Electric motor core/lamination question - Page 1
Forum section: electronics / general

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 30, 2019, 12:28:51 am
content: I kinda got on a tangent watching random Youtube vids on how different types of electric motors and generators and how they work. Got me kind of intrigued at how excitation based ones work as they require no magnets and there are different ways to get power to the coils such as brushes or even inductively by having a separate generator on same shaft where the stator/rotar roles are reversed.Anyway one thing I noticed is the way a lot of the coils are wound. What I noticed is that the core they are wound around is basically like a star, and the metal is shared for all coils. How does this work exactly? I would have figured that each coil/core would be individual so that it acts as it's own magnet/pickup. If the metal is shared among all the coils, won't one coil interfere with the other? Or am I not understanding how magnetism works, or is there a lot more at play here? I guess to simplify my question, imagine you have a metal bar and on one end you have one coil and on the other end you have another coil wound the same direction, and both coils are on a different phase. How does one coil's magnetism not mess up the other coil? Or does the magnetism stay local within it's own coil section of the bar?

author: amyk
author_rank: Super Contributor
timestamp: August 30, 2019, 12:22:36 pm
content: Three words: lines of flux.https://en.wikipedia.org/wiki/File:Gramme_Ring_-_Four_Pole_Stator_Field_Flow.jpgIn fact if you have two coils opposing each other, a pole will naturally appear in the middle of the two; this is how ahttps://en.wikipedia.org/wiki/Dahlander_pole_changing_motorworks.Related video which may help explain this:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2019, 12:31:11 pm
content: Interfere, absolutely; indeed this is beneficial allowing us to put more copper on, by placing it on opposite sides of the rotor/stator, not just one side.The windings are spaced apart in such a way that their electrical and mechanical phase angles match, so there is no destructive interference.It is an important point to note, that the coils indeed are not fully independent, and couple into each other -- a customer had such a problem a few years ago, when their TRIAC switches kept burning out. Load was a two-speed motor, and switching one channel would cause the other to fail. The spike from switching on one winding was transformed through the windings into a spike that overstressed the other switch. Solved with bigger TRIACs and better snubbing.Tim

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 30, 2019, 05:21:42 pm
content: Oh interesting, so essentially even though it's a continuous core you can have multiple magnetic fields sharing it and it basically splits it into separate magnets.That way the low/high speed motor works is interesting too, I figured there would be extra windings for each pole but guess it makes sense now that the pole just gets created naturally from the winding next to it if all windings are set to same orientation. If there is an N the S gets created naturally. Guess there's a whole science as to what determines where based on shape of metal etc. The fact that it's laminated probably helps "direct" it I imagine.Is there an advantage to this vs if they just put a bunch of individual bars along a big plastic (or other non magnetic material) wheel? I guess by making the entire thing metal there's less saturation losses? Or is it just the fact that it's easier to manufacture, as you basically have a press that makes a bunch of star shapes and then they are laminated together.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2019, 02:55:04 am
content: Y'mean like a science toy, just a bar electromagnet (or no bar at all, just a loop)? For single phase, it doesn't matter much; synchronous machines are often constructed that way, with salient poles (the pole shape is mushroom-like to cover more air gap and better support the winding, but the rotor cross section is significantly smaller than for other designs).Magnetic field spreads out in steel just as well as it does in air; sticking a magnet to one side doesn't automatically preclude sticking more magnets elsewhere. You get a distribution of forces and fields, just as you ever would.It is possible to learn this intuitively, but it takes much less experience to, say, plug everything into FEMM and see the fields more exactly.Laminated iron is used for reduced AC losses. It's used even in synchronous machines (DC field rotor) because there is residual AC in fringing fields (the change in reluctance over stator slots induces harmonic energy), and transients (a synchronous machine is still an induction machine during startup, or for rotation within a cycle; there are shorting bars in the rotor to generate AC losses that dampen this motion).I wonder what the cost breakdown is, between cast versus laminated silicon steel. Or if the properties are even comparable, e.g. maybe cast steel can't reach as high saturation flux. Probably the biggest problem is that it's harder to control material properties when it's a solid lump of something, versus a lower-loss stack that you can slot stuff into.Although automotive alternators do use cast poles on the rotor; it would be difficult to make the interleaved multi-spoke structure otherwise. But that's probably for many reasons, not necessarily performance related (alternators have poor efficiency), but all of them ultimately cost related (saving complexity in the winding may be a higher priority; flux density isn't a big deal because RPM makes up for it; etc.).Tim

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 31, 2019, 04:23:31 am
content: Yeah meant like if you just took a bar or metal, like even just a nail or laminations, anything really. You put two coils on it at each end but make both ends south, or both ends north. But from what I gather it would in fact work, and you would just get the opposite pole in the middle. That's the part I didn't realize could actually happen so why I was wondering why a single chunk of metal can share multiple coils running at different polarities/phases. But now it all makes sense.

author: GlennSprigg
author_rank: Super Contributor
timestamp: August 31, 2019, 02:28:05 pm
content: Unless I mis-read something in other peoples responses, and here I'm addressing theactual need/usage of 'Laminations'....Laminated layers of ferromagnetic materials in transformers/motors etc, is to greatlylimit that dreaded "Eddy-Current" effect. The 'Core' of say a transformer, is utilized dueto it's magnetic properties, BUT, it is also an electrical conductor! The alternating magneticfields ALSO induce a circling electrical current flow within the Core itself. By using Oxidecoated laminations, instead of a 'solid', this interrupts those Eddy Currents.