Digital Simulation Of Sinusoidal Pwm Inverter Fed Im Drive

IEEE 1999 International Conference on Power Electronics and Drive Systems, PEDS’99, July 1999, Hong Kong.

DIGITAL SIMULATION OF SINUSOIDAL PWM INVERTER FED INDUCTION MOTOR DRIVE IEE Agnivesh Gupta,Mernber BHEL,Ramachandral’uram

M.Veera charY,Member IEEE Dept of Electrical Enpeering JNTU College of Engineering ANANTAPUR - 515 002 E-mail: [email protected]

Abstract: Computer simulation of sinusoidal Pulse width Modulated Voltage Source Inverter fed 4.1kV, 4-pole induction motor performance operating at constant Volts/Hertz is presented using conventional per-phase equivalent circuit. sinusoidal Pulse width Modulation strategy for the inverter was introduced into the motor performance analysis program allowing the computation of the drive system performance over the entire speed range. Deep bar effect of the rotor is incorporated. Harmonic motor losses are evaluated, compared to a loss model. Various performance characteristics of variable speed induction motor are plotted.

I. Introduction Variable speed drives (VSD’s) employing squirrel cage induction motors, Pulse Width Modulation (PWM) inverters are well known. Developments in fast switching power devices, operational advantages of VSD’s such as smooth speed control, substantial saving in energy over wide range of speeds etc.,have made the use of PWM inverter fed induction motors are better suited for industrial applications. Simulation studies of high power induction machines with 6-step Voltage Source Inverter (VSI) reported in real]. In that study it is found that due to the presence of supply harmonics the machine performance deteriorated such as increased copper losses, development of pulsating torque’s, noise and vibration, reduction in efficiency and rated power etc.,. F d e r the studies established that the induction motors fed from such non-sinusoidal supplies needs certain redesign considerations for variable speed applications.

To improve motor performance certain harmonics magnitude is suppressed using well-known Pulse Width Modulated Inverter Supplies. Many different P W M strategies have been reported in the literame [SI which include both on-line and off-line patterns for fundamental voltage magnitude control and harmonic reduction or elimination. Sinusoidal P W M technique is most widely employed in the inverters along with the filters to provide almost sinusoidal output voltages even though the realization of the control circuitry is complex and microcomputer implementation requires larger memory requirements.

Hyderabad Andbra Radesh

INDIA.

performance is compared with 6-step VSI, conventional supply [1J . Simulation models are developed both for the Sinusoidal PWM waveform generation and induction motor performance evaluation.

II. MathematicalModel A. Model for sinusoidal Pulse Width Modulation

The sinusoidal PWM is generated by comparing a high frequency d e r with a sinusoidal reference supply of desired fkequency. The switching instants are calculated for line to pole voltages (Vm, Vb & V,) by solving the following equation

Where 4 = 0, 2 d 3 and 4 d 3 for a$ & c phases respectively. With the above-calculated pole voltages phase and line voltages are obtained from the following equations

v, =

vu0

- 50 (3)

Harmonic analysis is incorporated to find the fundamental and harmonic voltages with constant Modulation Index to fkequency ratio, which results in constaut VoltdHertz controL B.Modeling of Induction Motor using Equivalent circuit

The steady-state performance model of the induction motor is developed based on the conventional per phase equivalent circuit as shown in Fig(1). The steady-state performance model equations fkom the ‘T’ equivalent circuit are obtained as follows the impedance offered to K’h harmonic (k=1,5,7,..)

Z, = R, + j X ,

In this paper an attempt has been made to investigate the impact of Sinusoidal P W M voltage waveforms on the variable speed squirrel cage induction motor performance operating at constant VoltslHertz operation and 0~803~57~9-8/99/$10.000 199QIEEE

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(4)

With the Fourier voltage components obtained in the preceding section the various currents (stator, rotor), torque’s and copper losses (stator, rotor) are computed and performance is predicted using the following equations

IK Results & Conclusions Simulationstudies have been made to predict the steady-state performance of induction motor supplied from Sinusoidal P W M voltages. Equivalent circuit parameters based on actual no-load and short circuit tests given in Table - I. Tabled: 4. IKV, Y-CONNECTED INDUCTIONMACHINE 0.00431 0.02701 0.13097 0.14730 4.94000

x 2

X-

Sk= (K k (1 - S)) K

P,=

ZIiR, k4.5.7..

C.Loss Model Equations

The loss model suggested in Ref. [7] gives the computation of inverter induced additional harmonic losses from the known values of fundamental copper losses. The stator, rotor harmonic copper losses can be obtained as

Various performance characteristics are generated with V/F control policy. The results are presented for different o p e r a ~ gftequencies 5 Hz to 50 Hz (F1=21) of the induction machine operating at constant Volts/Hertz are shown in Fig(2) - Fig(6). The p.u. Copper losses (stator & rotor) are increasing with frequency of operation as shown in Fig(3). But the increase in the rotor copper losses are high at high frequencies on account of increased rotor bar resistance as compared to stator copper losses. The total loss (1.0 P.U) at rated conditions with pure sinusoidal supply from tests comprises 3.96% of machine rating. The losses obtained in this method is compared with the values obtained using loss model [7] as shown in Fig(4). The computed stator harmonic copper losses are equal to the losses obtained with loss model. There is a slight deviation in the computed rotor harmonic copper losses with that obtained using loss model on account of no. of harmonics considered. The variation of efficiency with sinusoidal PWM (Fig(5)) and sinusoidal supply is tabulated for comparisonas Frequency

(14)

q with Fundamental

qwith Sinusoidal

m.Simulation The induction motor steady-state performance is determined for hdamental, harmonics and then neglecting saturation in the machine the total performance of the machme is obtained as the sumn-m‘on of individual performances. Simulation is carried out at different frequencies when the A4 is operating at constant Volts/Hertz taking the deep bar effect in the rotor into account [SI. A comprehensive simulation program was developed which uses the following routines * Generation of SinusoidalP W M waveforms * Harmonic analysis of the Sinusoidal P W M voltage waveforms * Deep bar effect calculationat fundamentaland harmonic frequencies * Performance evaluationinvolves determinationof Copper losses (fundamental8c harmonic) Torque’s = Efficiencies * Computation of harmonic copper losses using loss model

At rated conditions (fs = 50 Hz)’the machine efficiency with 6-step VSI supply [l] found to be 95.48% whereas with Sinusoidal P W M supply it is 97.35%. From the above simulationresults the reduction in the efficiency with 6-step inverter supply is very marginal(l.87%) as compared with sinusoidalPWM supply. The fundamental torque variation with frequency .and total electromagnetictorque developed wth Smusoidal P W M supplies tabulated in Table III. From the results the effect of steady harmonic torque’s on the net total torque developed is almost negligible. The predominant pulsating harmonic torque’s computed with Sinusoidal P W M supply is compared with pulsating torque obtained with 6-step VSI supply.

is shown in Fig(6). The fun@e$al

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1

Frequency

1

Table - I11 TORQUE

Fundamental

0 9994.417

Sinusoidal PWM 0.0037572 0.0077917 0.0116469 0.0156109 0.0195049

6-Step VSI 0.0032 0.0192 0.0427 0.0830 0.1130

N.SREENIVASULU, M.VEERACHARY: “Simulation Of Variable Speed Induction Motor Fed From PWM Supplies at Constant VoltdHertz operation”, National Conference on Electric Drives & Controls for Transportation Systems, VIDISHA, BHOPAL; India; Jan 16-18,1997.

I

Total

0.9999999

50

Frequency 10 20 30 40 50

I

M.ABOOST, PHOIVOS D.ZIOGAS: “Stateof-Art Carrier PWM Techniques - A critical Evaluation”; IEEE.Tran. Ind. AppL March 1988, pp.271-280.

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J.T.BOYS, S.J.WALTON ‘‘ A Loss Minimized Sinusoidal PWM Inverter’’; Pro. IEE, VOl-132, R b , Sep’1985, pp.260-268. . DE BUCK F.G.G, GISTERLINK P, DE BACKER D: ‘‘ A Simple But Reliable Loss Model For Inverter Supplied Indudon Motors”, 1EEE.Tran Ind. Appl., IA-20, Jan’1984, pp.190-202. M.LIWSCHIT2 GARM: “Skin Effect of Squirrel Cage Rotors”; AIEE. T m April-1954; pp. 255-258. RM.GREEN, J.T.BOYS: ‘‘ Inverter AC-Drive Efficiency”; IEE Pro. PtB, vol-129, March-1982; pp.75-81.

At rated conditions T5-1 with Sinusoidal PWM supply is 0.0195949p.u as compared to 0.113p.u present with 6-step VSI supply. The pulsating torque T7-1is almost zero whereas in 6-step VSI it found to be 0.023p.u. These substantial reductions in the pulsating torque’s will inturn reduce messes developed on the rotor shaft, which results in reduced vibration and noise levels.

APPENDIX List of symbols Stator Resistance Rotor Resistance Stator leakage reactance Rotor leakage reactance Magnetizing reactance Harmonic orders (1,5,7.) Fun. or Harmonic Voltage Harmonic slip Stator Current Rotor Current Stator Copper Losses Fundamental StCu.losses Rotor Copper Losses Fun. Rotor cu.losses Frequency Ratio Operating frequency Operating to Rated frequency Ratio

REFERENCES AGNIVESH GUPTA, M.VEERACHARY: “ Simulation of Variable Speed Induction Motor - Constant Volts/ Hertz OperaGon”; International Electric Machines And Drives ConferenceQEEE IEMDC); Wisconsin; U S A ; May 18-21, 1997.

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AGNIVESH GUPTA, M.VEERA CHARY: “ Simulation Of Variable Speed Squirrel Induction Motor at Constant Stator Current Operation”, International Conference on Power Electronics and Drive Systems (PEDS’97), Singapore, May 26 29, 1997.

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M.VEERA CHARY, AGNIVESH GUPTA “ Steady State Analysis of Equal Area Pulse Width Modulated Inverter Fed Variable Speed Squirrel Cage Induction Motor”; International Conference On Contribution of Cognition to Modeling, Lyon, FRANCE; July 6-8, 1998.

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