Re: comparative efficiency test


"Camilo" <camilogil@xxxxxxxxxxxxxx> escreveu na mensagem
news:1194730566.552533.72740@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Sounds like a daft method if you ask me - unless your motor is very low
rpm.

is daft but cheaper, I have no money to buy a dynamometer and
initially I wouldn´t need precision, also I was supposing a case (I
know there are electric motors with very high efficiency, close to
100%) ... anyway, the fact is, when I´ll need measuring precision
obviously I will use a dynamometer or torque meter but I just wanted
to know about a cheaper test (that I forgot to inform to you)

The torque of any motor depends on speed.
At zero speed, the stall torque is maximum.
At no load the torque is zero and it achieves the maximum RPM.
Between those two extremes the torque falls down almost
linearly.

DC motors have a characteristic curve.
You can see the trivial characteristic curve for small DC motors
on a large catalogue here:
http://www.transmotec.com/PDF/Catalogues/Motors_DC_Catalogue_1W-100W.pdf
There is point for maximum efficiency.
Therefore, efficiency is highly variable for any motor, depending
on the speed (RPM).

The torque depends on the quality of the magnets.
Nice strong magnets produce high torques and very high speeds
at no load. Torque depends also on current squared.

With a string on the shaft, used to lift a weight, you can measure
the maximum torque at zero speed, only.
The best will be a large wheel, with lots of inertia and compare
the two motors (time to achieve a given RPM and take the values
for voltage at motor terminals. You also need a very good and
expensive current meter, or else use a by-pass and only measure
a tiny portion of the total current with a normal Ampere meter,
but this is very difficult if you cannot accurate measure the
resistances to have the same ratio in both cases).
Small DC motors mostly depends on the magnets and some
new kind of magnets are several times better then the old ones
for power, and also they are more efficient.
Large DC motors don't have magnets, but coils in the stator.
The more current the stator produces the better for torque.

I guess you cannot produce a small DC motor better then
the specialists do, like those of the link above (they have the
curve for efficiency ploted).
http://www.transmotec.com/default.aspx


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