Magnetic reaction force and magnet strength

Electric Motors and Controllers
Post Reply
larsb
1 kW
1 kW
Posts: 314
Joined: Dec 10, 2014 5:12 am
Location: Gothenburg, Sweden

Magnetic reaction force and magnet strength

Post by larsb » Mar 18, 2018 4:41 am

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.)
Last edited by larsb on Mar 18, 2018 11:49 am, edited 1 time in total.


larsb
1 kW
1 kW
Posts: 314
Joined: Dec 10, 2014 5:12 am
Location: Gothenburg, Sweden

Re: Magnetic reaction force and magnet strength

Post by larsb » Mar 18, 2018 8:59 am

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)

User avatar
Miles
100 GW
100 GW
Posts: 11452
Joined: Mar 16, 2007 4:15 pm
Location: London UK

Re: Magnetic reaction force and magnet strength

Post by Miles » Mar 18, 2018 9:13 am

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...

larsb
1 kW
1 kW
Posts: 314
Joined: Dec 10, 2014 5:12 am
Location: Gothenburg, Sweden

Re: Magnetic reaction force and magnet strength

Post by larsb » Mar 18, 2018 11:13 am

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)?

User avatar
fechter
100 GW
100 GW
Posts: 12419
Joined: Dec 31, 2006 3:23 pm
Location: California Bay Area, USA

Re: Magnetic reaction force and magnet strength

Post by fechter » Mar 18, 2018 1:06 pm

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.
"One test is worth a thousand opinions"

larsb
1 kW
1 kW
Posts: 314
Joined: Dec 10, 2014 5:12 am
Location: Gothenburg, Sweden

Re: Magnetic reaction force and magnet strength

Post by larsb » Mar 18, 2018 1:45 pm

I'll have to read up on the basics and further on.

Can you recommend any specific book on bldc motor design?

User avatar
Miles
100 GW
100 GW
Posts: 11452
Joined: Mar 16, 2007 4:15 pm
Location: London UK

Re: Magnetic reaction force and magnet strength

Post by Miles » Mar 18, 2018 1:57 pm

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=jju ... es&f=false

larsb
1 kW
1 kW
Posts: 314
Joined: Dec 10, 2014 5:12 am
Location: Gothenburg, Sweden

Re: Magnetic reaction force and magnet strength

Post by larsb » Mar 20, 2018 1:38 pm

Found Duanes book here:
http://www.avislab.com/blog/wp-content/ ... rsion2.pdf

It was quite expensive at Amazon, 750 usd!

Post Reply