101seminartopics - 123seminarsonly.com...2. rf lighting the very first proposal for rf lighting, as...

101seminartopics.com

1. INTRODUCTION

RF light sources follow the same principles of converting electrical

power into visible radiation as conventional gas discharge lamps. The

fundamental difference between RF lamps and conventional lamps is that RF

lamps operate without electrodes .the presence of electrodes in conventional

florescent and High Intensity Discharge lamps has put many restrictions on

lamp design and performance and is a major factor limiting lamp life. Recent

progress in semiconductor power switching electronics, which is

revolutionizing many factors of the electrical industry, and a better

understanding of RF plasma characteristics, making it possible to drive lamps

at high frequencies.


2. RF LIGHTING

The very first proposal for RF lighting, as well as the first patent on

RF lamps, appeared about 100years ago, a half century before the basic

principles lighting technology based on gas discharge had been developed.

Discharge tubes

Discharge Tube is the device in which a gas conducting an electric

current emits visible light. It is usually a glass tube from which virtually all the

air has been removed (producing a near vacuum), with electrodes at each end.

When a high-voltage current is passed between the electrodes, the few

remaining gas atoms (or some deliberately introduced ones) ionize and emit

coloured light as they conduct the current along the tube. The light originates

as electrons change energy levels in the ionized atoms. By coating the inside of

the tube with a phosphor, invisible emitted radiation (such as ultraviolet light)

can produce visible light; this is the principle of the fluorescent lamp.

We will consider different kinds of RF discharges and their

advantages and restrictions for lighting applications.


3. DISCHARGE TYPES

There are three practical ways to energize RF light sources, though

there are more ways to create RF plasma

3.1 CAPACITIVE RF DISCHHARGE

Capacitive RF discharge may be energised by RF electrodes placed

inside or outside the discharge vessel .The current path in a capacitive RF

discharge plasma is closed by displacement currents in the RF electrode

sheaths (whether the electrodes are inside or outside the discharge vessel).

Capacitive RF discharges operate at gas pressure considerably lower than

atmospheric pressure and are exited by an RF electric field E with frequency

lower than 1GHz wavelength much larger than the discharge size L,( L).

(Figure 1)


Due to electron depletion in the sheaths, the sheath

impedance is much larger than the plasma impedance. Therefore, the voltage

applied to the lamp is mainly dropped in the sheaths, and sheath impedance

controls the discharge current of a capacitive RF discharge. A significant dc

voltage is developed in the RF sheaths as a result of RF voltage rectification,

and this leads to an acceleration of the plasma ions into the electrodes (or

wall). This has important consequences that limit the application of the CRFD

for RF lighting. The additional power loss of ion acceleration reduces RF lamp

efficiency.

The CRFD power P is proportional to Vrf.,therefore, to achieve a

significant lamp power; one must use a high RF voltage and/or a high

frequency . To reduce ion sputtering, the RF voltage, Vrf applied to the lamp

electrodes should be lower than 100-200V. Since large RF voltages are

prohibited because of ion sputtering, the practical application of a CRFD for

lighting is limited to low power and/or relatively high frequency operation.

(Figure 2)


Example of a CRFD –based lamp is shown in figure 2. This is an RF

–driven sub miniature (a few millimetres in diameter) fluorescent lamp with a

cold cathode. Increasing its driving frequency from 50kHz in standard

application to 40 MHz results in a significant reduction in power loss at the

electrode sheaths. Sheath power loss decreases with frequency.

3.2 INDUCTIVE RF DISCHARGE

In an inductive RF discharge, the plasma RF current is closed within

the plasma without forming RF sheaths. The electric field that maintains the

discharge is induced by an RF current flowing through an induction coil

outside or inside the plasma. Inductive RF discharges (IRFD) or inductively

coupled plasmas (ICP) operate over a wide range of gas pressure and

frequency for which L.

The utility of an IRFD as a light source is defined by its power

transfer efficiency =Pp/(Pp+Pc), where Pp and Pc are the power delivered to

the plasma and that dissipated in the inductor. To obtain an RF lamp efficiency

equal to, or better than electroded discharge lamps, should be no less than

about 90%. Power transfer efficiency depends upon many factors, such as

filling gas, gas pressure, discharge topology and geometry, driving frequency,

and inductor construction. Lamp power also has a significant influence on

power –transfer efficiency. Contrary to capacitive RF discharges, where the


fraction of RF power transferred to the plasma falls with increasing discharge

power, usually increases with power in an IRFD.

Figure 3(RF lamp with closed ferrite core)

Typical example of IRFD realisation is shown in figure 3.the closed

ferrite cores increase the coupling between the coil and plasma, thus enhancing

IRFD efficiency.

3.3 WAVE –SUSTAINED DISCHARGES

Wave-sustained RF discharges (WRFD) are maintained by

electromagnetic waves that are incident on the plasma surface or propagate

along a plasma boundary .the wavelength in a wave-sustained RF discharge is

comparable to the plasma size ( L), which implies a relatively high RF

driving frequency. Wave discharges are usually maintained by microwave

power sources at frequency in the GHz range. However, in some surface wave

discharges with a long plasma column working as a slow wave structure, the

length of the propagating waves is much shorter than in a vacuum, and the

driving frequency may be much smaller (10-100 MHz.). The application of


microwaves is advantageous for the excitation of high –pressure HID light

sources where relatively high- power density is needed to achieve a near –

equilibrium plasma.

4. CHOICE OF FREQUENCY AND DISCHARGE TYPE

A few frequencies allocated for industrial applications such as

13.56,27.12, and 40.68 MHz in the RF frequency band, and to 2.45GHz in the

microwave band, the frequency rage between 2.2 – 3.0 MHz (2.65MHz is

standard) has reduced restrictions on EMI and has been specifically allocated

for RF lighting devices.

An RF generator (RF ballast) is the essential yet most expensive part

of a modern RF lighting system. Electronic ballasts for driving electroded

fluorescent lamps operate at a few tens of kilohertz. For such frequencies,

ballasts efficiencies is rather high (90-95%), its cost is quite reasonable, and

EEMI levels comply with regulations that are more tolerant of lower

frequencies. With increasing frequency, EMI radiations grows, regulations are

more stringent, ballast efficiency decreases, and ballast cost increases. This is

why there is no hope for microwave RF lamps for general lighting .To the

contrary, decades of development of RF light sources shows a preference for

inductive RF discharge and reduction of RF frequency.

With a limited choice of available frequencies for industrial and

residential applications, the inductively coupled plasma source seems to be the


most practical way to make a commercially viable lamp with alight output of

over 1000 lm.

5. COMMERCIAL RF LIGHT SOURCES

Considering the difficulties created by fluorescent lighting, it should

come as no surprise that researchers have been trying to develop a practical

electrode-less light source for years .The main obstacle to the development of a

commercial RF lamp was the lack of efficient and economical electronic

components to drive the lamp frequencies as high as 60 Hz, the level necessary

to produce visible light more efficiently.

Within the past 10 years, the following RF lamp designs have

achieved varying degrees of commercial use:

MICROWAVE- POWERED SULFUR LAMPS


(Figure 4)

This light source is remotely energised by microwave power source

of about 1.5kW at 2.45GHz generated by a magnetron. 1.5kW magnetron,

similar to those used in microwave ovens, generates power at 2.45GHz and

delivers energy to a quartz glass bulb through a short wave guide (Fig.4).

Filled with argon at a small dose of sulphur, the bulb (about 3cm in diameter)

is rotated for discharge stability within the resonant cavity. Providing about

135,000 lumens, or 95 lm/W, and a life rating of 15,000 hr, the compact

sulphur lamp also features low –infrared and ultraviolet emission and good

colour stability.

SPHERICAL EXTERNAL- COIL INDUCTION LAMP


( Figure 5)

This type of lamps employs a 4.5 –cm diameter fluorescent bulb

driven inductively at 13.56MHz from an RF power supply housed in a base

unit (Fig.5). An induction coil wrapped around the lamp energises the neon gas

and a small quantity of mercury contained with in the lamp .In turn, a screen

cage surrounds the lamp to reduce EMI emissions to an acceptable level. The

system operate at 27 W, and the efficiency is 37 lm/W. Typical applications

would be difficult –to-reach locations, such as a bridge or a room with a high

ceiling.

RE-ENTRANT CAVITY INDUCTION LAMP


(Figure 6)

These lamps are operating at 2.65MHz and are available in three

wattages –55W, 85W,and 165W.all three are shaped standard incandescent

lamps (Fig.6). The 85W model is 11cm in diameter and 18cm long.

An induction coil is wound on a ferrite core with an internal copper

heat conductor connected to the lamp base and located in the centre of the

lamp. A heat conductor removes heat from the re-entrant cavity and the

induction coil. A 40 cm coaxial cable delivers power from the electronic

ballast to the base of the lamp. By separating the two components, the ballast

operates cooler, which extents its life. With its vibration resistance, an

efficiency of over 75 lm/W and a 100,000-hr average rated life, this induction

lamp is particularly useful for applications where regular maintenance of

lighting equipment is difficult .A typical application of 165W induction lamps


would be pole-mounted luminaries for dusk –to-dawn illumination on a

campus.

SELF- BALLASTED RE-ENTRANT CAVITY LAMP

(Figure 7)

This compact fluorescent RF lamp has a reentrant topology and

is integrated with an electronic ballast operating at 2.65 MHz. The lamp power

is 23 W at 48 lm/W, and lamp life is rated up to 15,000 hours. Significant

efforts have been made in this lamp for suppressing magnetic and electric

components of EMI to comply with existing regulations.


LOW- FREQUENCY EXTENTED-COIL INDUCTION LAMP

(Figure 8)

This lamps consist of a 5.4 cm diameter Pyrex glass tube constructed

with a rectangular or stretched –donut shape (Fig.8). Two ferrite coils located

on the shorter sides of the rectangular lamp provide the energy coupling. The

power transfer efficiency of this unit is as high as 98%. For example, the

150W lamp offers 80 lm /W efficiency. The lamps operating frequency of

250kHz minimizes the problems associated with EMI, and the ballast design is

much simpler than an RF system working at 2.65MHz. This induction lamp is

particularly useful for applications on bridges, tunnels, high-mounted street

luminaries, and similar difficult-to-reach locations.


6. ADVANTAGES

Absence of electrodes.

Some RF elctrodeless lamps on the market today reach 100,000 hours.

Maintenance is low.

Gas pressure is optimized for maximal efficiency in RF lamps.

It have instant and harmless starting and are more convenient for

dimming

Efficiency is high.


7. DISADVATAGES

Cost is higher than fluorescent and incandescent lamps.

With increasing frequency, efficiency will decrease, so microwave RF

lamps not used for general lighting.

It is woks efficiently for some particular frequencies, if frequency

changes there are problems with electromagnetic interference (EMI).


8. FUTURE SCOPE

Induction lamps are suitable for a range of installations, including

general lighting within a plant, as well as out door areas. Energy conservation

and environmental concerns will inevitably bring about a new generation of

compact residential RF lamps. Induction lighting is an economical choice for

many plants. While it costs two or three times more than a HID lamp and

ballast system, induction lighting last six times longer. Payback, in

maintenance savings alone, can be as fast as one year for hard-to-reach

applications and two to three years for general ambient lighting.


9. CONCLUSION

It has taken nearly a century from the first ideas and the first RF lamp

proposals to make commercially viable RF lamps. The elimination of

electrodes opens up great opportunity for increased durability, light output and

efficiency, and it removes many of the lamp-shape restrictions of conventional

electrode discharge lamps .The initial cost of RF lighting products is the major

barrier to the widespread RF lamps, but with further development of the many

components of RF lighting technology, the range of applications should

increase.


10. BIBLIOGRAPHY

(1) Bright idea: Radio frequency Light sources By V.A.GODYAK from

IEEE Industry applications magazine, May/June 2002.

(2) RF lighting tunes in improved Illumination By Joe Knisley from

www.ecmweb.com.

(3) Website www.plantservices a good fit.com.

(4) Website www.ibl:gov.com

(5) McGraw Hill Encyclopedia for Science and Technology Volume-14.


ABSTRACT

After years of research and development, radio frequency light

sources are just now becoming a mainstream lighting option. RF light sources

follow the same principles of converting electrical power into visible radiation

as conventional gas discharge lamps. The fundamental difference between RF

lamps is that RF lamps operate without electrodes [anode and cathode].

There are three practical ways to energize RF light sources, though

there are more ways to create RF plasmas. These three ways correspond to

different types of interaction of electromagnetic fields with the bounded

plasma and to different kinds of RF discharges. They are: capacitive, inductive

and wave sustained discharges.

The most suitable frequency range is 2.2 - 3.0 MHz [2.65MHz is the

standard] for RF lighting devices. An RF generator (RF ballast) is the essential

yet most expensive part of a modern RF lighting system.


CONTENTS

1. Introduction

2. RF Lighting

3. Discharge types

Capacitive RF discharge

Inductive RF discharge

Wave-sustained RF discharge

4. Choice of frequency and discharge type

5. Commercial RF light sources

6. Advantages

7. Disadvantages

8. Further Scope

9. Conclusion

10. Bibliography


ACKNOWLEDGEMENT

I extend my sincere gratitude towards Prof . P.Sukumaran Head of

Department for giving us his invaluable knowledge and wonderful technical

guidance

I express my thanks to Mr. Muhammed kutty our group tutor and

also to our staff advisor Ms. Biji Paul for their kind co-operation and

guidance for preparing and presenting this seminar.

I also thank all the other faculty members of AEI department and my

friends for their help and support.

101seminartopics - 123seminarsonly.com...2. rf lighting the very first proposal for rf lighting, as...

Documents