powerpointsinduction motor drives 2

1、PowerElectronicsChap. 14Induction Motor DrivesChap.14Induction Motor DrivesOutlinesu Introductionu Basic Principles of Induction Motor Operationu Induction Motor Characteristics at Rated Valuesu Speed Control by Varying Stator Frequency and Voltageu Impact of Nonsinusoidal Excitation on Induction Mo

2、torsu Variable-Frequency Converter Classificationsu Variable-Frequency PWM-VSI Drivesu Line-Frequency Variable-Voltage Drivesu Soft Start of Induction MotorsChap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)14.4.4 Induction Motor Capability Most induction motor

3、s can be operated up to twice the rated speed without any mechanical problems.Chap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF) Most induction motors can be operated up to twice the rated speed without any mechanical problems.Below the rated speed:Constant-Tor

4、que RegionBeyond the rated speed:Constant-Power RegionHigh speed operation:fslConstant-RegionChap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)Below the rated speed:Constant-Torque Regionk f2Tfem6agsl Iffag is maintained constant, the motor can deliver its rate

5、d torque by drawing itsfsl .rated current at a constantChap.14Induction Motor DrivesSpeed Control by Varying StatorFrequency and Voltage (VVVF)Beyond the rated speed:Constant-Power Region= k3 f fagEagVs= constantfagfff2Tkem6agsl= 4ppwk fffVslsls3agk13wTemsl2fChap.14Induction Motor DrivesSpeed Contro

6、l by Varying Stator Frequency and Voltage (VVVF) Slip is constant.k13wTemsl2f= k132k21(2p f ) =s2fpfwrPem(1- s)ws= k15f= Tem wr= constant The output torque capability decreases as the motor speed increases.Chap.14Induction Motor DrivesSpeed Control by Varying StatorFrequency and Voltage (VVVF)High s

7、peed operation:Constant- fslRegion Depending on the motor design,fagis reduced so much that themotor approaches its pull-outtorque.k13wT= constantfslemsl2f1Tkem,max162f The output torque in this regionis limited by the maximum torque produced by the motor.Chap.14Induction Motor DrivesSpeed Control b

8、y Varying Stator Frequency and Voltage (VVVF)14.4.5 Braking in Induction MotorsMechanical brakingThe energy associated with inertia is wasted.BrakingVariable-frequency controlThe energy associated with inertia is saved.Chap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltag

9、e (VVVF)Mechanical brakingThe energy associated with inertia is wasted.BrakingVariable-frequency controlThe energy associated with inertia is saved.The generation mode with aTemk9fslTemnegativeisused to realize variable-frequency braking.Chap.14Induction Motor DrivesSpeed Control by Varying Stator F

10、requency and Voltage (VVVF) The negative Temcauses the motor speed todecrease quickly and some of the energy associated with the motor-load inertia is fed into the source connected to the stator.Brakingf0f1Chap.14Induction Motor DrivesImpact of Nonsinusoidal Excitation on Induction MotorsThe three p

11、hase voltages or currents produced by PWM inverters contain harmonic components with higher frequency.Harmonic Motor CurrentsHarmonics VoltagesHarmonic Power LossesTorque PulsationsChap.14Induction Motor DrivesImpact of Nonsinusoidal Excitation on Induction Motors14.5.1 Harmonic Motor CurrentsIf the

12、 stator voltage is known, the harmonic components in the motor current can be obtained by using the principle of superposition and the harmonic equivalent circuit.For calculating the harmonic currents, the magnetizing components can be neglected and the harmonic current magnitude is primarily determ

13、ined by the leakageVIreactance at the harmonic frequency. hhhw(L+ L)lslrChap.14Induction Motor DrivesImpact of Nonsinusoidal Excitation on Induction Motors VhIhhw(L+ L)The magnitudes of harmonic currents can be reduced by increasing the frequencies of the harmonic voltages.lslr14.5.2 Harmonic Power

14、LossesThe additional power losses (copper loss, core loss and stray loss) due to harmonic currents are in a range of 10- 20% of the total power losses at the rated load.= h=2Additional copper lossDP(R+ R )I 2cusrhChap.14Induction Motor DrivesImpact of Nonsinusoidal Excitation on Induction Motors14.5

15、.3 Torque PulsationsHarmonics in the stator excitationPulsating-torque componentsTroublesome speed fluctuations & shaft fatigueChap.14Induction Motor DrivesVariable-Frequency Controller ClassificationsThe basic requirements for variable-frequency converters Ability to adjust the frequency according

16、to the desired output speed. Ability to adjust the output voltage so as to maintain a constant air gap flux in the constant-torque region. Ability to supply a rated current on a continuous basis at any frequency.Chap.14Induction Motor DrivesVariable-Frequency ControllerClassificationsPulse-width-mod

17、ulatedvoltage source inverter with a diode rectifier(Chap.8)Diode rectifier (Chap.5)Square-wave (six-step) voltage source inverter with a thyristor rectifierLine-voltage-commutated controlled converter (Chap.6)Current source inverter with a thyristor rectifierChap.14Induction Motor DrivesVariable-Fr

18、equency PWM-VSI Drives A PWM inverter controls both the frequency and the magnitude of the voltage output. An uncontrolled diode bridge rectifier is generally used at the input.Chap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives In a PWM inverter, the harmonics in the output voltage appea

19、r as sidebands of the switching frequency and its multiples.Triangular Carrier PWMChap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives14.7.1 Impact of PWM-VSI HarmonicsHigh leakagereactanceHarmonic voltages are at high frequency.Motor current ripple is small.Iron losses (eddy current & hys

20、terisis)Harmonic voltages have high amplitudes.dominate.14.7.2 Input Power Factor & Current Waveforms The input ac current drawn by the rectifier of a PWM-VSI drive contains a large amount of harmonics. The power factor at the utility system is essentially independent of the motor power factor and t

21、he drive speed.Chap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives14.7.3 Electromagnetic BrakingPower flow directionInduction Motorduring electromagnetic brakingEnergy DissipationEnergy EfficientVariable Frequency ConverterChap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives14.7

22、.4 Adjustable-Speed Control In VSI drives, the speed can be controlled without a speed feedback loop, where there may be a slower acting feedback loop (dc current feedback) through the process controller.Chap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives14.7.4 Adjustable-Speed Controlws=

23、 wr,ref+ k18TemTo keep fag constantVs = k19ws+ k20TemRamp limiterTo limit the motor current stays below the current limit when there is not ramp limiter.Chap.14Induction Motor DrivesVariable-Frequency PWM-VSI Drives14.7.5 Induction Motor Servo DrivesChap.14Induction Motor DrivesVariable-Frequency PW

24、M-VSI Drives In servo drives, the torque developed by the motor should respond quickly and precisely to the torque command without oscillation, since these drives are used for position control.Accurate knowledge of motor parameters are needed, where adaptive control with parameter estimation is ofte

25、n utilized.Chap.14Induction Motor DrivesVariable-Frequency PWM-VSI DrivesControl diagram of rotor flux oriented vector control of induction motorsSpeed EstimatorInduction MotorTorque EstimatorTw-emT *i-i*emsaww* sqiASRATREquivalentsqisbACR3s/2rInverteryrDCMotorisdi*y*isdscrAYR-yrqFlux Linkage Estima

26、torChap.14Induction Motor DrivesVariable-Frequency PWM-VSI DrivesSimplified rotor flux oriented vector control of induction motorsSpeed EstimatorInduction Motorww*-i*isawASRsqisqEquivaisblentACRInverter3s/2riDCyry*i*isdMotorrsdsc1/LmqFlux Linkage EstimatorChap.14Induction Motor DrivesLine-Frequency

27、Variable-Voltage Drives In some applications, line-frequency variable-voltage drive is a cheaper solution than VVVF drive.is proportional to V 2 for a value of rotor speed The torque Tems(equal to the line frequency) and fixedStable Operationffsl .determined byPoint= kV 2Tem21sThe operation point wo

28、uld not be stable if the load torque remained constant with speed.Chap.14Induction Motor DrivesLine-Frequency Variable-Voltage Drives For a load requiring a constant torque with speed, a motor with a higher resistance, which has a large value of slip at which the pull-out torque is developed, should

29、 be used.PrfslStable Operation Point%P=rPf - femsl Speed control by controlling the stator voltage results in a very poor energy efficiency at low speeds because of high rotor losses caused by large slips. Hence, this technique is limited to low power applications.Chap.14Induction Motor DrivesReduce

30、d Voltage Starting ofInduction Motors (“Soft Start”) To reduce the large starting currents, the motor can bestarted at reduced voltages obtained from the following circuit.After start up, the thyristors will be shorted out.Chap.14Induction Motor DrivesSummary1.Induction motors are the workhorse of i

31、ndustry because of their low cost and rugged construction.In a three-phase induction motor, the resultant field distribution in the air gap is sinusoidal and rotates at a synchronous speed.The speed of an induction motor can be controlled by varying the stator frequency, which controls the synchronous speed and, hence, the motor speed, since the slip is kept small.For braking in an induction motor to reduce its speed, the stator frequency is decrea