PLC Applications for speed control of induction motor through VFD

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 02 | Feb -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1221
PLC APPLICATIONS FOR SPEED CONTROL OF INDUCTION MOTOR
THROUGH VFD
Ms. N. M. Rao1, Vidya Gaikwad2, Prachi Kashid3 , Pavan Dandale4
1Assistant professor, Electrical Dept, AISSMS’S IOIT, Pune, India.
24th year student, Electrical Dept, AISSMS’S IOIT, Pune, India.
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - The paper is about controlling the speed of a
three phase induction motor using variable frequency
drive(VFD) through programmable logic controller(PLC).
Programmable logic controller is an industrial controlling
device and is used to automate machinesand factoryassembly
lines. 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 changed accordingly. VFD
employed in this experiment is based on V/f method of speed
control in which flux remain constant.
Key Words: Variable frequency drive, Programmable logic
controller, Induction motor, V/f method.
1.INTRODUCTION
Three phase induction motors are mostly used AC motors in
industry for various operations due to its low cost and
simplicity. Many factories use PLCs in automation processes
to diminish production cost and to increase quality and
reliability. By implementinga monitoringandcontrol system
for the speed of motor, the induction motor can be used in
high performance variable speed application. Variable
frequency drives are generally required because in many
applications it is not desired to run the motor at same speed
all the time due to surrounding circumstances. For example,
a pump delivering cooling liquid supply may require peal
load operation only for a requisite period of time and may
require only much less amount during remaining time of
day. VFD will allow the speed of the pump to run at a lower
rate in such case thereby enabling energysaving benefits [1].
Speed control methods of induction motors
 Stator voltage control method used have many
disadvantages such as it offers control over limited
range, its use introduces harmonics content and
thus reduces the input supply power factor and can
be used for only low power drives like fan, blowers
etc.[2]
 Stator current control method used have
disadvantages such as low starting torque as
compared to stator voltage control method.
 Stator frequency control method used have
disadvantages such as at low frequencies reactance
will be low leading to high motor currents, more
losses and reduces efficiency, after saturation of
magnetic circuit, motor parameters will changes
leading to inaccurate speed-torque
characteristics.[2]
 All the above mentioned methods of speed control
have considerable disadvantages therefore V/f
method of speed control is adopted which has
advantages such as starting current decreases,
however starting torque becomes more.
1.1 Variable frequency drive
A variable frequency drive is used forapplications where
speed control is important factor due to load changes
wherein the speed needs to be increased or decreased
accordingly. VFD reduces energy consumption and energy
costs, extents equipment life and reduces maintenance cost.
Figure 1: Block diagram of variable frequency drive
1.2 Programmable logic controller
It is used in electrical system toimprovethereliabilityand
efficiency of electrical equipment (electrical motor) inn
automation processes. In this application PLC of Allen
Bradley is used which communicate with VFD and in turn
control the speed of induction motor.
Main parts of PLC are

A. Processor –
 It is brain of PLC
 It execute various logical and control function
 Consist of number data processing equipment that
facilitates I/O processing.
B. Memory-
 Contains all the programs
 Hold all the data of PLC
 Coordinate I/O operation
C. I/O Module-
 Provide a connections to the process that is to be
controlled
Figure no.2: Block diagram of PLC
2. EXPRIMENTAL WORK
A. Experimental setup
1. The Allen-Bradley PowerFlex 4M AC drive is the
smallest and most cost effective drive which
provides powerful motor speed control in compact,
space saving design
OVERVIEW OF THE PROPOSED MECHANISM
A. V/f method
Variation in supply frequency leads to change in
speed. Product of flux and supply frequency is
directly proportional to the induced stator voltage.
When stator drop consider zero then
E≈V
V α f ϕ
If induction motor is to be operated at different
frequencieswithpracticallyconstantvalueofpower
factor, efficiency, a constant absolute slip, overload
capacity and, then with the iron unsaturated, it is
essential that the supply voltage V be varied with
the change in frequency accordingly. Both output
voltage and frequency vary as the speed if the
automatic correction provided.
Figure no.5: Torque speed characteristics of Induction
Motor
As induction motor follow the relation
Tα V/f
So for constant V/f ratio a constant torque can be achieved.
However, this control scheme focus to ensure that any
particular torque can be obtained at the same flux as
operating at normal frequency and voltage.
Ns=120f/p
From above relation the induction motor speed can be
changed by varying the frequency of given becausepolesare
inbuilt and cannot contribute in speed control. So for
constant V/f ratio torque developed is constant in entire
operation. It is the only focus of this method Fig. shows the
speed-torque characteristic of IM with V/f control.
Figure no.3: Torque –speed characteristics with V/f control
Figure no.4: Block diagram of complete setup
Power Flex 4M AC is used in this experiment which is cost
effective and smallest which provides powerful speed
control.

Table -1: VFD specifications used in experiment
Motor rating 0.4 kw(.5 H.P)
Output 3 phase,0-240
AC voltage range
0-230V
Continuous AMP
2.3 A
Overload Amp
3.45,60 sec
The PLC used in this project is Allen Bradley Micro Logix
1400 series. The main parts of this PLC are CPU, power
supply, output module and input module.
Table-2: PLC specifications
Number of I/O 20 inputs,12 outputs
Power supply voltage 24 V DC
Power supply inrush current 24 V DC,15 A for 20 ms
Power consumption 60 W
Table-3: Motor specifications
KW 0.37(0.5 HP)
RPM 2820
Frequency 50±5%
Voltage 415±10%
Ambient temperature 50˚C
Ampere 1.1
Efficiency 88.6%
Conduction of experiment
Allen Bradley PLC ,Allen Bradley Power Flex 4M VFD, a 3
phase induction motor was connected for conduction of
experiment in which for various values of input supply
frequencies, the correspondingvaluesofmotorspeedinRPM
were obtained as shown in table 4.
Input supply frequency in
Hz
Motor speed in RPM
15 900
25 1410
30 1750
45 2500
3. CONCLUSIONS
The present work describe a mechanismtomonitorVFD by
PLC. A complete study of each and every equipment along
with their specification is done. Software platform namely
Allen Bradley ladder logic programming RS Linux is
implemented. A 0.5 Hp 3-Phase Induction Motor was taken
for experimental set-up offered various control modes of
motor operation. A PLC program is made and verified in RS
Linux software. Therefore, the control method discussed in
this paper can be applied in every industry where 3 phase
induction motor is used.
REFERENCES
[1] Rinchen Geongmit Dorjee, Monitoring and Control of
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] Power electronics by P.S. BHIMRA, Third edition, khanna
publication, ISBN No. 81-7409-056.
[3] Ashfaq Husain (2002),”Electricmachines”byDhanpatRai
publication co.(pvt)Ltd. Educational and Technical
Publishers.
[4] “Programmable Logic Controller Principles and
Applications (2013 edition)” by John W. Webb, Ronald A.
Reis by PHI Publishers.

PLC Applications for speed control of induction motor through VFD

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