Magnetic reaction force and magnet strength

larsb

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I have thought a while about a fundamental question on the motor and force generation from the magnetic fields.

The coils magnetic fields generated are turned on to interact with the permanent magnets in a BLDC

This should mean that the force and torque of a motor are directly related to the available permanent magnet field strength and therefore has a maximum achievable level where the permanent magnet field is "overpowered"

on a simplified level i think the magnet field strength should be the first design consideration - but it isn't in the literature i've read.

Why?

Isn't it possible to achieve higher fields from the coils than even an N52 neodym?

(so the limiting factor for torque should then be the magnets.)
 
I don't see an explanation in that thread.. Only a lot of confusion and some mix-ups.

There should be an easy answer to my question?

(I know that unbalanced radial forces are present in some motor configurations due to the geometry, it's not really what i ask about)
 
Surely, the limiting factors for torque are core saturation and heat dissipation?

The reason I posted the link to that post is that thinking of the situation as two opposing magnets is intuitive but misleading, as that thread shows...
 
OK, this touches some thoughts i've had on cored and coreless motors:
what is the function of the core with regards to the force, is there lorentz force acting on it or how does it contribute?

And my question still remains as i didn't mean it related to electromagnets, more like forces and opposing forces:
When one field is less than the other, do you get more force from increasing the greater field (= is it not following newton third law)?

Or is it that the coil fields will never be larger than the PM fields (or am i coming from totally wrong assumptions)?
 
I think generally the core iron will saturate before you reach the limit of the magnets. In a coreless design, there won't be saturation, but you are then thermally limited by the copper. The torque will be function of the flux coupled between the rotor and stator, so both contribute.

If the magnets are "too strong" you will have increased eddy current losses. Magnets are expensive too, so most designs avoid using more than necessary.

For a given amount of power input, the flux you get from a copper coil will be much greater with a core, until you reach saturation.
 
Brushless Permanent Magnet Motor Design - Duane C. Hanselman

But start with the first chapter of:
Electric Motors and Drives - Austin Hughes
https://books.google.co.uk/books?id=jjuTYtKokc8C&printsec=frontcover&dq=fundamentals+austin+hughes&hl=en&sa=X&ved=0ahUKEwjlyNaWhZ_LAhVHthoKHaAuDtIQ6AEIJjAA#v=onepage&q=fundamentals%20austin%20hughes&f=false
 
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