The no-load RPM value doesn't tell you much by itself. The stall torque
and current are better properties to know, but they still don't tell the
whole picture. What you want to know is how fast the motor will turn
(RPM) and how much current (AMP) will be used for a given workload
(Torque). In other words, it would be best to look at the RPM and AMP
draws for each motor over the expected range of workloads for your
application. Back in my warship days, we spent many an evening
measuring motor performance/cost to find just the right drive and pump
motors for our fleet of ships. (Which is why I still have a couple boxes
full of different types of motors in the workshop.)
In general, I would expect that any motor designed to efficiently spin
an airplane propeller (such as the Great Planes motor) would not work
well when used to spin a pump impeller or move a mechanical load. Yes,
you can add lots of gearing to make it move the load, but the gearing
significantly reduces the overall efficiency. Also, motors designed to
spin an airplane prop don't have to be overly concerned with
over-current problems, which allows them to be made lighter. That is,
since the prop is not expected to hit something hard (unless you've
really had a bad flying day), stall currents will never be experienced.
If you adapt them to workloads that are capable of causing the motor
to stall, they could generate more heat than the motor parts can handle.
For example, door lock actuators are cheap little motors that are
mechanically stopped when the arm reaches the extreme position. They
work very reliably for their intended purpose. But, if you keep applying
the current for even just a few seconds, the motor winding wire will
simply melt. A classic case of designing the motor to work well only
within a given performance envelope to save money.
So, in addition to examining the RPM/AMP/Torque performance curves, you
should also consider the intended purpose of the motor to determine if
there could be design limitations.
On 7/28/2015 12:07 AM, Doug Conn wrote:
Hello –
We’ve had a few discussions regarding brushed vs brushless motors. The
most important point I remember was that brushed and brushless ran at
similar energy efficiencies under load. I’m wondering about efficiency
with respect to size and weight.
Let’s compare this brushless motor
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLWY0&P=7
<http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLWY0&P=7>
to this brushed motor
http://www.andymark.com/CIM-motor-FIRST-p/am-0255.htm
Both are in the 300 watt range. At 12v, the unloaded brushless motor,
rotates at 32k rpm and the brushed motor at 5.3k rpm. Of course, the
brushless motor would need a lot more gearing to make it usable in a
tank. That aside, could I expect roughly the same performance from both
motors ? It’s hard for to believe that because the brushless motor is
1/14 of the brushed motor weight and 1/7 of the volume.
Is there another factor (besides cost) I need to take into account when
comparing a brushed to a brushless motor ?
Thanks,
Doug
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