Analysis of Variable Frequency Three Phase Induction Motor Drive

Transcription

1 World Academy of Science, ngineering and Technology Analyi of Variable Frequency Three Phae Induction Motor Drive Thida Win, Nang Sabai, and Hnin Nandar Maung Abtract AC motor drive are widely ued to control the peed of conveyor ytem, blower peed, pump peed, machine tool peed, and other application that require variable peed with variable torque. The complete ytem conit of an ac voltage input that i put through a diode bridge rectifier to produce a dc output which acro a hunt capacitor, thi will, in turn, feed the PWM inverter. The PWM inverter i controlled to produce a deired inuoidal voltage at a particular frequency, which i filtered by the ue of an inductor in erie and capacitor in parallel and then through to the quirrel cage induction motor. II. POCDU FO FQUNCY CONTOLLD INDUCTION MOTO DIV A. Variable Frequency AC Motor Drive The traditional variable-frequency drive (known a a voltper-hertz (V/Hz) change the motor frequency and voltage uing olid-tate control unit. Keyword Pule-width modulated inverter, diode rectifier, three-phae induction motor. A I. INTODUCTION modern adjutable peed AC machine ytem i equipped with an adjutable frequency drive that i a power electronic device for peed control of an electric machine. It control the peed of the electric machine by converting the fixed voltage and frequency of the grid to adjutable value on the machine ide. There are many type of inverter, and they are claified according to number of phae, ue of power emiconductor device, commutation principle, and output waveform. Thi reearch interet in three-phae inverter circuit that change DC input voltage to a three-phae variable-frequency variable-voltage output. Three-phae inverter are alo ued in application in which AC with a controllable frequency i required. In thi application, three-phae AC i rectified into DC and then filtered to minimize the ripple content. The DC link i generally ued for thi purpoe. Thi i a variable DC obtained by employing three-phae full controlled power tranitor bridge. Thi controlled DC i converted into controlled pule by mean of a voltage to frequency converter. Thee controlled pule are fed to the inverter bridge for producing the variable voltage variable frequency output. Thi output i fed to the three-phae induction motor for controlling it peed. Thida Win i with the lectrical Power ngineering Department, Mandalay Technological Univerity, Myanmar ( malthida80@gmail.com). Nang Sabai wa with the lectrical Power ngineering Department, Mandalay Technological Univerity, Myanmar ( :nangabai@gmail.com). Hnin Nandar Maung i with the lectrical Power ngineering Department, MandalayTechnologicalUniverity, Myanmar ( hninnandarmg@gmail.com). Fig. 1 A block diagram of a variable-peed-control ytem The baic tep for thi proce are hown in the block diagram of Fig. 1, and the circuit i known a a DC link converter. The firt tep i to convert 60-Hz AC into DC power. The econd tep i to convert thi DC power back into AC at the deired frequency. B. Tranitor Baed Variable-Frequency Induction Motor Drive The modern trategy for controlling the AC output of uch a power electronic converter i the technique known a Pule- Width Modulation (PWM), which varie the duty cycle of the converter witch(e) at a high witching frequency to achieve a target average low frequency output voltage or current. In principle, all modulation cheme aim to create train of witched pule which have the ame fundamental volt econd average a a target reference waveform at any intant. The major difficulty with thee train of witched pule i that they alo contain unwanted harmonic component which hould be minimized. Three main technique for PWM exit. Thee alternative are: 1. Switching at the interection of a target reference waveform and a high frequency triangular carrier (Double dged Naturally Sampled Sine-Triangle PWM).. Switching at the interection between a regularly ampled reference waveform and a high frequency triangular carrier (Double dged egular Sampled Sine-Triangle PWM)). 647

2 World Academy of Science, ngineering and Technology Switching o that the amplitude and phae of the target reference expreed a a vector i the ame a the integ area of the converter witched output over the carrier interval (Space Vector PWM). lo and the tator copper lo, and produce a high pitch accoutic noie. While any increae in flux beyond the value i undeirable from the conideration of aturation effect, a decreae in flux i alo avoided to retain the torque capability of the motor. Therefore, the variable frequency control below the frequency i generally carried out by reducing the machine phae voltage, V, along with the frequency in uch a manner that flux id maintained contant. Above the frequency, the motor i ope at a contant voltage becaue of the limitation impoed by tator inulation or by upply voltage limitation. And per unit frequency k i K = f / f Fig. Baic circuit topology of pule-width modulated inverter drive Where f = operating frequency f = frequency of the motor Fig. 4 Single-phae equivalent circuit of polyphae induction motor Fig. 3 egular aymmetrically ampled pule width modulation C. Variable Frequency Control of Induction Motor Synchronou peed, N Where, f = upply frequency p = pole =10f/p Synchronou peed i directly proportional to the upply frequency. Hence, by changing the frequency, the ynchronou peed and motor peed can be control below and above the normal full-load peed. The voltage induced in the tator,, i proportional to the product of the lip frequency and air gap flux. The motor terminal voltage can be conidered proportional to the product of the frequency and the flux, if the tator drop i neglected. Any reduction in the upply frequency without a change in the terminal voltage caue an increae in the air-gap flux. Induction motor are deigned to operate at the knee point of the magnetization characteritic to make full ue of the magnetic material. Therefore the increae in flux will aturate the motor. Thi will increae the magnetizing current, ditort the line current and voltage, increae the core D. Operation below the ated Frequency (K< 1) It i generally preferred to operate the motor at a contant flux. The motor will operate at contant flux if I m i maintained contant at all operation point. From Fig. 3 can write the equation at the condition of motor operation: 1 I m = =. X f π L m Where L m = magnetiziting inductance When the motor i ope at a frequency f, then I m = = K.X m m 1. K. f πl m By the comparion of quation, I m will tay contant at a value equal to it value if = K. So the flux will remain contant if the back emf change in the ame ratio a the frequency, in other word, when (/f ) ratio i maintained contant. Motor operation for a contant (/f) ratio and at a frequency f, 648

3 World Academy of Science, ngineering and Technology I = S Where S= K. + K.ω ω Kω ( KX ) r = / ( KS) + X Note that ω i the ynchronou peed at the frequency. Now the developed torque i 3 T = I /S Kω T= 3 ω / ( KS) ( KS) X + Now, i maintained contant for a given frequency. The power tranferred acro the air-gap will be maximum at a lip S m for which K.X =± / S m (or) S m = ± KX T max =± 3 ω. X So, for a variable frequency control at a contant flux, the breakdown torque remain contant for all frequencie, both during motoring and regenerative breaking.alo, the examination of quation how that for a contant (SK),the _ rotor current I and torque T are contant. Now, if i take a a reference vector, then the phae lag of I i given by Q r =tan -1 (K..X / ) Since Qr i alo contant for a given (SK), the motor current will alo be contant. Thu, the motor operate at contant value of torque, I 1 and I when the flux and (KS) are maintained contant. The phyical ignificance of SK, Kω ωr ω SK = = ω ω Where ω = Kω ω Note that ω i the peed, which i the difference in the frequency f (or ynchronou peed Kω ) and the rotor peed ω r. W i the drop in motor peed from it no-load peed (Kω ) r _ when the machine i loaded. From quation, a contant value of (KS) implie the motor operation at a contant lip peed ω. So, it become clear that for any value of T, the drop in the motor peed from it no-load peed (Kω ) i the ame for all frequencie. Hence, the machine peed-torque characteritic for 0 < < S m are parallel curve. The nature of peed-torque curve for the variable frequency operation at a contant flux are hown in Fig. 4 both for motoring and braking operation. Fig. 5 Speed torque curve with variable frequency control. Tet and eult of the drive Fig. 6 Complete block diagram of the drive The pule width modulated (PWM) inverter for variable peed drive of induction motor circuit drive mall induction motor up to about 0.5 hore power, 380 volt, variable frequencie. The frequency may be adjuted from 16 Hz to 60 Hz. So, the motor peed can be varied from 464 rpm to 1740 rpm. The complete ytem of thi thei conit of an AC voltage input that i put through a diode bridge rectifier to produce a DC output which acro a hunt capacitor, thi will, in turn, feed the PWM inverter. The PWM inverter i controlled to produce a deired inuoidal voltage at a particular frequency to the quirrel cage induction motor. 1. Laboratory Tet Arrangement Performance tet and reult of variable frequency drive of three-phae induction motor are expreed a follow: Supply voltage Supply frequency 380 volt 50 Hz 649

4 World Academy of Science, ngineering and Technology Motor rating 0.5 hp Number of pole 4 Normal peed 1450 rpm Number of peed up tep 45 Maximum peed 1740 rpm Minimum peed 464 rpm Maximum frequency 60 Hz Minimum frequency 16 Hz TABL I SPD-UP TSTING OF VAIABL FQUNCY DIV OF TH-PHAS INDUCTION MOTO No. of Step Frequency (Hz) Speed (rpm) Fig. 8 Photo of the Completed Drive Circuit. Output Voltage Waveform of the Inverter Circuit The output voltage waveform of the inverter circuit i hown in Fig. 7. Thi form tep ine wave, with 10 degree phae hift to each other. When the drive i teted with the digital cope, the output frequency of the drive i 55 Hz and the output voltage i 7 V. The drive i adjuted with the frequency to control the peed of the induction motor. Fig 9 Output voltage waveform of PIC18F45 controller circuit Fig. 7 Photo of Output voltage waveform of Inverter Fig. 10 Output voltage waveform of gate driver circuit 650

5 World Academy of Science, ngineering and Technology III. CONCLUSION To control the peed of a three-phae induction motor in open loop, upply voltage and frequency need to be varied with contant ratio to each other. The author of thi paper directly contributed to the electronic deign of the inverter and controller. Alo the author implemented the ytem in it entirety and experimentally verified it operation at a wide range of peed. ACKNOWLDGMNT The author would like to expre her deep gratitude to her teacher, Profeor Dr. Khin Aye Win, Yangon Technological Univerity, Myanmar for her guidance, help, upport and haring idea. The author i deeply grateful to her upervior Dr. Salai Tluang Za Thang, Lecturer of lectrical Power ngineering Department, Mandalay Technological Univerity, Myanmar for hi cloed guidance, accomplihed uperviion and uggetion for thi paper. FNCS [1].Krihnan, 001. lectric Motor Drive (Modeling, Analyi, and Control), Prentice Hall, Inc. [] Ned Mohan, Torre M. Undeland, William P. obbin, 1995, Power lectronic Converter, Application and Deign, Wiley, New York. [3] ichard Valentine, Motor Control lectronic Handbook, McGraw-Hill, New York. [4] Sigh, M. D. and Khanchandani, K. B Power lectronic, Tata McGraw-Hill Publihing Company Limited, Newdelhi. [5] Ham N. J., Hammerton C. J. and Sharple. D., 000. Power Semiconductor Application, Tata McGraw-Hill Publihing Company Limited, Newdelhi. Thida Win received her B. degree in lectrical Power ngineering from Mandalay Technological Univerity and M. degree in lectrical Power ngineering from Yangon Technological Univerity, Myanmar, then following three year teaching in Technological Univerity, Myanmar. Her interet include Power lectronic Device and it application. 651