Electric-driven Air Compressor Troubleshooting

The electric-driven air compressor has a problem and not starting in the auto mode. Do the troubleshooting and identify the issue.

Suppose this electric-driven air compressor refuses to start when the switch is in the “Auto” position, but starts up immediately when the switch is placed in the “Hand” position.

Electric-driven Air Compressor

Electrical Air Compressor

The first test performed by a technician is to measure AC voltage between test points A and F with the switch in the “Auto” position. There, the meter registers 117 volts AC. You are then called in to help:

Electric-driven Air Compressor Troubleshooting

Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. no coincidental faults), determining whether or not each fault could independently account for all measurements and symptoms in this circuit.

Identify the below mention faults are possible or impossible?

PSHH failed open
PSH failed open
PSL failed open
“Hand” switch position failed open
“Auto” switch position failed open
OL contact failed open
Auxiliary “M” contact failed
Contactor “M” coil failed open

Also, comment on whether or not the initial test between points A and F was a useful one (i.e. did it provide any new information to help diagnose the problem?).

Answer:

The initial test between points A and F was useless. We already knew from the symptom of the compressor running in “Hand” but not in “Auto” that the fault must be an open, and it must lie between the Hand/Off/Auto switch and test point A somewhere.

An open fault anywhere between points A and F would of course drop the full control voltage, so the measurement of 117 volts AC should come as no surprise.