Comparison of air core and iron core choke

¹ Synonyms for an output choke are output reactor, output inductor or output coil.

Air core choke

Air core variant is characteristic for the absence of magnetic core. Main advantage is the linear behaviour and no saturation effect. Limitations might be larger clearances to metallic objects resulting in larger footprint.

Comparison of air core and iron core choke

In this paragraph we summarize main differences between air core and iron core chokes.

• Footprint:
  • Iron core choke results in smaller footprint. It can be placed close to other metallic objects. That is a major advantage when the choke shall be integrated, e.g. installed inside the VFD enclosure.
  • Air core choke shall respect magnetic clearances to objects that could encircle the flux. The choke manufacturer typically indicates two distances: one applicable for smaller magnetic objects and another one for larger magnetic structures (e.g. a steel fence, armoured concrete etc).
• Weight
  • Air core choke is naturally lighted due to absence of the iron core.
  • Iron core choke is heavier but requires overall less space (as explained above).
•  Installation
  • Iron core chokes are easy to integrate in a metal enclosure. It can be the enclosure of the VFD or its own in case of stand alone component. Iron core chokes are often installed indoor.
  • Air core chokes can be installed indoor or outdoor. Due to the magnetic clearances the outdoor design might be preferred.
• Core saturation:
  • Air core choke does not saturate. That is a major benefit especially if the choke shall provide its function in a fault condition (e.g. current limiting function).
  • Iron core choke saturates. The degree of saturation depends on the choke design. A typical saturated inductance is approximately 20% of the nominal non-saturated inductance. The minimum saturated inductance might be a parameter specified by the purchaser (min. saturated inductance required for the functionality). Specially designed iron core choke can achieve higher relative saturated inductance. An example might be an iron core with an air gap. Such design reduces the saturation effect but typically brings other challenges.
• Leakage flux:
  • Iron core choke has very small leakage flux as majority of the total flux flows through the iron core. It means less interaction with surrounding equipment and less disturbance. Main benefit is the overall compact design (see footprint above).
  • Air core choke has a significant stray magnetic field. That poses restrictions when it comes to installation in the vicinity of magnetic objects. The magnetic field also acts on the human. People shall not be exposed to a excessive magnetic field (limitation for the level and time duration). Areas with strong magnetic field shall not be accessible by the personnel.
• Cost:
  • Air core choke tends to be cheaper on the component level. However, there might be extra cost on the system level (e.g. due to larger footprint or necessity to use special non-magnetic steel for other components).
  • Iron core choke usually has higher cost due to the additional material. However, on the system level it might be offset by the compact design.