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DESCRIPTION
The 110-Z3 is a proportional, zero voltage switched, two leg controlled, three phase power
controller capable of delivering 0-99% of the applied line voltage to directly connected electric
heaters. It will not control high inrush loads (tungsten lamps), transformers or motors. This is
accomplished by time proportionally switching SCR's on and off. Modular construction allows
for simple and solderless field serviceability. There are four field replaceable items:
printed circuit board
fuses
SCR packages
MOV's (metal oxide varistors)
THEORY OF OPERATION
The standard input signal (4-20mA) is applied to the blue input terminal block, processed and fed to
optical couplers. The output of each opto coupler controls its respective SCR. An I²T fuse connected in
series with the input provides over current protection in the event of external wiring shorts, or too large of
a heater load. Additionally, a MOV (metal oxide varistor) is connected in parallel with each SCR,
providing voltage spike protection to the controller.
Zero voltage switching controls proportionally turn on and off each full cycle of the power line. By
varying the number of AC power line cycles, the SCR provides power to the heaters. With a variable
time base, the optimum number of cycles turned on/off is achieved. This method produces less RFI line
noise than phase angle fired SCRs. Power factor is 100%, while harmonic distortion is limited. Charts
below, left and center show 25% power output using variable time based zero voltage switching. The
chart on the right shows 50% output.
INDEECO 110-Z3 variable time base SCR firing. Note how half cycles can be turned on and off
providing 0.08 second resolution on 60Hz power lines.
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