■ PHOTO 5.
from the graph is
that the motor does
not start turning until
around nine or 10
seconds into the test,
which is where
sufficient voltage has
built up from the
PWM. The purple
RPM line continues
on to a maximum of
around 8,300 RPM.
The green line represents the
amps required to run the motor. As
you can see, it starts right off at about
. 3 amps and slowly climbs until the
motor just starts to turn, at which point
there is a blip, after which it steadily
climbs to a max of around 1.6 amps.
The red line represents the voltage from the battery. As the RPM and
amps increase, the voltage from the
battery drops in concert, starting off at
12. 85 and dropping to around 12. 6.
A couple of additional points to
note: Once the motor starts turning
(represented by the purple RPM line) it
tracks (is proportional to) the voltage
which, in this case, is the gray throttle
line. This is a characteristic of a DC
motor. Up near the top of the graph at
close to 100% throttle, you can see all
the measured inputs getting very
bouncy and jagged. I would surmise
that this is because the magnetic fields
are breaking down in the windings,
but I am not a DC motor expert.
■ PHOTO 6. Real time ‘run’ graph.
comes with an electronic scale which
would normally be used to measure
the thrust of a spinning propeller;
useful I guess, if you want to know if
you have sufficient thrust to fly. But in
our case, we have no propeller and
we really want to measure torque.
The Internet and the advice from
a friend are wonderful things. After
much searching, I came across a
device called a “Prony Brake” which
was developed by French mathematician Gaspard de Prony (1755-1839)
and can, in fact, measure tourque. To
keep things simple and in line with the
rest of the construction, I made my
Prony Brake out of wood, which you
can see in Photos 7 and 8.
The construction is fairly straightforward; a hole the same diameter of
the motor shaft is drilled through a
piece of wood and at exactly six inches from that on the other side (other
plane), a small screw is partially
screwed in. A saw cut is made though
the shaft hole, extending quite far
down the wood. (You will need to look
at the pictures to understand this). A
small hole is then drilled through the
wood on the same plane as the screw,
for a bolt, spacer, and wing nut.
The operation of the Prony Brake is
as follows: Open the wing nut so it is
very loose. Slide the slit in the wood
over the motor shaft until the shaft clicks
into the pre-cut hole and then loosely
tighten the wing nut; not so tight though
that the brake does not freely rotate on
■ PHOTO 7. Prony Brake construction.
All right, one last
parameter to measure: Torque. The
Power Analyzer Pro
■ PHOTO 8. Prony
February 2007 15