vvc control

7/30/2019 Vvc Control

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CHAPTER 2: FREQUENCY CONVERTERS 81

Con trol circu it

The control circuit, or control card, is the fourth main compo-

nen t of th e frequen cy convert er a nd h as four essential t asks:

cont rol of th e frequen cy convert er semi-condu ctors.

data excha nge between the frequency converter and periph-erals.

gath ering and reporting fault messages.

car rying out of protective fun ctions for t he frequency convert -

er a nd m otor.

Micro-processors ha ve increased t he speed of th e cont rol circuit,

significan tly increasing the nu mber of applica tions su ita ble for

drives and r educing t he n um ber of necessar y calculat ions.

With microprocessors the processor is integrated into the fre-

quency convert er a nd is always able to determine th e optimu m

pulse patt ern for each opera ting sta te.

Fig. 2.29 shows a PAM-controlled frequency converter with

intermediate circuit chopper. The control circuit controls the

chopper (2) and t he invert er (3).

Uf

1 2 3

Fig. 2.29 Th e principle of a control circuit u sed for a chopper-

controlled int erm ediate circuit

Contr ol circuit forchopper frequency

PI voltage r egulator

Sequencegenerator

Control cir cuit for PAM frequency converter


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This is done in accordance with the momentary value of the

inter mediat e circuit voltage.

The int ermediat e circuit voltage cont rols a circuit th at fun ctions

as a n a ddress coun ter in th e data stora ge. The storage ha s the

out put sequences for t he pu lse pat ter n of th e invert er. When th e

int erm ediat e circuit volta ge increases, th e coun ting goes fas ter,

the sequence is completed faster and the output frequency

increases.

With respect to the chopper control, the intermediate circuit

voltage is first compa red with th e ra ted va lue of the r eference

signa l a volta ge signa l. This volta ge signa l is expected t o give

a correct output voltage and frequency. If the reference and

intermediate circuit signals vary, a PI-regulator informs a cir-

cuit that the cycle time must be changed. This leads to an

adjustmen t of the int ermediat e circuit voltage t o th e referen ce

signal.

PAM is th e tr ad itiona l technology for frequ ency inver ter cont rol.

PWM is the more modern technique and the following pages

deta il how Danfoss ha s ada pted PWM to provide par ticular an d

specific benefits.

Danfoss co ntrol principle

Fig. 2.30 gives t he cont rol procedur e for Dan foss inverters.

The control algorithm is used to calculate the inverter PWM

switching an d t ak es th e form of a Voltage Vector Cont rol (VVC)

for voltage-source frequency converters.

82 CHAPTER 2: F REQUENCY CONVERTERS

Fig. 2.30 Control principles used by Danfoss

Software Hardware (ASIC) Inverter

VVC Synchronous60 PWM Motor

VVCplus Asynchronous SFAVM 6 0 P WM

Controlalgorithm PWM


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VVC cont rols the amplitu de a nd frequ ency of th e voltage vector

us ing load and slip compen sa tion. The angle of th e voltage vec-

tor is deter mined in relat ion to th e preset motor frequen cy (ref-

eren ce) as well as th e switching frequ ency. This provides:

full rated motor voltage at r at ed motor frequency (so th ere is

no need for power redu ction)

speed regulat ion r an ge: 1:25 without feedback

speed accur acy: 1% of ra ted speed with out feedback

robust against load changes

A recent developm en t of VVC is VVCplus un der which. The am pli-

tu de and an gle of th e voltage vector, as well as th e frequ ency, is

directly controlled.

In addit ion to th e pr oper t ies of VVC , VVCplus provides:

improved dyna mic properties in the low speed ran ge

(0 Hz-10 Hz).

improved motor magnetisation

speed cont rol ra nge: 1:100 with out feedback

speed accur acy: 0.5% of th e rat ed speed without feedback

act ive resonance dampening

torque cont rol (open loop)

operat ion at th e current limit



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VVC control principle

Under VVC the control circuit applies a mathematical model,

which calculates the optimum motor magnetisation at varying

motor loads using compensat ion par am eters.

In a ddition t he synchronous 60 PWM procedur e, which is int e-

grat ed into an ASIC circuit, deter mines t he optimum switching

times for the semi-conductors (IGBTs) of the inverter.

The switching times are determ ined when:

The num erically lar gest pha se is kept at its positive or n ega-

tive poten tia l for 1/6 of the per iod t ime (60).

The two oth er phases are var ied proportiona lly so th at t he

resulting output voltage (phase-phase) is again sinusoidal

an d r eaches th e desired am plitu de (Fig. 2.32).


0,00

0,5 UDC

0,5 UDC

3600 60

60

120 180 240 300

Fig. 2.31 S ynchronous 60 PWM (Danfoss VVC control) of one

phase

UDC = interm ediate circuit voltage


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Un like sine-cont rolled PWM, VVC is based on a digita l gener a-

tion of the required output voltage. This ensures that the fre-

quency converter output reaches the rated value of the supply

voltage, the motor current becomes sinusoidal and the motor

opera tion corr esponds to those obta ined in direct mains conn ec-

tion.

Optimum motor magnetisation is obtained because the fre-

quency converter takes the motor constants (stator resistance

and inductance) into account when calculating the optimum

out put voltage.

As t he frequency convert er cont inues to measu re t he load cur -

ren t, it can r egulate th e out put voltage to ma tch th e load, so th e

motor volta ge is ada pted to th e motor type an d follows load con-

ditions.

0,00

0,50

1,00

0,50

1,00

U-V V-W W-U

3600 60 120 180 240 300

Fig. 2.32 With the synchronous 60 PWM principle the full output

voltage is obtain ed d irectly

Switching pattern of phase UPh ase volta ge (0-point h alf the int ermedia te circuit voltage)Combined voltage to motor


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VVCplus control principle

The VVCplus cont rol principle uses a vector m odulat ion p rinciple

for cons ta nt , voltage-sourced PWM inver ter s. It is based on an

improved motor model which makes for better load and slip

compensat ion, becau se both th e active an d the r eactive cur ren t

components are available to the control system and controlling

th e voltage vector a ngle significant ly impr oves dyna mic perfor-

mance in the 0-10 Hz range where standard PWM U/F drives

typica lly ha ve problems.

The inverter switching pattern is calculated using either the

SFAVM or 60 AVM pr inciple, to keep th e pu lsat ing torqu e in

th e a ir gap very small (compa red t o frequen cy convert ers u sing

synchronous PWM).

The user can select his preferred operating principle, or allow

the inverter to choose automatically on the basis of the heat-

sink t emper a tu re. If th e tempera tu re is below 75C, th e SFAVM

pr inciple is used for cont rol, while above 75 th e 60 AVM prin -

ciple is applied.

Table 2.01 gives a br ief overview of the two principles:

The control principle is explained using the equivalent circuit

diagram (Fig. 2.33) and th e basic cont rol diagram (Fig. 2.34).

It is import an t to remember t ha t in th e no-load st at e, no curr ent

flows in t he r otor (i

= 0), which mea ns t ha t t he n o-load volta ge

can be expressed a s:

U = U L = (RS + jSLS) is


Table 2.01 Overv iew: SFAVM versus 60 AVM

Max. switchingSelect ion frequency of Proper t ies

inverter

SFAVM Ma x. 8 k Hz 1. low t or qu e r ipp le com pa red t o t he syn ch ron ou s

60 PWM (VVC)

2. no gearshift

3. high switching losses in inverter

60-AVM Ma x. 14 kH z 1. r ed uced swit ch in g los ses in in ver t er (by 1/3comp ar ed t o SFAVM)

2. low torque ripple compa red to the synchr onous60 PWM (VVC)

3. relatively high torque ripple compared t oSFAVM


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in which:

RS is the st at or resistan ce,

is is the motor ma gnetisation curr ent,

LS is th e stat or leakage inductance,

Lh is the m ain indu cta nce,

LS (=LS + Lh) is the sta tor inductan ce, an d

s (=2fs) is the a ngular speed of th e rotat ing field in t he a ir

ga p

The no-load voltage (UL) is determined by using the motor

da ta (ra ted volta ge, cur ren t, frequ ency, speed).

Under a load, th e active cur ren t (iw) flows in t he rotor. In order

to enable th is cur ren t, an additiona l voltage (UComp) is placedat th e disposal of th e motor:

The additional voltage UComp is determined using the no-load

and active currents as well as the speed range (low or high

speed). The voltage value and the speed range are then deter-

mined on th e basis of th e motor dat a.


iw

LR

RrLh

is

LSRS

+

UL

Uq

UComp

Fig. 2.33b Equivalent circuit diagram for three-phase AC motors

(loaded)

iw

LR

Rr

is

LS

Lh

RS

U = U L Uq

Fig. 2.33a Equivalent circuit diagram of three-phase AC m otor

loaded)


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f

frequency(internal)

fs

presetreference

frequency

f

calculatedslipf

requency

ISX

reactivecurrentcomponents(calculated)

ISY

activecurrentcomponents(calculated)

ISX0,

ISY0

no-loadcurrentofxandyaxes(calcula

ted)

Iu,

Iv,

Iw

currentofphasesU,

VandW

(measure

d)

R

s

statorresistanc

e

R

r

rotorresistance

angleofthevoltagevectors

0

no-loadvaluetheta

load-dependent

partoftheta(compensa

tion)

T

C

Temperatureof

heatconductor/heatsink

UDC

voltageofDCintermediatecir

cuit

UL

no-loa

dvoltagevector

US

stator

voltagevector

UComp

load-dependentvoltagecomp

ensation

U

motor

supplyvoltage

Xh

reacta

nce

X1

stator

leakagereactance

X2

rotorleakagereactance

s

stator

frequency

LS

stator

inductance

LSs

stator

leakageinductance

LRs

rotorleakageinductance

is

motor

phasecurrent(apparen

tcurrent)

iw

active

(rotor)current

E

xp

lana

tions

for

Fig

.2

.33(

page

87)an

dFig

.2

.34(page

89)


9/14

xy

PWM-ASIC

abxy ab

2

3

ISX0

ISY0

ISX

ISY

IuIv

Iw

UL

UDC

TC

I0 0

L

fs

f

f

p

U

U

Ucomp

f

f

f

=

3~

Rectifier

Interve

r

Motor

Switching

logic

Voltage

vector

generator

(noload)

Load

compen-

sator

Slip

compen-

sation

Ramp

Mains

~

BasisVVCplus

Motor-

model


Fig

.2

.34

Bas

iso

fVVCp

lusc

on

tro

l


10/14


As shown in F ig. 2.34, th e motor model ca lcula tes t he r a ted n o-

load va lues (cur ren ts a nd a ngles) for t he load compen sa tor (ISX0,

Isyo) and t he volta ge vector gener at or (Io,o). Knowing th e actua l

no load values makes it possible to estimate the motor shaft

torque load m uch more a ccur at ely.

The volta ge vector genera tor calcula tes th e no-load voltage vec-

tor (UL) an d th e an gle (L) of the voltage vector on the basis of

the stator frequency, no-load current, stator resistance and

indu cta nce (see Fig. 2.33a). The resu lting voltage vector ampli-

tude is a composite value having added start voltage and load

compen sa t ion volta ge. The volta ge vector L is th e sum of four

ter ms, an d is an absolut e value defining the an gular position of

the voltage vector.

As t he r esolution of the t het a component s () an d th e stat or fre-

quency (F) determ ines th e out put frequen cy resolution, t he va l-

ues ar e represented in 32 bit resolution. One () theta compo-

nent is the no load angle which is included in order to improve

the voltage vector angle control during acceleration at low

speed. This resu lts in a good cont rol of th e cur ren t vector since

the torque current will only have a magnitude which corre-

sponds to th e actua l load . Without th e no load angle componen t

the current vector would tend to increase and over magnetise

th e motor without producing torque.

The measu red motor cur ren ts (Iu , Iv and Iw ) ar e used t o ca lcu-

late the reactive current (ISX ) and active current (ISY) compo-nents.

Based on the calculated actual currents and the values of the

voltage vector, the load compensator estimates the air gap

torque and calculates how much extra voltage (U Comp) is

required t o ma inta in th e magnet ic field level at th e ra ted value.

The angle deviation () to be expected because of the load on

th e motor sh aft is corr ected. The output voltage vector is repr e-

sented in polar form (p). This enables a direct overmodulation

an d facilita tes t he linka ge to the PWM-ASIC.

The voltage vector control is very beneficial for low speeds,

wher e th e dynamic per form an ce of th e drive can be significan t-

ly improved, compared to V/f control by appropriate control of

th e voltage vector a ngle. In addition, st eady st at or perform an ce

is obta ined, since the cont rol system can m ake better estima tes

for t he load t orqu e, given th e vector valu es for both voltage and

current, than is the case on the basis of the scalar signals

(amplitude values).


11/14


Fie ld-orien ted (Vec tor) control

Vector cont rol can be designed in a nu mber of ways. The ma jor

difference is the criteria by which the active current, magne-

tising cur ren t (flux) an d torque values a re calculat ed.

Comparing a DC motor and three-phase asynchronous motor

(Fig. 2.35), highlights the problems. In the DC, th e valu es tha t

are important for generating torque flux () and armature

cur ren t ar e fixed with r espect to size and pha se position, based

on the orientation of the field windings and the position of the

carbon br ushes (Fig. 2.35a).

In a DC motor the armature current and flux-generating cur-

rent are at right angles and neither value is very high. In an

asynchronous motor the position of the flux () and the rotor

current I1 depends on th e load. Fu rt her more un like a DC motor,

th e phase an gles and curr ent a re not directly measur able from

th e size of th e sta tor.

Using a ma th ematical m otor m odel, th e torque can, h owever, be

calculat ed from th e relationsh ip between t he flux and t he st at or

current.

U

IL IM

IM

I S

I

M ~ I sinG

G D

I

I

a ) b)

U i

Fig. 2.35 Com parison between DC and AC asynchronous m achines

DC machine

Simplified vector dia gram of asyn-chronous ma chine for one load point


12/14


The measured stator current (IS) is separated into the compo-

nent th at generat es th e torque (IL) with the flux ()at right

an gles to these two var iables (IB). These generat e th e motor flux

(Fig. 2.36).

Using th e two cur ren t component s, torque an d flux can be influ-

enced independent ly. However, as t he calcula tions, which use a

dynamic motor model, are quite complicated, they are onlyfina ncially viable in digita l drives.

As this technique divides the control of the load-independent

state of excitation and the torque it is possible to control an

asynchronous motor just as dynam ically as a DC motor pr o-

vided you have a feedback signal. This method of three-phase

AC cont rol also offers the following advantages:

good reaction to load cha nges precise speed regulation

full torque at zero speed

performa nce compar able to DC drives.

L

T ~ IS L sin

IM

U

IW

IB

IS

Fig. 2.36 Calculation of the current com ponents for field-oriented

regulation

: An gu la r v elocit y

IS: S ta t or cu r ren t

IB: Flux-generat ing current

IW: Active curr ent/rotor curr ent

L: Rotor flux


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V/f characte rist ic and flux vec tor con trol

The speed control of three-phase AC motors has developed in

recent year s on th e basis of two differen t cont rol principles:

normal V/f or SCALAR control, an d

flux vector contr ol.

Both methods have advantages, depending on the specific

requ iremen ts for dr ive perform an ce (dyna mics) an d accura cy.

V/f cha ra cter istic cont rol ha s a limited speed regulat ion r an ge of

approximately 1:20 and at low speed, an alternative control

str at egy (compensa tion) is required. Using t his t echn ique it is

relat ively simple to ada pt th e frequen cy convert er t o the m otor

and the technique is robust against instantaneous loads

th roughout th e speed ran ge.

In flux vector drives, the frequency converter must be config-

ur ed precisely to the m otor, which requ ires det a iled kn owledge.

Additiona l componen ts a re a lso requ ired for t he feedback signa l.

Some advant ages of th is type of contr ol are:

fast r eaction t o speed chan ges and a wide speed ran ge

better dynamic reaction t o cha nges of direction

it provides a single cont rol stra tegy for th e whole speed ran ge.

For the user, the optimum solution lies in techniques which

combine th e best properties of both str at egies. Chara cter istics

such as robustness against stepwise loading/unloading across

the whole speed range - a typical strongpoint of V/f-control - as

well as fast reaction to changes in the reference speed (as in

field-oriented control) are clearly both necessary.

Danfoss VVCplus is a control strategy that combines the robustpropert ies of V/f cont rol with th e higher dyna mic perform an ce of

th e field-orient ed cont rol principles and ha s set n ew stan dar ds

for dr ives with speed cont rol.



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VVCplus Sl ip compe nsat ion

Independently of the actual load torque, the magnetic field

strength of th e motor a nd th e shaft speed are maint ained at t he

speed r eferen ce comm an d value. This is done u sing of two equa l-

ising fun ctions: slip compensa tion an d the load compensa tor.

The slip compensation adds a calculated slip frequency (f) to

th e ra ted speed signal in order t o ma intain th e required refer-

ence frequ ency (Fig. 2.31). The r ise in st a tor frequency is limit-

ed by a u ser-defined ru n-up t ime (ra mp). The estima ted slip val-

ue is tak en from the estimat ed value of th e torque load a nd t he

actua l magnetic field str ength so th e ma gnetic field weaken -

ing is also taken in to considera tion.

The stationary behaviour of the control system is illustrated

together with th e torque/speed gra phs in Fig. 2.37.


2000

2

10

20

24[Nm]

1000 2000 3000 4000 [rpm]

Fig. 2.37 Torque/ speed characteristics (R ated torque 10 N m )

Rated torque

vvc control

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