On 3/15/2017 4:45 PM, James wrote:
On 15/03/17 18:07, Jeff Liebermann wrote:
On Wed, 15 Mar 2017 16:45:57 +1100, James
wrote:

On 15/03/17 16:15, Jeff Liebermann wrote:
On Wed, 15 Mar 2017 15:14:56 +1100, James
wrote:

On 15/03/17 13:43, Jeff Liebermann wrote:
On Tue, 14 Mar 2017 07:54:26 -0700, Joerg

wrote:

On 2017-03-13 20:00, Jeff Liebermann wrote:
On Mon, 13 Mar 2017 12:38:07 -0700, Joerg

wrote:
55V at 500mA. This is encouraging.

That's 27.5 watts out of a 3 watt dynamo. I was impressed, until I
converted 136 km/hr and found that it was 84.5 mph. With a rocket
assisted bicycle, I might be able to do that.

Well, yeah, they just wanted to see where the limit is. I guess the
enameled copper wire inside would smoke out if you kept that
speed for long.

Only the resistive part dissipates power in the wi
P = I^2 * R = 0.5^2 * 2 = 0.5 watts
So, it won't be the wire that gets hot. However, the cores in
saturation are going to get warm. Offhand, I don't know how to
calculate how hot.

Do you mean eddy currents in the core?

Nope. I meant hysteresis losses. Eddy currents do contribute to
losses by "bucking" the build up and collapse of the magnetic field,
but most of the heat is produced by hysteresis losses:
https://www.quora.com/What-is-hysteresis-loss-Where-does-the-loss-actually-occur

See an induction heater or stove for an extreme case of heat being
generated by eddy and hysteresis currents:
https://en.wikipedia.org/wiki/Induction_heating

Hysteresis losses are different from saturation. Saturation shouldn't
occur if the core has been adequately designed to accommodate all the
permanent magnet flux & MMF.

Most (not all) dynamos are designed to provide some form of
self-regulation. This made sense back in the days when the load was
just a simple 6v incandescent light bulb. It wouldn't do to have
Kamakazi downhill speeders producing enough revolutions to have the
dynamo belch 12VAC and burn out the bulb.

Fast forward to today, and we no longer use incandescent lights on
dynamos. Most (not all) LED lamps have built in regulators and really
don't need to have the dynamo perform any additional regulation.
However, the industry is conservative, and things change very slowly.
Kinda like the automobile industry requiring 25 years to get rid of
the buggy whip socket. So, we still have dynamos that intentionally
designed to NOT produce a linear increase in output for high RPMs.

Fortunately, the problem is not universal.
http://www.pilom.com/BicycleElectronics/HubDynamo.htm
Notice the 2nd graph of Hub vs Bottle Power. The Shimano DH-3D71 hub
dynamo produces a linear increase in output power with no saturation
visible. (However, at my cruising speed of 15 km/hr, it only produces
4 watts and is therefore only a slight improvement over the common
bottle dynamo).

Hystersis loss is core saturation loss. What happens is that at some
point, adding additional current to the coil and core does produce an
increase in the magnetic field. Magnetic domains will change
direction when current is applied, but are not terribly thrilled with
the idea. Their resistance to this change in current and direction is
hysteresis loss. This additional current (and power) has to go
somewhere since it can't be used to build a changing magnetic field.
So, it gets converted into heat.

Eddy current losses are the result of changes in magnetic field, not
changes in applied current. When the magnetic field finally decides
to change direction and AFTER hysteresis losses are produced, the
resultant magnetic field breaks up into small magnetic loops or eddys.
Adjacent eddys fight each other resulting in repulsion. The energy
required to overcome this repulsive force are the eddy current losses.

As I understand it, the favored core material is some form of mu-metal
or permalloy which saturate nicely at low currents:
https://en.wikipedia.org/wiki/Mu-metal
I'm not sure this is the material as some description suggest that
it's "cold rolled dynamo silicon sheet steel" or the same stuff used
in the E/I laminations of a common AC power transformer.

It's after midnight. I give up for tonite.


You seem to be confusing current and magnetic flux.

What is used to alter the characteristics is the inductance of the
alternator coil.

That's how I've seen it explained, with appropriate equations. Since AC
frequency increases with speed, the inductive reactance due to the coils
increases with speed. That fights the tendency for more voltage from
higher magnet-to-coil velocity.


--
- Frank Krygowski