| ??? 02/20/05 08:58 Read: times |
#88000 - Difference in calculation & Actual Param Responding to: ???'s previous message |
Mr.Prahlad You aid in first posting
since transformer has got magnetising current and is itself an inductive load thus the thyristor is never fired at zero cross but always fired with a minumum 2500uS delay past zero cross at this firing position maximum current is obtained and as the firing delay increases the current obtained goes on reducing. The maximum firing delay is 7500uS giving minimum welding current. you said 2500us firing delay gives max current & 7500us firing delay gives min current but table you have posted 2500us delay shows/give min curr & 7500 us delay shows/give max current ?? But firing angle increase decrease currr which appears corr. My other question is How have calculated theoratically that at what firing angle what will be RMS current ie (firing -> rms theoratical table/equation/realtion ship) becuse RMS current depend upon load like if 2 oms gives 5 amp at 90 deg firing then 4 ohms laod will some around 2.5 amp. In your case load cosist of following (1) resistance between welding arm/points which depend upon material to be welded , its thickness and pressure applied between welding points. You may say that this is in milli-ohm range but a diff of say 0.1 ohm may make the dirrence (2) transformer resistive & inductive load. with diff transformer it will vary. And even if transformer manufac give these values it may not be correct for u case as they are calculated using countinuous sine wave but you case is for single half cycle with delayed firing , steep voltage pulse at the time of firing (which may be like a fast rising pulse) and both of above may produce hormonics which is firing angle dependent. Tranformer equiva resis is calculated with single 50/60 Hz supply. More it is very for main frequecy change of around 0.5 - 1 hz(1-2 %) . This will change transformer inductance considerably. More 1-2 % change in main frequenc corrs ponds to error of 2-4 degree in fired time(time for which curr/volg is applied) 90 degree firing at 50hz -> firing at 5 msec form zero crossing which means load curre applied for 5 msec. But at 49 hz half cycle time is 10.2 msec , you will fire at 5 msec delay after zero crossing for 90 deg firing and your laod current will be applied for 5.2 msec. Moreover it is very common for mains volt from supplier to fluctuate in 5-10 % range ( varies with time , day , month , place) which can introduce considerable chnage in RMS value from those calculted. You may have calculated theoratical RMS values for diff firing angle taking a fixed mains voltage amplitue , frequency and transformer & weld load which may not be the same at the time of your actual experiment. At least you can measure main voltage& frequency and correct firing angle accordingly. U can apply some methodlog to caluc equilvalent load at diffir firing angle by applying low value ac mains voltage. or more simply apply 5 V stepped down AC main & calculated RMS current for 4-5 firing angle value around calculated one and then measure AC mains values and calculate Equivalent RMS at that AC mians value at those firing angle and then use firing angle for which these Equivalent RMS value (from 5 V ac mains) matches desired RMS. Beacuse even if you calibare machine at manu time , parameter effecting RMS value will vary at actual place. |
