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Building an 05 Motor Dyno
By Jay Turner
Introduction
There has been a lot of interest
lately in electric motor dynos. I’m not certain why this is, except that many
racers seem to feel that if they had a dyno they could make their boats run
faster. The truth is, a dyno may or may not be of any help to the average FE
racer.
Most hobby dynos available for
car motors are not well suited for boat racing. The dynos seldom will allow high
amp testing, and that is where we need to test our marine motors ( 40 to 50 amps
). Also, they usually supply incorrect voltage to the motor, complicating a true
test of the motor. They are also expensive, and the vast majority of fat
electric racers would be better off spending that money on top grade cells or
motors.
A simple dyno will tell the racer
how many amps his motor will draw at a certain voltage and rpm. From this he can
calculate the power produced, but what good is that really? The dyno will also
tell him that his 15-turn 05 motor doesn’t produce as much power as his
11-turn motor does, but he didn’t need a dyno to know that. Even if he knows
the maximum power and rpm, he will still have to experiment with props in the
boat on the water to find the right one. So, what good is a dyno to the average
FE racer?
For the average racer, the
biggest advantage to a dyno is in fine tuning his motor, either with brushes,
springs, or timing. He can also compare two or more similar motors ( ROAR motors
come to mind ) and see which one is putting out the most power. If the racers is
expecting to run substantially faster by using his dyno, he’ll probably be
disappointed. If he wants to get the last little bit of power out of his motor,
then the dyno may be of help.
Parts Needed
I built a simple dyno for testing
6 and 8 cell motors several years ago. This apparatus can be built for under
$120, perhaps a lot under if the racer already has some of the parts. It can be
modified slightly for other motors and cells counts. The parts needed are:
1 x 10 x 24 wood plank
AstroFlight Whatt
Meter or equivalent
Digital Mini-Tach -
Tower or GloBee, etc.
On/off Toggle switch
Airplane Prop hub for
05 shaft
Variety of small
airplane props [ 4.5 x 2, 5 x 3, 6 x 2, 7 x 3 ]
Prop Shaft Adapter -
Hobby Lobby
Wire and Connectors
Battery Pack to match
boat.
Misc. Wood and glue
Assembly
Mount the motor on a spacer
block high enough off the wood plank for a 7” diameter prop to clear. You
could use one of the plastic motor tester mounts, but these lack strength if
you are looking for a high power 6 to 8 cell motor. I used an aluminum motor
mount out of a boat.
Attach the motor/mount/spacer
block securely ( use screws from underneath the plank ) near one end of the
plank, but position it so the prop does not extend past the end of the plank. I’m
right handed, so I put the prop on the left end of the plank. The tach will be
securely glued ( silicone works well ) to a block of wood positioning it about 1”
to the right of ( behind ) the prop and high enough so that the prop arc passes
in front of the tach input port.
You can mount the Whatt Meter to
the plank with servo tape, positioned a bit further to the right of the motor
mount assembly. Attach the on/off switch to the board between the pack and Whatt
meter and mark the OFF position plainly. A strip of velcro will hold a battery
pack near the right end of the board. I used LightSpeed connectors and 14-gauge
wire between the motor and switch, the switch and meter, and meter and battery.
This allows the motor, Whatt Meter and pack to be removed easily.
The airplane prop will produce a
lot of thrust, so I recommend that you glue a small brick or other heavy object
to the left end of the plank. This will keep the dyno from “taking off” when
testing a powerful motor on 8 cells.
Running the Dyno
Before we start to test the first
motor, I want to stress the fact that you CAN be hurt while operating the
dyno if you are not careful. That airplane prop can cut you badly, so ALWAYS
wear safety glasses, stay to the right of the prop arc, and NEVER allow
anyone to stand anywhere near the left end of the dyno! This type of dyno has
been used for many years, and I haven’t heard of anyone seriously hurt by one,
so don’t be the first.
Securely mount the test motor to
the motor mount, then attach the first prop. I suggest using one of the smallest
props first. Make sure the adapter and prop are securely attached, then plug the
battery pack into the meter. The meter should read no volts; if it reads
voltage, the switch is in the ON position - turn it OFF now. Only
when you are certain that the pack is turned OFF can you then plug the
motor into the meter. Now the prop won’t start up and startle you when you
plug in the motor.
Turn on the dyno. The prop will
make quite a bit of noise - this is normal. Stop the motor after a few seconds
and unplug the pack, then check the tightness of the prop and adapter. If all is
well, turn on the tach and then the dyno. Record the rpm and the amps and volts
from the meters, then shut the dyno off. Short runs are best so that the motor
temperature stays cool. Top off the same pack on the peak charger ( do this
between every test ) and change to the next larger prop. Repeat the
procedure for several props, but stop when the amp draw exceeds 50 amps - no
need to go higher for most applications.
What does it all mean?
Now that we have some numbers,
what do we do with them? I could turn this article into a complex technical
discussion, but that won’t help many racers to use their dyno. For now, I’ll
explain how to use the dyno to compare motors or motor tuning changes.
For an example I’ll make up
some motor data from the dyno for two different motors:
Motor A:
6 x 2 prop 25,000 rpm 15 amps 7.0 volts
7 x 3 prop 15,000 rpm 40 amps
5.8 volts
Motor B:
6 x 2 prop 30,000 rpm 20 amps 6.8 volts
7 x 3 prop 20,000 rpm 55 amps
5.1 volts
The comparison here is easy,
since we can see that Motor B turned both props faster than did Motor A - see
the chart below. Since each prop absorbs the same amount of power at the same
rpm, spinning it faster means that more power is produced. Motor B also drew
more amperage from the battery, another indication that it was producing more
power.
We can do the same thing when we
are testing different tuning setups, although the differences will be much
smaller. For an example, Setup A is with copper brushes and medium springs,
while Setup B is with silver brushes and heavy springs:
Setup 1:
6 x 2 prop 25,000 rpm 15 amps 7.0 volts
7 x 3 prop 15,000 rpm 40 amps
5.8 volts
Setup 2: 6 x 2 prop
24,000 rpm 20 amps 6.8 volts
7 x 3 prop 17,000 rpm 45 amps
5.6 volts
Again we can see that the Setup 2
turns the props at different speeds. While this brush/spring combination lost
some rpm at low amp draw, it spun the 7 x 3 prop faster so it made more high rpm
power. This means that the “2” setup is better for racing where we want
maximum power, but the “1” setup is probably better for sport running since
it draws less amperage and will run longer.
There are other ways to use the dyno data, but
that can be covered in a different article. I hope that this helps you to
understand what a dyno will and won’t do, and if you build one I hope you
enjoy analyzing and learning about our hobby motors
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