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Power Generation Riddle No.15 - Old excitation systems
Can some please explain the old excitation systems not the static type I have come across some a 1930 generators that have dc exciter on them with brushes and slip ring connections to the main rotor, from what I can see there are two sets of coils in the exciter stator one set of coils are connected in series with the commutator though the brush set, one of the cables connected to the main rotor slip rings is connected to the coils and the other onto the other DC brush set connected onto the exciter going to the other slip ring on the main rotor. The other set of coils are connected to the field flash circuit which is  also in parallel with this is the dc control (AVR) I presume that increasing and decreases the voltage on these coils they induce a flux that cuts into the commutator to excite the main rotor and make power. What are your thoughts.

Cheers Grant
Author : Grant - From: New Zealand
Thu, November 1st, 2012 - 23:33
Figure below shows one old and classic generator exciting system arrangement consisting of a main exciter with manual or automatic control of the field can be classified in a general way as “slow response” systems. The “regulator” in this case detects the voltage level and includes a mechanical device to change the control rheostat resistance. One such directacting rheostatic device (the “Silverstat” regulator) consists of a regulating coil that operates a plunger, which in turn acts on a row of spaced silver buttons to systematically short out sections of the rheostat.

In application, the device is installed as shown in Figure below. In operation, an increase in generator output voltage will cause an increase in dc voltage from the rectifier. This will cause an increase in current through the regulator coil that mechanically operates a solenoid to insert exciter field resistance elements. This reduces excitation field flux and voltage, thereby lowering the field current in the generator field, hence lowering the generator voltage. Two additional features of the system in Figure are the damping transformer and current compensator. The damping transformer is an electrical “dashpot” or antihunting device to damp out excessive action of the moving plunger. The current compensator feature is used to control the division of reactive power among parallel generators operating under this type of control. The current transformer and compensator resistance introduce a voltage drop in the potential circuit proportional to the line current. The phase relationship is such that for lagging current (positive generated reactive power) the voltage drop across the compensating resistance adds to the voltage from the potential transformer. This causes the regulator to lower the excitation voltage for an increase in lagging current (increase in reactive power output) and provides a drooping characteristic to assure that the load reactive power is equally divided among the parallel machines.

Author : Hamid - From: Iran
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