Frequency, voltage & interconnected system
The frequency of the produced emf for a p polar generator is given by f = p/2 n, here n is speed of the generator in rps or f = p/120 n, whenever n is in rpm. Frequency of the produced voltage is standardized to 50 HZ in our country and in many European countries. In USA and Canada it is 60 Hz. The table shown below gives the rpm at which the generators with distinct number of poles are to be driven in order to produce 50 Hz voltage.
A modern power station has more than one generator and such generators are associated in parallel. Also there exist a large number of power stations spread over an area or a country. A regional power grid is formed by interconnecting such stations via transmission lines. In another words, all generators of various power stations, in a grid are in effect associated in parallel. One of the benefits of interconnection is obvious; assume due to technical problem the generation of a plant becomes nil or less then, a part of the demand of power in that region still can be made from the other power stations associated to the grid. One can therefore avoid complete shutdown of power in a region in situation of technical problem in a specific station. It can be shown that in an interconnected system, with more number of generators associated in parallel, the system voltage and frequency tend to fixed values irrespective of degree of loading present in the system. This is the other welcome benefit of inter connected system. Interconnected system though, is to be controlled and monitored cautiously as they might give rise to instability leading to collapse of the system.
All electrical appliances (i.e., fans, refrigerator, TV and so on) to be connected to A.C supply are thus designed for a supply frequency of 50 Hz. The frequency is one of the parameters that decide the quality of the supply. It is the duty of electric supply company to view that frequency is maintained close to 50 Hz at the consumer premise.
It was indicated earlier that a maximum of few hundreds of volts (say around 600 to 700 V) could be developed in a D.C generator, the restriction is imposed principally due to presence of commutator segments. In the absence of commutators, current day produced voltage in alternator is much higher, usually around 10 kV to 15 kV. It can be illustrated that rms voltage induced in a coil is proportional to φ and n that is., Ecoil ∝ φ n here φ is the flux per pole and n is speed of the alternator. This can be justified by perception as well: we know that mere rotating a coil in absence of magnetic flux (φ) is not going to induce any voltage. Presence of flux without any rotation will fail to induce any voltage as you need rate of change of flux linkage in a coil. To organize the induced voltage one has to control the d.c field current like speed of the alternator gets fixed by frequency constrain.