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AUXILIARY WINDING
The auxiliary winding is another winding inside the stator slots. It's electrically
independent from the main stator winding and its function is to supply an independent
source of power to the AVR for a constant voltage.
There are two big advantages when a generator is supplied with the aux winding:
1) The voltage is constant during a short circuit: In a generator without the aux winding
the AVR is taking power from the main terminals. When you have a short circuit on the
main terminals of the generator the AVR is not able to get power from the main terminal.
For this reason the AVR doesn’t work and the excitation system is de-powered. The
generator switches off and you are not able to recognize that there is a short circuit on the
generator terminals. If you want to be sure to recognize that there is a short circuit, the
generator should be able to supply during a short circuit, a short circuit current equal at
least 3 times the nominal current. You can use electronic devices connected to the AVR
(like the Varicomp device in the M8B series) or the aux winding. When you use an
electronic device there is always the possibility that the electronic device doesn't work
and there is also a little delay before its activation. With the aux winding you don't have
these problems: the voltage is simply always the same. With a constant voltage supplied
to the AVR you have a short circuit current equal 3 times the nominal current.
2) The voltage is constant during a transient phase: In a generator without the aux
winding the AVR is taking power from the main terminals. When you have a load
suddenly applied to the generator the voltage of the generator decreases to a value
typically equal to 80 - 90% of the nominal value (depending on the value of this load).
This voltage drop depends only on the electromagnetic characteristics of the generator
and doesn't depend on the way to supply power to the AVR (from the main terminals or
from the aux winding). But a generator with the aux winding is better after the first phase
when the voltage is less than the nominal value, the voltage that the AVR is taking from
the aux winding is equal to the nominal value, also if in the main terminals there is a
reduced value. On a generator without aux winding, the AVR is taking a voltage that is
less than the nominal value! For this reason in the generator without aux winding just
when the AVR needs more power because it has to increase the voltage to the nominal
value, it is taking less power because the voltage is less than the nominal value. On a
generator with aux winding the voltage is equal to the nominal value also in this situation
so the AVR can work better than without aux winding. For this reason the transient
behavior of a generator with aux winding is better because the reaction of the AVR is
stronger. In the end, the generator is able to come back to the nominal value of the
voltage in a period of time shorter than a generator without aux winding or on the other
hand you can apply a bigger load with the same period of time of reduced voltage.