first and, then, the discharge cycle. Before starting, assume a pulse has
just been developed across the load. Hence, storage capacitor Cst is fully
discharged. Also, there is no current flowing through charging choke Lch.
c. At the start of the charging cycle, storage capacitor Cst begins to
charge to the 6KV (6,000-volt) source.
Current flow through the choke in
Part A of Figure 61 sets up a magnetic field. The charge cycle is slow at
first because of the opposing effect of the choke.
Remember, a coil
produces an induced voltage which opposes changes in current flow.
Since
current is increasing, the induced voltage produced by the expanding
magnetic field opposes the supply voltage.
d. Now, as Cst reaches its initial charge of 6KV, current flow starts
to drop off, causing the magnetic field of the choke to collapse.
Once
again, the self-inductance of the choke opposes this change in current flow.
The collapsing magnetic field produces an induced voltage which keeps
current flowing in the same direction as before. Part B of Figure 60 shows
that this induced voltage, which is 6KV, is in series with the power supply,
and so aids the charging current. Notice, now that 12KV is applied to the
storage capacitor instead of the 6KV at the start of the cycle. Therefore,
Cst charges to 12KV or double the voltage of the source.
e. Part C of Figure 60 shows what happens in the circuit if the switch
never closes. Cst won't stay charged because Lch and Cst form a resonant
circuit.
Therefore, capacitor voltage
and
inductor
current
vary
sinusoidally as shown in Part C. Notice that capacitor voltage is maximum
when inductor current is zero. (See dotted vertical lines in Part C.) This
means that the induced voltage of Lch is maximum when inductor current is
zero.
Recall that the amount of induced voltage depends not only on the
size of the inductor, but also on how fast the current is changing. And,
current changes fastest when it is going through zero. So Cst charges to
twice the source voltage because of the additional induced voltage in series
with the source voltage.
75
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