THEORY OF OPERATION
4-7
Versapulse Select Service Manual
0621-499-01 01/94
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4.3 COOLING
Refer to the Coolant System Simplified Diagram, figure 4.1. As the lamps in the laser head are flashed, heat
energy is produced in the flash lamps, rods, and housings. The cooling system transfers the heat energy from
the flash lamp, rod and housings to the outside air. It includes a de-ionized water coolant loop with a forced
air heat exchanger. A speed controlled fan forces air through the heat exchanger. The cooling system also
provides cooling for the two beam dumps.
The coolant is maintained nonconductive (de-ionized) to minimize corrosion in the cooling loop.
Coolant flow is monitored as a part of the 24 VAC loop to the main contactor. If flow is not sufficient the
system will not start, and if already started it will shut off.
Coolant temperature is monitored by the control electronics. The software checks the coolant temperature at
regular intervals, providing overtemperature disable (F52 and "OVERHEATING displayed at touch screen)
when the coolant temperature reaches 35˚C. The disable clears when coolant temperature falls to 32˚C.
Coolant conductivity is monitored by the control electronics. The software checks the coolant conductivity at
regular intervals, and will disable the system (F6) if the conductivity exceeds software limits. Note that the
conductivity is effected by the action of the D/I filter. If the conductivity is excessive, running the system in
its standby condition will allow water circulation through the D/I filter, eventually lowering the conductivity
to an acceptable level. Conductivity will tend to be higher when the unit is first turned on, then decrease as
the D/I filter removes charged particles. The D/I filter is a consumable item, and should be changed after
about six months of use, or sooner if coolant conductivity problems occur.
The coolant pump operates continuously and at a constant speed from turn on to turn off. It circulates the
coolant through the closed loop. A fan drives air over the heat exchanger. Fan speed is controlled by the Fan
Speed Control PCB. The coolant pump must never be run dry.
The differential fan speed controller uses phase angle control of the AC line voltage to control the speed of the
fan. The fan speed is controlled to minimize fan noise. The speed controller monitors air temperature into
the fan and the coolant temperature at the point where it is hottest (where it exits the cavity module and is
returned to the heat exchanger). As the coolant temperature rises above the ambient air temperature, the
controller acts to increase the fan speed. The line voltage for the fan is supplied through a solid state relay on
the PCB. When the relay is on, the line voltage is connected to the fan. When the relay is off, the line voltage
is disconnected from the fan. Control circuitry on the PCB switches the relay on for only a portion of each AC
cycle - as coolant temperature increases above air inlet temperature the relay is left on for a longer portion of
the AC cycle, increasing the electrical power to the fan (the fan spins faster).
(Refer to 8-17) The DC voltage output of U4-8 sets the duty cycle for the SCR1 and SCR2 solid state relay
circuit. U4-1 provides a negative voltage proportional to coolant temperature. U4-7 provides a positive
voltage proportional to air inlet temperature. When coolant temperature and air inlet temperature are the
same, the two voltages are equal in magnitude, and cancel each other out. The output of U4-14 and the
setting of potentiometer R7 sets the minimum duty cycle. As coolant temperature increases, the output of
U4-1 increases, and the control voltage out of U4-8 increases. The result is that the solid state relay remains
on for a longer portion of the AC cycle, and the fan speed increases.
12/95
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