powerpointsinduction motor drives 1

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 DrivesIntroductionk (speed)2 , Powerk(speed)3 In a centrifugal pump, Torque12Low efficiencyHigh efficiency In comparison

3、 with a throttling valve to control the flow rate, the variable-speed-driven pump can result in significant energy conservation.Chap.14Induction Motor DrivesBasic Principles In a large majority of applications, induction motor drives incorporate a three-phase squirrel-cage motor, since it is simple,

4、 low-cost and rugged.Chap.14Induction Motor DrivesIntroduction Squirrel-cage induction motors are the workhorse of industry because of their low cost and rugged construction.Induction Motor DrivesAdjustable- speed drivesSpeed controlSophisticated controlfans, compressors, pumps, blowers, etc.Compute

5、r peripherals, machine tools, robotics, etc.Servo DrivesChap.14Induction Motor DrivesBasic PrinciplesBasic relationship between frequency & speed of IMp = 4p = 2= 2p( p2) =2 (2p f ) =2 ww(rad s)Electrical synchronous angular speeds1fppMechanical synchronous angular speedw120= 60 s =nfs2ppStator freq

6、uency(Hz)Motor speed(r/min)Number of PolesChap.14Induction Motor DrivesBasic Principlesfag The air gap fluxrotates at a synchronous speed. Itinduces a counter-emf (air gap voltage) Eag .Nsfag= Lmim= k3 f fagEag The torque in an induction motor is produced by interactionfagofand the rotor current. So

7、 there must be a relativefagmotion betweenand the rotor.Chap.14Induction Motor DrivesBasic Principles The relative motion is called “Slip”.s = ws- wrSlipwsSlip frequencySlip speed= wslw= w-w= swf= sffslsrsslwsChap.14Induction Motor DrivesBasic Principles The induced rotor voltage results in the roto

8、r currents at the slip frequency.fslfagEr= k3Induced rotor voltageEr= Rr Ir+ j2p fsl Llr IrChap.14Induction Motor DrivesBasic Principles The relationships among the currents in induction motorMagnetizing currentRotor currentNsfagIm= k8fagk5fag= Lmim2p fsl LlrIrfslRrI= I+ I+ I 2I 2IsmrsmrChap.14Induc

9、tion Motor DrivesBasic Principles The relationship of power in induction motorAir gap powerPower loss in rotorP= 3 fR I 2P= 3R I 2agrrfslrrrf- fslElectromechanical output power= P- P= 3RI 2PemagrrrfslPrfsl%P=rPf- femslChap.14Induction Motor DrivesBasic Principles= Pag= PemElectromagnetic torqueTemww

10、drs Under the following condition,2p fsl Llrqr90Rr0= k fIsin dk fk f2TIfem4agr4agr6agslChap.14Induction Motor DrivesBasic PrinciplesVsEagf2p fsl Llr )Rr Ir(Rrfslk3fagfStator frequencyV/f constant controlChap.14Induction Motor DrivesCharacteristics at Rated Values In the commonly used induction motor

11、s,is keptfslsmall, hence, the dashed portions of the torque are notused due to high power loss.Dashed portions are not usedPrfsl%P=rPf- femslLinear portions are usedChap.14Induction Motor DrivesCharacteristics at Rated Values If an induction motor is started from the line-voltage supply without a po

12、wer electronic converter, it would draw 6-8 times its rated current at start-up.Chap.14Induction Motor DrivesCharacteristics at Rated Values If an induction motor is started from the line-voltage supply without a power electronic converter, it would draw 6-8 times its rated current at start-up. In t

13、he commonly usedfslinduction motors,is keptsmall, hence, the dashed portions of the torque are not used.Chap.14Induction Motor DrivesCharacteristics at Rated ValuesTem - Tload Available acceleration torquefor themotor to accelerate from standstill. The intersection determines the steady-state point

14、of operation.Chap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)14.4.1 Torque-Speed Characteristics For small values offsl , keeping fag constant results ina linear relationship between Temandfsl.Temk9fsl= 4pfslww=fslsslfpsk10wslTemwsl1 = wsl2 = wsl3 = wsl4Chap.

15、14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF) In many loads such as the centrifugal pumps, compressors, and fans, the load torque varies by the square of the speed.k (speed)2Tload1kwTw w w wem10slsl1sl2sl3sl4Chap.14Induction Motor DrivesSpeed Control by Varying

16、 Stator Frequency and Voltage (VVVF)Example 14-1 A four-pole, 1hp, 380-V motor is supplying its rated power to a centrifugal load at a 50-Hz. Its rated speed is 1425 rpm. Calculate its speed, slip frequency, and slip when it is supplied by a 228-V, 30-Hz source.SolutionCalculation at rated condition

17、Keeping Vsfconstant,Resultsnr= 873s = 3%= 0.9fslHzrpmChap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)14.4.2 Start-up Considerations For a inverter-driven induction motor, it is important to keep the current draw from becoming large during start-up.Chap.14Indu

18、ction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF) For a inverter-driven induction motor, it is important to keep the current draw from becoming large during start-up.k4fag Irk5fag fslTemIr For a constant fag ,Irk11 fsl By VVVF control,Tstartf( f)startslratedTratedChap.14

19、Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)14.4.2 Start-up Considerationsf In practice, the stator frequencyis increased continuouslyat a preset rate, which does not let the current exceed a specified limit until the final desired speed has been achieved. This

20、rate is decreased for higher inertia loads to allow the rotor speed to catch up.Chap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF)14.4.3 Voltage Boost Required at Low Frequencies In induction motors of normal design, 2p fLlrRr ( ffsl ).Is= Ir - jImEagIrHence,wi

21、ll be in phase with.Chap.14Induction Motor DrivesSpeed Control by Varying Stator Frequency and Voltage (VVVF) In induction motors of normal design, 2p fLlrRr ( f- jImfsl ).Is= IrEagIrHence,will be in phase with.Vs= Eag + (Rs + j2p fLls )IsVs= Eag + (2p fLls )Im + Rs Ir + j(2p fLls )Ir - Rs Im Chap.14Induction Mo