Re: I was new to this list. / SRM - long?

["Lee A. Hart" <leeahart@earthlink.net>]

Eric Chang wrote:
> In a recent discussion (argument) with a physics professor, I brought
> up the EVDL explanation of why there are no large BLDC motors: there
> is a field scaling problem.  He told me that this is not quite
> correct; it is due to the low reluctance of most rare earth magnetic
> materials corrupting the magnetic circuit if the path length is too
> long.

I think there are elements of truth in both. PM magnets do have higher
reluctance than the best magnetic materials. But some motor designs are
less sensitive to field distortion than others. Most PM motors are
designed so field strength matters more than field position. Just as a
series resistor in an electrical circuit stabilizes the current, the
extra reluctance stabilizes the field strength.

But VR motors are strongly affected by both field strength and field
position. They require low reluctance to maintain control and good
performance. This is where all the effort goes in trying to make a good
VR motor.

> force of a magnetic circuit is a function only of the magnetic
> permeability... It is not a function of its elctrical conductivity.

That just means magnetic and electrical losses are separate. You can
have a motor design with an extremely efficient magnetic circuit, but
that requires very long copper paths and thush high electrical losses.
Conversely, you can design a motor with very short, efficient windings
but long magnetic paths. But most designs tend to be "square", with an
effort made to keep magnetic and electrical roughly equal.

> Heat generation in an SRM rotor, however, is via a different physical
> principle, i.e. climbing up and descending a unipolar hysteresis curve.

No; this is the same principle as in most motors. What you are thinking
about is where heat from the magnetic and electrical losses need to be
dissipated in the rotor or stator.

Synchronous PM motor:
	electrical loss	100% in stator
	magnetic loss	100% in stator
Variable Reluctance motor:
	electrical loss	100% in stator
	magnetic loss	split between stator and rotor
Brushed DC PM motor:
	electrical loss	100% in rotor
	magnetic loss	100% in rotor	

So if you are in marketing, you can claim any of these are "best".
Synchronous, because all the heat is being dissipated in stationary
parts which are easy to cool. VR because the losses are split between
rotor and stator, so there's more surface area to dissipate the heat.
Brushed PM DC becauses the spinning rotor acts as its own cooling fan.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
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