24VNA6 / 25VNA4: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
5
Variable Frequency Drive (VFD)
The VFD converts single phase 60 Hz line power into a variable
frequency, variable voltage, 3-phase output to drive the compressor at
the speed requested by the control system. This function is performed
by first converting the incoming line power to an intermediate high
voltage DC supply. The high voltage DC supply is then converted to a
3-phase output by a variable frequency inverter for driving the
compressor. The VFD also provides high voltage DC supply power to
the fan motor.
The VFD contains numerous protection modes for maximizing
reliability and preventing faults in wiring or the compressor and fan
components from causing failures within the VFD.
VFD Filter Board
The VFD filter board blocks electrical noise generated within the VFD
from back-feeding onto the line power feed to the equipment. It contains
non-replaceable fuses which will protect the VFD in the event of a
severe power surge condition.
Reactors
Reactors are part of the VFD incoming power conversion circuit. The
reactors enable boosting of incoming line power when necessary and
also provide power factor correction.
High Pressure Switch (HPS)
The HPS is a normally closed switch connected to VFD control circuitry.
The HPS opens when the compressor discharge pressure reaches 670
psig. When the switch opens, a control input to the VFD is disabled and
causes the compressor and fan to shut down. When compressor
discharge pressure decreases to 470 psig, the switch closes and allows
equipment operation to resume.
Compressor Brushless Permanent Magnet Motor
(BPM)
The compressor uses a highly efficient, brushless permanent magnet
motor. Motor speed is synchronized to, and controlled by the frequency
of the drive provided by the VFD.
VFD + Compressor BPM
Compressor runs at variable speeds controlled by the VFD.
The VFD controls acceleration and deceleration speeds of the
compressor. Speed changes are typically limited to 60 rpm per second.
The VFD will automatically limit compressor speed if compressor load
current reaches operating limits.
Fan Brushless Permanent Magnet Motor (BPM)
The fan uses a highly efficient, brushless permanent magnet motor.
Motor speed is synchronized to, and controlled by an on-board variable
frequency inverter (VFI) integrated with the motor and located within
the motor housing. The VFI uses the high voltage DC supply provided
by the VFD to drive the fan, and a low voltage control signal from the
VFD to determine fan speed. The fan provides a feedback signal to the
VFD indicating fan speed.
VFD + Fan BPM
The fan runs at a variable speed over a range of 200 rpm to 800 rpm
depending on air movement requirements.
The fan on-board VFI will automatically limit fan speed if fan load
current reaches operating limits.
Crankcase Heater
Some equipment has a crankcase heater band installed around the
compressor case near the bottom of the compressor. The heater is
energized when the outdoor ambient temperature drops below 65°F and
is de-energized when the outdoor ambient temperature goes above 85°F.
Heating the compressor prevents liquid refrigerant from accumulating in
the compressor by ensuring that the compressor is not the coldest part of
the system. The crankcase heater operates independently of indoor
equipment and System Control and functions as needed any time the
outdoor equipment is powered.
Some equipment has an internal stator heating function without a
crankcase heater. The compressor windings will occasionally be
energized during the OFF cycle to start the stator heat operation, thus
maintaining a sump temperature that is essential for compressor
reliability. The compressor will not run during this process. The
equipment that utilizes the stator heating function does not include a
crankcase heater, have PCM SW version 5.0 or higher, and utilizes new
VFD hardware and software. The VFD part numbers HR46VP002,
HR46VQ002, and HR46VR002 include the stator heating function.
With either crankcase heating or stator heating the system must be
powered on for a minimum of 24 hours prior to starting the system to
ensure that liquid refrigerant is not present in the compressor.
CONTROL SUB-SYSTEM
The control sub-system provides the connection point for low voltage
field communication and control wiring. The control sub-system
controls all operation of the equipment.
Primary Control Module (PCM)
The PCM is the heart of the equipment control system. The PCM is the
connection point for all control components except for the high pressure
switch and crankcase heater (when present).
The PCM receives equipment control requests from a communicating
Infinity Control or from a non-communicating control (traditional
thermostat) in an emergency operating mode.
The PCM receives signals from numerous sensors, including:
• 4 thermistors: OAT, OCT, OST, PDT
• 2 pressure transducers: SPT, DPT.
The PCM controls numerous actuators and signals, including:
• 3 control solenoids: PEV, RVS and LLS
• 2 electronic expansion valves: EXV-H, EXV-VI
• VFD Control Interface
• Emergency Mode 24VAC Control Signals: O and W
The PCM performs numerous diagnostics and supports 120 diagnostic
codes. Each of these codes is described in the Troubleshooting section.
Diagnostics cover the following:
• Line voltage and communication
• Equipment configuration (model plug, etc.)
• Compressor and fan operation
• Sensors
• Actuators
The PCM contains indicators and displays to provide operating status
and conditions. The PCM also supports communication with the Carrier
Service Tech App through the Bluetooth Module which provides
detailed diagnostic information and reprogramming capability.
