Carrier 50MQ - Manual (Page 6)

START-UP
The 50MQ units are equipped with a crankcase
heater. It is recommended that heater be energized
a minimum of 24 hours before starting unit. To
energize heater only, turn the thermostat to “Off’
position and close electrical disconnect to 50MQ
unit.
Heat Anticipator Settings for Room Thermostat
(HH07AT070)
— Set anticipator settings for room
thermostat according to Table 3. These settings
may be changed slightly to provide a greater degree
of comfort for a particular installation.
Accessory Outdoor Thermostat provides adjustable
outdoor control of accessory electric heater over
10 kw. This thermostat makes contact when a drop
in outdoor temperature occurs. It energizes a
second stage of electric heat when the outdoor
temperature setting is reached, provided the room
thermostat is on the second stage of heating. Refer
to heat load of building for correct outdoor
thermostat setting. The accessory emergency heat­
er relay is manually operated to lock out compres­
sor and bypass the outdoor thermostat for electric
heater operation during heat pump shutdown. See
88EH electric heater Installation, Start-Up and
Service Instructions for installation of outdoor
thermostat and emergency heater relay.
To Start Unit — (Ensure crankcase heater has been
energized for 24 hours.) Adjust the thermostat as
follows;
1. Set selector switch at “Off.”
2. Set fan switch as desired (“Fan”) (“Auto.”).
3. Turn on main disconnect switch(es) to unit.
4. Set thermostat dial to the desired temperature.
5. Set selector switch at “Heat” or “Cool.” Check
system refrigerant charge. See Refrigerant
Charging.
50MQ Unit Single-Phase Compressors of the split
capacitor (PSC) type require an equalized system
pressure to start. When supply voltage is within
permissible limits and compressor does not start,
give compressor a temporary capacitance boost.
Use an 88-108 microfarad start capacitor with a
bleed resistor wired across the terminals. Connect
wires with insulated probes to each capacitor
terminal. Touch probes to each side of run
capacitor or to compressor motor terminals R and
S. Start compressor;
pull probes away after 3
seconds Discharge start capacitor. (Two or more
bumps may be necessary to start compressor.) Run
compressor for 20 to 30 minutes, then shut off and
allow system pressure to equalize. Try restarting
without boost capacitor. If after 2 attempts (with­
out boost capacitor) the compressor does not start,
add an accessory start capacitor relay package.
UNIT CONTROLS
High-Pressure Relief Valve (Safety Control) is
located in compressor. Relief valve opens at a
Table 3 — Thermostat Anticipator Settings
FIRST-STAGE
ACCESSORY
ELECTRIC
HTR (Kw)
SECOND-STAGE
UNIT
ANTICIPATOR
SETTING
ANTICIPATOR
SETTINGS i
240 V 208 V
5.0 3.75" 4 *
50MQ022, 7.5 5.6 56 !
027,032, .4
10
0 7 5 56
037 15.0 1 1.3 .4 ;i
20
0 15.0 .4 1
pressure differential of approximately 600 psi
between suction (low side) and discharge (high
side) to allow pressure equalization. The valve also
permits pressure equalization when high-side —
low-side pressure differential reaches 90 psig at
shutdown. Compressor can then start unloaded. A
hissing sound during pressure equalization indicates
pressures are equalizing and does not indicate bad
valves.
Internal Current and Temperature Sensitive Over­
load (Safety Control) resets automatically when
internal compressor motor temperature drops to a
safe level (overloads may require up to 45 minutes
to reset). When an internal overload is suspected of
being open, check by using an ohmmeter or
continuity tester. If necessary, refer to Carrier
Standard Service Techniques Manual, Chapter 2,
for complete instructions.
Defrost Control, consisting of a defrost timer,
defrost thermostat and defrost relay, interrupts
normal system heating operation if enough frost
forms on outdoor coil to impair unit performance.
Defrost control simultaneously stops outdoor fan,
de-energizes reversing valve solenoid to return
system to cooling cycle (outdoor unit as con­
denser, indoor unit as evaporator), and activates
accessory electric heater. Unit can defrost every 90
minutes, but will defrost only if required.
For the heat pump to defrost, 2 conditions are
necessary:
1. Defrost timer contacts must be closed.
2. Refrigerant temperature from outdoor unit
must be cold enough to cause defrost ther­
mostat contacts to close. Contacts close at 35
(+0, -6) F.
Every 90 minutes of elapsed running time, the
defrost timer contacts close for 10 seconds. If the
defrost thermostat contacts are closed, the unit
defrosts. The defrost timer limits defrosting period
to 10 minutes. Normally the frost is removed and
the defrost thermostat contacts will open to
terminate defrosting before 10 minutes have
elapsed. Defrost thermostat contacts open at 65
(+0, -6) F. When defrosting is terminated, the
outdoor fan motor and reversing valve solenoid are
energized returning unit to heating cycle.
HEAT PUMP CIRCUITS shown in Fig. 8 are
refrigerant flow diagrams for heating and cooling
cycles.