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		Do DC Drives Still Have A Place?  SCR based DC drives are low in price and high in reliability. Even though AC drives are attractive substitutes for DC drives and motors the large base of installed DC drives ensures a viable market for DC drive replacements and repair for years to come. The electronic component which most industrial DC drives are designed around is the Silicon Controlled Rectifier (SCR). SCRs are inexpensive and reliable.				Typical Stud Mounted SCR Used In DC Drives
		Hockey Puck SCR		Molded Case SCR Package
		The SCR is switched on, or "fired", by applying a positive voltage to the gate (the small lead on the top of the SCR at the top right). When the current passing through the SCR reaches zero, as it does every half AC cycle, the SCR naturally "switches off" and will not conduct again until another positive voltage is applied to the gate. By firing the SCR earlier in the cycle a higher average DC voltage is output. This type of voltage control is called Phase Angle Control.

Phase Angle Firing DC Voltage Control Using SCRs:
AC Wave Form	Full-Wave Rectified Wave Form
During one complete alternating current (AC) cycle current flows in a positive direction for half a cycle and in a negative direction for half a cycle.		Rectification redirects current flow during half of the cycle so that current always flows in one direction. This is called direct current (DC).
Firing The SCR Earlier or Later During The Cycle Raises and Lowers the Average Output Voltage
When the SCR is fired late in the cycle the period of time during which current flows is short and a low average DC voltage results. (The shaded area represents the time period during which current is flowing.)		By firing the SCR earlier in the cycle the period of time during which current flows is long and a high average DC voltage results.

Unlike the higher priced transistors used in an AC drive, an SCR cannot be easily "turned off". Generally, the control system just waits on the voltage across the SCR to pass through zero so that the SCR naturally quits conducting. Should a high current flow through the SCR be detected, which can result from a shorted output circuit, like what may happen when a DC motor armature fails, the SCR cannot be switched off. DC drive designers must rely on fast acting fuses in the power circuit, to protect equipment from catastrophic damage.

More information on DC drives to follow...