Page 328 - Solutions Manual to accompany Electric Machinery Fundamentals

SOLUTION First, assume that SCR is triggered. When that happens, current will flow from the power
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supply through SCR and the bottom portion of transformer T to the load. At that time, a voltage will be
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applied to the bottom part of the transformer which is positive at the top of the winding with respect to the
bottom of the winding. This voltage will induce an equal voltage in the upper part of the autotransformer
winding, forward biasing diode D and causing the current to flow up through capacitor C. This current
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causes C to be charged with a voltage that is positive at its bottom with respect to its top. (This condition
is shown in the figure above.)
Now, assume that SCR is triggered. When SCR turns ON, capacitor C applies a reverse-biased voltage
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to SCR , shutting it off. Current then flow through the capacitor, SCR , and the load as shown below.
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This current charges C with a voltage of the opposite polarity, as shown.

SCR will cut off when the capacitor is fully charged. Alternately, it will be cut off by the voltage across
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the capacitor if SCR is triggered before it would otherwise cut off.
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In this circuit, SCR controls the power supplied to the load, while SCR controls when SCR will be
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turned off. Diode D in this circuit is a free-wheeling diode, which allows the current in the load to
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continue flowing for a short time after SCR turns off.
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