Harvest Cycle - Scotsman MXG 327 Service Manual

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ATTENTION. In case, after 15 minutes
from the beginning of the freezing cycle,
the temperature of the evaporator sensor
probe is higher then 0°C (32°F) (shortage
of refrigerant, inoperative hot gas valve,
etc.) the P.C. BOARD switch OFF
immediately the
simultaneous blinking of the WARNING
RED LED.
The length of this second portion of the freezing
cycle is pre-fixed and related to the setting of the
first four DIP SWITCH keys.
The DIP SWITCH keys setting is made in
consideration of the type of condenser used.
In Table B are indicated the various lengths of the
second portion of freezing cycle (Time mode) in
relation to the different combinations of the DIP
SWITCH KEYS.
In Table A herebelow are illustrated the DIP
SWITCH key combinations for the three different
factory.
The electrical components in operation during
the freezing cycle are:
COMPRESSOR
FAN MOTOR (in air cooled version)
WATER PUMP
and during the second phase of freezing cycle
(Time mode) they are joined by the
ELECTRONIC TIMER
At average 21°C amb/15°C water temp. t he
refrigerant head pressure, in the course of the
freezing cycle, ranges between 12 and 10 bars
(175-145 psig) being controlled by the tempera-
ture sensor probe located within the condenser
fins (air cooled version) or, on the condenser
tube coil (water cooled version).
On the air cooled version, the condenser tempe-
rature sensor, when senses a rising of the
condenser temperature beyond the pre-fixed
limit, changes its electrical resistance and send a
low voltage power flow to the Micro Processor of
P.C. BOARD which in turn energizes, through a
TRIAC, the FAN MOTOR.
When the opposite situation occures, i.e. the
condenser temperature gets below the pre-fixed
limit, the temperature sensor changes again its
electrical resistance reducing therefore the current
flow to the P.C. BOARD to cause the fan motor
temporary cut-off.
NOTE. On this model the condenser sensors
are used just to switch off the unit in case the
condensing temperature rise up to more then
70 ° C (for air cooled version) or more then
62 ° C (for water cooled version) for one of the
following abnormal reasons:
CLOGGED CONDENSER (Air cooled
version)
FAN MOTOR OUT OF OPERATION (Air
cooled version)
INSUFFICIENT FLOW OF COOLING
WATER (Water cooled version)
AMBIENT TEMPERATURE HIGHER THEN
° °
40 C (100 F)
unit with the
it causes the total and immediate SHUT-OFF
of the machine in order to prevent the unit
from operating in abnormal and dangerous
conditions.
When the ice maker stops on account of this
protective device, there is a simultaneous
glowing of the RED LED, warning the user of
the Hi Temperature situation.
After having efiminated the source of the
condenser hi-temperature, to restart the
machine just switching OFF and ON the unit
at main line switch.
The ice machine resumes its normal operation
by going through the 5 minutes water filling
phase.
MXG refrigerant operating pressures (suction/
discharge) may change according to models.
See related table at pag. 25.
The total length of the freezing cycle ranges f rom
20 to 25 minutes.
DEFROST OR HARVEST CYCLE (Fig. D)
As the electronic timer has carried the system
throughout the second phase of freezing cycle,
the defrost cycle starts.
ATTENTION. In case the unit be able to
° °
reach 0 C (32 F) evaporating temperature
within 15 minutes, but after 45 minutes
from the beginning of the freezing cycle it
has not yet reached the evaporator tem-
° °
perature of -15 C (5
straight into the defrost cycle omitting
the timed portion of the freezing cycle
relied to the setting of the first four DIP
SWITCHES.
NOTE. The length of the defrost cycle is
related to the length of the second phase of
freezing cycle T2. (Time to drop the
evaporating temperature from 0 ° C (32 ° F) -
small Red LED blinking-to-15 ° C (5 ° F) small
Red LED ON steady.
It is possible to extend the length of the defrost
cycle by changing the setting of DIP SWITCH
7 and 8 as shown on table at page 28.
The electrical components in operation during
this phase are:
COMPRESSOR
CONTACTOR COIL
WATER INLET VALVE
HOT GAS VALVE
WATER DRAIN VALVE
and the
WATER PUMP
on the first 30 seconds.
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F) the machine goes