Page 332 - Solutions Manual to accompany Electric Machinery Fundamentals

SOLUTION
(a) When SCR is turned on, it will remain on indefinitely until it is forced to turn off. When SCR is turned
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1
on, capacitor C charges up to V DC volts with the polarity shown in the figure above. Once it is charged,
SCR can be turned off at any time by triggering SCR . When SCR is triggered, the voltage across it
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2
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drops instantaneously to about 0 V, which forces the voltage at the anode of SCR to be -V DC volts,
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turning SCR off. (Note that SCR2 will spontaneously turn off after the capacitor discharges, since V DC /
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R < I for SCR .)
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2
H
(b) If we assume that the capacitor must be fully charged before SCR can be forced to turn off, then the time
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required would be the time to charge the capacitor. The capacitor charges through resistor R , and the
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time constant for the charging is  = R C = (20 k)(15 F) = 0.3 s. If we assume that it takes 3 time
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constants to fully charge the capacitor, then the time until SCR can be turned off is 0.9 s.
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(Note that this is not a very realistic assumption. In real life, it is possible to turn off SCR with less than
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a full V DC volts across the capacitor.)
(c) SCR can be turned on again after the capacitor charges up and SCR turns off. The capacitor charges
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1
through R LOAD , so the time constant for charging is
 = R LOAD C = (250 )(15 F) = 0.00375 s
and SCR will turn off in a few milliseconds.
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(d) In this circuit, once SCR fires, a substantial period of time must pass before the power to the load can be
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turned off. If the power to the load must be turned on and off rapidly, this circuit could not do the job.

(e) This problem can be eliminated by using one of the more complex parallel commutation circuits described
in Section 3-5. These more complex circuits provide special paths to quickly charge the capacitor so that
the circuit can be turned off quickly after it is turned on.
S1-12. Figure S1-7 shows a single-phase rectifier-inverter circuit. Explain how this circuit functions. What are
the purposes of C 1 and C 2? What controls the output frequency of the inverter?

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