machines iv assignment
DESCRIPTION
Induction Machine DynamicsTRANSCRIPT
FACULTY OF
ENGINEERING, SCIENCE AND BUILT ENVIRONMENT
DEPARTMENT OFELECTRICAL POWER ENGINEERING
ELECTRICAL MACHINES IV
REPORT TITTLE:
INDUCTION MACHINES DYNAMICS
5/3/2013
Lecturer Name:
Student Name: Mr. M.C. Leoaneka
i
Abstract
This experiment is proposed the dynamic simulation of a three phase induction motor
based on the theoretical calculation and the computer simulation.
The dynamic simulation plays a vital role in the validation of the design process of the
motor drive system and it is needed for eliminating inadvertent design mistakes and
resulting error in the prototype construction and testing. This report gives the simulation
of a dynamic performance of induction motor performed by the aid of the PSIM, a
computerized software package version 9.2.1.
ii
Acknowledgement
First and foremost I would like to thanks the “Almighty God”. Without his help and
blessing I would not have been able to finish this report.
I would like to express my most sincere gratitude to All Durban University of Technology
Lecturers who have provided me with information. All my colleagues and friends who
direct or indirectly have contributed for the completion of this report.
I would also like to thanks my family, specially my dear mother Isabel Jose for her
unlimited support and love and my girl friend Halala Zulu that have been so patient and
helpful.
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ContentsAbstract...................................................................................................................................................... ii
Acknowledgement.................................................................................................................................... iii
List of Symbols.........................................................................................................................................vi
1- Introduction...........................................................................................................................................1
1.1- Objectives......................................................................................................................................1
2.0- The theory..........................................................................................................................................2
2.1- Methods of Monitoring Mynamic Condition in an Induction Motor..........................................2
2.1.1- Thermal Monitoring................................................................................................................2
2.1.2- Torque Monitoring..................................................................................................................2
2.1.3- Noise Monitoring....................................................................................................................3
2.1.4- Vibration Monitoring...............................................................................................................3
2.2.0- Behviour of an Induction Motor under Dynamic Condition...................................................3
2.2.1- Inrush Current........................................................................................................................3
2.2.2- Voltage Dip.............................................................................................................................4
2.2.3- Frequency Dip........................................................................................................................4
2.2.3- Acceleration Time..................................................................................................................4
2.2.4- Torque.....................................................................................................................................5
2.2.5- Reactive Power and Starting Power Factor.......................................................................5
2.3.1- Mitigate the Problem of Dynamic Condition.......................................................................5
3.0- The Methodology..............................................................................................................................6
3.1- The theoretical Calculations........................................................................................................6
3.1.1- Starting Current Calculations...............................................................................................7
3.1.2- Zth & Vth Calculation.............................................................................................................7
3.2.0- Thevenin Equivalent Circuit......................................................................................................7
3.2.1- Rotor Current Calculations...................................................................................................8
3.2.2- Starting Torque Calculations................................................................................................8
3.2.3- Slip at Maximum Torque...........................................................................................................8
3.2.4- Maximum Torque......................................................................................................................8
4.0- Induction Motor Modelling................................................................................................................9
4.2- Discussion of the results............................................................................................................13
4.2.1- Moto Current (Ia)..................................................................................................................13
4.2.2- Motor Speed.........................................................................................................................13
4.2.3- Motor Torque........................................................................................................................13
4.2.4- Torque * Speed (Motor Power)..........................................................................................13
iv
4.3-Motor with load (0-100Nm) simulation results..........................................................................14
4.3- Discussion of the results............................................................................................................16
4.3.1- Moto Current (Ia)..................................................................................................................16
4.3.2- Motor Speed.........................................................................................................................16
4.3.3- Motor Torque........................................................................................................................16
4.3.4- Torque * Speed (Motor Power)..........................................................................................16
5- Conclusion..........................................................................................................................................17
7- Recommendations.............................................................................................................................17
9- Reference............................................................................................................................................18
10- Appendix...........................................................................................................................................19
10.1- Appendix (a)..............................................................................................................................19
10.2-Appendix (b)...................................................................................................................................20
10.3- Appendix (c).....................................................................................................................................21
v
List of Symbols
Vl Line Voltage
Vph Phase Voltage
f Supply frequency
Ls Stator inductance
Rs Stator resistance
Lr Rotor inductance
Rr Rotor resistance
Lm Magnetizing Inductance
Xls Stator inductative Reactance
Xlr Rotor Inductive Reactance
Xlm Magnetizing Rectance
Ns Synchronous Speed
S Slip
SmT Slip at Maximum Torque
Tst Starting Torque
Ist Starting Current
Z1 Total Impedance
Tem Maximum Torque
Vth Thevenin Voltage
Zth Thevenin Impedance
J Moment of Inertia
vi
1- Introduction
An induction motor is simply an electric transformer whose magnetic circuit is separated
by an air gap into two relatively movable portions, one carrying the primary and the
other the secondary winding. Alternating current supplied to the primary winding from an
electrical power supply induces an opposing current in the secondary winding, when the
latter is short-circuited or closed through external impedance. Relative motion between
the primary and secondary structure is produced by the electromagnetic forces
corresponding to the power thus transferred across the air gap by induction.
The essential features which distinguish the induction machine from other type of
electric motors is that the secondary currents are created solely by induction, as in a
transformer instead of being supplied by a dc exciter or other external power sources,
as in synchronous and dc machines.
1.1- Objectives
The purpose of this experiment is to study and analyse the behaviour of and induction
motor under the dynamic condition, when the motor is initially unloaded and later is
loaded and the load is increased from one value of inertia to the other. Then comparing
the results obtained from theoretical calculation and those obtained from the practical
simulation.
1
2.0- The theory
2.1- Methods of Monitoring Mynamic Condition in an Induction Motor
Condition monitoring is defined as the continuous evaluation of the health of the plant
and equipment throughout its service life. It is of vital importance to be able to detect
faults while they are still developing.
The followings are one of the condition monitoring method applicable to induction motor:
2.1.1- Thermal Monitoring
The thermal monitoring of electrical machines is accomplished either by measuring the
local or bulk temperature of the motor,or by parameter estimation. A stator current fault
generates excessive heat in the shorted turns and heat promulgates the severity of the
fault until it reaches a destructive stage.
2.1.2- Torque Monitoring
All types of motor faults produce the sidebands at special frequencies in the air gap
torque. However it is not possible to measure the air gap torque directly. The difference
between the estimated torques from the model gives an indication of the ecxisteVnce of
broken bars. From the input terminals, the instantaneous power includes the charging
and discharging energy in the windings. Therefore, the instantaneous power cannot
represent the instantaneous torque. From the output terminals, the rotor, shaft, and
mechanical load of a rotating machine constitute a torsional spring sytem that has its
own natural frequency.
2
2.1.3- Noise Monitoring
Noise monitoring is done by measuring and analyzing the acoustic noise spectrum,
acoustic noise from air gap eccentricity in induction motors can be used for fault
detection. However, the application of noise measurements in a plant is not practical
because of the noisy background from the other machines operating in the vacinity.
2.1.4- Vibration Monitoring
All electric machines generate noise and vibration, and the analysis of the produced
noise and vibration can be used to give information on the condition of the machies.
Even very small amplitude of vibration of machine frame can produce high noise.
2.2.0- Behviour of an Induction Motor under Dynamic Condition
2.2.1- Inrush Current
This is the initial current seen by the motor during the starting operations. The inrush
current directly relates to mechanical stress of the bearings and
belts on the motor load (Cohen, 1995). The resistive or copper losses are proportional
to the square of current, I2R, and therefore affect the efficiency. The power lost is
dissipated as heat, causing thermal stress to the machine and affecting its upkeep cost
and overall lifetime.
3
2.2.2- Voltage Dip
The allowable amount of voltage dip is usually dependent on the size of
the network and the load torque characteristics. The latter is due to the fact that the
torque is approximately proportional to the square of the voltage. According to (IEEE
Std 399- 1997, 1998), the allowable voltage dip range can vary between 80% and 95%
of the rated value.
The minimum voltage dip for NEMA type B motors is approximately 80%, given a static
prime mover torque, so as to achieve the 150% of rated torque required to accelerate
the rotor during starting (NEMA, 2009).Tables describing general use and the locked-
rotor starting kVA are given Appendix C. In power systems a common voltage dip limit
is 94% of rated voltage. Shunt capacitors and other reactive power compensators are
often used to improve voltage response.
2.2.3- Frequency Dip
To maintain system stability it is important to retain as close to the fundamental
frequency of the system as possible. The frequency dip is usually not considered as
important as the voltage dip.
2.2.3- Acceleration Time
The time it takes to approximately reach the rated speed of the
motor. It is often indicative of other parameters such as torque and current. Faster
acceleration time is desired, but often means that high-rated current and other
undesirable affects occur. However, a longer acceleration can mean that the applied
torque is too low and that a still significant current is applied over a longer span of time
resulting in a still too high amount of thermal stress to the motor.
4
2.2.4- Torque
The speed-torque curve is used to represent the required torques of the motor for
different speeds. During start-up the initial locked rotor starting torque must be met to
overcome the potential energy at standstill, and the accelerating torque must be
exceeded to maintain acceleration or the motor will stall (IEEE Std 399-1997, 1998),
(Larabee, Pellegrino, & Flick, 2005), and (Kay, Paes, Seggewiss, & Ellis, 1999)
2.2.5- Reactive Power and Starting Power Factor
It is important to take into account the high reactive power consumed by the motor. In
such a case, the rating of the upstream equipment may need to be rated higher than the
steady-state condition (Kay, Paes, Seggewiss, & Ellis, 1999). The reactive power during
start-up is closely related to the voltage dip. Typical values of the power factor are about
0.20 for motors under 1000 HP (IEEE Std 399-1997, 1998). The locked rotor kVA per
HP is defined for each NEMA code letter, see Appendix C, which can help determine
the expected starting reactive power corresponding with the starting power factor
(Chapman, 2005).
2.3.1- Mitigate the Problem of Dynamic Condition
In order to mitigate these problems several methods are being used such as:
Full voltage, reduced voltage, incremental voltage, soft-starter, and variable frequency
drives.
Another starting technique is to control the applied torque of the attached prime mover,
such as the fluid coupling method. Other methods not discussed below include single-
phase starting of a three-phase motor that can be found in work by (Badr, Alolah, &
Abdel-Halim, 1995). The reader is also referred to (Ansari & Deshpande, 2009) for a
review the problems associated with unbalanced voltage starting.
5
3.0- The Methodology
3.1- The theoretical Calculations
Induction Motor
data
Formulae & Quantity calculations
VL= 380V
f = 50Hz
4 poles (p=2)
Rs= 0.54Ω
Ls= 0.29mH
Rr= 0.056Ω
Lr= 0.54mH
Lxm= 31mH
J= 0.001kgm2
XLs= 2π*f*Ls = 2π*50*0.29*10−3= 0.091Ω
XLr= 2π*f*Ls = 2π*50*0.54*10−3= 0.169Ω
XLm= 2π*f*Ls = 2π*50*31*10−3= 9.74Ω
ns = fp
= 502
= 25rad/sec, Ns= 60*25 =
1500r.p.m
Vph = VL√3
= 380√3
= 219.4V
At starting S=1, hence Rr (1−S )
S = 0
Table 1: Calculations
0.194ohm 0.056ohm0.091ohm 0.1696ohm
9.74ohm r2(1-s)/sVph= 219.4V
Rs Xs
Xm
Rr Xr
Figure 1: Circuit Diagram
6
3.1.1- Starting Current Calculations
Z1= (Rs+jXs) +(Rr+ jXr )×( jXm)Rr+ jXr+ jXm
= (0.194 + j0.091) +(0.056+ j0.169 )∗( j 9.74)0.056+ j 0.169+ j 9.74
= 0.357
∟46 ˚Ω
Istarting = VphZ 1
= 219.40.357
= 614.56 A
3.1.2- Zth & Vth Calculation
Zth = (Rs+ jXs )×( jXm)Rs+ jXs+ jXm
= (0.194+ j0.091 )×( j 9.74)0.194+ j 0.091+ j 9.74
= 0.212∟26.26˚ = (0.190 + j 0.0938) Ω
Vth = Vth Xj Xm
Rs+ jXs+ j Xm = (219.34) X j 9.74
0.194+ j0.091+ j 9.74 = 217.28V
3.2.0- Thevenin Equivalent Circuit
0.19ohm 0.056ohm0.0938ohm 0.1696ohm
Vth= 217.28V
Rth Xth Rr Xr
Figure 2: Thevenin Equivalent Circuit
7
3.2.1- Rotor Current Calculations
Ir = Vt hZt
= 217.28
0.19+ j 0.0938+0.056+ j 0.1696 = 602.86A
3.2.2- Starting Torque Calculations
Tst = 3∗Vt h2∗Rr
2πns∗[(Rt h+Rr)2+¿ = 3׿¿ = 389.62Nm
3.2.3- Slip at Maximum Torque
SmT = Rr
√(Rt h)2+(Xr+Xr)2 =
0.056
√(0.19)2+(0.1696+0.0938)2 = 0.1724
3.2.4- Maximum Torque
Tem = 3×Vt h2
4 π ×ns ׿¿ = 3×217.28
2
4 π ×25׿¿ = 875.67 Nm
8
4.0- Induction Motor Modelling
To simulate the circuit under dynamic condition the schematic diagram in the figure (3)
below has been drawn. Then, relevant data are being inserted into the programs.
For simulating the first experiment the figures below gives the detailed of the data
inserted: Figure (4) gives all the parameters of the motor, figure (5) gives parameters
required in the simulation control. Figure (6) gives the data for the external load
connected to the motor; figure (7) gives the parameter for the voltage steps. And figure
(8) gives the data for the supply power.
Figure 3: Induction Motor Schematic Diagram (100-250Nm load)
9
Figure 4: Induction Motor Input Data Figure 5: Simulation Control Input Data
Figure 6: Step (2-Level) Input Data Figure 7: Mechanical Load Input Data
10
Figure 8: Supply Power Input Data
11
Figure 9: Current, Speed, Torque and Torque*Speed wave forms
0K
-0.5K
-1K
0.5K
1K
I_a
Motor Current
0
-500
500
1000
1500
2000
Speed
Motor Speed
0
-200
200
400
600
800
1000
Torque
Motor Torque
0 2 4 6 8 10
Time (s)
0K
-200K
200K
400K
600K
800K
1000K
1200K
Torque*Speed
Torque * Speed
12
4.2- Discussion of the results
When simulating the motor under these conditions we observe in the waves forms that:
4.2.1- Moto Current (Ia)
The motor reaches a maximum current values of 914.8A at 8msec, after 8msec the
currents starts decreasing and reaches a value of 52.68A at 7.3sec, then after 8 secs at
nearly 8.2secs the motor reaches a steady value of 63.1A which continues with up until
the load changes again.
4.2.2- Motor Speed
The motor starts accelerating from standstill and reaches the synchronous speed of
1493RPM at 8 secs, and continues with it until there is any a load change or
disturbances in the supply Voltage.
4.2.3- Motor Torque
The motor reaches a maximum torque value of 833.37Nm at 94msec and then starts
decreasing and nearly 3.2sec it reaches 4995Nm. Then, the torque starts increasing
again and after 6 sec it reaches its maximum value again. At 6.24secs starts decreasing
and at 8.7sec it reaches a steady value of 241.6Nm.
4.2.4- Torque * Speed (Motor Power)
The motor reaches its maximum power of 1.06MW at 6.2sec, and after that the power
starts decreasing, and at 8, 8 sec the power goes to a steady value of 361.96KW.
13
4.3-Motor with load (0-100Nm) simulation results
In this part of experiment we simulate the motor when the load changes from 0-100Nm, all other motor data remains the same just on the voltage step see figure 7 that we change the values.
A
I_aIM
V
Speed
V
Torque
Motor_Load
380V
Vs50Hz
Torque_Step
1000
Simulator control
Figure 10: Induction Motor Schematic Diagram with load (0-100Nm)
14
Figure 11: Current, Speed, Torque & Torque * Speed wave forms
0K
-0.5K
-1K
0.5K
1K
I_a
Motor Current
0
500
1000
1500
2000
Speed
Motor Speed
0
-200
200
400
600
800
1000
Torque
Motor Torque
0 2 4 6 8 10
Time (s)
0K
-200K
200K
400K
600K
800K
1000K
1200K
Torque*Speed
Torque * Speed
15
4.3- Discussion of the results
When simulating the motor under these conditions we observe in the waves forms the
following:
4.3.1- Moto Current (Ia)
The motor reaches a maximum current values of 914.8A at 8msec, after 8msec the
currents starts decreasing and reaches a value of 36.9A at 6sec, then after 6 secs starts
decreasing a bit more and from 8.54sec and so on it gets a steady value of 43.3A,
assume without load.
4.3.2- Motor Speed
The motor starts accelerating from standstill and reaches the maximum speed o of
1500RPM at 6 sec, then continuous with this speed until there is any a load change or
disturbances in the supply Voltage.
4.3.3- Motor Torque
The motor reaches a torque of 499.6 Nm at 2.52sec, and then at 4.89sec it reaches its
maximum torque of 825.95Nm. After 4.98secs the torque starts decreasing and at
6.89sec it reaches a minimum torque of 0.603Nm.Then it starts increasing again and
reaches a steady torque value of 97.2Nm at 8.74sec.
4.3.4- Torque * Speed (Motor Power)
The motor reaches its maximum power of 1.06MW at 6.2sec, and after that the power
starts decreasing, and at 8, 8 sec the power goes to a steady value of 361.96KW.
16
5- Conclusion
It has been seen that after these experiments that the load plays a vital role in the
acceleration time of the motor, so the bigger the load the longer is the time for the motor
to reach its normal speed. That’s why induction motors suffers a lot when the load
suddenly changes.
7- Recommendations
It is recommended that in order to mitigate the problems with dynamics several methods
are being used such as:
Full voltage, reduced voltage, incremental voltage, soft-starter, and variable frequency
drives.
17
9- Reference
Books:
1- MEHTA, V. K. & MEHTA, R. 2002. Principles of electrical machines: for degree, A.M.I.E., diploma and other engineering examinations, New Delhi, S. Chand.
2- BIMBHRA, P. S. 1995. Generalized theory of electrical machines, New Delhi U6 - ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info:sid/summon.serialssolutions.com&rft_val_fmt=info:ofi/fmt:kev:mtx:book&rft.genre=book&rft.title=Generalized+theory+of+electrical+machines&rft.au=Bimbhra%2C+P.+S&rft.date=1995-01-01&rft.pub=Khanna&rft.externalDocID=12284 U7 - Book U8 - FETCH-dut_catalog_122841, Khanna.
3- MEHTA, V. K. & MEHTA, R. 2002. Principles of electrical machines: for degree, A.M.I.E., diploma and other engineering examinations, New Delhi, S. Chand.
Internet research
4- http://www.drivetechinc.com/articles/IM98VC1.pdf
5- http://www.montefiore.ulg.ac.be/~vct/elec047/dyn_of_ind_mac.pdf
6- http://www.jatit.org/volumes/research-papers/Vol5No6/16Vol5No6.pdf
18
10- Appendix
10.1- Appendix (a)
VARIABLE SPEED DRIVE
19
10.2-Appendix (b)
20
10.3- Appendix (c)
Motor Soft Starter
21