This YouTube video https://www.youtube.com/watch?v=nlG2dsYRagI shows how an electric engine is made but I don't understand how friction between the stator and rotor is dealt with.

At 3:50, we see the rotor inserted inside the stator and at 4:32 we can see a hand turning the shaft. I don't see any lubrication, how can this be possible ?

At 3:34, we can see a calibration of something and the woman says in french that the rotor will turn around steel bearings, I guess the lubrication will be made there : In the connection between rotor center and the steel bearings but still we can see that there's no space between the rotor and the stator.

  • $\begingroup$ 3:50 doesn't have the end caps with support bearings installed. 4:32 does. You never see him spinning it without the end caps because he can't. If the rotor and stator were intended to have contact when spinning then end bearings would be pointless. $\endgroup$
    – DKNguyen
    Commented May 13, 2022 at 13:34
  • 1
    $\begingroup$ Try getting a motor and taking it apart to see. $\endgroup$
    – Solar Mike
    Commented May 13, 2022 at 13:39
  • $\begingroup$ At 4:00 you can see that the larger bearing has seals between the inner and outer sections. It is pre-packed with high-melting-point grease during manufacture. The rotor has vanes to circulate air for cooling, and generally works in filthy conditions. You really don't want dust getting sucked into the bearings, or the lubricant getting onto the electrics (where it will encourage built-up of dirt and therefore heat). $\endgroup$ Commented May 13, 2022 at 21:37

1 Answer 1


... there's no space between the rotor and the stator.

There is always an air gap between the rotor and stator.

Iron has a permeability of about 1000 times that of air so the trick for maximum efficiency is to keep the flux in iron and minimise the air gap in the magnetic path where losses would occur. Minimumising air gap will require higher tolerances and therefore will be more expensive to manufacture.

  • $\begingroup$ What do you think about the video at minutes 3:55 ? $\endgroup$
    – Aminos
    Commented May 13, 2022 at 12:09
  • 3
    $\begingroup$ @Aminos Of course you can't see an airgap at 3:55 because the end caps with the rotor bearings aren't installed yet so the rotor is just sitting at the bottom of the stator. There's no magic here when sliding the rotor inside the stator. You just slide it in when you put the end caps on it raises up the rotor concentrically . I've done it at work. You can't see the airgap until the end caps with the bearings are installed which means you can't see airgap unless you look through a hole in the motor case and even you might not be able to see it through a camera because the airgap is very small. $\endgroup$
    – DKNguyen
    Commented May 13, 2022 at 13:30
  • $\begingroup$ Watch at 3:52 for a few seconds. I can see quite a lot of radial movement. $\endgroup$
    – Transistor
    Commented May 13, 2022 at 14:22

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