| RFI Radio Frequency
Interference can become a real headache. No matter what type of
motor you choose, try to find one that is of the "Fully Shielded" types, to avoid
RFI. Here, the cheaper motors that you may find
are less shielded, or worse, not even protected at all. Often those that do cause some Servo
Chatter, may only need a set of small "RF Noise" suppressing capacitors to reduce this annoying
effect. R/C Noise or R/C Chatter, is
caused by sparks from the motor's windings & brushes. Usually a pair of small
"capacitors" from an electronics' parts store ( Radio Shack ), rated at .01 to
.1 MFD. ( Microfarad ), that are "Non-Polarized" types, stop this
problem. Non-Polarized, means the Caps. have no - Negative or + Positive
polarity. So they can be attached to each motor terminal, with the other lead,
soldered directly to the can (motor metal casing). This forms the "RF
Shield" needed to reduce its noise, or ground it out, which causes "Speed
Controls" or their Servos ( Or Rudder Servo ), to Go Hard Over, Wiggle or make them -
Go Nuts Sometimes!
Testing Motor Total Current
Load
Clamp the motor in a bench
vice, then use a set of pliers to lock its drive shaft - Tight. So it
won't spin as its full voltage 3 - 6 or 12 volts ( whatever its voltage rating is ), is
applied. Then apply your DC Amp Meter- leads, across one motor terminals
and the other connected to the battery. Caution - be sure you use a "DC Amp or Multi-Metter" that reads
Amperes, capable of handling more than the motor draws?
Here a good meter may be
purchased to read over 15 Amps DC, from Radio Shack or any electronic or Electrical Supply
parts store.
Use a set of jumper leads
to provide the battery, for a few seconds. As you watch, check the meter
reading, with the motor shaft locked tight, under full voltage.
You will
see what the motor draws, but understand for a split second as the voltage is applied, the
motor will draw even more. That's called Surge Current, as it attempts to
start turning. So you will see the amp |
|
meter jump up, for just a
split second to a higher Amp load, then fall back to reveal what its demanding from the
battery. And what the motor will apply to its speed control, under full
load. Note that he higher the load is from the motor, the less time
your battery will last, determining how long the model is run!
Here also, a modeler that's
building very small scale boats, find small motors don't draw anything close to 1/2 Amp in
total load. But here again, because of their scale hull size and limited space for
the battery, speed control and other items., this test theory works.
Some Final Thoughts
1. Allow your motor to turn
If you load the motor too much (with a big
propeller) the amp draw will go up and your duration will come down
(decrease). Make sure your drive
train turns freely.
2. Less turns, more amp draw
equals
higher rpm!
Match the motor to the job you plan to
accomplish. Remember than just dropping in a R/C car motor is not the best
idea. Although I covered some of these motors, the basic knowledge of these types of
motors is the same for other types.
3. Small propellers at high revs are not
efficient
They'll cause lots of cavitation. (with a
characteristic noise) and the boat runs less efficiently. Most scale
propellers I
have seen do not recommend that they be turned more than 5,000 rpm. Keep this in
mind.
4. If the motor and battery are too hot
This means that you are probably using a
propeller too big for the setup. If the motor is too loaded then the amp draw goes up and
duration will come down. The solution is to use a smaller prop, or a gear drive. |
