Embed Size (px)
Citation preview
Speed Control of MultilevelInverter-Based Induction MotorUsing V/F Method
Smrati Singh, Piyush Sharma, Arpit Varshney and Ankit Kumar
Abstract In this paper, the speed of a three-phase induction motor is controlled byusing modified cascaded five-level inverter and we compared the total harmonicdistortion of the modified cascaded five-level inverter with the conventional three-level inverter. To reduce the total harmonic distortion, multicarrier PWM is used.An open-loop speed control has been achieved by using V/f method. The simula-tion result gives that the modified cascaded five-level inverter effectively controlsthe motor speed and enhances the drive performance through reduction in totalharmonic distortion (THD).
Keywords Modified cascaded five-level inverter (MCFLI) � Induction motor �Multicarrier PWM � THD � V/f control
1 Introduction
Adjustable speed drives (ASDs) are the essential and endless demand of the industriesand researchers. They are widely used in the industries to control the speed of con-veyor systems, blower speeds, machine tool speeds, and other applications. In many
S. Singh (&) � A. VarshneyElectrical and Electronics Engineering Department, SRM University, NCR Campus,Modinagar, Uttar Pradesh, Indiae-mail: [email protected]
A. Varshneye-mail: [email protected]
P. SharmaElectrical Engineering Department, SS College of Engineering, Udaipur, Rajasthan, Indiae-mail: [email protected]
A. KumarSchool of Electrical, Electronics and Communication Engineering, Subharti University,Meerut, Indiae-mail: [email protected]
© Springer India 2015K.N. Das et al. (eds.), Proceedings of Fourth International Conference on SoftComputing for Problem Solving, Advances in Intelligent Systems and Computing 335,DOI 10.1007/978-81-322-2217-0_20
231
industrial applications, traditionally, DC motors were the workhorses for the ASDsdue to their tremendous speed and torque response. But, they have the inherentinconvenience of commutator and mechanical brushes, which undergo wear and tearwith the passage of time. In most cases, AC motors are preferred to DC motors, inparticular an induction motor due to its low cost, low maintenance, lower weight,higher efficiency, improved ruggedness, and reliability. All these features make theuse of induction motors mandatory in many areas of industrial applications [1–5].
The advancement in power electronics and semiconductor technology has trig-gered the development of high-power and high-speed semiconductor devices inorder to achieve a smooth, continuous, and step-less variation in motor speed.Applications of solid-state converters/inverters for adjustable speed induction motordrive are widespread in electromechanical systems for a large spectrum of industrialsystems. Voltage or current converters, as they generate discrete output waveforms,force the use of machines with special isolation and in some applications largeinductances connected in series with the respective load. Also, it is well known thatdistorted voltages and current waveforms produce harmonic contamination, addi-tional power losses, and high-frequency noise that can affect not only the powerload but also the associated controllers. All these unwanted operating characteristicsassociated with pulse-width modulation (PWM) converters could be overcome withmultilevel converters. Nowadays, multilevel inverters are the promising alternativeand cost-effective solution for high-voltage and high-power applications includingpower quality and motor drive problems. Multilevel structure allows raising thepower-handling capability of the system in a powerful and systematic way.
In this paper, three-level and a modified cascaded five-level inverter Simulinkmodels are developed for induction motor drives. A modified cascaded five-levelinverter has less number of switches as compared to conventional five-levelH-bridge inverter. THD of this inverter has been compared with the conventionalthree-level inverter and gives a reduced amount of THD. The simulation of three-level and modified cascaded five-level inverter-fed induction motor models iscarried out in MATLAB. The FFT spectrums for the outputs are analyzed to studythe reduction in the harmonics.
2 Multilevel Inverter
Multilevel voltage source converters are emerging as a new breed of power con-verter options for high-power applications. The multilevel voltage source converterstypically synthesize the staircase voltage wave from several levels of dc voltages.There are some multilevel voltage source converters which are
1. Diode clamp2. Flying capacitors3. Cascaded inverters
232 S. Singh et al.
Increasing the number of voltage levels in the inverter without requiring higherrating on individual devices can increase power rating. The unique structure ofmultilevel voltage source inverters allows them to reach high voltages with lowharmonics without the use of transformers or series-connected synchronizedswitching devices. The harmonic content of the output voltage waveform decreasessignificantly. Ac loads require constant or adjustable voltages at their input termi-nals. When such loads are fed by inverters, it is essential that output voltage of theinverters is so controlled as to fulfill the requirements of AC loads. This involvescoping with the variation of DC input voltage, for voltage regulation of inverters andfor the constant volts/frequency control requirement. There are various techniques tovary the inverter gain. The most efficient method of controlling the gain (and outputvoltage) is to incorporate PWM control within the inverters. The carrier-based PWMschemes used for multilevel inverters are one of the most straightforward methods ofdescribing voltage source modulation realized by the intersection of a modulatingsignal (duty cycle) with triangular carrier waveform [6, 7].
3 Modified Cascaded Five-Level Inverter
In modified cascaded five-level inverter, an auxiliary circuit is added in the simpleH-bridge inverter. The output voltage of simple H-bridge inverter is the three-levelvoltage waveform, and to make it a five-level waveform, the auxiliary circuit isconnected, which contains two back-to-back IGBT switches in series with thediodes. Figure 1 shows a single-phase modified cascaded five-level inverter.
For +Vdc/2, switches S4 and S6 are on, for +Vdc, switches S1 and S4 are on, for –Vdc/2, switches S2 and S3 are on, for –Vdc, switches S2 and S5 are on, and for zero,either switches S4, S1 or switches S2, S4 is conduct shows in Table 1. Therefore,five-level inverter output voltage is obtained. Multicarrier sinusoidal PWM law hasbeen adopted to generate the gating pulses for modified cascaded five-level inverter.
Fig. 1 Single-phase modified cascaded five-level inverter
Speed Control of Multilevel Inverter-Based Induction Motor … 233
4 Adopted Pulse Width Modulation
Figure 2 shows the pulse width modulation for generating gate pulses of single-phase MCFLI. To obtain the five-level PWM, reference sine wave is compared withtwo triangular carrier waves of high frequency (about 1.6–2 kHz). First, Vref iscompared with the carrier 1 as Vref [Vc1 up to Φ1. After Φ2, Vref is compared withVc2 and similar outputs received. If modulating index is (Ma) > 0.5, the output willbe a five level. Modulating index is Ma = Am/2Ac, where Am is amplitude ofmodulating (reference) signal and Ac is the amplitude of carrier signal.
5 Speed Control of Induction Motor
It is very important to control the speed of induction motors in industrial andengineering applications. Efficient control strategies are used for reducing operationcost too. Speed control techniques of induction motors can be broadly classified intotwo types—scalar control and vector control. Scalar control involves controlling themagnitude of voltage or frequency of the induction motor (Fig. 3) [8–11].
Having known the torque–speed characteristic of the motor, its speed can becontrolled in three ways:
(i) Changing the number of poles(ii) Varying the input voltage at fixed frequency(iii) Varying both the input voltage and frequency accordingly
Table 1 Switching statesOn switching state Va Vb Vab = V0
S4, S1 Vdc 0 +Vdc
S4, S6 Vdc/2 0 +Vdc/2
S4, S3 0 0 0
S2, S1 Vdc/2 Vdc/2 0
S2, S5 0 Vdc –Vdc
S2, S3 0 Vdc/2 –Vdc/2
Fig. 2 Pulse width modulation for generating the gate pulses
234 S. Singh et al.
To maintain the torque capability of the motor close to the rated torque at anyfrequency, the air gap flux, uag, is maintained constant. Any reduction in the supplyfrequency without changing the supple voltage will increase the air gap flux, andthe motor may go to saturation. This will increase the magnetizing current, distort theline current and voltage, and increase the core loss and copper loss, and it makes thesystem noisy.
The air gap voltage is related to uag and the frequency f as
Eag ¼ K1uagf ð1Þ
Vs � K1uagf ð2Þ
or
uag ¼ constant � Vs
fð3Þ
where K1 is a constant.
6 V/F Control of Induction Motor
Figure 4 shows the block diagram of closed-loop V/f control of three-phaseinduction motor. The speed error is processed through a PI controller and slip speedregulator. The slip speed regulator sets the slip speed command xsl, whose maxi-mum value is limited to limit the inverter current to a permissible value [12–15].
Fig. 3 Torque–speed characteristic of induction motor
Speed Control of Multilevel Inverter-Based Induction Motor … 235
The synchronous speed, obtained by adding actual speed xr and slip speed xsl,determines the inverter frequency. The reference signals for the closed-loop controlof the machine terminal voltage V�
s are generated from frequency f using a functiongenerator. It ensures nearly a constant flux operation up to the base speed and theoperation at a constant terminal voltage above the base speed. A step increased inspeed command x�
m produces a positive speed error. The slip speed command x�sl is
set at a maximum value. The drive accelerated at a maximum permissible invertercurrent and producing the maximum available torque until the speed error isreduced.
7 Simulation Results
Figure 5 shows the MATLAB/Simulink model of closed-loop V/f control of three-phase induction motor. It consists of DC source, three-phase inverter, and three-phaseinduction motor as open loop, and in addition to that, it has PI controller and limiter.
Fig. 4 Closed-loop volts/Hzcontrol of induction motor
Vdc
g
A
B
C
+
-
Universal Bridge
actual speedgate pulses
Subsystem
Scope2
12.38
Constant2 Tm
m
A
B
C
3 HP - 220 V
60 Hz - 1725 rpm
<Electromagnetic torque Te (N*m)>
<Stator current is_a (A)>
<Stator current is_b (A)>
<Rotor speed (wm)>
<Stator current is_c (A)>
Fig. 5 MATLAB/Simulink model for closed-loop V/f control of induction motor
236 S. Singh et al.
Now, the simulation circuit is run with closed-loop control that shows speed of theinduction motor. Here, a subsystem shows the generation of gate pulses, and a uni-versal bridge is used as a three-phase inverter. Figure 6 shows the Simulink model forgenerating the pulses.
Figures 7, 8 and 9 show the motor speed, torque, and stator current of a three-phase inverter-fed induction motor. Reference speed is set at 157 rad/s. It reachesthe steady state at 0.3 s.
1
gate pulses
-K-
V/fSaturation
Repeating
Sequence
>
Relational
Operator2
>
Relational
Operator1
>
Relational
Operator
NOT
Logical
Operator5
NOT
Logical
Operator4
NOT
Logical
Operator2
f(u)
Fcn3
f(u)
Fcn2
f(u)
Fcn1
PI
Discrete
PI Controller
157
Constant1
Clock
1
actual speed
Fig. 6 MATLAB/Simulink model for pulse generation
Fig. 7 Speed of three-levelinverter-fed I/M
Fig. 8 Torque curve ofthree-level inverter-fed I/M
Speed Control of Multilevel Inverter-Based Induction Motor … 237
Figure 10 shows the PWM generation for a modified cascaded five-level inverterfor one phase. PWM generation is considered the more important in the inverterdesign, and several multicarrier techniques have been developed to reduce thedistortion in multilevel inverters, based on the classical (SPWM) with triangularcarriers.
The PWM pattern adopted in modified cascaded five-level inverter makes theinverter producing output voltage with three levels (zero and half supply dc voltagepositive and negative, respectively) at modulation index (Ma ≤ 0.5) and five levels(zero, half, and full supply voltage positive and negative, respectively) at modu-lation index (Ma > 0.5).
Figure 11 shows the modified cascaded five-level inverter for a single phase.Here, only six controlled switches are used to get five levels which reduced thecomplexity of the circuit and the total harmonic distortion as compared to con-ventional inverter.
Fig. 9 Stator current ofthree-level inverter-fed I/M
6Out6
5Out5
4Out4
3Out3
2
Out2
1Out1
Sine Wave1
Sine Wave
Scope2
RepeatingSequence
Interpolated
>=
RelationalOperator1
>=
RelationalOperator
PulseGenerator1
PulseGenerator
NOT
LogicalOperator9
NOT
LogicalOperator8
NOT
LogicalOperator7
OR
LogicalOperator6
OR
LogicalOperator5
NOT
LogicalOperator4
AND
LogicalOperator3
AND
LogicalOperator2
AND
LogicalOperator1
AND
LogicalOperator
Divide1
Divide
15
Constant
Add1
Add
Fig. 10 MATLAB/Simulink model for pulse generation of MCFLI
238 S. Singh et al.
Figure 12 shows Simulink model for the open-loop V/f control of the modifiedcascaded five-level inverter-fed induction motor. Here, remaining two phases havebeen obtained by giving a 120° phase shift in modulating signal. The scope isconnected through bus selector that shows speed, torque, and stator current of theinduction motor.
Figures 13, 14, 15, 16, 17, 18 and 19 show the phase voltage, line voltage, motorspeed, torque, stator current, and THD of a modified cascaded five-level inverter.By comparing the three-phase inverter and a modified cascaded five-level inverter,we can say that the distortion in five-level inverter voltage is less. The current
2
ConnectionPort1
1
ConnectionPort
Out1
Out2
Out3
Out4
Out5
Out6
sinusoidal PWMgenerator
g m
C E
s6
gm
CE
s5
gm
CE
s4
gm
CE
s3
gm
CE
s2
gm
CE
s1
Vdc1
Vdc
iout
To Workspace3
Diode2
Diode1i+ -
Current Measurement2
i+
+
-
Current Measurement1
Fig. 11 MATLAB/Simulink model for modified cascaded five-level inverter
Scope1
Connectiction Port
Phase C
Phase B
Phase A12.38
Constant2 Tm
m
A
B
C
3 HP - 220 V
60 Hz - 1725 rpm
<Rotor speed (wm)>
<Electromagnetic torque Te (N*m)>
<Stator current is_a (A)>
<Stator current is_b (A)>
<Stator current is_c (A)>
Connectiction Port
Connectiction Port
Fig. 12 MATLAB/Simulink model for modified cascaded five-level inverter-fed I/M
Speed Control of Multilevel Inverter-Based Induction Motor … 239
waveforms are closed to sinusoidal. The speed and torque ripples are very less ascompared to three-phase inverter. Dynamic response is also better for five-levelinverter, which can be observed from the speed and torque waveforms. Figures 18and 19 represent the harmonic spectrum analysis of a five-level inverter. In thiscase, the total harmonic distortion is 12.30 % in phase voltage and 1.55 % in linevoltage. Table 2 shows the comparison of THD in three-level inverter and modifiedcascaded five-level inverter.
Fig. 13 Phase voltage of a modified cascaded five-level inverter
Fig. 14 Line voltage of a modified cascaded five-level inverter
240 S. Singh et al.
0 0.2 0.4 0.6 0.8 1-50
0
50
100
150
200
Time in sec
Sp
eed
in r
ad/s
ec
Speed CurveFig. 15 Speed of a modifiedcascaded five-level inverter-fed I/M
0 0.2 0.4 0.6 0.8 1-100
0
100
200
Time in sec
To
rqu
e in
N-M
Torque CurveFig. 16 Torque of a modifiedcascaded five-level inverter-fed I/M
0 0.1 0.2 0.3 0.4 0.5 0.6-150
-100
-50
0
50
100
150
Time in Sec
Cu
rren
t in
Am
p
Stator CurrentFig. 17 Stator current of amodified cascaded five-levelinverter-fed I/M
Speed Control of Multilevel Inverter-Based Induction Motor … 241
8 Conclusion
A three-level and a modified cascaded five-level inverters have been simulated. Thefollowing features of the system have been demonstrated in the simulation study.The less total harmonic distortion has been achieved in phase voltage and linevoltage with modified cascaded five-level inverter. Speed of the induction motorhas been controlled and achieved the steady-state response in 3 s in the case ofthree-level inverter and in 25 s in the case of modified five-level inverter.
Fig. 18 Harmonic spectrumof phase voltage and voltageof modified cascaded five-level inverter
Fig. 19 Harmonic spectrumof line
Table 2 Comparison ofTHD Parameter THD (%) in
three-levelinverter
THD (%) inmodified cascadedfive-level inverter
Phase voltage 22.50 12.30
Line voltage 4.27 1.55
242 S. Singh et al.
Appendix
Parameter Values
Induction motor: RS = 0.435 Ω, LlS = 4e − 3H, R0r ¼ 0:816X, L0lr ¼ 2e� 3 H,
Lm = 69.31e3 H, p = 4, power = 3 hp, line–line voltage (rms) = 220 V.PI controller: Ki = 50, Kp = 2.
References
1. Rashid, M.H.: Power Electronics: Circuits, Devices and Applications, 2nd edn. Prentice-HallInc., Englewood Cliffs (1993)
2. Bose, B.K.: Power Electronics and Motor Drives. Academic Press, an imprint of Elsevier,Massachusetts (2006)
3. Rodriguez, J., Lai, J.S., Peng, F.Z.: Multilevel inverters: survey of topologies, controls, andapplications. IEEE Trans. Ind. Appl. 49(4), 724–738 (2002)
4. Ogbuka, C.U., Agu, M.U.: A modified closed loop V/F controlled induction motor drive. Pac.J. Sci. Technol. 10(1) (2009)
5. FloraViji Rose, S., Manikandan, B.V.: Simulation and implementation of multilevel inverterbased induction motor drive. IEEE Trans. Ind. Appl. IA-9(3) (2009)
6. Tolbert, L.M., Peng, F.Z., Habetler, T.G.: Multilevel converters for large electric drives. IEEETrans. Ind. Appl. 35, 36–44 (1999)
7. Stemmler, H.: Power electronics in electric traction applications. In: IEEE Conference ofIndustrial Electronics, Control and Instrumentation, IECON’93, Vol. 2, No. 7, pp 07–713(1993)
8. Selvaraj J., Rahim, N.A.: Multilevel inverter for grid-connected PV system. IEEE Trans. Ind.Electron. 56(1) (2009)
9. Manasa, S., Balaji Ramakrishana, S., Madhura, S. Mohan, H.M.: Design and simulation ofthree phase five level and seven level inverter fed induction motor drive with two cascaded H-bridge configuration. Int. J. Electri. Electron. Eng. (IJEEE) 1(4), 25–30 (2012)
10. Murugesan, M., Sivaranjani, S., Asokkumar, G., Sivakumar, R.: Seven level modifiedcascaded inverter for induction motor drive applications. J. Inf. Eng. Appl. 1(1), 36–45 2011
11. Tembhekar, T.D.: Improvement and analysis of speed control of three phase induction motordrive including two methods. In: Second International Conference on Emerging Trends inEngineering and Technology, ICETET-09 (2009)
12. Holmes, D.G., McGrath, B.P.: Multicarrier PWM strategies for multilevel inverters. IEEETrans. Ind. Electron. 49(4), 858–867 (2002)
13. Sundararajan, K., Nachiappan, A., Veerapathiran, G.: Comparison of current controllers for afive-level cascaded H-bridge multilevel inverter. Int. J. Comput. Eng. Res. (ijceronline.com) 2(6), 55–62 (2012)
14. Rose, S.F.V., Manikandan, B.V.: Simulation and implementation of multilevel inverter basedinduction motor drive. INCACEC 2009, 1–8 (2009)
15. Pravin, S.E., Starbell, R.N.: Induction motor drive using seven level multilevel inverter forenergy saving in variable torque load application. ICCET 2011, 352–357 (2011)
Speed Control of Multilevel Inverter-Based Induction Motor … 243
