Iron loss

[MIgardener]

 
  I keep running across posts that mention Iron loss. What exactly is iron loss?
 

[SparWeb]

It refers to the eddy currents.  Some kinetic energy is lost when the field around a magnet builds up the field lines in a nearby metal, and you don't get the energy back if you move the magnet around.  I personally think of it as little "cells" in the metal that contain arrows.  The magnet makes all the arrows line up.  Once the magnet is moved, the arrows spin around either to a new direction or randomly if the magnet is taken away.  Not sure if that's the right way to picture it, but at least it helps imagine the "eddies" spinning around.  It generates heat.  An eddy-current brake is designed to do this very effectively.

The eddy current loss happens in all metals, steel, iron, copper etc.  but in a motor like the one you are converting, the magnets are interacting with the laminated iron stator.  If you try to measure a torque from iron loss (pretty hard on a small motor) it's really hard to separate out the iron loss, the friction in the bearings, and some kind of "average" that overcomes the cogging, so they are sometimes all lumped together when we discuss it.

You should try googling the high-school science demonstrations where the teacher drops a ball bearing down a copper tube, then drops a magnet down the same tube...  takes a while for the magnet to come out the bottom.

[MIgardener]

It refers to the eddy currents.  Some kinetic energy is lost when the field around a magnet builds up the field lines in a nearby metal, and you don't get the energy back if you move the magnet around.  I personally think of it as little "cells" in the metal that contain arrows.  The magnet makes all the arrows line up.  Once the magnet is moved, the arrows spin around either to a new direction or randomly if the magnet is taken away.  Not sure if that's the right way to picture it, but at least it helps imagine the "eddies" spinning around.  It generates heat.  An eddy-current brake is designed to do this very effectively.

The eddy current loss happens in all metals, steel, iron, copper etc.  but in a motor like the one you are converting, the magnets are interacting with the laminated iron stator.  If you try to measure a torque from iron loss (pretty hard on a small motor) it's really hard to separate out the iron loss, the friction in the bearings, and some kind of "average" that overcomes the cogging, so they are sometimes all lumped together when we discuss it.

You should try googling the high-school science demonstrations where the teacher drops a ball bearing down a copper tube, then drops a magnet down the same tube...  takes a while for the magnet to come out the bottom.

  Thanks

[Flux]

Strictly there are 2 parts to iron loss. There is a part due to eddy currents, which happen with any conductor. Motor cores are laminated to reduce this to a fairly small level and the thinner the laminations the less the loss. If you make a core of solid iron the eddy losses become enormous, normally the thickness of the laminations is a compromise between loss and cost of manufacture and  loss will not be great in a commercial motor core at 60Hz but high frwequency machines need much thinner lams in the core.

The other loss component is hysteresis and is a property of the alloy and can't be avoided, it will be the main component in a motor core. It is a loss within the material when you take the magnetisation over a cycle, the domains in the core line up under the magnetic field and line up the other way when the flux reverses. Not all the energy stotred in the aligned domains is recovered when the field reverses, there is a loss at the molecular level and this depends on the material.

For something like hard steel the loss is high and the rising and falling magnetisation are widely separated ( fat B H loop).

Pure iron has a much thinner loop and a lot lower loss. Special alloys can have very thin loops with far less loss but the best in this respect tend to saturate early so there is a compromise in the choice of material. You can look at hysteresis in the same way as a pendulum in a clock case, it will dispalce air in one direction then in the other but the air doesn't reverse instantly and there is frictional loss within the air molecules.

Flux

[boB]

One more way to view eddy current loss is to realize that the magnetic core should be a good conductor of magnetism but NOT a good conductor of electricity.  The less it is a conductor of electricity, the less current will be created in the material and the less that conductor will look like a shorted winding that draws current from the source when that source is changing voltage.  Separating the core material like Flux says (laminating) helps it from being one big conductor of electricity.

Some other type of cores accomplish a similar thing by having the magnetic material be separated into small grains or powder inside a non-conductive glue (the binder) like powered iron.

boB