Question

Sketch a diagrammatic arrangement of a static or self excited alternator.

Static Excitation System (or Functional Type)

The principle of the static or self-excitation system is that a 3-phase transformer with two primaries, one is shunt and the other in series with alternator output feeds current from its secondary windings through a 3-phase rectifier for excitation of the main alternator rotor.

On no-load, the generator excitation is provided by shunt connected primary which is designed to give sufficient main rotor field current for normal alternator voltage at no-load.

Reactor coils give an inductive effect so that current in the shunt winding lags main output voltage by 90°. Build-up of voltage at starting is assisted by capacitors which provide a resonance condition with reactors.

On load, the generator current contributes additional excitation current via the series primary coils. Variation in load current directly alter excitation and rotor field strength to keep voltage approximately right.

In this generator, the excitation system make use of load current from the alternator to supply that component of excitation current needed to maintain voltage as load increases. This component of excitation is a function of the load. Field current is thus forced to adjust rapidly as load changes. Voltage disturbances accompanying application or removal of load are greatly reduced. Statically excited alternator have better recovery from voltage disturbance.

State why the voltage dip is less in a self excited alternator than in brushless or conventional alternator.

Self excited compounded alternator [functional voltage regulating device] employs control by measuring alternator terminal voltage as well as by sensing changes in loading conditions by current transformer

A 3 winding 3 phase transformer adds together 2 component of current

The current obtained from the generator terminals is constant [for constant voltage] and lags the generated voltage by 90deg because of the effect of reactor.

The component obtained from the current transformer is proportional to and in phase with the load current.

The vector addition of these 2 current in the correct proportion give the correct excitation current under all operating conditions.

The common function is the use of the terminal a.c. voltage for establishing the no load voltage and using the load current as a means for providing the extra excitation needed to balance such thing as armature reaction and leakage reactance.

A low voltage dip requires a low sub transient reactance of the alternator winding but too low a reactance may involve high short circuit current

The self excited compounded alternator takes care of both and maintains a low voltage dip as well as keep the short circuit current within range of available protective gear.

In self excited conventional generator with error operated voltage control system only terminal voltage is measured and which governs the excitation control

The absence of load sensing makes it vulnerable to greater voltage dip and long voltage time.