Reciprocating compressor duty combines cyclic torque with a pressure-dependent start. Two packages with the same running power can impose very different motor requirements if one starts fully unloaded and the other can retain discharge pressure.
Define three pressure states
| Start state | Motor question | Control question |
|---|---|---|
| Normal unloaded start | Is torque margin adequate with unloaders operating? | Is unloader position proved before start? |
| Residual-pressure restart | Can the train accelerate after a short interruption? | Is equalisation time enforced? |
| Failed-unloader case | Must the motor start, trip or remain inhibited? | How is failure detected and alarmed? |
Do not let “unloaded start” remain a note without a valve sequence, pressure boundary and permissive.
Use the compressor torque trace
Request torque against crank angle during starting and normal operation, together with flywheel effect, train inertia and minimum starting speed. Compare the motor torque-speed curve at the predicted terminal voltage. Confirm coupling and shaft limits for peak cyclic torque.
Check acceleration and heating together
The motor must accelerate before its permissible stall or start time is reached. A weak grid can extend acceleration by reducing available torque. Repeated restart attempts add heat even when the compressor never reaches running speed.
For VFD starting, review low-speed cooling, current limit, minimum frequency, torsional restrictions and any prohibited continuous-speed bands. A smooth electrical ramp can still cross a mechanical resonance.
Integrate auxiliaries and protection
Lubrication, cooling water, separator levels, suction and discharge valves, unloaders and vibration protection belong in the start permissive. Stall protection should coordinate with the expected acceleration trace, while overload protection addresses running duty.
Information to include in the enquiry
Send compressor type, gas and pressure states, unloading sequence, torque trace, train inertia, starting frequency, supply study, hazardous-area classification and required restart philosophy. LEADGO can check the motor and starting method against these cases.