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Ballast transformer
Posted by Rockinriley14 on 3/22/2004, 5:20 pm thanks!
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Re: Ballast transformer
Posted by Bert Hickman on 3/24/2004, 12:06 am, in reply to "Ballast transformer" Hi, It sounds like you have found a ballast transformer that's used in conjunction with high pressure mercury vapor, sodium vapor, or metal halide lamps (sometimes called High Intensity Discharge or HID lamps). These are special current limited transformers that can provide the relatively high voltage (~2-3 kV that's initially needed to "start" the lamps when they are cold) and also provide a low to moderate voltage (several hundred volts at a higher current level) once the lamps have warmed up and vaporized the active metal inside the lamps. These transformers are sometimes packaged in a cylindrical style in order to slide inside hollow aluminum poles for street lighting applications. You can find out more about them by looking up HID ballasts at Advance's site: These transformers often use moderate voltage capacitors in conjuction with special windings to help reduce the amount of current they draw from the power mains (by improving their power factor). Unfortunately, these are relatively specialized transformers - they are not of much use for Tesla Coiling purposes without rewinding. Best regards, -- Bert --
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Re: Ballast transformer
Posted by Rockinriley14 on 3/26/2004, 11:07 pm, in reply to "Re: Ballast transformer" --Previous Message-- thanks again!
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Re: Ballast transformer
Posted by Bert Hickman on 3/27/2004, 9:10 am, in reply to "Re: Ballast transformer" Hi, Most transformers are constructed to have tight magnetic coupling (0.98-0.99) between the primary and secondary. This is done to minimize the amount of voltage drop in the secondary as you increase load current - transformers with high coupling coefficients have good load regulation. However, ballast transformers are designed to have a comparatively lower primary-to-secondary coupling coefficient (~0.90 - 0.95). The coupling coefficient is the portion of the magnetic flux from one winding that is linked to the other winding. In an ideal transformer this would be 1.00 (100%). Reducing the amount of coupling will effectively limit the amount of short circuit current that will be available from the secondary winding. This current limiting function is inherent in the proper functioning of the ballast, and is essential for stabilizing the arc inside a mercury or sodium vapor lamp. The technique is electrically identical to adding an inductor in series with the primary of an ideal transformer. In a ballast, the manufacturer combines the current limiting inductor and transformer into a single "high-leakage inductance" transformer. NST's, MOT's, welding transformers, and doorbell transformers are all made in this fashion. Although you might be able to use the ballast as an isolation transformer, it would have very poor voltage regulation as you changed loading - not a desirable function for an isolation transformer. However, you can often change this characteristic by modifying the magnetic paths within the transformer. Often, the primary and secondary windings have a small magnetic shunt (a small number of laminations) that divert some of the magnetic flux that would otherwise link the two windings. The magnetic leakage path provided by the shunt reduces the effective coupling between windings - by removing these shunts you can increase the coupling between windings and reduce the leakage flux. This will make the transformer behave more as an ideal transformer and have improved load regulation. Tesla Coilers often do this to NST's in order to force them to deliver higher short circuit current. Best regards, -- Bert --
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