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Power Electronic Riddle No.3 - Fluorescent lamps electronic ballast
What is difference between inductive and electronic ballasts performance in fluorescent lamps?
Author : Larisochka - From: Ukraine
Thu, April 29th, 2010 - 10:51
Fluorescent lamps can be operated on a conventional ballast (CB) and a starter.
In this case the ballast functions as an inductive resistor; it comprises a lag ballast which consists of a laminate iron core and a copper-wire winding.
Conventional ballasts are the cheapest kind of ballasts, but they do give rise to significant losses of energy due to the generation of heat.
Low loss ballasts (LLB) are comparable to conventional ballasts, except that their core material is of a higher quality and they have thicker copper wires to reduce the loss of energy in the control gear. Low loss ballasts are only slightly more expensive than conventional ballasts, so they are frequently used in lieu of the latter.
Electronic ballasts (EB) differ in weight, form and function from conventional, inductive ballasts. They consist of a filter, which prevents any reactive feedback onto the mains supply, a rectifier and a high-frequency inverter.
Electronic ballasts have an integrated ignition device, which means that no additional ignitor is required. They ensure a flicker-free start and switch off automatically if the lamp is defective, which prevents the ignitor being activated time and again; switching and operation are as trouble-free as with incandescent lamps.
Operating the lamps at 25–40 kHz presents a number of advantages, above all, enhanced luminous efficacy. This in turn means that the luminous power is achieved, but at a lower energy consumption.
At the same time there is considerably less power loss. The high operating frequency of the lamps also prevents stroboscopic and flicker effects, and magnetic interference and humming, all of which are associated with conventional ballasts.
Electronic ballasts are to a large extent insensitive to voltage and frequency fluctuations. They can be operated at both 50 and 60 Hz and over a voltage range of between 200 and 250 V. As they are also designed to be run on direct current, fluorescent lamps with EBs can be operated on batteries, should there be a current failure, thereby simplifying the provision of emergency lighting.
Electronic ballasts are, however, more expensive than inductive ballasts.
If fluorescent lamps are operated using inductive ballasts it is necessary to provide a separate starter. The starter first preheats the lamp electrodes. Once the electrodes are sufficiently heated the starter breaks the circuit. This induces a voltage surge in the ballast which in turn ignites the lamp.
The simplest form of ignitors are glow starters. They comprise bimetal electrodes encased in a glass tube filled with inert gas.
Switching the lamp on produces a luminous discharge between the electrodes in the starter, which in turn heats up the electrodes. During this process the bimetal electrodes bend inwards until they touch, thereby closing the heater or filament circuit of the fluorescent lamp.
After a short time the starter electrodes cool down and separate. This disconnection induces a voltage surge in the ballast which in turn ignites the lamp. When the lamp has been ignited it is only the operating voltage of the lamp that is applied to the starter. This is insufficient to produce a luminous discharge in the starter. The electrodes therefore remain open, which avoids the lamps being permanently heated.

Glow starters are the starters most frequently used and they are the most economical. They do have one drawback; they repeatedly try to ignite the lamp in the event of it being defective. This gives rise to noise and flickering lamps. Moreover, ignition problems may arise in the case of under voltage or low ambient temperatures due to the fact that the preheating times are inadequate.
Safety starter switches are similar to glow starters. They switch off automatically after repeated attempts to ignite the lamp, thereby ensuring that defective lamps are not subjected to continuous ignition. To operate the starter again it is necessary to reset the safety switch manually.
Thermal starters have contacts that are normally closed when the lamp is switched on. The contacts are disconnected by means of an additional heating element which heats up a bimetal strip or dilating wire. The starter only opens when it has been sufficiently preheated and since the preheating time is prolonged if the temperature or the voltage conditions are not ideal, ignition is not always trouble free.
As there is no need for an initial warm up period to the point of contact, the result is that thermal starters ignite faster than glow starters. Thermal starters are more expensive than glow starters.
Some versions require a separate heating current supply through the ballast.
Electronic starters open and close the preheating circuit without any mechanical contacts. They ensure a quick and safe start under a much wider range of conditions; in the case of defective lamps the re-ignition process is terminated.

Author : Hamid - From: Iran- Firouzabad in Fars
Fri, April 1st, 2011 - 19:13
Which kind of ballast generates less harmonics?. 
Author : Sinnadurai Sripadmanaban - From: Srilanka
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