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Below is a squirrel cage induction motor's rotor and on the left shows only the shorted aluminum rotor bars; and on the right it shows the lamination disks are added to the rotor.

enter image description here

As far as I know the rotor can still rotate(as on the left) without those laminations. Those shorted bars are actually the structure which creates rotation. They form coils and pass currents and hence create torque and rotation ect.

But why are these limitations(on the right side) needed in construction of the rotor?

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  • \$\begingroup\$ I always thought those were just for mechanical support. \$\endgroup\$
    – Hearth
    May 6, 2018 at 16:33
  • \$\begingroup\$ Oh you mean to be able use bearings for the shaft? \$\endgroup\$
    – user16307
    May 6, 2018 at 16:34
  • \$\begingroup\$ @user16307 Rotor core is a common constructional feature for all rotating machines. There are slots in the rotor core in which each of the bars is inserted and then shorted from the outside. The core is mounted on the shaft. \$\endgroup\$
    – akshayk07
    May 6, 2018 at 16:59

5 Answers 5

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An induction motor is basically a regular power transformer but the secondary is "shorted" and is allowed to rotate. Would you expect good efficiency if you had big air gaps in a regular power transformer? Maybe you don't know how power transformers work?

The bottom line with power transformers is that you want the primary magnetism (stator) coupled to the secondary winding (rotor) as much as possible and you therefore avoid large air gaps that would bypass the secondary winding.

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Magnetic flux must pass through the rotor. The rotor conductors are surrounded by iron so that magnetic flux has an easy path to flow. Some other material could be used to support the rotor cage and connect it to the shaft, but iron is more effective. Without iron a higher magnetomotive force would be required.

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Laminations (usually thin silicon steel 0.2 to 0.7 mm) are used to reduce eddy current losses

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As others have mentioned, there are two main reasons as to why there are laminations that surround the aluminum bars. The first is to carry the magnetic field lines, since iron (and other ferromagnetic materials) are much better at carrying magnetic fields than air. Now that we've established the need for an iron core, the question remains why the core is laminated. This is to reduce eddy current losses, since thinner layers reduce the cross-sectional area of the iron, increasing resistance, thus reducing eddy currents and corresponding losses.

I hope my basic-level understanding helps.

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Motors are generally constructed of laminations, sheets of steel, to reduce eddy currents.

That's the basic scheme, and that makes perfect sense for the stator (oscillating / rotating magnetic field), and for the rotor of universal or DC motors (of which, an active winding rotates around it, driven by a commutator; same outcome for the core).

But consider the squirrel-cage induction motor. While the magnetic field does rotate through the rotor, it rotates slowly: at the difference, the slip frequency. This can be some single-digit Hz, and the corresponding skin effect in steel might be 1cm or more! And, we're putting shorting bars on the thing, for the express purpose of generating eddy currents; what gives?

Things are even more perplexing in series-wound motors. In the universal motor case, we still might be applying AC (the stator polarity reverses every half-cycle), but since rectifiers are cheap and abundant, we might well ask whether a solid core is practical, and worth the cost of a rectifier. For the DC case, the stator could indeed be solid metal.

Similarly, synchronous and PMAC machines might use solid metal rotors. In the synchronous case (wound field), we might still desire at least some modest rate of change, or indeed to use the rotor field as a frequency subtraction mechanism (see also: resolvers and synchros); but in the permanent-magnet case, surely it couldn't matter?

I don't have exact answers for these, but we can come up with some possibilities. Likely, you would need to do the real economic analysis to figure out which factors are dominant in any given case -- so, I don't feel bad about offering hand-waves as a starting point.

The below reasons are geared particularly towards the AC induction motor and rotor, but various reasons will apply to all machine types, or similar reasoning can be applied to them.

  • Control of skin depth. In a solid-rotor induction machine, skin depth depends on the slip frequency. If we used a solid rotor across the board, we would have a sensitivity to scale: a small (100W?) motor will behave very differently from a large (10kW+?) one. In particular, if magnetic field only occupies the surface, say 1cm or so, of a solid core, how would we even design a few-pole induction motor of, much of any power at all? (Even a 100W motor, typically has a rotor a couple cm across.)

  • Linearity; consistency of behavior and performance. If we have the skin depth varying with slip frequency, then the amount of core participating, also varies. That is, as frequency goes up, effectively the rotor's magnetic core cross-section decreases. But flux density is fixed by the stator (applied voltage and frequency), so as slip increases, we will quickly run out of available rotor cross-section -- that is, saturation occurs, and now things get... weird. We end up with a situation where, flux density is maximum towards the surface of the rotor, until at some depth, enough saturation has occurred and flux density declines (and below which, we get skin effect as normal). This runs part of the rotor at much higher flux density than usual, greatly increasing losses; effective permeability decreases (some of the rotor is acting as air gap), increasing magnetizing current; and the torque curve varies with applied voltage, frequency and slip in a complex multi-variate manner. Accordingly--

  • The ability to tune performance as a design parameter. There are different induction motor prototypes/standards, optimized for various purposes. The efficiency and torque curves vary, for tradeoffs in starting current/LRA, slip (a "softer" motor handles fluctuations in RPM better), size/cost, etc.

    Synchronous machines in particular, are tuned by way of introducing shorting bars to their [electro]magnets: in this case, instead of a constant slip due to load torque, RPM remains synchronous but the relative phase between applied voltage (or more precisely, say, instantaneous stator magnetic field) and rotor angle is the analogous parameter, and the phase shift acts just like the angle applied to a torsion spring. And like a spring, there is a restoring force, and a mass (rotor inertia), which would oscillate if we didn't apply some damping to it. Shorting bars on the rotor are used to provide exactly this damping force.

    Likewise, we use shorting bars, instead of a cylinder or what have you, because it gives better performance, and more design parameters. The thickness and depth of the bars can be adjusted to tune performance, while the space between them ensures there is always facing core area, minimizing air gap between rotor and stator.

  • Materials. Core materials are mass produced in sheet form: with the prevalence of AC power in our world, laminations are almost universally required. Electrical steel is a mass-produced product, available in standard grades and thicknesses. It's easily punched to shape (well, "ease" is relative, it is more abrasive than mild steel for example -- but tooling cost and speed are rolled into overall production cost), and stacks can be made relatively cheaply.

    In contrast, solid pole pieces would have to be cast whole, or machined or forged from blanks. They might suffer from issues such as crystal size or orientation (directional solidification, forge grain), material segregation (the FeSi or other alloy may be nonuniform), or metallurgical limitations (identical alloys might not be pourable, or may require well-equipped foundries to handle, e.g. inert-gas or vacuum casting).

    Offhand, I don't think FeSi alloys have any particular limitations, and you do see cast pole pieces in several common applications (e.g. linear / voice-coil motors -- speakers and such). These may still be reasons that limit other applications (perhaps e.g. the variable-bias rotor of an alternator would have requirements at odds with parts that can be produced this way, etc.).

  • Dimensional accuracy. This is a huge one, and one that applies to a range of manufactured products, actually. Consider the options to make a solid-metal rotor with specific geometry. We might need to turn the OD, drill and bore the ID (to slip or shrink fit onto a shaft), and mill the slots (where the windings/magnets go), to within adequate tolerances. What if we need pockets to place magnets in? They could be cast (cored out) as a start, but the finish and dimensional tolerance of casting is far too poor; machining is required. Alternately we could drill, bore, broach, etc. such features, but -- clearly, getting any kind of accuracy on a high-aspect hole is not going to be simple, nor cheap.

Then, consider a part like so:

enter image description here

Source: Motor core-Shanghai Pressing Steel Plate Processing Company

In a design like this, I believe the rectangular slots are for placing permanent magnets, in what looks like a Halbach array -- which might maximize flux density at the surface while minimizing magnet volume, a worthy goal as magnets are generally more expensive than electrical steel (or, the good ones (NdFeB) certainly are, while the cheap ones (Sr ferrite?) aren't nearly strong enough). Such spaces have to be machined to a tight fit on the magnets, then glued in place (or even, perhaps, pressed in place at near zero clearance). The thin webs bounding these spaces, might ideally be zero (core wrapping around a magnet "short-circuits" useful flux) but must be retained for mechanical strength, and such webs are not at all easy to machine, let alone accurately the entire height of a part.

But punching shapes from sheet, and making stacks -- punching can be done extremely accurately and repeatably.

Another example is the microwave oven magnetron: now, I don't know if it's still produced this way, but back around WWII, the design and production of them was a top concern, and they devised a method of stacking punched plates, and furnace-brazing them together. They might be extruded now for all I know (as I understand it, extruding copper alloys is a challenging process, but thanks to modern materials, is just feasible), but in any case, a dimensionally accurate, prismatic section is required, and punched stacks can indeed do the job.

When perfectly prismatic shapes aren't required, we have an added tweak: the stack can be gently twisted, making helical slots or cavities. This can be used to smooth out the torque as the motor rotates.

I recall handling one "servo motor" before, which was by all indications just any old PMDC motor as such -- if maybe a relatively large one given ratings -- but, between the number and spacing of windings/commutator poles, a gentle helical twist along the rotor, and probably other less apparent tweaks -- it really was as smooth as silk, and applying a small voltage easily got it moving, slowly and smoothly; or shorting the terminals and giving the rotor a spin found a smooth yet impressively stiff brake; etc. Most motors aren't so concerned with torque pulsations or what have you, but when smooth operation is required, oh yeah, they can definitely do a good job!

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