size is proportional to (v/f).in normal transmission if a bridge rectifier is used then the frequency will be doubled 'so size considerably reduces. then why is it not adopted
Yes, the input to a transformer can and often is pulsating DC. But not quite in the way you're asking about. One cannot take a full-wave rectified AC and directly use a transformer to transmit power. At least not on a steady-state basis. The transformer core needs to be reset. And without a high-frequency switching circuit usually with an inductor to help reset the core, the transformer would saturate probably on the 2nd cycle of the AC power line. The best example is a Power Factor Corrector (PFC) DC-DC Converter, which takes a full-wave rectified signal and chops that at high-frequency and then feeds it to a transformer or a transformerless dc-dc converter to produce a regulated DC output. The goal in the case of the PFC is to have the converter follow the rectified line voltage so that the input current drawn from the AC line also follows the AC line voltage and the power factor is raised up to usually 0.9 up to 0.999 pf.
Transformer operates in the linear region of B-H curve of core material used, to maximum utilization of linear region, by possible maximum flux density(B) obtained by needed amper turns(AT), of magnetization by needed magnetizing current( constant V/f, means working flux density in core would be constant) of B-H curve, pulsating D.C voltage, will reflect on secondary perfectly, as the drawn magnetizing current would be according applied voltage praposnate magnetizing current, and hence AT. Primary voltage would reflect on secondary, as per turn ratio(V1/V2=N1/N2), giving same shape wave form, and can be used as isolating transformer. If pulsating D.C voltage is not with in the linear working rang of B-H curve, depending on positive or negative saturation the waveform would be clipped on positive or negative side, of voltage wave form, producing inrush of magnetizing current(refer my paper," Laboratory methods to generates inrush current pattern, for power transformers".)...
Bhupendra, get real. Unless an air core is used, the core will saturate within the first few cycles of the AC power line with the full-wave rectified AC line. This is simply due to the core not being allowed to reset. The core would be driven positive to the peak, then back to zero, but never in the other direction (negative) to reset the core. This might be OK if there were sufficient time between input pulses, like a 1/2 wave rectified AC input. In that case, there would be 50% of the total period (1/Fo) for reset. But a full wave has no time to reset in the case of Full wave rectified AC as the input. However, even that can be mitigated by chopping-up the input with a high-frequency converter.
Depended, on time constant(T=L/R),and working frequency which are not given.....
Bhupendra, the frequency and time constant don't really need to be given in this problem. The frequency was said to be AC power line which can be 50 or 60 Hz. And the main limitation here is core saturation and not T=L/R. The question was put as to why full-wave rectified AC power is not used as an input to transformers to get the gains in smaller size and weight, etc that come from the doubled frequency. And the simple answer is: without additional converter functions, that the transformer core cannot reset and the core will saturate within a few cycles of the start time, t0.
CAN. But the core will saturate and lead to heat . So it needs be reset ,
An even bigger problem is that one will get almost no output from the transformer secondary once the transformer saturates. And an even bigger problem than that is the transformer primary will look almost like a short-circuit to the primary, causing potentially destructive high currents that may damage parts or start a fire.
I have worked with three winding transformer and pulsating DC given to the primary winding. The core use to get saturated when the input AC or pulsating DC was given to the primary of the transformer directly. But if you connect the whole circuit with the auxiliary winding for feedback the core is not saturated and you will get a proper output.
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