The Anatomy of Stray Flux in 3 Phase Reactors

Docta Reacta: Hey, hey, nice to be back in front of the class at the Power Magnetics Institute of higher learning for your increased enlightenment and edification as I illuminate you with some technical elegance! Hmmm…. Overboard I hear, ringing in my ears!

The discussion of stray flux in the design and application of 3 phase reactors is insufficiently addressed by systems engineers when trying to deploy them in more sophisticated applications in tighter spaces.

It is inherent in the geometry of the 3 Ø (Three Phase) Core Type Core that the only path for zero-sequence flux is external to the core.  For 3 Ø – three phase reactors, this means that there is stray magnetic flux (third harmonic) on the sides of the reactor. See the diagram below.


Stray Flux in a 3 Phase Reactor

Stray Flux in a 3 Phase Reactor



So from a practical standpoint, if several inches of clearance is not maintained on the side of the reactor to a steel enclosure, it could cause heating of the steel wall and/or vibration. That vibration is the noise you sometimes hear with poorly constructed 3 phase reactors. The flux can be sensed by holding a steel screwdriver next to the reactor when energized. (I was going to mention the precaution of doing that: be sure to maintain sufficient electrical clearances.  Also, the flux tends to grab the screwdriver out of your hand. But I know you technical folks are plenty bright so I will pass on that one).

It is also worthy of note that this fact has implications for the framing of the reactor. Structural steel pieces tying the top and bottom yoke together on the side may heat up.  The most popular method of securing the top and bottom yoke is a steel bolt in the window between the coils, precisely because it does not heat up.

So there you have it for your enlightenment!  Please keep the cards and letters coming in the fan mail.  And of course, if you have any questions, please get in touch with me.  I would be delighted to hear from you!

Until next time!


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  • David Jones

    I keep seeing these failing, at times they have lost their physical integrity at the structural steel pieces like you mentioned in the post. I have one I am observing now that is vibrating but seems to be structurally sound as of now. How much clearance around the reactor is necessary?