Question { 4135 }

In all the generators, the excitation supply is DC. What is
the significance of DC excitation? What will happen if AC
supply is given as excitation supply?

Answer

Not all excitation systems use DC. There are excitation
systems which use high frequency AC as the input to the
generator field. There are a couple of reasons for this.
One is to prevent slip rings (if they are installed) from
excessive wear. Second, the regulator can be built smaller
and react quicker with less power loss. There are other
reasons. Will leave the rest for you to ponder.

Question { 25974 }

What is the difference between a Vacuum Circuit Breaker and
a Load Break Switch?

Answer

Both can be used to isolate a load.

Both can have protective relays associated with them
although load breaks usually are fused for quicker
instantaneous isolation times (At or less than 3 cycles)

Vacuum breakers usually are used for isolating at a
switchgear (specific equipment or portions of the
distribution system) They make or break their circuit under
a vacuum. Load breaks normally do this in the air.

Load breaks have heavy contacts which are designed to open
under full load repeatedly.

Vacuum breakers can have the same but are normally not
except for vacuum contactors.

Question { United Phosphorus, 42937 }

why circuit breaker closing time is always higher then
tripping / opening time.

Answer

Neither of you are correct and for that matter who really
cares which is greater?
The opening and closing times of circuit breakers are
based on mechanical variables such as charging spring
tension, contact gap, trip mechanism delay etc. The goal
for a circuit breaker is to make or break the contacts as
quickly as possible to either slam the device on line or
open the contacts quickly enough to minimize the time that
an arc exists at the contacts.
What is significant are two measurements that provide an
indication of what the actual operating characteristics of
the circuit breaker are.
First, connect the contacts in series with one another
and a timer such that the circuit goes timer, 1st contact
set, 2nd contact set, 3rd contact set timer. (In one out
the other). Time the opening and closing of the circuit
breaker contacts. This will provide the fastest set of
contact make/break time.
Second, connect the contacts so that they are in
parallel with eachother record the value (This is the
average open/close time).
Once you have done this several times, say about 20,
take an average of each of the first/second measurement
times. Each manufacturer of circuit breakers has published
values for what those readings should be (average.
The significant readings of circuit breaker testing are
not usually related to the timing of the contacts, yes the
contacts can cause a motor to briefly single phase for less
than maybe 2 complete power cycles but not enough to really
matter, the significant measurements are those of mico-ohm
or millivolt drop across the contacts and closing spring
tension.