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Why ac current frequency is 50hz?
The generation of AC currents and relationship to frequency has to do with the speed of the unit that is turning the generator (prime mover). The formula for frequency is f = Number of revolutions per minute of the engine (N) x Number of magnetic poles (P) / 120 Speed the prime mover up for a higher frequency, slow the prime mover down for a lower frequency. This is why the generator has to be wound for the voltage that it is going to produce and the frequency that it is to operate on. This has probably got to do with Iron. Most transformers consist of large amounts of iron that need time in order to change polarity of a magnetic field. Faster frequency would make the transformer less efficient. The iron would not manage to become fully magnetized. Slower frequency would make the transformer need more iron, hence becoming heavier. If the size of the iron core were the same and we just lowered the frequency, then as soon as the magnetic field is buildt up, then the rest of the energy until next half wave will be transferred into heat. It is simply a trade off between efficiency and practicality. Today's high speed transformers operate at speeds between 20-80 Khz and is only doable due to a special core material of powdered iron that is much easier to magnetize. Electricity in the early days was DC only. However, some decisions were made to use AC because DC is, at high voltages, very dangerous to humans, something that was demonstrated during the first few 'victims' of the electric chair that in the beginning operated at DC current only.
How do you use transformers on DC supply?
In order to pass DC current through a transformer you would need some basic knowledge of what a transformer actually is. A transformer is simply put a coil of copper wire around metal plates of iron. When you put current onto the coil, the metal plates will build up a magnetic field. During the build up of this magnetic field, the current will be 'low' When the magnetic field is maximized, the coil will work as a shortcut on a DC power source. In order to use a transformer, ie transform voltage up or down, we need a fluctuating magnetic field. This can not be done without electronics of some variety, and the resulting voltage will be AC, not DC. The easiest way to use a transformer with a DC power source is to make a circuit that electronically change polarization of voltage back and forth. Depending on transformer, this can be as low as 50 times per second (EU standard 50 Hz AC 220 Volt, US standard 60 Hz AC 115 Volt) or higher like 100.000 times per second (100 Khz) as is in many high speed switch power supplies. It all depends on the transformer you use and what you want to use it for. If say you want to make a power source for an electric drill and you want to use a car battery as the source, then I would forget it. An electric drill need a lot of juice and in order to do so via a transformer, this transformer would need to be very very large. Every ampere of 12 Volt would only give you approx 0.07 Ampere of 115 Volt. A drill easily drain 800 to 1000 Watts. Approx 8 Ampere at 115 Volt. Following this, an ideal transformer with hardly any loss would eat away 100 Ampere every 8 ampere on output 115 Vac. With the loss factor (some of the transformed voltage will be given away as heat) and you would probably need approx 110 Ampere 12 Volt in order to generate 8 Ampere 115 Vac. A normal 60-65 Ampere car battery would then last a mere half hour fully charged. Another problem is that most batteries are not designed for such a load over any prolonged time (more than 30-60 seconds). Systems have been and are buildt that can withstand such a drain but they consist of many batteries connected in paralell in order to even the drain between all batteries.
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