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Electronic Ballast
 

High-frequency operation

High-frequency (HF) operation of >20KHz, fluorescent lamp efficiency is increased by 10% i.e. conversion rate of UV radiation increases. Thus advantage of high frequency operation is that the lamp requires less input power for the same light output as it operates at high frequency. This leads to an energy saving. Thus a typical fluorescent lamp of standard four feet lamp and 26mm diameter delivers lumen of 2450. This lamp at 50 Hz consumes 36W. The same lumen are available at 32W with frequency >20KHz. Thus lamp lumen per watt efficiency at 50Hz is 68 Lumen per watt and with high frequency is 76.5 Lumen per watt

Similarly, the current requirement for same lamp at 50Hz is 430mA and with high frequency it reduces to 320mA for same lumen. This leads to reduced heat and stress of lamp.

Also losses in the ballast are as low as 3-4W against 5.5-15W in the case of magnetic ballast. In electronic ballast additional energy savings are possible by control features of electronic designs (Dimming).

What are harmonics?

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency e.g. if fundamental frequency is 50Hz, then the 2nd harmonic is 100Hz, the 3rd is 150Hz etc.

Non-linear loads that draw current in abrupt pulses rather than in smooth sinusoidal manner create harmonics. Non-linear loads consist of inductive or capacitive loads. These pulses cause distorted wave shapes, which in turn cause harmonic currents to flow back into other parts of the power system..

Harmonics are measured in terms a parameter called Total Harmonic Distortion (THD). It takes into account all the harmonics and is expressed as percentage value of fundamental.

Harmonics are having adverse effects such as overloading of transformers (de-rating) and rotating equipment, tripping of circuit breakers and fuses, neutral overloading etc.

thd

Ballasts are non-linear loads. Low power factor and harmonics generation are problems that evidence themselves on the lamp side. One of these is the lamp current crest factor, which is the ratio of the peak lamp current to the rms lamp current. A sine wave has a crest factor of 1.41. The service life of a fluorescent lamp is significantly shortened when the crest factor exceeds the lamp manufacturer's recommendation, usually 1.7 for most lamps

Simple Inverter Ballast

Electronic ballasts were first introduced as simple inverter ballasts. They perform the basic function of starting the lamp and controlling the lamp current. Power factor correction is not incorporated. This is a basic type of electronic ballast, which consists of various electronic circuits for each element shown in the block diagram below. This is still prominent in integrated Compact Fluorescent lamps (CFL's).

Block diagram of Simple Inverter ballast

simple_inverter

An AC input is fed to AC-DC converter, which converts the AC voltage to DC. DC is Inverted into high frequency AC using an inverter, which is fed to the lamp using lamp circuit.

Simple Inverter ballast has disadvantages like very high THD (>120%), low power factor (0.5). The current waveform is highly distorted as shown in fig. This leads to high VA loads besides, adverse effects of harmonics.

Waveforms for various types of ballasts

wave_forms

Passive Power Factor Corrected (PPFC) Ballast

In order to overcome disadvantages of low power factor in the Simple Inverter Ballasts, Passive Power Factor Corrected (PPFC) Ballast was developed. PPFC ballast consists of additional circuit for power factor correction called as Valley Fill Circuit or Power Factor Correction (PFC) Circuit.

PPFC ballast

ppfc

As shown in block diagram above, AC to DC Converter converts AC current into DC. It is pulsating DC, having ripple content. The pulsating DC is fed to Valley Fill Circuit/ Power Factor Correction circuit (Please refer fig.). Capacitors C1 & C2 work for power factor correction. They keep current flowing continuously in both half cycles.

This results into input line current drawn continuously from the supply instead of spikes as in the case of Simple Inverter ballast. Please refer waveforms of fig. Thus, this circuit brings input line current waveform close to input voltage waveform. i.e. minimum phase shift exists between them. Thus power factor is corrected (to about 0.95).

Power Factor Correction Circuit in PPFC Ballast

ppfc_pf

In an attempt to correct the power factor, current waveform is distorted as shown in fig. Hence, the Total Harmonic Distortion (THD) is more (30%) in case of PPFC ballast. Since the current waveform is distorted, crest factor is also increased (>2). This power factor correction can be achieved by combination of L and C (in lieu of valley fill circuit) for passive power factor correction. But this is very bulky to use.

Input Filter controls the generation of Electromagnetic/Radio frequency Interference within the stipulated limit and also protects the ballast from those emanating from supply system. On the lamp side, protections are provided against faulty lamp conditions (short circuit, open lamp, transient voltages etc).

The ballast is designed to operate at rated voltage of lamp i.e. 240V. As the input voltage varies, the variations are transferred as they are to the lamp through the ballast i.e. input wattage to lamp is varied (wattage decreases with decreased voltage and vice versa). The light output also varies directly.

Summarizing, following effects can be observed
  • THD is controlled up to 30% and severity of effects is some what reduced. However, Lamp Current Crest Factor is more (>2). High peaks of Lamp current affects the lamp life. (In Inverter ballast the lamp current crest factor is generally <1.7)
  • KVA demand charges for electricity will be more as KVA requirement is increased.
  • Power Factor is high (0.95)