59
Table 39B — 5K Thermistor Temperature (°C) vs Resistance
SERVICE
Economizer Assembly —
Each circuit on
30XW175,200,350,400 units has an economizer assembly. The
30XW150,325 units do not have an economizer and have one
main electronic expansion valve. The 30XW150,325 units are
controlled the same way as units with a separate economizer
assembly. See Fig. 42.
Electronic Expansion Valve (EXV) —
See Fig. 43
for a cutaway view of the EXV. High-pressure liquid refriger-
ant enters valve through the top. As refrigerant passes through
the orifice, pressure drops and refrigerant changes to a 2-phase
condition (liquid and vapor). The electronic expansion valve
operates through an electronically controlled activation of a
stepper motor. The stepper motor stays in position unless pow-
er pulses initiate the two discrete sets of motor stator windings
for rotation in either direction. The direction depends on the
phase relationship of the power pulses.
The motor directly operates the spindle, which has rotating
movements that are transformed into linear motion by the
transmission in the cage assembly. The valve cone is a V-port
type which includes a positive shut-off when closed.
The large number of steps and long stroke results in very ac-
curate control of the refrigerant flow. The stepper motor has ei-
ther 4260 (main) or 2785 (economizer) steps.
MAIN EXV CONTROL — Each circuit has a thermistor lo-
cated in a well in the discharge line of the compressor (DGT)
and another one located in the compressor motor cavity (SGT).
Each circuit also has discharge and suction pressure transducer.
Discharge and suction pressure as measured by the transducers
are converted to saturated temperatures. The main control logic
for the EXV uses discharge superheat to control the position of
the EXV. The difference between the temperature of the
discharge gas and the saturated discharge temperature is the
superheat. The EXV module controls the position of the elec-
tronic expansion valve stepper motor to maintain the discharge
superheat set point.
TEMP
(C)
RESISTANCE
(Ohms)
3
14,026
4
13,342
5
12,696
6
12,085
7
11,506
8
10,959
9
10,441
10
9,949
11
9,485
12
9,044
13
8,627
14
8,231
15
7,855
16
7,499
17
7,161
18
6,840
19
6,536
20
6,246
21
5,971
22
5,710
23
5,461
24
5,225
25
5,000
26
4,786
27
4,583
28
4,389
29
4,204
30
4,028
31
3,861
32
3,701
33
3,549
34
3,404
35
3,266
36
3,134
37
3,008
TEMP
(C)
RESISTANCE
(Ohms)
38
2,888
39
2,773
40
2,663
41
2,559
42
2,459
43
2,363
44
2,272
45
2,184
46
2,101
47
2,021
48
1,944
49
1,871
50
1,801
51
1,734
52
1,670
53
1,609
54
1,550
55
1,493
56
1,439
57
1,387
58
1,337
59
1,290
60
1,244
61
1,200
62
1,158
63
1,118
64
1,079
65
1,041
66
1,006
67
971
68
938
69
906
70
876
71
836
72
805
TEMP
(C)
RESISTANCE
(Ohms)
–32
100,260
–31
94,165
–30
88,480
–29
83,170
–28
78,125
–27
73,580
–26
69,250
–25
65,205
–24
61,420
–23
57,875
–22
54,555
–21
51,450
–20
48,536
–19
45,807
–18
43,247
–17
40,845
–16
38,592
–15
38,476
–14
34,489
–13
32,621
–12
30,866
–11
29,216
–10
27,633
–9
26,202
–8
24,827
–7
23,532
–6
22,313
–5
21,163
–4
20,079
–3
19,058
–2
18,094
–1
17,184
0
16,325
1
15,515
2
14,749
TEMP
(C)
RESISTANCE
(Ohms)
73
775
74
747
75
719
76
693
77
669
78
645
79
623
80
602
81
583
82
564
83
547
84
531
85
516
86
502
87
489
88
477
89
466
90
456
91
446
92
436
93
427
94
419
95
410
96
402
97
393
98
385
99
376
100
367
101
357
102
346
103
335
104
324
105
312
106
299
107
285
6" MINIMUM
CLEARANCE FOR
THERMISTOR
REMOVAL
1.188 in.
2.315 in.
1/4-18 NPT
Fig. 40 — Dual Leaving Water Thermistor Well
(00PPG000008000A)
a30-4080
Fig. 41 — Typical Remote Space Temperature
Sensor (33ZCT55SPT) Wiring
7
8
TB6
SEN
SEN
SENSOR
a30-4081
O-RING
BRASS NUT 3/8 - 24 FOR
ASSEMBLY ON BRASS WELL
Fig. 39 — 5K Thermistor
(30RB660036 Thermistor Kit)
a30-4079