Manitowoc J-1300, Руководство по обслуживанию

Страница: 94 / 165

Section 6 Electrical System
6-43
DISCHARGE LINE THERMISTOR
NOTE: The discharge line thermistor is not used on
later production J-model ice machines. The part has
been removed due to the redundancy of safety limits
3 and 4. Any fault that would stop the ice machine on
safety limit 3 or 4 would also stop the ice machine on
safety limit 1 or 2.
As an example, consider an expansion valve that is
overfeeding refrigerant, causing low discharge line
temperatures in the harvest cycle. This would stop the
ice machine on safety limit 3. But, due to decreased
harvest temperatures, the ice machine will stop on
safety limit 2 (harvest cycle exceeds 3.5 minutes)
with safety limit 3 removed.
When replacing an old style control board with a new
style control board, the thermistor will no longer be
used.
Function
The discharge line thermistor senses the compressor
discharge line temperature. This is used in
conjunction with the control board safety limits to
stop the ice machine if the discharge line temperature
falls below 85
°
F (29.4
°
C) or rises above 255
°
F
(123.9
°
C).
Specifications
100,000 Ohms
±
2% at 77
°
F (25
°
C)
CAUTION
Use only Manitowoc thermistors.
Check Procedure
Thermistors generally fail because of moisture or
physical damage. Manitowoc J-Model discharge line
thermistors are encased in a specially-designed,
moisture sealed aluminum block. This eliminates
physical damage and moisture concerns.
Verify that the thermistor resistance is accurate and
corresponding to the high and low temperature
ranges.
1.
Disconnect the thermistor from terminals 1A and
1B on the control board.
2. Connect the ohm meter to the isolated thermistor
wire leads.
3. Acquire a temperature meter capable of taking
readings on curved copper lines. Attach the
sensing device to the compressor discharge line
next to the thermistor aluminum block.
Important
Do not simply “insert” the sensing device under the
insulation. It must be attached to and reading the
actual temperature of the copper line.
4. With the ice machine running, verify that the
temperature of the discharge line corresponds to
the thermistor resistance reading as stated in the
chart on the next page.
Check the thermistor at high temperatures (during the
freeze cycle) and at low temperatures (during the
harvest cycle). It is normal for the compressor
discharge line temperature to rise during the
freeze cycle and drop during the harvest cycle.
NOTE: If the ice machine is inoperable, remove the
thermistor and place it (for a short time) in an ice
water bath, and then in a boiling water bath, to verify
its accuracy. See the chart on the next page.
5. If the thermistor would fail closed , the ice
machine would stop on safety limit #4 15 seconds
after contact #4 on the control board closes
(compressor starts).
6. If the thermistor would fail open , the ice
machine would start and run through two normal
freeze and harvest sequences. During the third
harvest sequence, the ice machine would stop on
safety limit #3.