Osram HQI-E 150 W/NDL CL Technical Information download pdf (Page 8)

8

3 Ballasts

f

or dischar

g

e lamp

s

C

PF

C

... PF

C

ca

p

acito

r

L

a ...

l

am

p

U

S

... supply volta

g

e

C

h ... chok

e

U

N

K

Dr

La

Ch

L

a

C

P

FC

U

S

Fig. 4: Discharge lamp with inductive ballast

(

ignition

u

nit has been le

f

t out, the various

p

ossibilities are

f

ea

-

tured in chapter 4 “I

g

nition and start-up o

f

dischar

g

e

l

amps”

)

C

hartin

g

the equations results in the curves shown in

Fi

g

. 5. The di

ff

erence between lamp watta

g

e and the

product of lamp voltage and lamp current is called

lam

p

p

ower

f

actor. It reaches values between 0.7 and

0.95

d

epen

di

n

g

on t

h

e operat

i

n

g

mo

d

e.

Th

e ye

ll

ow

curve was

g

enerate

d

b

y us

i

n

g

a

high

er

l

amp power

fac

t

o

r

λ

L

[(

to be more exact: 1.05*

(

1-n

/

3

)].

T

ypical volta

g

e and current wave

f

orms as shown in

Fig. 6 show that while the current is

(

approximately

)

sinusoidal, volta

g

e is not. A

f

ter the current zero cross

-

in

g

, the volta

g

e initially increases

(

so-called re-i

g

nition

peak

)

to then fall to a relativel

y

constant value

(

saddle

)

(

see also chapter 6.2.2 and Fi

g

. 30

)

. Volta

g

e remains

approximatel

y

the same be

y

ond the maximum o

f

the

current, an

d

h

as t

h

e same zero cross

i

n

g

as t

h

e current.

S

o there are areas with hi

g

h volta

g

e which count

towards the e

ff

ective value o

f

the volta

g

e but don’t

contr

ib

ute to t

h

e watta

g

e as t

h

e current at t

h

at po

i

nt

i

n

time is nearl

y

zero. This results in lamp power

f

actors

deviatin

g

f

rom the value 1

.

I

f

the lamp volta

g

e is equal to zero, the volta

g

e drop

across t

h

e c

h

o

k

e

i

s t

h

e ent

i

re supp

l

y vo

l

ta

g

e, an

d

t

h

e

choke

sho

rt-

ci

r

cui

t

cu

rr

e

nt

is

r

eached

.

This

is

t

he

m

a

x-

imum current that can

f

low throu

g

h the choke inas-

m

uc

h th

e

cu

rr

e

nt h

as

no

DC com

p

onent

(

see cha

p

ter

6

.2.9 for effects of direct current com

p

onents

).

T

he

f

ollowin

g

curves are typical

f

or 150 W and apply in

t

h

e same way to ot

h

er watta

g

es

.

Voltage in V

Current in A

Time in ms

Fi

g

. 6:

G

raph showin

g

lamp volta

g

e and current of a

150 W lam

p

when o

p

erated at a choke

(

a

pp

lies in the

same wa

y

to other watta

g

es

)

Fig

. 5:

L

amp current

I

L

,

l

amp watta

g

e

P

L

o

v

e

r t

he

r

a

t

io

o

f

lamp volta

g

e to suppl

y

volta

g

e U

L

/U

S

;

Z=99

Ω

fo

r

a

150

W

l

am

p

S

ince the dischar

g

e reacts to increasin

g

lamp current

with fallin

g

volta

g

e

(

which would cause the current to

rise indefinitel

y

until the fuse blows or another part o

f

the circuit fails

)

, the lamp current must be limited b

y

a ballast durin

g

operation. This usually consists o

f

an

inductive circuit

(

choke

)

, althou

g

h in rare cases up to

400 W capacitive circuits are also possible

(

althou

g

h

this usuall

y

results in a shorter service life

)

. In the

blended lam

p

(

HWL

)

, the resistance of the filament

serves as a series resistor

f

or the hi

g

h-pressure mer

-

cury

di

sc

h

ar

g

e

l

amp.

I

n most cases, a

ddi

t

i

ona

ll

y to t

h

e

current-

li

m

i

t

i

n

g

e

l

ement, an

ig

n

i

t

i

on

d

ev

i

ce

i

s nee

d

e

d

to start dischar

g

e

(

see chapter 4 “I

g

nition and startin

g

discharge lamps”

)

.

In modern luminaires

,

an electronic ballast

f

ul

f

ils the

f

unction o

f

i

g

nitin

g

the lamp, limitin

g

the lamp current

an

d

contro

lli

n

g

t

h

e

l

amp watta

g

e

.

3.1 Inductive ballasts

(

chokes

)

Th

e vo

l

ta

g

e across t

h

e e

l

ectroma

g

net

i

c

b

a

ll

ast

i

n

-

creases as the current increases

,

there

f

ore a stable

workin

g

point can be achieved in the series connec-

tion o

f

the dischar

g

e lamp and the choke

.

Describin

g

the relationships o

f

current and volta

g

e

requires a s

y

stem o

f

di

ff

erential equations which can

-

not

g

enerally be solved. The

f

ollowin

g

approximation

f

ormulas describe how the lamp current and lamp

watta

g

e depend on the relationship o

f

lamp volta

g

e to

supply volta

g

e

[

3

]:

P

L

..... lamp watta

g

e in W

U

S

..... supp

l

y vo

l

ta

g

e

i

n

V

n ..... ratio o

f

lamp volta

g

e

U

L

to supp

l

y vo

l

ta

g

e

U

S

Z

..... c

h

o

k

e

i

mpe

d

anc

e

(G

l. 4.1

)

n

n

Z

U

P

S

L

3

1

2

⎟

⎠

⎞

⎜

⎝

⎛

−⋅=

[]

n

)

n

225 ,01

2/1

2

−−

(

Z

U

I

S

L

[]

n

)

n

225 ,01

2/1

2

−−

(

≈

whereb

y

:

(

1-n

/

3

)

....approximation for the lam

p

p

ower factor

λ

L

(

Gl. 4.2

)

Lamp power P

in W

L

Lamp current I in A

180

160

140

120

100

80

60

40

20

0

2,5

2

1,5

1

0,5

0

0

0,5

1

U /U

L

S

L

5

%

hi

g

her lam

p

p

ower

f

acto

r

P

L

I

L

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 55 56

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