Electronic Ballast PDF

11. ELECTRONIC BALLAST 11.0 Conventional Tube Light System : 11.1 Striking Voltage : This is the voltage required to form an arc discharge across the tube. Normally this voltage requirement depends on the condition of the gas inside (ageing factor). NOTE : Older the Tube,...

11. ELECTRONIC BALLAST 11.0 Conventional Tube Light System : 11.1 Striking Voltage : This is the voltage required to form an arc discharge across the tube. Normally this voltage requirement depends on the condition of the gas inside (ageing factor). NOTE : Older the Tube, higher is the requirement of striking voltage. 11.2 Current Limiting : Any gas discharge device is to be connected with a series impedance, so as to limit current through the device for safer operation. Once the arc is established, the voltage across the tube is relatively maintained constant. To produce a constant power/light output, it is necessary to maintain the current constant irrespective of supply voltage variations. Here the need of an impedance device termed Ballast comes in. When the supply is given, the current is made to flow through the choke and filaments through a starter. The current is established and the filaments are heated up. After sometime, the starter opens and the current is made to stop abruptly. This causes a sudden collapse of magnetic field in the choke and produces high voltage surge in the order of 600- 15O V which is sufficient enough to produce arc across the tube. The; current is reduced to a constant level. The starter gets inoperative then. Now the path of the current is through the choke and tube. It gives inference that as and when the current is increased, the power fed into the tube increases and hence the brightness. Correspondingly the loss in the choke is also increased since it is connected in series. Ideal inductance will Consume no power. But practically, there is some resistance present in the choke which results in I2R losses. There is also hysteresis and eddy current losses in the core of the choke. Hence there will be certain amount of power (18W) consumed and wasted as heat in the. choke which results in loss. A good quality choke will produce less loss compared to the low quality one. This device is inductive in nature & makes the current to lag and hence the power factor is less than unity around 0.5. When the supply voltage is changed from its normal value, then the current through choke is affected and hence the brightness. Moreover, the change in the voltage is directly applied across the choke, since the voltage across the tube is constant. 11.0.3 Power Measurement : The power fed into the system is nothing but the sum of power loss in the choke and the power fed into the tube. The power fed into the system Ps = V1. I1 COS θ Where V1 – is the RMS value of the supply voltage. I1 – is the RMS current measured. COS θ – is the power factor which is property of the choke, and nothing but the Cosine of the angle difference between the voltage and current wave form. Note : Generally copper choke will produce a power factor of 0.5. 58 11.0.4 Efficiency : We say the system is efficient, when it draws less power for the same light output. The efficiency is increased by 1. Reducing the loss in the choke. 2. Increasing the efficiency of the system by new techniques i.e. by increasing the luminous output per unit input of power. 3. Drawing appropriate current from the power line which reduces the loss in the line (line loss) i.e. improving the power factor. 11.1 Role Of Electronic Ballast : By the use of electronic ballast, we could achieve the ideal goal of higher efficiency system. This operates on a totally different principle and hence the series loss in the choke is eliminated. But some meager amount of power is required {about 1.5W) for its own operation. By operating the the light at higher frequency i.e. from conventional supply of 50Hz to say 20,000 Hz, we can achieve, higher luminous efficacy. The improvement of efficiency has been calculated as 13%. Hence, less amount of power (87%) is required to produce the same light output when we use the high frequency. Since the Electronic Ballast operates at high frequency, the generation of high voltage is easy and possible even at very low supply voltages. Hence the starting is instantaneous and enables the tube light to operate at very low supply voltage. The electronic ballast has no moving mechanical parts, no wear and tear and hence the life expectancy is higher. The parts can he replaced even if there is a failure. The ballast operates by converting the AC supply into DC. Hence, the current wave will not be purely sinusoidal and results in distortion. This distortion produces HIGH Radio frequency signal. Good ballast will have a suppressor to arrest the RF produced inside. Electronic ballast can withstand voltage range and fluctuations because the current regulation is much better. Electronic ballasts are ideal, for tube light application if reliability is established. Good electronic ballast will have the RF.suppression circuit, spike suppression circuit, over load protection circuit as extra features. With these facilities the reliability will improve at an extra cost. The modern T5 fittings are coming with good quality electronic ballast integral to the unit. But the ENERGY SAVING, higher power factor and low voltage starting would out weigh the initial higher costs of the electronic ballast 11.2 Principle Of Electronic Ballast : The electronic ballast provides all the necessary control actions required by the tube.light. The block diagram of the Electronic ballast is shown in Fig.2. The second block is an oscillator. This oscillator frequency will be some where in the range of 15 kHz to 50khz. Oscillator output power is amplified to the level required. Now the source is ready at 20khz (15-45Khz) instead of 50hz. This high frequency source, is used to light the tube with a smaller choke in series. 11.2.1 Protection Circuits : The Electronic ballast has the protection circuit against overload, short circuit, Input transients, Spikes and dips. These protection circuits are additional features only but these protection circuits play very important role in the reliability & in the life of the ballast. 59 11.3. Energy Saving Aspects : The energy saving has two aspects. 1. Reducing the level of energy being wasted. 2. Achieving better efficiency by new techniques for the same application. In the electronic ballast, both the approaches are done & hence the energy saving is considerable. 11.3.1 Reduced Wastages : With Electronic ballast, the series loss encountered by the conventional ballast is avoided fully. Hence the total energy wasted as heat in the conventional ballast is fully saved as energy, which is around 10-18 Watts depending upon the quality of the Ballast. The Electronic Ballast requires only about 1.5-2.0 watts for its own operation. There by the saving is from 8.5 – 16.5 watts in the normal supply voltage conditions. 11.3.2 New Techniques : It is a fact that the fluorescent lamp when operated with high frequency in the order 10 – 50kHz, the efficiency (i.e. The light output per unit of input power) will increase by about 13- 14%. At high frequencies, the number of peak light outputs per unit time will increase. At every peak the phosphor coated on the wall of the tube starts giving maximum glow. In between the peaks the light output will start falling down towards zero. The phosphor requires a minimum amount of time for the light to decay to zero. If the frequency of applied voltage is high, then the peak to peak time gap is reduced and the decay of light output will become insignificant due to smaller time interval & hence the average light intensity is increased. It is not possible to get the increased light output by increasing frequency beyond certain level. There after the light output will become constant. Because the time gap interval is so small further increase in frequency can't increase the average light output. When the frequency is increased from 50 Hz to 10 kHz, the efficiency is increased from 1 to 13% and remains at 13% itself up to 50 kHz. 11.3.3 We can summarise the energy saving as follows: 1. The loss in conventional Ballast is fully eliminated. 2. The conventional Ballast will produce higher loss when the voltage is increased. Hence additional saving is obtained in the high voltage existing areas, particularly in the Industries. 1. Indirect Loss: 1. Heating of Ballast will increase the load on air conditioner & hence the extra load on air conditioners is saved with Electronic Ballast. 2. Since the conventional Ballast produces poor power factor of 0.5, the line current requirement is very high & hence the line losses in the wiring system. With electronic ballast this loss is reduced due to unity power factor. These indirect losses are reduced in the Electronic Ballast. 3. Electronic ballast operates by converting the 50Hz into high frequency, there by the lumen output is increased by about 13% OR less amount of power is required to produce the same lumen output. 60 11.4 Economy Analysis : There are so many ways by which we can recover the investment made in the Electronic ballast. The savings are 1) The direct power saving. 2) Power factor is unity. Hence reduced VA RATING. The KVA demand is reduced thereby saving in penalty. 3) The saved VA can be used for other constructive work. or a lesser capacity generator is required to run the plant & hence saving in investment. 4) Heating is reduced. Load on air conditioners is reduced. 5) The life of the lamp is increased due to high frequency operation. Hence saving in inventory of Tube lights. Considering only the direct electricity saving, leaving all other indirect Savings, we can workout the economics as below. Energy consumed by Conventional Ballast for 1 x 40 w tube at 240V 54W a) The Energy Consumed by Electronic Ballast for same lumen output @ 240V is 35W b) The direct Saving of Energy is 19W Table – 1. Electronic Conventional Comments Ballast 40W Ballast 40W Power consumed 35 W 54.0W Saving I9W Cost of Ballast Rs. 350/- Rs. 120/- Extra invest Rs. 230/- Annual operating cost at Rs. 471.5 Rs728.00 Saving 15 Hrs/day and 300 days Rs. 256.50 a year @ Rs.3.00/unit With the above example, we can conclude that the extra investment made for the Electronic ballast can be recovered well within one year period leaving all other indirect savings& benefits, which is also within the guarantee period of the Device. The life expectancy of the Electronic ballast is minimum 10 years. Hence anybody can take a decision of going in for the changes immediately, by considering the other operational /maintenance/finance conditions. 11.5 Operational Problems With Electronic Ballast: Following are the operational problems : 1). RF & Electromagnetic interference 2) Power factor 3) Light output 4) Wave form Distortion/Harmonics. 61 11.5.1 RF & EMI: The main problem faced by the user in the operational side is Radio frequency & Electromagnetic Interference created by the Electronic Ballast. Radio Frequency Interference : Radio frequency interference is bound to be there whenever there is a distortion of current waveform. This is a common feature with all the electronic equipments such as TVs, Radios, Power Supply Units, Computers, Instruments, Thyristor Panels, Mixies, Motors with brushes etc. The level at which the RF emission produced depends upon the amplitude and the amount of distortion of the current wave form. Hence all the equipments connected to electrical system will produce RF signal, the spectrum of which may vary. Measuring the RF signal is very difficult with oscilloscope etc. We need to have" RF free chamber and sophisticated RF measurement equipments, which are available only at few places in India. The RF signal sometimes may interfere with radio receiver, if the RF spectrum falls on the range allotted for radio transmission purposes. Some people use radio receivers to test whether the Ballast produces RF or not. This is not a very correct method. The radio receivers are extremely sensitive and are designed to pick up even very weak signals. Bringing the Radio closer to the Ballast or any electrical gadget will produce some interference which is an indication of presence of RF signal in that particular frequency tuned in the receiver. But this will not give an indication about the strength of the signal. The RF produced interferes with other equipments by two means. 1) Radiated Emission from the Ballast. 2) Conducted Emission through the power line. Radiated Emission : The ballast produces RF signal in the form of electromagnetic waves. This is radiated in the air. The strength of the signal will be maximum at the source and will be reduced with respect to the distance from source. Radiated emission can be controlled by using suitable enclosure and the suppressor at the source itself. Conducted Emission : One part of the RF produced is conducted through the mains cable and is transmitted to the other surrounding areas up to the transformer. This is most dangerous one, which causes interference in the other equipment connected in the same power line, some where away in the premises. This conducted emission is also to be measured and should he with in the standard specified limits. This conducted emission can be suppressed by using suitable suppressors at the source. It is to he noted that even though the interference is heard on the receiver, the Ballast is said to he RF suppressed, if the strength of the RF signal radiated/conducted is below the FCC CLASS -A standard. 11.5.2 Power Factor : It is an important factor to he considered for efficient power distribution. It determines how effectively the power is transferred from source to the load. In general, when the voltage waveform and current waveform are in phase, the power transfer will be effective and we say that the power factor is unity. When there is a phase difference between the voltage waveform and current waveform, then the power factor is said to be less than unity and is equal to the Cosine of the phase angle difference, Cos θ (applicable only for the sinusoidal waveforms) 62  Power Factor = cos θ (Where θ = is the phase difference between volt & current waveform in degrees.) When the power factor is less than unity for the constant voltage line the current drawn will be increased for the same load. This excess current is to flow through the cable. This excess unwanted current will produce more drop on the power line and hence the power loss, called as line loss. This line loss is proportional to the square of the current drawn through the power line. Hence in order to minimise the loss on the line, it is preferred to have unity power factor for the ballast. This power factor in no way. affects the Electricity Bill for the user. But the low level power factor will contribute a heavy line loss on the grid of the Electricity Distribution System and is a loss to the Nation. Also it is important to note that all the Electrical Generators, Transformers etc., are rated for the KVA. lt is the product of voltage and current. We should not draw more than the KVA Rating of that equipment. For example, when a shop is having 5 KVA Generator set @ 230V it can draw 5000 Watts of power at unity power factor, (i.e. Voltage = 230V & Current = 21.73 Amps.) But when we connect 0.5 power factor load, the maximum current one can draw is only 21.73 Amps. But the load will be 230 x.0. 5 x 21.73 - 2500 Watts. Hence, we can utilise only 2500 Watts even though we have the generator capacity for 5000 watts. Thus KVA plays a very important role in the industry. This overall KVA demand in every half an hour should not exceed the allotted value. If it exceeds the limit, then the industry will be penalised on the tariff. Maintenance of the KVA within the limit can he easily achieved by improving the power factor near unity. Here the importance of Electronic Ballast for Tube Light comes in. 11.5.3 Light Output: The light output from the fluorescent tube operated in the Electronic ballast is essentially equal to the conventional system. Measurement of absolute value of lumen is very very difficult. "Lumen is defined as. the light energy emitted within unit solid angle by a uniform point of source Of unit luminous intensity of one candella". A relative value of lumen can be measured OR compared with that of the standard system. The Lux level can be measured with the Lux meter, kept at a particular distance. Relative measurement can be taken as explained below. Consider a standard system with standard ballast connected to the tube light at the rated voltage. Now measure the Lux level using the Lux meter, by keeping it at a particular distance, preferably at one meter from the geometrical centre of the tube light. Now connect the Electronic ballast to the same tube light, at the rate voltage and measure the Lux value read on the meter as in the previous case, without disturbing the geometrical arrangement. The difference in reading can be expressed in terms of percentage less or percentage more than that of the standard ballast system. Note : 1) As far as possible avoid reflection coming on to the lux meter. 2) Avoid external light falling on the Lux meter. 3) Maintain the voltage constant while taking measurement. 11.5. 4 Wave Form Distortion : Irrespective of the load conditions, the ideal power lines should provide a pure sinusoidal voltage at the rated frequency & voltage. Ideally all loads are expected to take only the sinusoidal current. But practically, the ballast consumes current as pulses. Hence the 63 current wave form is non sinusoidal. Wherever the non sinusoidal current wave form is demanded by the load, the wave form distortion is caused. This is referred as wave form distortion OR Harmonic distortion. The non sinusoidal current wave form can be resolved into the fundamental and its harmonics. Usually their level is expressed in terms of the percentage of the fundamental value. Generally all the odd harmonic will have the effect on the distribution system. Hence these levels are to be within the limits. However ISI limitation on the various harmonics are listed below: 2nd 5% 3rd 30% 5th 7% 7th 4% 11.6 Maintenance Problems : The maintenance problems are listed below : 1. Routine maintenance 2. Replacement of the Device 3. The reliability of the Device 4. Serviceability of the Device 11.6.1 Routine Maintenance : With respect to Electronic ballast, there is no need of routine maintenance. Only maintenance required is monitoring the life of the tube by seeing its glow & its end blackening and to be replaced if need be. 11.6.2 Replacement of Ballast: Replacement of ballast when it is defective is simple & wiring is less complicated than the conventional system, since there is no third component like starter. 11.6.3 Serviceability: The Electronic ballast can be serviced and reused. Only the failed one is to be replaced which works out cheaper. Conventional ballast, is to be thrown out for a new one, when it goes defective. 11.6.4 Reliability: This is the most important factor which governs the population of the Electronic ballast. In spite of all the merited benefits, the higher failure rate restricts the population of Electronic ballast widely. The reliability of the ballast depends on the following : i) The parameters taken into consideration while designing the ballast. ii) The circuit design & component selection. iii) The manufacturing Quality control. iv) The power line condition at the user point. 64 i) Parameters which are to be considered while designing the ballast are a) The voltage fluctuations b) The expected spike level on the power line c) The atmospheric temperature conditions. d) Environmental conditions. a) Particularly in Indian conditions, where the power distribution line runs into several thousand kilometers, maintaining the voltage at remote point is very very difficult. Hence the circuit design should take care of the voltage range to which the ballast has to operate. Generally this range is from 160V-280V. b) Majority of the failure of electronic ballast is caused by the spike voltage existing on the power line. Hence the design should take care of the spikes which are anticipated in the power line. c) The temperature rise inside the ballast should not exceed about 5° - 10° above the ambient. Hence this temperature raise is to be limited & restricted within the limits to avoid failure of components. Good heat sinking arrangement is also to be considered while designing the ballast. d) The environmental conditions such as corrosive atmosphere, moisture etc., are to be considered in the design and to be carried out in the manufacturing. ii) The circuit design : Considering all the above into account, the design should be made in order to get the maximum efficiency in the total system with prime purpose of reliability. All the components are to be selected carefully and sufficiently for its reliability. iii) Manufacturing & Quality control : Even though the design is perfect, improper manufacturing can cause a failure to the major extent. The manufacturing methods should give importance to quality at all levels and stages of manufacturing. iv) Power line condition : As discussed earlier, the power line condition is very very bad in the industrial atmosphere and is also carried over to the other domestic areas. There is no check on the instruments being installed & its effect on the power line. Hence all the gadgets connected to power line attribute pollution in the form of surges, spikes, dips, wave form distortion. etc. In heavy industries, loads driven using thyristors etc. produce spike voltages up to 6000V. These levels will vary from factory to factory. This is the main cause. of failure in electronic ballasts. However much we protect the ballast, the spike level exceeding the expected design value will cause the ballast to fail. 65 11.7 Conclusion :. Even though the electronic ballast was introduced in the market in the early 1980's, the population of electronic ballast in real use is much less than expected. The main reason was due to its poor reliability. The failure rate of electronic ballast when introduced were in the range of 30-40%. Now this has been reduced to 2-10% by the quality conscious manufacturers. Intial failures were due to to the poor design, the poor manufacturing methods & due to the power line conditions existing in the industries. The power line condition attributes to a major factor for the ballast failure. Low cost ballast can't provide all the protection necessary for the ballast. Power & light output comparison charts of electronic ballast and conventional ballast are given, which are self explanatory. (Fig. 5 and Fig.6) Failure of 2-3% can't be avoided even when best design & manufacturing process is used. This is mainly due to the poor power line conditions. Considering the energy saving & low operational cost, the electronic ballasts are. best substitute for conventional ballast in the in the long run. 66 67