Hi Friends,
In a Home UPS available in India I have noticed that charging the battery (150 – 200 AH, Lead Acid) is done by using the same transformer tapped at nearly 130V. AC mains (generally 220 V) is given to that tap controlled by SCR/TRIAC, fired at required angle so that charging current is maintained at certain level. I have undertaken to make such a charging system using PIC MCU. When testing, it is seen that the project works well except a problem at a particular point. If the firing angle is set near zero crossing point ( say at 10 degree and 190 degree of a cycle), a horrible humming is produced in the transformer and sometimes the AC mains fuse is blown without any charging current. But beyond that point the circuit works smoothly.
I guess that due to lagging of current behind voltage and firing of thyristor this humming is produced. I request you to offer your valued comment please.
an unaccurate zero crossing detector results in occasional overflow to 180 degree half waves
FvM said:A special proble with small phase angles can be that triggering might become unreliable, or an unaccurate zero crossing detector results in occasional overflow to 180 degree half waves.
Firing at 10 degrees you are applying 98% of 220 volts to the primary of a 130 volt transformer!!
a pulse train would be more reliable.
I have examined zero crossing pulse as well as the triggering pulse using CRO and observed that they are very steady and clear. I could not get your observation of 'overflow to 180 degree half waves'
If it happens however at low phase angles, I suspect there's a problem with the triggering circuit, though.
Well, could you take some scope waveforms? You got us playing a guessing game here.
Please include, if the scope has such a funcion, the RMS value. Also measure that the phase angle in the positive and negative semicyles is identical
Sorry to place such a problem. I have an old analogue Scope that has no provision to measure the parameters you asked.
Are you using full wave or half wave zero crossing signals ?.
Hint : Always fire at the most after 9 milliseconds AND not less then 4 milliseconds of the zero cross signal detected by the micro controller .
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