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Electrical Theory & Applications for HVACR
Chapter 3: Motors
Page 61
A start capacitor can be removed from a start winding circuit by a centrifugal switch, current relay,
PTC, potenƟal relay (voltage relay), or an electronic Ɵming device.
IDENTIFYING HERMETIC MOTOR TERMINALS
Technicians will oŌen have to idenƟfy run (R), start (S), and common (C) terminals or check hermeƟc
motor windings for an open, a short, or a grounded condiƟon.
Note: When checking hermeƟc compressors, remember that terminals are under high pressure and
could blow out of the shell, causing serious injury or death.
If any of the following condiƟons is found, the motor must be replaced:
Any set of terminals measure 0 ohms (the winding is shorted)
Any set of terminals measure infinity (the winding is open)
Each terminal must be checked to ground. If a measureable resistance is detected, the motor is
grounded.
If the internal overload is open, there will be measureable resistance between start and run
terminals and infinity between common to start or run. The compressor should be allowed Ɵme
to cool and rechecked.
Fig. 3‐18(a)
COMMON
Fig. 3‐18(b)
RUN
Fig. 3‐18(c)
START
Fig. 3‐18(d)
Step 1:
Ohm each set of terminals with all wiring
removed and record the informaƟon as shown in
Figure 3‐18(a).
Step 2:
The terminals with the highest ohm
measurement (between run and start) idenƟfy
the other terminal as the common, shown in
Figure 3‐18(b).
Step 3:
Place one test lead of the ohmmeter on
the common terminal and the other on the
terminal with the lowest measurement. This is the
run terminal. See Figure 3‐18(c).
Step 4:
The remaining terminal with the
middle resistance measurement is the start
terminal, shown in Fig. 3‐18(d).