Turbo Compressor Without Frequency Converter: Complete or Partly Completed Machinery?
A turbo compressor delivered without a frequency converter is not automatically partly completed machinery. The decisive question is whether the missing converter is technically essential for the intended compression function.
The Practical Situation
A manufacturer supplies a turbo compressor to an OEM customer or plant integrator. The delivery scope includes the compressor housing, rotor, high-speed motor, bearings, sensors and electrical interface — but not the frequency converter.
The customer will select or integrate the frequency converter later as part of the complete system. At first sight, this may appear to be partly completed machinery. However, the classification is not automatic.
The Classification Question
The central issue is whether the turbo compressor can perform its intended function in the delivered condition. For machinery classification, functionality is more important than optimisation, sales wording or the fact that the product will later be integrated into a larger system.
Scenario 1
Frequency converter used for optimisationIf the converter mainly improves efficiency, speed control, soft start or process integration, the compressor may still be complete machinery.
Scenario 2
Frequency converter technically requiredIf the compressor cannot start, reach stable speed or perform its compression function without the converter, partly completed machinery may be justified.
Complete Machinery Route
If the turbo compressor can technically start, rotate and compress gas in the delivered condition, or with standard connection requirements, it may be classified as complete machinery.
- CE marking under machinery legislation
- EU Declaration of Conformity
- Operating instructions
- Risk assessment
- Technical documentation
Partly Completed Machinery Route
If the frequency converter is technically essential for start-up, stable operation and the intended compression function, the turbo compressor may be classified as partly completed machinery.
- Declaration of Incorporation
- Assembly instructions
- No CE marking under machinery legislation
- Clear integration requirements
- Defined interfaces to converter and control system
Interaction With Functional Safety
Turbo compressors may require safety functions such as overspeed protection, temperature monitoring, pressure limitation, vibration monitoring, emergency shutdown, safe start-up sequence and fault reaction logic.
If these functions are implemented in the external frequency converter or control system, the interfaces must be clearly defined. This is where CE conformity and functional safety interact.
Interaction With Process Safety
In applications such as hydrogen, fuel cells or gas treatment, the turbo compressor is not only a machinery safety issue. Process safety aspects may include flammable gases, ATEX zones, pressure, leakage, hot surfaces, ignition sources and abnormal operation.
CE Compliance
We assess whether the compressor is complete or partly completed machinery and define the correct documentation and conformity route.
Functional Safety
We review safety functions, control interfaces, shutdown logic, overspeed protection, monitoring requirements and validation needs.
Process Safety
We assess ATEX, pressure, hydrogen, gas treatment, ignition source and process interface risks linked to the compressor application.
Example: Fuel Cell Turbo Compressor
A turbo compressor is supplied for a fuel cell system without the external frequency converter and system controller. The compressor cannot start or maintain stable speed without the specific converter and control logic.
In this case, partly completed machinery may be a reasonable classification, provided that the missing elements are technically essential and the integration requirements are clearly described.
Typical Mistakes
1. Assuming “without frequency converter” always means partly completed machinery. The converter must be technically essential for the intended function.
2. Treating an unsafe machine as partly completed. Unsafety and incompleteness are different issues.
3. Leaving interfaces undefined. External converter and control requirements must be documented.
4. Ignoring functional safety. Safety functions implemented externally must still be specified and validated.
5. Ignoring process safety. Hydrogen, ATEX and pressure risks can significantly change the assessment.
Recommended Assessment Approach
A structured assessment should define the intended function, delivery scope, dependency on the frequency converter, applicable legislation, safety functions, external interfaces and documentation route.
The classification decision should be recorded in the technical documentation and supported by technical reasoning, especially in OEM and high-speed compressor applications.
Need Support?
We support machine manufacturers, importers and plant integrators with complete vs. partly completed machinery assessments, turbo compressor and blower classification, CE conformity assessment, functional safety interface review, ATEX and process safety relevance checks, PED screening, EMC review and OEM documentation.
Request a Turbo Compressor CE Review