Compressor Will Not Stay Running: Tracing the Oil-Pressure Rung
The compressor control ladder has four rungs. Rung 1: L1 - STOP pushbutton (NC) - high-pressure cutout (NC, manual reset) - low-pressure cutout (NC) - oil-pressure switch (NC, held closed only when oil pressure is proven; a 90-second timer opens the contact if it is not) - CR1 coil - N. Rung 2: a CR1 normally-open contact is wired in parallel with the START pushbutton (NO); this parallel pair is the only path around the STOP button. Rung 3: CR1 normally-open contact in series with the motor starter coil M. Rung 4: an M auxiliary contact in series with an amber RUN lamp. Symptom: the operator presses START, CR1 picks up and seals in, M energizes and the compressor starts; at 90 seconds CR1 drops out and the compressor stops. Oil pressure at the compressor reads 22 psig; the OEM requires 25 psig minimum oil pressure differential.
1. Trace the circuit and explain exactly why the compressor stops at 90 seconds. What is the timed switch actually proving, and is it doing its job?
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Trace:
- Pressing START completes rung 1 through STOP, the HP cutout, the LP cutout, and the oil-pressure switch to CR1. CR1 picks up.
- CR1's seal-in contact on rung 2 holds rung 1 energized after START is released, so the circuit does not depend on the operator holding the button.
- CR1's contact on rung 3 energizes M, the compressor starts, and the RUN lamp lights from the M auxiliary on rung 4.
- The only timed element anywhere in the ladder is the 90-second timer on the oil-pressure switch. At 90 seconds CR1 drops, which drops M.
Conclusion: the seal-in held for 90 seconds, so the STOP button, HP cutout, LP cutout, seal-in contact, and control power are all good. The oil-pressure switch opened because oil pressure was never proven within the time window.
What the switch proves: differential pressure, not gauge pressure. The gauge reads 22 psig against a 25 psig minimum differential - 3 psi short. If the gauge is a case-pressure gauge, the differential is even lower once suction pressure is subtracted. Either way it is not proven.
Is it doing its job? Yes - this is a correct protective trip, not a nuisance. The fault is upstream of the switch: low oil level, a worn oil pump, a clogged oil strainer, diluted or foamed oil, air in the oil pump suction, or wrong rotation on a replaced motor. Do not blame the switch until a second gauge confirms the differential.
2. The night mechanic wants to jumper the oil-pressure switch to finish the production run. Evaluate that proposal and state what you would do instead.
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Refuse it, and remove any jumper found.
Why:
- The oil-pressure switch is the machine's only protection against running with no oil film. At the pressures and speeds of an industrial ammonia compressor, bearing and crank damage can occur in minutes, well before any temperature alarm responds.
- The failure mode is not just an expensive rebuild. A seized or scored compressor can progress to a mechanical failure that breaches the refrigerant boundary - an ammonia release in an occupied plant.
- A jumper left in place is invisible to the next shift. It converts a protected machine into an unprotected one and is a reportable safety violation in most plants.
The one defensible use of a jumper is a supervised, momentary functional check of the rest of the control circuit, made by a qualified person, logged, and removed before leaving the panel. That is not what is being proposed.
Instead:
- Confirm the differential with a second, calibrated gauge on a separate port.
- Check oil level, oil temperature (cold oil will not build pressure), and the oil pump suction for air or foam.
- Check the oil strainer and pump for wear, and confirm rotation if the motor was recently replaced.
- Correct the cause; if the machine cannot make oil pressure, it does not run. Carry the load with another machine or accept the production loss - the product is cheaper than the compressor.
3. If instead CR1 did not pick up at all when START was pressed, describe how you would use a voltmeter to locate the open and the reading that proves each section is good.
Read the worked answer
Method: measure across each device while the circuit is complete (START held or seal-in closed). A device with full control voltage across it is the open; a closed device reads approximately zero volts across it.
Work from L1 through the rung:
- L1 to neutral at the control transformer secondary - should read full control voltage (e.g., 120 V). No reading means a blown control fuse, tripped control breaker, or open transformer; nothing downstream can work.
- Across STOP (NC) - press and hold START. Reading ≈ 0 V means STOP is closed and passing. Full voltage means the STOP button contact or its wiring is open (including a wire off the back of the button).
- Across the HP cutout (NC, manual reset) - full voltage means it is tripped and needs resetting, or the contact is bad. Zero volts means it is closed.
- Across the LP cutout (NC) - same logic. It is a common culprit: suction may be pumped down and holding the LP cutout open, which is normal operation, not a fault.
- Across the oil-pressure switch - the timer holds it open until the compressor runs, so before starting it should show full voltage. If it shows zero with the compressor not running, someone has jumpered it.
- At the CR1 coil terminals - full voltage line-side to neutral with no pickup means the coil is open or the return leg is broken.
Also check, in parallel with the above: the overload relay reset button/contact (a manually reset overload will hold the whole rung out), and the seal-in contact on rung 2 if the machine starts but drops when START is released. Voltage measurements across closed contacts are safer and more informative than ohming a live circuit.