Dec 1, 2014

Voltage Build Up in Self Excited Generator


Shunt generator:

Whenever the shunt Generator is run at a continuous speed, there will be an e.m.f. is generated attributable to residual magnetism within the main poles. This little e.m.f. circulates a field current that successively produces further flux to bolster the initial residual flux. This method continues and also the generator builds up the traditional generated voltage following the open circuit characteristic as shown in Fig-a.
The field resistance Rf are often portrayed by a line passing through the origin. This is represented in Fig-b. The 2 curves are often shown in a single diagram as they need an equivalent ordinate which is shown in fig-c.
From the time when the field circuit is inductive, there's a delay within the increase in current upon closing the field circuit switch. The rate at that current will increase depends upon the voltage obtainable for increasing it. Assume at any instant, the field current is i (= OA) and is increasing at the rate di/dt. Then,
Rf = total field circuit resistance
L = Inductance of field circuit
At the thought of instant, the whole e.m.f. obtainable is AC . The quantity AB of the e.m.f. AC is absorbed by the drop iRf and also the balance portion BC is available to overcome L di/dt. Since this surplus voltage is available, it's attainable for the field current to extend on top of the value OA. However, at point D, the obtainable voltage is OM and is all absorbed by i Rf drop. Consequently, the field current cannot increase extra and also the generator builds up stops.

Finally to conclude, that the voltage build of the generator is given by the purpose of intersection of open circuit characteristic and field resistance line. In fig-c D is point of intersection of the 2 curves. Therefore the generator can build up a voltage OM.

Series Generator:

In the course of primary operation, with not any current nevertheless flowing, a residual voltage are generated specifically as within the case of a shunt generator. The residual voltage can reason a current to flow through the complete circuit once the circuit is closed. There’ll then be voltage build up to an equilibrium point specifically analogous to the build of a shunt generator. The voltage build up graph are like that of shunt generator except that currently load current (instead of field current for shunt generator) are taken on X-axis.

Compound Generator:

When a compound generator takes its series field flux sustaining its shunt field flux, the machine is claimed to be cumulative compound generator. Once the series field is coupled in reverse so its field flux opposes the shunt field flux, then the generator is said to be differential compound.

The easiest way to build up voltage during a compound generator is to begin underneath no load conditions. At no load, solely the shunt field is effective. Once no-load voltage build up is achieved, the generator is loaded. If under load, the voltage rises, the series field connection is cumulative. If the voltage drops considerably, the association is differential compound.


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