IMM AGS-2
81
Mechanical high pressure equipment protection control
The high pressure equipment protection control is a single pole, pressure-activated switch
that opens on a pressure rise. When the switch opens, the control circuit is de-energized,
dropping power to the compressor and fan motor contactors. The switch is factory set (non-
adjustable) to open at 310 psig (2137 kPa)
±
7 psig and reclose at 200 psig (1379 kPa)
±
7
psig. Although the high pressure switch will close again at 200 psig (1379 kPa), the control
circuit will remain locked out and it must be reset through the MicroTech II control.
The control is mounted on the rear of the compressor. See page 71.
Compressor motor protection
The compressors are supplied with two types of motor protection. Solid state electronic
overloads mounted in the control box sense motor current to within 2% of the operating
amps. The MUST TRIP amps are equal to 140% of unit nameplate compressor RLA. The
MUST HOLD amps are equal to 125% of unit nameplate RLA. A trip of these overloads
can result from the unit operating outside of normal conditions. Repeat overload trips
under normal operation can indicate wiring or compressor motor problems. The overloads
are manual reset and must be reset at the overload, as well as through the MicroTech II
controller.
The compressors also have a solid state Guardister
circuit that provides motor over
temperature protection. The Guardister circuit has automatic reset and gives a Starter Fault
(F75) that is cleared through the starter display and must also be reset through the
MicroTech II control.
Head pressure control (standard)
The MicroTech II controller automatically cycles the condenser fans in response to
condenser pressure. Each fan in a circuit is cycled independently for 4, 5 or 6 steps per
circuit, depending on the unit size. This maintains head pressure and allows the unit to run
at ambient air temperatures down to 35
°
F (1.7
°
C). The settings are adjustable through the
controller.
Each fan added has a decreasing percentage effect, so the control pressure band is smaller
when more fans are on and largest with only one or two fans on.
Unit operation with the standard control is satisfactory down to outdoor temperatures of
35
°
F (-1.7
°
C). Below this temperature, the VFD option is required to regulate the speed of
the first fan on the circuit to adequately control the discharge pressure. The VFD option
allows unit operation to 0
°
F (-17.8
°
C) outdoor temperature, assuming no greater than 5-
mph wind.
Head pressure control (optional low ambient)
The optional low ambient control includes a variable frequency drive (VFD) on the first fan
on each circuit. The remaining fans cycle based on discharge pressure. This control must
be used for operation in ambient temperatures below 35
°
F (1.7
°
C) down to 0
°
F (-17.8
°
C).
NOTE
: VFD and standard fan cycling will provide proper operating refrigerant discharge
pressures at the ambient temperatures listed for them, provided the coil is not affected by
the existence of wind. Louvers must be utilized for low ambient operation if the unit is
subjected to winds greater than 5 mph.
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.
