ac vector drives 1 revision

7/29/2019 AC Vector Drives 1 Revision

1/26

School of Electrical and Electronic Engineering

AC Vector Controlled Drives

Induction Motor Drives

Greg AsherProfessor of Electrical Drives and Control

[email protected]


2/26

Part I

Revision of Induction motors

1.1 Introduction

1.2 The Equivalent circuit

1.3 Understanding the physics of operation1.4 Variable voltage-frequency operation

(The V-f PWM drive)


3/26

1.1 Introduction

THE 3 PHASE INDUCTION MACHINE

60% of world's generated energy rotating machines

>90% of this induction machines

The induction machine consumes more of worlds generated electricitythan 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)


4/26

1.1 Introduction construction of cage IM

C

C B

BA

A

A A

AA

Iron

Albars

End rings

Rotor

(side view)

VA

Stator has 3 windings AA, BB, CC wound 120 apart in space

Stator windings connected to 3-phase mains ate = (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


5/26

1.1 Introduction - speed of rotating fields

fe P (poles) rads-1 rpm

314 3000

1500

1000

750

600

157

105

78

63

2

4

6

8

10

314

314

314

314

314

50

50

50

50

50

s sefe 2 =

Rotating field set up by stator currents rotates atsynch speed s

N

S

N

N

S

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-polemachine, or a 4-pole machine or .etc.


6/26

1.1 Introduction

Concept of torque increasing with rotor slip

HighRr

Rotor bars see magnetic field rotating past them (conductors in moving field)

Currents induced in rotor bars to establish torque; rotor travels at inattempt to catch up with rotating field

Have ;

Bigger slip, bigger torque

slrs = 0then == slrs

r

srss

=

re

s

e

sl

re

s

R

sVP

R

VPT

22

2

3

2

3 ==sT sl =

r

TLowRr

Slope =

re

s

R

VP

2

3

2

s = 1 s = 0s = 0.5

Rated Operation Irat(Stator current increases with slip)


7/26

1.2 Per phase equivalent circuit

Im

RS

VS

IRlS sRR lR

LM

IS

s

)s(RR

s

R RR

R + 1Power

losses

Mechanical

power

Vs ,IS rms stator volts, current per PHASE (not line-line)

IR rms rotor current referred to the primary (also component ofIs flowing to cancelmagnetic field of rotor currents)

Im rms component of stator current which magnetises machine (sets up rotating field

L0 magnetising inductance

lrls rotor leakage inductance, stator leakage inductance

Lr rotor self inductance,

Ls stator self inductance,

Rs stator resistance, Rrrotor resistance

Stator an rotor leakage coefficients

ror lLL +=

sos lLL +=

o

rrLl=

oss

Ll= ( ) srsr

+++= )1(111


8/26

1.2 Per phase equivalent circuit full speed range

r

Tstart

4Irat5Irat

3Irat

T

s=0s=0.5s=1

2Irat

Irat

( )

++

+

=

222

2

2

3

rser

s

s

e

r

lls

R

R

VP

s

RT

Leakage effects reduce torque for a given slip, also causing maximum torque andshape of torque curve at large slips

Torque-slip curve now given by:

Tacc

P1

Typical fan-pump load shown When motor switched to mains:

- motor goes to P1- motor too large or too small?

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

Real T-speed curve Final speed determined by load


9/26

Part 1.2

Understanding the physics of operation

Series of animations which can be viewed on web CT


10/26

1.3 Rotating field and rotating flux


11/26

1.3 Rotating field, f lux and applied voltage


12/26

1.3 Rotating f ield and induced rotor currents


13/26

1.3 Torque on induced currents


14/26

1.3 Field due to rotor currents - cancelled!!


15/26

1.3 Stator & rotor current fields increasing load


16/26

1.3 Stator current components


17/26

1.3 Effect of rotor leakage -1


18/26

1.3 Effect of rotor leakage - 2


19/26

1.4 Variable frequency (and voltage) operation

( )

++

+

=

222

2

2

3

rser

s

s

e

r

lls

RR

VP

s

RT

put Vs

= kesince : this keeps Im (and field) constant when

applied frequency changesooe

sm

L

k

L

VI

( )

sl

r

e

s

rs

sl

r

e

ssl

r

RR

llRR

kPRT

and

2

3

2

2

2


20/26

1.4 Field weakening esp. at higher speed

In Vs = ke, k is such that Vrated(eg 415V) occurs ate-rated(eg 50Hz)

IfVrated

is the maximum voltage of the converter, thenIm

and the field must

reduce if we wish e

> e-rated

Seen that as field of flux 1/e; hence T 1/

efor a given current (Ir)

Eventually, leakage effects impose

Te= constant Te2 = constantT = constant Field weakening region often

called Constant Power

Frequencies to 2e normal

Employed if load also hasconstant power characteristic(so that good motor-loadmatching can be got)

1 4 Th PWM t


21/26

1.4 The PWM converter

E

580V

IDC

Variable Vs and e synthesized by modulatingthe transistor switching pattern

Motor speed

rmay be +ve or ve depending on phase sequence ofVS Regeneration occurs whenr> e

Is = Is rated

Is = -Is ratede

Is = 0Generatingregion IDC

Under region, current reverses intoDC link,, charging C

Voltage increases!

1 4 Th PWM t ti


22/26

1.4 The PWM converter - regeneration

IDC

E

Called dynamic braking

If E rises toEnom+E, then transistor turned on. IfEfalls toE

nom-E, then turned off

Cheap but energy wasteful, especially if load hasmany braking instances

IDCIDC

Called PWM rectifier or active font-end

Can draw near sinusoidal currents

form supply

Can inject reactive power into supply

Line inductors required to decouplesupply voltage from PWM output

1 4 Open- loop V-f control


23/26

1.4 Open loop V f control

(where accurate speed-holding not required)

Vm

Ramp generator ramps feto fe* at rate k (fe = kt )

K reduced (or set to zero)ifIDC> Imax or E > E+E

Irat

A

B

e1

2Irat3Irat

e2

Irat

2Irat

e2e1

+

-

6

reduce k

setfe*

set

Imax

fe

V

f

PWM

E+E +

-

Ramp generator

with slope k

Voltage-frequency

characteristic

1 4 Open loop V f control Low speed voltage boost


24/26

1.4 Open- loop V-f control - Low speed voltage boost

fe

Field weakening

Aim is to adjustVs to keepIm constant

-moesss

ILjRIV +=

When e is not small

-

moess ILRI


25/26

1.4 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 longperiods under constant speed

V-f drive operation based on steady state sinusoidal operation onlycontrolling 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 controlof IMs


26/26

Part II

Revision of Induction motors

Understand concept of slip and operation on mains supply

Understand physics of torque production

Understand how field of rotor currents is cancelled by

extra stator current (load component)

Know how to derive parameters from manufacturers data

Understand what a PWM converter does

Know the principles and limitations of V-f control

ac vector drives 1 revision

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