EFFECT OF PWM VOLTAGE PULSES ON ADJUSTABLE SPEED...

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 2, February 2014

300

ISSN: 2278 – 7798 All Rights Reserved © 2014 IJSETR

EFFECT OF PWM VOLTAGE PULSES ON ADJUSTABLE

SPEED DRIVES

M.Aravinth1*, S.P.Rajkumar

2

1P.G Scholar, Department of Electrical and Electronics Engineering [P.G] Power Electronics and Drives,

Sri Ramakrishna Engineering College, India.

2Professor & Head, Department of Electrical and Electronics Engineering [U.G], Sri Ramakrishna

Engineering College, India

ABSTRACT: Adjustable speed drives allow for more precise speed control of induction motor at high power

factor and fast response characteristics compared to older technologies. However due to the high switching

frequencies as well as the high ‘dv/dt’ in the output, increased dielectric stresses are formed in the insulation

system of the motor they supply. The presence of semiconductor switches like IGBTs and MOSFETs in PWM

drives has increased the occurrence of steep fronted voltage surges in motors used in ASDs. This project is

to study the line, initial intercoil and interphase voltage distribution of random wound ASDs. Simulation on

the proposed motor drive system is done using MULTISIM.

Keywords: Adjustable speed drives, IGBTs and MOSFETs

1. INTRODUCTION

Adjustable Speed Drives (ASD’s) have become very popular variable speed control devices used in

commercial, industrial and some residential applications. These devices have been available for about 20 years and

have a wide range of applications ranging from single motor driven fans, pumps and compressors, to highly

sophisticated multidrive machines. They operate by varying the frequency of the AC voltage supplied to the motor using solid state electronic devices. These systems are fairly valuable but provide a higher degree of control over the

operation and in many cases, reduce the energy use enough to at least offset if not more than pay for the increased

cost. ASD’s allow precise speed control of a standard induction motor and can result in significant energy savings and improved process control in many applications. It can control the speed of a normal squirrel cage NEMA type B

Induction motor. For a surge impinging on the motor winding, the winding presents itself like transmission line.

2. MODELLING OF A TYPICAL RANDOM WOUND INDUCTION MOTOR

Most of the motors have very low winding resistance, exhibiting essentially a low loss transmission line. Inter-

turn capacitive coupling to earth overrides any mutual inductive couplings. The core might be represented as a

magnetic member with a very low susceptance. The stator core would provide the return path for the surge current since the active conductors are insulated. Further the stator windings are either connected in delta or in star with

insulated neutral. Hence for most part of the winding, the surges would travel without attenuation and also get

reflected at coil junction at stator and overhang portions of a coil. The deviation from uniform theoretical transmission line model values due to actual construction can aggravate the situation. Such a deviation is very pronounced in stator

windings using random windings [3]. As far as the inter-turn coupling is concerned, for the high speeds of switching of

IGBTs (typically <200ns), the inter turn capacitive coupling can be safely assumed to be much greater than inter-turn

mutual inductive coupling.


301


A.Winding Model

1.5HP mush-wound, Delta connected induction motor with the following stator specifications: Number of

stator slots = 36; Number of coils per phase = 6; Conductors/slot = 50*; Stator bore diameter = 0.13m; Stator core length = 0.13m; Enameled wire = 21 SWG

(*Actual design value is 104 conductors per slot. Since the investigation involves the first few turns of a coil, the

motorette has been made with 50 conductors per slot. Hence, for simulation also the same data has been used).

For the physical parameters of the motor winding, calculations yield the following coil parameters: Coil Resistance, Rs=7.7 mΩ;

Inductance, L=28μH;

Earth Capacitance, C=100pF: Winding Insulation Resistance, Rp= 1GΩ.

Fig.1 Winding model of the Induction motor

B. Circuit Diagram

Fig.2 Winding diagram of Induction motor

The inter-turn model assumes that in the slot region, six conductors placed on the periphery of the impulse

conductor can totally shield the conductor from all other conductors and from the core. Capacitance of the conductor subtended on the encircling conductors is used for slot region and a dielectric permittivity of 2.5 is chosen. However,

due to the loose arrangement of the turns in the overhang where the effect of air overrides the enamel insulation, air

permittivity is used. All the unit distance values are converted into real values, using the overhang and slot lengths of the conductors.


302


3. SIMULATION CIRCUITS AND RESULTS

The simulation details of the Winding model are explained. The initial intercoil voltage distribution with

various frequencies with various rise times is simulated and the results are discussed. MULTISIM 2007 is used as a simulation tool.

A. Multisim Block of Winding Model

Fig. 3 Multisim block of the induction motor winding

B. Simulation Output for Icv In B-Phase At Various Carrier Frequencies And Rise Times

Icvb-2kHz-50,200,400,800ns

Fig. 4 Output waveform for Initial InterCoil Voltage Distribution in B-phase for 2 kHz


303


Icvb-8kHz-50,200,400,800ns


Icvb 20kHz-50,200,400,800ns


Icvb-50kHz-50,200,400,800ns

Fig.7 Output waveform for Initial InterCoil Voltage Distribution in B-phase for 50 kHz


304


Merged Output for B - phase

-20

-15

-10

-5

0

5

10

15

20

25

30

35

133

667

110

0613

4116

7620

1123

4626

8130

1633

5136

8640

2143

5646

9150

2653

6156

9660

3163

6667

0170

3673

7177

0680

4183

7687

1190

4693

8197

1610

051

1038

610

721

1105

611

391

1172

612

061

1239

612

731

1306

613

401

1373

614

071

1440

614

741

1507

615

411

1574

616

081

Sample (Time Base)

------- - 2kHz

------- - 8kHz

------- - 20kHz

------- - 50kHz

Vo

ltag

e in

(v

olt

s)

Fig .8 Merged output waveform for initial voltage distribution in B-phase

D. Simulation Input For IICVD in Y-Phase At Various Rise Times And Carrier Frequencies

Icvy-2kHz-50,200,400,800ns

Fig. 9 Input waveform for initial voltage distribution in Y-phase

Icvy-8kHz-50,200,400,800 ns

Fig.10 Input waveform for initial voltage distribution in Y-phase


305


Icvy-20kHz-50,200,400,800ns


Icvy-50kHz-50,200,400,800ns


E. Simulation Output For Iipcv Distribution In B &Y-Phases At Various Rise Times And Carrier Frequencies

INTERPHASE COIL VOLTAGE DISTRIBUTION

FOR 2kHz – 50,200,400,800 ns

Fig 13 Output waveform for interphase coil voltage distribution in B & Y-phase at 2 kHz


306



FOR 8kHz – 50,200,400,800 ns

Fig.14 Output waveform for interphase coil voltage distribution in B & Y-phase at 8 kHz


FOR 20kHz – 50,200,400,800 ns



FOR 50kHz – 50,200,400,800 ns



307


TABULATION FOR INITIAL INTERPHASECOIL VOLTAGE

DISTRIBUTIONSl. No CARRIER FREQUENCY in

(kHz)

RISE TIMES in

(ns)

MAXIMUM INTER

COIL VOLTAGE

DISTRIBUTION in

(Kilo Volts)

1. 2 kHz (a) 50 ns

(b) 200 ns

(c) 400 ns

(d) 800ns

(a) 9.8 kV

(b) 8.8 kV

(c) 7.8 kV

(d) 6.4 kV

2. 8 kHz (a) 50 ns

(b) 200 ns

(c) 400 ns

(d) 800ns

(a) 9.8 kV

(b) 9.6 kV

(c) 8 kV

(d) 6.4 kV

3. 20 kHz (a) 50 ns

(b) 200 ns

(c) 400 ns

(d) 800ns

(a) 9.8 kV

(b) 9.5 kV

(c) 7.8 kV

(d) 6.4 kV

4. 50 kHz (a) 50 ns

(b) 200 ns

(c) 400 ns

(d) 800ns

(a) 9.8 kV

(b) 8.9 kV

(c) 8.4 kV

(d) 8.2 kV

F. Inferences from Oscillogram & Tabulation

1. Rise time of PWM pulses determine the maximum interphase coil voltages.

2. These voltages can reach exceedingly from dangerous levels as shown in the tabulation.

3. The tabulation it can be inferred that carrier frequency has little effect in determining the maximum interphase coil

voltages, However it should be noted that the number of pulses impinging on the line & terminal will increase with the carrier frequency; thus increasing the number of high voltage pulses.

4. CONCLUSION

The initial coil voltage distribution and interphase coil voltage have been simulated using MULTISIM and the

performance results are observed. The simulated results have shown that the Pulse rise times and Carrier Frequency

play a major role in the initial voltage distribution. Obtained results confirm that these over voltages can easily become

causing premature insulation failures in the random wound motors.


308


REFERENCES

[1] Stefan Grubic, Jose M. Aller, Bin Lu and Thomas G. Habetler(2008), “A Survey on Testing and Monitoring Methods for Stator Insulation Systems of Low-Voltage Induction Machines Focusing on Turn Insulation

Problems‖, IEEE Transactions on Industrial Electronics, vol. 55, no. 12.

[2] Sang Bin Lee, Karim Younsi and Gerald B. Kliman (2005), ―An Online Technique for Monitoring the

Insulation Condition of AC Machine Stator Windings‖, IEEE transactions on Energy Conversion, vol. 20, no. 4.

[3] Jeffery L. Kohler, Joseph Sottile, and Frederick C. Trutt, (2002),“Condition Monitoring of Stator Windings in

Induction Motors: Part I—Experimental Investigation of the Effective Negative-Sequence Impedance Detector‖, IEEE transactions on Industry Applications, vol. 38, no. 5.

[4] Yuseph Montasser, Mostafa I. Marei, and Shesha H. Jayaram,(2008), ―Low-Power High-Voltage Power

Modulator for Motor Insulation Testing‖, IEEE Transactions on Industry Applications, vol. 44, no. 4. [5] S.Ponnuswamy Rajkumar, J.Sudesh Johny, A. Ebenezer Jeyakumar (2011), ―Identification of Impending

Interturn Faults in Random Wound Induction Motors Used in Adjustable Speed Drives‖, WSEAS

TRANSACTIONS on CIRCUITS and SYSTEMS.

[6] Puranik K.K (2013), ―Important aspects of Inter turn Insulation in High Voltage Motors‖, Research Journal of Engineering Sciences, ISSN 2278 – 9472 Vol. 2(5), 15-18.Res. J. Engineering Science.

[7] Yuseph Montasser (2006), ―Design and Development of a Power Modulator for Insulation Testing‖ A thesis

presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Master of Applied Science in Electrical Engineering.

[8] Persson E,(1992) ―Transient Effects in Application of PWM inverters to Induction Motors‖ - IEEE-IAS

28.1095-1101.

[9] Kaufhold M, Borner G, Eberhardt M, Speck J,(1996) ―Failure Mechanisms of the Interturn Insulation of Low Voltage Machines Fed by Pulse Controlled Inverters‖,IEEE Electrical Insulation Magazine vol 12, no. 5.

[10] R.A. Hanna, and S.W. Randall (2000), ―Medium Voltage Adjustable-Speed Drive Retrofit of an Existing

Eddy-Current Clutch Extruder Application‖, IEEE Trans. On Industry Applications, vol. 36, pp. 1750-1755.

M.Aravinth was born in Nagarcoil, Kanyakumari, India, in 1986. He is pursuing M.E Degree in

Power Electronics and Drives at Sri Ramakrishna Engineering College, Coimbatore, India. He

received the B.E. degree in Electronics and Instrumentation Engineering from Anna University of

Technology Coimbatore, India in 2012. He finished his Diploma in Electrical and Electronics Engineering in PSG Polytechnic College Coimbatore, India, in 2005. His area of Interest includes

Electrical Machines and Automation.

EFFECT OF PWM VOLTAGE PULSES ON ADJUSTABLE SPEED...

Documents

Transcript of EFFECT OF PWM VOLTAGE PULSES ON ADJUSTABLE SPEED...