characteristics and
Page | 30
Global Journal of Advanced Engineering Technologies
ISSN (Online): 2277-6370 & ISSN (Print):2394-0921
·
·
·
with increase in frequency flux and torque also
get reduced[6].
Stator current control method used have
following disadvantages such as low starting
torque as compared to stator voltage control
and if constant current is provided through
current source inverter then there is
generation of unwanted harmonics in the
system and torque pulsations[6].
Static rotor resistance control method used
have following disadvantages such as reduced
efficiency at low speeds, speed changes very
widely with load variation, unbalances in
voltages and currents if rotor circuit
resistances are not equal, Speed above normal
value is not possible and this method cannot
be implemented in squirrel cage induction
motor[6].
All the above mentioned methods of speed
control have considerable disadvantages
therefore V/f method of speed control is
adopted which has following advantages
starting current decreases, however starting
torque becomes more, stable operating region
of motor is increased, at base speed, the
voltage and frequency achieve their rated
values[5,6].
II. V/F METHOD
In this method, V/f ratio remains constant which leads
to constant flux. Result of constant flux is that the
starting torque is inversely proportional to frequency
and maximum torque is independent of frequency(
except at low frequency). Thus maximum torque is
constant in this case.
Other advantages are Power factor at starting is
improved, therefore power input to motor is more,
starting current decreases, however starting torque
becomes more, stable operating region of motor is
increased, at base speed, the voltage and frequency
achieve their rated values and acceleration and
deceleration of the motor can be controlled by
controlling the change of supply frequency to the motor
with respect to time [5].
A) Variable frequency drive
A variable frequency drive is a motor controller that
drives an electric motor connected to it by changing the
frequency and voltage supplied to it. Working of VFD
can be understood in three stages. First stage of VFD is
the converter, second stage is to get rid of the AC
www.gjaet.com
Volume 5, Issue 1- 2016
ripples on a DC bus by using filter and the last stage is
passing the DC obtained through inverter after which
the output is a supply of variable frequency and
variable voltage which is fed to the induction motor.
The VFD used in this experiment is Allen Bradley
Power Flex 4m.
b) Why VFD?
Variable frequency drives are used extensively in
industries because they reduce energy consumption and
energy costs, extend equipment life and reduce
maintenance cost and provide a tight process control.
III. PROGRAMMABLE LOGIC CONTROLLER
A PLC is a solid state, industrial computer that
performs discrete or sequential logic in a factory
environment. it was originally developed to replace
mechanical relays, timer, and counters. A sequence of
instructions is programmed by the user of PLC
memory. Its purpose is to monitor crucial process
parameters and adjust process operations accordingly.
PLC saves Material cost, Energy, installation cost and
ease of maintenance, trouble shooting, labour cost by
reduced wiring and associated errors, less space, no
moving parts-rugged and possibility of reprogramming.
Some value added benefits greater life and reliability,
tremendous
flexibility,
advanced
function,
communication, shorter project time, diagnostics and
easier storage, archiving and documentation.
Application:
Packaging, material handling, power generation,
security system, paint lines, bottling and canning,
machining, HVAC/building automation, automated
PLC used in the experiment is Allen Bradly Compact
Logix programmed by RSLogix 5000.
IV. EXPERIMENT
A) Objective:
The main purpose of this experiment is to control the
speed of a three phase induction motor using VFD
through PLC.
B) Experimental set-up
The Allen Bradley Power Flex 4M AC drive is the most
cost effective and smallest drives which in a compact
and space saving design provide a powerful motor
speed control. Its specifications are shown in the table
below.
Page | 31
Global Journal of Advanced Engineering Technologies
ISSN (Online): 2277-6370 & ISSN (Print):2394-0921
Volume 5, Issue 1- 2016
TABLE 1: SPECIFICATIONS OF VFD
DESCRIPTION
Dimensions HxWxD
Weight
Number of I/O
Power
supply
voltage
Power supply inrush
current
Power consumption
DETAILS
90x180x87 mm
0.9 Kg
2 digital & 2 analog
I/O cards
24V DC
15A for 20 ms
50W
The PLC used in this experiment is Allen Bradley
Compact Logix series. The basic parts of RSLogix
5000 series are Power supply, analog and digital I/O
module
TABLE 2: SPECIFICATION OF PLC
Rated Output
0.75kW (1Hp)
Rated Voltage
240V AC, 1Rated Current
4.2A
Rated torque
3.5kg
C) Ladder logic programming
This experiment was conducted based on ladder logic
programming which is done on a software installed on a
personal computer(RSLogix 5000) according to which
the PLC takes the inputs, processes them according to
the program and gives the output to the VFD which
again processes this input within the drive and finally
controls the speed of the three phase induction motor.
Figure 1: Ladder Logic programming
www.gjaet.com
Figure 2: Plot of frequency vs speed
Above graph shows that the graph of frequency vs
speed comes out to be linear except at lower frequency
showing linear variation of speed with frequency.
Figure 3: Plot of V/f ratio vs. frequency
Above graph shows the variation of V/f ratio vs
frequency. So, this proves that VFD works on the
principle of V/f method keeping flux constant.
Figure 4 : Plot of voltage vs. frequency
Above graph shows the variation of voltage with
frequency so this also proves the linear variation of
voltage with frequency.
Page | 32
Global Journal of Advanced Engineering Technologies
ISSN (Online): 2277-6370 & ISSN (Print):2394-0921
D) Another application
The result obtained above is used for a simplified
approach to control the speed of exhaust fans used in
cooling towers in Power Plants and the Ladder logic
programming used is as shown under
Volume 5, Issue 1- 2016
Computer Science and Software Engineering, ISSN:
2277 128X, Volume 3, Issue 12, December 2013.
[4] S. Takiyar and B. K. Chauhan, Hybrid Method for
Customized Control of Induction Motor, International
Journal of Computer and Electrical Engineering, Vol.
5, No. 4, August 2013.
[5] Z. W. Ma and T. Zeng, “Stability improvement of
V/Hz controlled PWM inverter fed Induction motors
Drive,” IEEE industrial Electronics and Applications’
1st conference, Singapore, pp. 1-4, May 2006.
[6] Power Electronics by P.S. BHIMRA, third edition,
khanna publication, ISBN No. 81-7409-056.
V. CONCLUSION
Being an economical motor, three phase induction
motors are used extensively in industries today. So, if
speed of three phase induction motor can be controlled
then wherever there is a use of three phase induction
motor its speed can be controlled. Therefore, the control
method discussed in this paper can be applied to
everywhere were three phase induction motor is used.
REFERENCES
[1] Rinchen Geongmit Dorjee, Monitoring and Control
of a Variable Frequency Drive Using PLC and SCADA
.ISSN: 2321-8169, International Journal on Recent and
Innovation Trends in Computing and Communication,
Volume: 2 Issue: 10.
[2] YASAR BIRBIR, H. SELCUK NOGAY, Design and
Implementation of PLC-Based Monitoring Control
System for three-Phase Induction Motors Fed by PWM
Inverter, International Journal of Systems Applications,
ENGINEERING & DEVELOPMENT Issue 3, Volume
2, 2008.
[3] Madivalappa.B, M. S. Aspalli, Speed Control of
Three Phase Induction Motor by Variable Frequency
Drive, International Journal of Advanced Research in
www.gjaet.com
Page | 33