I am starting a new project with IPM type BLDC with outer rotor topology. As some of the common feature and differences are well known to us but those are not sufficient so I need help from you all.
Main drawbacks are cooling, inertia, and mecanical integration due to bearing on only one side of the rotor shaft.
Advantages are manufacturability (winding can be easily automated) and more torque comparing to equivalent size inner BLDC.
On top of inertia limiting your accelerations, there is also the problem of mechanical constraints. As you would have a bigger rotor as compared to an inner-rotor configuration, you must take greater care of mechanical limits of your magnet ring or magnet blocs. The peripheral speed (v in m/s) of your magnet can be much higher even if you run at the same mechanical speed (N in rpm), due to the increased diameter (D in m).
v = N*D*pi/60
Main drawbacks are mechanical integrity and cooling. As the shaft is supported from one side bearing, the mechanical balance is difficult to maintain.
As others rightly pointed out, main drawbacks are cooling and mechanical stability.
Cooling:
the winding being in the inner portion of the machine, they have less surface area to dissipate the losses, either via air cooling or water jacket (located at the ID of the stator yoke). Such an issue is partially counteracted by the fact that you would need less current to produce the same torque (since the air gap radius is larger than in its inner rotor counterpart), but some problems may still arise...
Mechanical stability:
You would generally have only one bearing to support the rotor (otherwise manufacturing becomes quite complicated), so the whole rotor structure is cantilevered and tends to bend (at standstill) or open up at the "free end" during rotation due to centrifugal force. Unless you want to add a lot of structural elements (thus mass and cost) to stiffen the rotor, these risks usually limit the rotor length and hence the torque capability for a given motor diameter.
Windage Loss:
this might not be an issue in most of the applications, but if your motor runs at high speed you will have significantly higher windage losses compared to the inner-rotor counterpart, simply because an outer rotor has two surfaces that rotate (at inner and outer diameter) and depending of speed and size this could be a huge loss contributor.
Benefits:
As already said, from the EM point of view the main benefit is that the airgap radius is larger than the inner-rotor counterpart (for a fixed motor outer diameter) and this helps increasing the torque capability per unit length and current.
Also interesting and helpful is that in an outer rotor the centrifugal force pushes the magnets against the rotor yoke (if you have an SPM), so you do not need any retaining ring/sleeve. For the same reason, since you are considering and IPM, you can limit a lot the iron ribs that are needed to contain the magnets against centrifugal forces (ideally, you could completely get rid of them), thus you gain a lot of magnet flux because you strongly limit the leakage flux in these iron ribs.
These are the pros and contra I experienced in my little effort on comparing inner and outer rotors.
Pretty long reply, hope this helps.
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