AC Vector Controlled Drives Induction Motor Drives 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

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

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)

Introduction – construction of cage IM B B’ A A’ Iron Al bars VA End rings Rotor (side view) 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

Speed of rotating fields Rotating field set up by stator currents rotates at synch speed s fe P (poles) rads-1 rpm 50 314 2 3000 4 157 1500 6 105 1000 8 78 750 10 63 600 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.

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 Low Rr Slope = High Rr Rated Operation Irat (Stator current increases with slip) r s = 1 s = 0.5 s = 0

Per phase equivalent circuit Im RS VS IR lS lR LM IS Power losses Mechanical power Vs , IS rms stator volts, current per PHASE (not line-line) IR rms rotor current referred to the primary (also component of Is flowing to cancel magnetic field of rotor currents) Im rms component of stator current which magnetises machine (sets up rotating field L0 magnetising inductance lr ls rotor 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 – 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 Tacc P1 Typical fan-pump load shown When motor switched to mains: - motor goes to P1 - motor too large or too small? r Tstart 4Irat 5Irat 3Irat T s=0 s=0.5 s=1 2Irat Irat P2 Smaller fan-pump load shown When motor switched to mains: - motor goes to P2 Lift, hoist load shown in green - constant due to gravitational force - slight increase due to friction etc

Rotating field and rotating flux

Rotating field, flux and applied voltage

Rotating field and induced rotor currents

Torque on induced currents

Field due to rotor currents - cancelled!!

Stator & rotor current fields – increasing load

Stator current components

Effect of rotor leakage -1

Effect of rotor leakage - 2

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

Field weakening esp. at higher speed In Vs = 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 Te= constant Te2 = constant T = constant 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)

The PWM converter IDC IDC Variable Vs and e synthesized 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! Is = Is rated Is = -Is rated e Is = 0 Generating region IDC

The PWM converter - regeneration IDC E 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 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

(where accurate speed-holding not required) 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 + - 6 reduce k set fe* set Imax fe V f PWM E+E Vm Ramp generator with slope k Voltage-frequency characteristic Irat A B e1 2Irat 3Irat e2 Irat 2Irat e2 e1

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

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

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