"The Natural Philosopher" wrote in message
...
On 23/01/14 11:38, John Williamson wrote:
On 23/01/2014 11:11, harryagain wrote:
"John Williamson" wrote in message
When you're making a multipole stator to get a sensible speed out of a
synchronous motor, fabricating a squirrel cage is trivial by
comparison.
Two drilled rings and a handful of rods, brazed or welded together in a
simple jig.

Most motors are small motors and much less efficient, more like 75%.
Very small ones, around 50%


What do you define as a small motor. To some plant designers, a small
motor is 500 horsepower. Fractional horsepower motors (200 - 500 2watts)
are still over 80%, and it's only in places like Hard Drives that the
efficiency drops, although in the case of laptop drives, the motor
efficiency needs to be higher than in desktop computers.


well I can point to to a 50W motor doing well over 90% efficiency.


It will cost you a bit more than a cheap ferrite can motor dong 75% of
course.

And it will be a PM motor with neodymium/other rare earth magnets.



Haryu is of course talking ******** as usdual. There is no technical
reason for a small motor to be any more or less efficient than a large
one. Its just that the value of efficiency in a motor to drive a
toothbrush is very low, whereas the value of efficiency in one driving a
train is rather larger.

The technical reason is that the air gap in small motors is larger
realtively speaking than in big motors.
Also friction losses arerelatively greater.





So, in the case of the latest central heating pumps, the *maximum*
saving in electrical consumption is about 100 watts for a system energy
consumption of up to 24,000 watts. With the waste heat from the motor
helping reduce the amount of primary fuel used on site, so the net
effect is to replace 100 watts of gas heat with 100 watts of electrical
heating.

The motors you quote as having efficiencies of 50% are only found in
toys and small servo mechanisms, so the energy savings are even lower.

Its very hard to get even those below 50%.

I think the worst model aeroplane motor I own, if driven without gears
into a highly unsuitable propellor will dip below 50%. on the right
battery and geared and not trying to extract the ultimate from a package
that isn't capable of it, I can easily get 65%-70%. That's on a brushed
motor with ferrite magnets that cost a couple of quid.

IF I want to go neodymium and brushless, then there are motors that will
top 90%. Naturally they cost more like £50.

Electric motor efficiency is NOT a constant. As current rises magnetic
losses tend to scale with the current, but resistive losses go up as the
square. So low current high RPM is the most efficient, until you get into
rising hysteresis losses and frictional losses. hysteresis means using
better , friction means using ball races instead of plain bearings. This
costs money, and high RPM means using a gearbox.

At some point on the various curves is the optimal point. I ran my 6V
motors on 12v, or even 14v, and geared them to draw less current than the
normally assumed maximum. They ran cool and delivered MORE power than they
did at 6V.

Best of all, they were more efficient the less current they drew, so
cruising around at half throttle netted me enormously long flight times.



My model locomotive would use 3 watts instead of 6. As it is now, if
it's the only thing running, the model railway doesn't even register on
the meter, so I'd say that was not in any way worth while. There are
other reasons to increase motor efficiency in models, though, and a
doubling of efficiency in an aeroplane motor would double the available
flight time, assuming the new motor and its control gear is no heavier
than the old one.


yes. But its hard to double a 60% efficient motor!

By and large the more efficient motors simply end up being more powerful
for a smaller package..with eejits driving them to the bleeding edge just
the same.

There's a lovely bit of physics that says that as a motor gets hotter, its
resistance rises, and that makes it get even hotter, and then as the
magnets get too hot, it draws even MORE current as it want to speed up to
create the same back EMF, so it gets even hotter still...it can happen in
seconds as the idiots ducted fan jet emits a tail of smoke and stops as
the controller blows up. If you are very lucky the battery then catches
fire and the radio control is lost as well.




Nevertheless it is being done. World wide.
http://en.wikipedia.org/wiki/Premium_efficiency

Which, if you bother reading the tables, shows that the new standards
are already reached by most if not all motors sold now.


Exactly. If you want efficiency design for it. Costs a wee bit more,
that's all. If electricity is expensive, then the cost repays itself. If
it isn't, its not worth doing.


But you have to plan for the future, not the present.