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Marvair MGA3090-3180 Wall Mount AC I&O Manual 05/2023 Rev.14
1.
Start-Up Voltage
: Measure the voltage at the compressor contactor during start-up. The voltage
must exceed the minimum shown in Table 5, section 2.2, or compressor failure is likely. A low
voltage condition must be corrected.
2.
Running Amperage
: Connect a clip-on type ammeter to the (common) lead to the compressor. Turn
on the supply voltage and energize the unit. The compressor will initially draw high amperage; it
should soon drop to the RLA value or less. If the amperage stays high, check the motor winding
resistances.
NOTE:
Feel the top of the compressor to see if it has overheated. If it is hot, the internal overload
may be open. You may have to wait several hours for it to reset.
3.
Motor Winding Resistances:
Using a digital volt-ohm meter (VOM), measure the resistance across
the compressor windings as shown below.
C
S
R
R
2
R
1
R
3
SINGLE
PHASE
R
3
> R
2
> R
1
R
3
= R
2
+ R
1
T
1
T
2
T
3
R
2
R
1
R
3
THREE
PHASE
R
3
= R
2
= R
1
Resistance can be measured as shown above. Any deviation from above values could indicate a
defective compressor.
4.
High Voltage/Insulation Test:
Test internal leakage with a megoh meter. Attach one lead to the
compressor case on a bare metal tube and to each compressor terminal to test the motor windings.
A short circuit at high voltages indicates a motor defect. Do not do this test under vacuum.
5. On single phase models, check the capacitor by substitution.
