@HyperFahd There is no one single formula to decide the VA rating of a machine (as far as I know) but there are several factors involved. Machine capacity in VA is just a relation of the machine's voltage and current. (Cont.)
@HyperFahd So manufacturers have to decide the required machine ratings and then build it accordingly. The voltage can be selected either by referenceing an international standard such as IEC 60038 for the whole manufacturer production line & catalogues (Cont)
@HyperFahd Or customly build the machine to a customer requirement. The desired machine voltage will decide the amount and thickness of insulating material the manufacturer will need to use to insulate his cables. (Cont)
@HyperFahd Also the voltage will decide the distance between one winding and another inside both motors & transformers. The manufacturer has also to decide the machine's frequency during design stage (Cont)
@HyperFahd Because the amount of magnetic flux that the core of a motor & transformer can handle depends on the ratio of Voltage to Frequency (V/f). If the core size & material can not hold the required amount of flux produced by the machine's current, it'll saturate (Cont.)
@HyperFahd Saturation means that the machine current will not increase (Because flux will be escaping outside the core) when the machine tries to draw more power, but the machine will overheat. (Cont)
@HyperFahd If you know the desired voltage, VA, and frequency then you know the desired current (Cont.)
@HyperFahd Based on desired current you will select the windings size (in mm2) and length to wind the machine. Also machine cooling method affects the amount of current that will flow without overheating, so it is another design factor. (Cont.)
@HyperFahd The buttom line is that no machine is designed with a single formula, but design engineers look into every component one at a time & design it to support the final desired power, voltage, current, ratio (for transformers), torque (for motors), etc.
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