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School of Electrical and Electronic Engineering

School of Electrical and Electronic Engineering. AC Vector Controlled Drives Induction Motor Drives Greg Asher Professor of Electrical Drives and Control Greg.asher@nottingham.ac.uk. Part I Revision of Induction motors. Equivalent circuit Power Flows Torque-speed characteristic.

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School of Electrical and Electronic Engineering

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  1. School of Electrical and Electronic Engineering AC Vector Controlled Drives Induction Motor Drives Greg Asher Professor of Electrical Drives and Control Greg.asher@nottingham.ac.uk

  2. Part I Revision of Induction motors • Equivalent circuit • Power Flows • Torque-speed characteristic

  3. THE 3 PHASE INDUCTION MACHINE • 60% of world's generated energy  rotating machines • >90% of this  induction machines • The induction machine consumes more of world’s generated electricity than any other piece of electrical equipment Power Range • 100-500W small fans • 1-50kW fans, pumps, conveyors, escalators • 500kW water pumping, coal cutting, • 1MW high speed train motor (eg. x4) • 10MW warship/cruise ship motor (X2)

  4. A A A B’ C’ C B A’ A’ A’ Rotor (side view) Iron End rings Al bars Introduction – construction of cage IM VA • Stator has 3 windings AA’, BB’, CC’ wound 120apart in space • Stator windings connected to 3-phase mains at e = (2) 50Hz mains • Fed by 3-phase currents 120 apart in time to create rotating magnetic field • Rotor has NO windings • It has a cage of Aluminium bars; currents will be induced in it

  5. N S N S S N Speed of rotating fields • Rotating field set up by stator currents rotates at synch speed s • If each phase spans 60° in space, then get 4-pole distribution • 1 rpm = 2 radians/minute = 2/60 radian/second (rads-1) Therefore 1 rads-1 = 60/2 10 rpm • Stator windings of an IM can only be wound in one way. P is fixed for an individual machine. An IM can either be a 2-pole machine, or a 4-pole machine or ….etc.

  6. Slope = Irat Rated Operation (Stator current increases with slip) Concept of torque increasing with rotor slip • Rotor bars see magnetic field rotating past them (conductors in moving field) • Currents induced in rotor bars to establish torque; rotor travels at in attempt to catch up with rotating field • Have ; • Bigger slip, bigger torque T LowRr HighRr r s = 1 s = 0 s = 0.5

  7. RS lR lS IR IS Power losses Mechanical power LM VS Im • L0magnetising inductance • lr lsrotor leakage inductance, stator leakage inductance • Lr rotor self inductance, • Ls stator self inductance, • Rs stator resistance, Rr rotor resistance • Stator an rotor leakage coefficients Per phase equivalent circuit • Vs,ISrms stator volts, current per PHASE (not line-line) • IRrms rotor current referred to the primary (also component of Is flowing to cancel magnetic field of rotor currents) • Imrmscomponent of stator current which magnetises machine (sets up rotating field

  8. Typical fan-pump load shown • When motor switched to mains: - motor goes to P1 - motor too large or too small? T 3Irat 2Irat 4Irat 5Irat P1 • Smaller fan-pump load shown • When motor switched to mains: - motor goes to P2 Tacc Irat Tstart P2 • Lift, hoist load shown in green - constant due to gravitational force - slight increase due to friction etc s=1 s=0.5 s=0 r Per phase equivalent circuit – full speed range • Leakage effects reduce torque for a given slip, also causing maximum torque and shape of torque curve at large slips • Torque-slip curve now given by: • Real T-speed curve • Final speed determined by load

  9. Rotating field and rotating flux

  10. Rotating field, flux and applied voltage

  11. Rotating field and induced rotor currents

  12. Torque on induced currents

  13. Field due to rotor currents - cancelled!!

  14. Stator & rotor current fields – increasing load

  15. Stator current components

  16. Effect of rotor leakage -1

  17. Effect of rotor leakage - 2

  18. put Vs = ke since : this keeps Im (and field)  constant when applied frequency changes T Tacc Only dependent on sl 1 0.25 0.5 0.75 Variable frequency (and voltage) operation Motor torque for given motor voltage Vs and frequency e: Torque expression becomes:

  19. T = constant Te= constant Te2 = constant Field weakening esp. at higher speed • InVs = ke, k is such that Vrated (eg 415V) occurs at e-rated(eg 50Hz) • If Vrated is the maximum voltage of the converter, then Im and the field must reduce if we wish e > e-rated • Seen that as field of flux  1/e ; hence T  1/e for a given current (Ir) • Eventually, leakage effects impose • Field weakening region often called “Constant Power” • Frequencies to 2e normal • Employed if load also has constant power characteristic (so that good motor-load matching can be got)

  20. Is = Is rated Generating region Is = 0 Is = -Is rated e E 580V IDC The PWM converter IDC • Variable Vs and esynthesized by “modulating” the transistor switching pattern • Motor speed r may be +ve or –ve depending on phase sequence of VS • Regeneration occurs when r > e • Under region, current reverses into DC link,, charging C • Voltage increases!

  21. IDC • Called “dynamic braking” • If E rises to Enom+E, then transistor turned on. If E falls to Enom-E, then turned off • Cheap but energy wasteful, especially if load has many braking instances E IDC IDC • Called PWM rectifier or “active font-end” • Can draw near sinusoidal currents form supply • Can inject reactive power into supply • Line inductors required to “decouple” supply voltage from PWM output The PWM converter - regeneration

  22. 6 B 2Irat E+E + 3Irat + setImax Irat A PWM - - e1 e2 reduce k fe Vm set fe* e1 e2 Ramp generator with slope k Voltage-frequency characteristic Irat 2Irat V f Open- loop V-f control (where accurate speed-holding not required) • Ramp generator ramps fe to fe* at rate k (fe = kt ) • K reduced (or set to zero) if IDC > Imax or E > E+E

  23. Aim is to adjust Vsto keep Im constant - Rs Rr/s Is • When e is not small  - Im Vs Vm Lm Vm • When e is small  - 1pu • The voltage boost Vb (normally 20-40V) is required to overcome the voltage drop due to Rs when e is small k Field weakening Vb fe Open- loop V-f control Low speed voltage boost

  24. Summary for PWM V-F drives • About 25-30% of IM drives are driven by PWM converters • Open-Loop V-f drive most common – 60% of total - many drives esp. pumps and fans are just switched on and left running for long periods under constant speed • V-f drive operation based on steady state sinusoidal operation – only controlling rms values • V-f drive has poor torque control and poor low speed performance - but OK for just starting loads requiring low torque at low speed • Need to control instantaneous values of current to get fast control of torque and flux (and hence speed) • This is done by “vector control” of IMs

  25. Part II Revision of Induction motors • Equivalent circuit • Power Flows • Torque-speed characteristic

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