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Marvair Scholar III™ VAISA I&S Manual 03/2019 Rev. 8
1.03 STANDARD CONTROLS
A.
Programmable Logic Controller (PLC) Microprocessor
Scholar III
TM
heat pumps and air conditioners use a factory installed PLC
microprocessor to control the operation, the safety switches and function
options. LED’s show operational status and provide assistance with diagnosis if
troubleshooting is ever required. Various control functions are field selectable. The
PLC is also capable of communicating to PLC’s in other Scholar units to allow run
time leveling and does not require additional equipment installed in the Scholar III
unit. The PLC microprocessor provides improved reliability because of the reduction
of components, the components utilized are more durable and the control box wiring
has been simplified. Pertinent statistical data about the life of the refrigeration system
can be accessed through the PLC.
The PLC microprocessor provides for the following control and operation functions:
• Anti-Short Cycle Timer
- Prevents the compressor from destructive short
cycling due to momentary power interruptions. One of three time intervals can
be field selected.
• Defrost Timer (heat pumps only)
- Adjustable defrost control that is based
upon both time and temperature. The time interval is adjustable from 30 to 90
minutes in one minute increments.
• BAS Control Relay
- Provides 24 VAC coil to control operation from Building
Automation System (BAS). Note - an additional BAS control relay can be
added when 120 or 240 VAC coils are required.
B.
High and Low Refrigerant Pressure and Loss of Charge Switches
These switches render the compressor and outdoor fan motor inoperative whenever
the limits of the pressure switches are exceeded. In the event of high pressure, the
Scholar III
TM
heat pump or air conditioner will turn off and lockout.
The low pressure (A/C) and loss of charge (HP) switch is bypassed for 8-1/2 minutes
during each start-up. The system will lockout if the low pressure switch opens three
times within one hour or if it does not reset within 8-1/2 minutes of the Scholar III
TM
heat pump or air conditioner start-up.
The high and low pressure and loss of charge switches are resettable at the wall
thermostat or by turning power off and then on to the Scholar III
TM
heat pump or air
conditioner. A fault LED located on the PLC indicates that a lockout has occurred
and whether it is due to high or low refrigerant pressure.
C.
Heat Related Controls
The
outdoor thermostat (heat pump only)
, factory set at 40°F, determines the
temperature at which the supplemental electric heat (or wet heat) turns on to heat the
classroom. The temperature set point can be adjusted in the field for local conditions.
When the outdoor thermostat activates wet heat, the compressor simultaneously turns
off.
