Q-40: What happens if you attempt to use an electric motor speed controller that’s either too big or too small (in amps)? How do you know which one to use? It seems each local hobby dealer has a different explanation.
A-40: I find it hard to believe that local hobby dealers have
different explanations. When operating any electric motor, you must know the
current drain (in amps). That figure can vary depending on the type of motor,
the voltage (number of battery cells), the propeller size, and the type of drive
(direct or gear/belt reduction).
It is important that you know the total current drain from your battery for your
particular setup (application). That is why I have repeatedly suggested that you
purchase an AstroFlight digital meter that can quickly indicate motor current,
voltage, and power (wattage).
With the motor current known, selecting the proper ESC should be easy. Let's say
you measure your Speed 400 direct-drive motor current, and it is determined to
be 10 amps. Using a 10-amp-rated ESC is okay. Most ESCs are rated for maximum
continuous current. Any ESC rated higher than 10 amps is fine for your
application.
However, keep in mind that the higher the ESC's current rating, the greater the
weight (and cost!) of that device. So you wouldn’t want to consider a
40-amp-rated ESC for use in a Speed 400-powered model where the motor is only
drawing 10 amps.
On the opposite side, any controller in this same example that is rated for less
than 10 amps might be expected to burn out. The lower the rating (less than 10
amps), the faster that burnout is going to occur.
With many individuals switching to Li-Poly battery power, it is also important
that you be aware of your ESC's voltage cutoff point. In the “Introduction to
Lithium-Polymer Batteries" article I wrote (published in the May 2004 <I>MA<P>),
I indicated how I switched from NiMH batteries to a two-cell Li-Poly battery for
my little Scratch-One electric-powered RC trainer. I used a Jeti JES-110 ESC in
that aircraft, but I didn’t make a point of indicating the cutoff voltage in the
article; it is 5.5 volts.
Normally, Li-Poly cells should not be taken lower than 3.0 volts per cell (or
6.0 volts for two cells in series). At 5.5 volts, that works out to 2.75 volts
per cell. That is slightly lower than I would have liked, but it's still in the
acceptable range.
New ESCs that are coming onto the market, such as FMA Direct's Super series and
certain types from Castle Creations, set the voltage cutoff automatically or
allow you to set that point yourself (a custom setting). This is the type of ESC
you should consider for the future. In the meantime, make sure you inquire about
any ESC's voltage cutoff point to verify that it is acceptable for your
particular battery application.