On 09/02/2010 09:23 PM, Michael Koblic wrote:
I have been trying to work this out from basic principles but the
answers are at best foggy:

Consider two electric motors (commutator) with identical inputs, say
250W, and identical RPM, say 15,000. What are the factors that determine
the motor efficiency and thus the output power?

If I understand the physics, the output power is directly related to
torque if rpm are held constant. What then, in the nature of the motor
construction, will increase its torque? Is the diameter of the rotor one
of the factors?

From the outside, the motor efficiency is (useful output power) /
(input power). The rest is lost, usually as heat (hopefully not as
noise, but you never know).

Your question about increasing the torque is misleading, though: you
don't want to know "what increases the torque" so much as "what
increases the _available_ torque at my desired output power level".
Diameter of the rotor is, indeed, one of the factors.

As an example of this, in the model airplane world there are two basic
choices in brushless motors: "regular" inrunner motors, that put the
field magnet on the motor shaft inside of the windings, and "outrunner"
motors that put the field magnets on the motor housing, which spins with
the motor shaft. Outrunner motors generate more torque at lower speeds
-- and lower efficiencies -- than inrunner motors. Why use them?
Because they don't require gearboxes, and as a system they're lighter
and more efficient.

So the question you wanted to ask was "what are the loss mechanisms?".
The answers that I know a

* Windage. The motor stirs the air, and maybe even has a built-in fan.
That takes power away from the output shaft.

* Friction. Bearings. 'nuff said.

* Hysteresis losses. Those coils are wound on iron, and it takes a
certain amount of energy to reverse the field in a piece of iron. That
energy heats up the laminations, but does you no good. Good magnet
steel has lower hysteresis, and good motor design puts enough of it in
that the hysteresis loss is lower. Poor motor design...

* Eddy currents. The iron those coils are wound on are conductive, and
the coils induce currents in the iron. The current takes energy to
establish, and swirls around and generates heat. Thinner laminations in
the armature reduces the effect, but thinner laminations increase cost
and decrease the amount of iron you can pack into a given space -- which
increases the hysteresis loss.

* Resistive losses. The armature is wound with wire (probably copper).
The motor current has to travel through this wire, which is resistive
and has losses. More wire takes more space, and requires more iron.
Better wire costs more.

Anything that you can do to reduce one of the above effects either costs
money, takes time to design, makes the motor bigger and/or heavier
and/or less robust, or makes some other effect worse.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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