Operating instructions

70 IMM AGSB-60
The head pressure control will provide proper operating refrigerant discharge pressures at the
ambient temperatures listed for it, provided the coil is not affected by the existence of wind.
Wind baffles must be utilized for low ambient operation if the unit is subjected to winds
greater than 5 mph.
Low ambient start
Low ambient start is incorporated into the MicroTech II controller logic. The MicroTech II
controller will measure the difference between freezestat and evaporator pressure and
determine the length of time that the compressor will be allowed to run (to build up evaporator
pressure) before taking the compressor off line. The danger of allowing the compressor to run
for too long before building up evaporator pressure is that the evaporator could freeze.
Phase/voltage monitor
The phase/voltage monitor is a device that provides protection against motor loss due to power
failure conditions, phase loss, and phase reversal. Whenever any of these conditions occur, a
Normally Closed contact opens in the external fault circuit of the starter, generating a F75 fault
code that then de-energizes all inputs. The F75 code is interrupted by the MicroTech II
controller as an external fault and must be cleared through the MicroTech II control.
When proper power is restored, contacts close and the fault must be cleared through both the
starter keypad and the MicroTech II control.
When three-phase power has been applied, the output relay should close and the "run light"
should come on. If the output relay does not close, perform the following tests.
1. Check the voltages between L1-L2, L1-L3 and L2-L3. These voltages should be within
2% of each other and within +10% of the rated three-phase line-to-line voltage.
2. If these voltages are extremely low or widely unbalanced, check the power system to
determine the cause of the problem.
3. If the voltages are within range, use a phase tester to verify that phases are in A, B, C
sequence for L1, L2 and L3. Correct rotation is required for compressor operation. If
incorrect phase sequence is indicated, turn off the power and interchange any two of the
supply power leads at the disconnect switch.
This can be necessary as the phase/voltage monitor is sensitive to phase reversal. Turn on the
power. The output relay should now close after the appropriate delay.
Compressor short cycling protection
The MicroTech II controller contains logic to prevent rapid compressor restarting. Excessive
compressor starts can be hard on starting components and create excessive motor winding
temperatures. The anti-cycle timers are set for a five-minute stop-to-start cycle and a 20-
minute start-to-start cycle. Both are adjustable through the MicroTech II control.
There is also a timer with a 5 minute default for minimum time between any two circuit starts.