| ??? 02/20/05 17:21 Read: times |
#88011 - Re:Diffrence in calculation and actual Responding to: ???'s previous message |
Dear Anil,
Thanks for your reply, 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. I think you are talking about first column in table. It is actually Firing Duration in (uS) and not Firing Delay Sorry for that typpo. My other question is
How have calculated theoratically that at what firing angle what will be RMS current For my theoretical table I used RMS formula SQRT(1/T * INTEGRAL.... I calculated RMS values considering Im=1 I call these values RMS factors then In order to avoid using decimal stuff with 8051 I scaled the values to Unsigned character scale with maximum value that I will fire being 255. The machine has a caliberate function using which the current at RMS factor 128 is measured and than using that as reference I am predicting firing angle for required current. (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 The resistance of workpiece is usually of the order of micro ohms thats why the secondary voltage of around 2V-10V produces currents upto 100,000 Amperes. (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. You have stated the causes for current variation very well thanks. Apart from the causes you listed there are several other factors that can cause weld current to deviate from desired value these are. 1. Change in composition of material being welded. 2. Sheet thickness variations. 3. Poor surface conditions. 4. Reistance rise of copper electrodes and busbars due to temperature rise by weld heat. Cu resistance changes approx. 0.35%/degree rise in temperature. 5. Electrode tip mushrooming due to continuous use. 6. Pressure change changes contact resistance. There are many more factors.... 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 are right the factors that I listed above and you listed can cause current to deviate from target value thats where my close loop feedback comes into picture the firing angle is increased or decreased accordingly to get desired current in next cycle and this process continues till end of weld. 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. Thats what the core issue of this thread is the values that I have calculated theoretically dont match with what I get practically when I do single cycle welding since with single cycle there is no feedback data available with multi cycle welding I get excellent result for example with 3 cycles I get error no more than 3% with 4 cycles 2% and beyond 5 cycle the error is always within 1%. Yes, this accuracy is by using theoretical table. 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. Along with current feedback in my system I have got voltage feedback too its exactly what you described using a 0-6V transformer. I use this voltage feedback for doing necessary adjustments for firing first cycle then on the current feedback takes control and it works excellently with multi cycle welds. The problem area is single cycle welds where I dont have any current feedback data available for doing correction. Once again thanks for your efforts in helping me. Regards, Prahlad Purohit |
