At 09:25 PM 15/10/2009, you wrote:
> > Among motor types, which give the best power ( energy per unit of
> > time )  to weight ratio ?
> > I am not going to consider reliability as a factor.
> > I am not including auxiliary items such as fuel tanks, air lines,
> > cabling, etc.The misleading answer may be "high pressure gas 
> operated motor".
>
>A prime example of this is the "topping cycle" gas turbines used to
>supply the propellant for the Space Shuttle main engines. These sit
>"before the combustion "chambers proper in the main engines and
>essentially preburn some incoming propellant. They can be highly
>efficin\ent because of the high presseures they operate at. (About
>4500 psi from memory).(Don't try this at home).
>
>The reason that this is misleading is that they are essentially a way
>to transfer the gas mechanical energy into propellant mechanical
>energy. All pumps are energy transferers - but a gas turbine is closer
>to the basic principles than some. That said, the mass and volumetric
>energy density of the power source can also be about as high as
>anything else available. Hydrocarbons contain about 10 kWh/kg (or
>about 36 MJ/kg). Hydrogen about 3 times that from memory. Maybe more.
>Hydrogen volumetric density is low, but you asked about mass.
>
>Brain says that electric motor probably get best results from
>permanent magnet units as no energy is used in magnetic field
>production BUT field stengths have an upper limit set by best
>available materials (various rare earth magnets) and if you were happy
>to waste energy a wound field MAY get higher values.
>
>Power out is primarily a function of the amount of power you can get
>into the motor :-). This is translated into creating interacting
>magnetic fields. Simplistically, increasing voltage and decreasing
>current will tend to allow higher energy densities as resistive losses
>decrease with square of voltage, but here is a point at which other
>factors take over.

I doubt this is true. The field strength is dependent on ampere-turns--
if you have a fixed weight = a fixed area cross-sectional window for the
windings, then the total resistance increases with the reciprocal of the square
of the wire cross-sectional area, so the current must drop proportional
to the wire cross sectional area to keep I^2*R power loss constant in 
the winding
volume. When you look at ampere-turns, it's thus a wash since the number
of turns increases proportional to the reciprocal of the wire cross-
sectional area. That's leaving volume and RPM constant and playing with
the wire size.

Some of the RC model motors look to have interesting power-to-weight
ratios.. a 1.8HP input (briefly, I should think) 14V motor with a ratio
of 5W per gram.... I even see a 1700W unit with 10W/gram pwr-in/mass!
Of course these use Neodymium rare-earth magnets to get that kind
of performance.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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