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e sufficient to cause conduction of the zener diode and to produce the necessary error signal
to Magnetic Amplifier for current control.
Zener diode starts conducting only at a designated voltage (zener voltage). The
voltage across the zener will be maintained even if the voltage input to the circuit is
increasing. Thus, it serves as a base for comparison.
Blocking Diodes (Dl and D2)
Diodes Dl and D2 are used to block the current from one zener to the other. Diode D1
prevent creepage of current from DT2 to DT l and D2 prevents current from DT1 to DT2.
This is achieved by the unidirectional property of diodes.
Working of Regulator:
The three phase output from the alternator is rectified by the bridge connected silicon
diodes. The DC excitation to the field is obtained by full wave rectification of alternating
current provided through the field transformer and the load windings of the magnetic
amplifier.
The voltage induced in the alternator winding is dependent on the speed of revolution
of rotor and on the excitation current. In the absence of voltage detector and magnetic
amplifier, the voltage of the alternator will rise indefinitely due to the positive feed back
limited only by saturation of stator. But as soon as the pre-set voltage is reached, the zener
diode in detector DT1 conducts and sends a "Control current" through the magnetic amplifier
windings 10-11. The flux produced by the control current is in such a way that it opposes the
flux produced by the load windings, thereby increasing the impedance of field circuit. This
increase in field impedance reduces the field current and brings back the output voltage to the
normal value required,
The current limiting is also achieved in a similar manner. When the pre-determined
load current is delivered by the alternator, the secondary voltage of the CT after rectification
by bridge RT2 will provide the necessary "error signal" for the magnetic amplifier. In this
case also the voltage drop across the resistance R 1 will be sufficient to cause the zener diode
in DT2 to conduct. The control current from this also passes through the same control
winding 10-11. The effect of this control current is to retain the current at the limited value
and to reduce the voltage. For a sustained over-load, the generator voltage will fall to the
battery voltage and relieve the alternator immediately, thereby reducing the chances of
damage due to the load.
2.2.4.1 CURRENT Vs VOLTAGE CHARACTERISTICS:
The equipment is designed to give DC side output upto a maximum current of 140
A. The graph at Fig. 15 shows the machine characteristics at 1500 rpm with a light load on
the machine.
GENERAL SERVICES : TRAIN LIGHTING 23