What exactly are Iron Losses?

[John in CR]

I've seen 2 types of losses mentioned for brushless motors, copper losses and iron losses. Copper losses seem easy enough, the heat generated due to the resistance of the copper as current flows through it. What are the iron losses, and where do they manifest themselves as heat, since all losses are what turns into heat? Are iron losses generally significant?

As a somewhat related topic, is just looking at no load current a reasonable method for comparing relative efficiencies of different motors? ie No work is done other than creating a bit of wind, so isn't that the basic loss of the motor (maybe mostly the iron loss?), no load amps x current.

John

 

[oofnik]

Iron loss is a little bit more complicated than copper loss. Whereas copper loss is purely resistive, iron loss encompasses several processes in which energy is lost. Most of it is due to the magnetic field being set up and collapsed at high frequency in the iron. A property called magnetic hysterisis is to blame - which is basically where the iron retains some of the magnetization energy within its field after collapsing. When the field is remagnetized the other way, energy has to push the iron back to the 'zero' state, then in reverse. Imagine pulling a spring to a length, letting go, and having it return to slightly longer than it's resting length instead of back to zero. Pushing the spring in also causes it to return to slightly shorter than zero. Do this 1000's of times and you can imagine some energy being lost.

The other major component of iron losses is due to eddy currents. This is minimized by cutting the iron into laminations to impede the flow of current, but some current still does manage to flow through the iron in a circular pattern. If you've ever been rafting or kayaking, you've probably noticed how water swirls around rocks in the river, creating eddy currents - which are roughly analogous to electrical eddy currents. The energy used to push electrons around in the iron in this way is completely wasted as heat.

No load current is just one indication of a motor's efficiency. One must also observe how the motor responds to real loads. A motor can have a very low no load current measurement but still suffer under high load. Pick up an electric motor design book if you'd like to learn more about why as I'm not sure myself

Hope this explains some of it!

 

[John in CR]

Here's an interesting little motor with no iron losses http://www.thingap.com/pdf/tgb2020ds.pdf

Thingap seems to have a lot of motors that would be great for ebikes with built in quality like temperature sensing. I believe these types of products need to be incorporated into ebike designs to achieve the bulletproof drivetrains all of us want.

John

 

[johnrobholmes]

Iron-less designs are no more efficient than well built iron laminated construction. The elimination of iron necessitates an increase in copper (losses), if all performance aspects are to be held constant.

Ironless motors are ideal for high rpm/ low torque construction, and where low power low speed resolution and control is most important.

In a nutshell, iron losses are from switching the magnetic field back and forth. There are also eddy current losses, or inefficiencies in the magnetic field around the iron.

 

[Microbatman]

Great question.

Now you got me thinking.

How does one minimize iron losses?

Thinner stator plates?
More pure iron in the stator?
More Iron in the stator?

 

[Miles]

Thinner laminations reduce eddy current losses.

For lamination materials see: http://www.protolam.com/page3.html

Also: http://www.burgessnorton.com/Soft%20Magnetic%20Composites.htm

 

[Microbatman]

Found a video of eddy current.

http://www.edumedia-sciences.com/en/a132-eddy-current-2


It looks like/my interpetation is that the power/current CLOSER to the inside of the loop is stronger than further away.

Am I correct?