I have scavenge many transformer, but how do I know its VA rating (in order to utilize it without damage it)For voltage rating, we can measure using Multimeter, but what about maximum capable ampere ? Or we can only estimate by judging size of it ? Would be glad if there is a way to measure other values too !
Do a load test to determine its approximately current rate. You need an AC voltmeter and AC current-meter and a high wattage variable resistor as its load. Suppose its secondary voltage at no load is 5 volts. Now apply the loading until it drops to about 4.8V ,read the current rate.
The VA rating depends on the amount of energy the transformer core can store per cycle. That depends on the magnetic properties of the core and the amount of material in it. Because most power transformers are constructed the same way from the same materials, the size is a very good indication of the VA rating. So I think the best way is to look on a component supplier's website, find some transformers that are about the same size as the unknown one, and read their VA rating.
The VA rating of a transformer can be estimated from the size of the core and its operating frequency. You'll need to measure (or estimate as closely as you can) the cross sectional area of the center leg of the core (if it has a center core and two outer legs) or one of the legs (if the core only has two legs and no center core). If your transformer is connected directly to the power mains, the operating frequency (F) will be either 50 Hz or 60 Hz depending upon where you live. Multiply the length and width of the core's cross section (in inches) and multiply them to get the core's cross sectional area (A, in square inches). The estimated turns/volt can be estimated as: X 10^8/(15.77*F*A*13000) The estimate VA capability can be estimated as: P(in VA) (A*F/9.6)^2 For example, suppose the center leg of your transformer measured 1 x 1 inch and it was designed to operate off 60 Hz. Then: X 100,000,000/(15.77*60*1*13000) 8.13 turns/volt (Useful if you want to rewind the secondary) and P (1*60/9.2)^2 (6.521)^2 39.06 Volt-Amperes Assuming your secondary voltage measurement was in RMS volts, then you can estimate the RMS current of the winding by taking your calculated VA and dividing by your measured voltage: Estimated amps P/V Good luck, and play safely!
