An Introduction to Electric Power Systems (Cont'd.)

By Ryan McConnell

The next thing to figure out is what is the watt rating of the motor?  Some motors state this directly, others do not.  Rather they state what the expected amp draw of a motor will be with various props.  The watts can be calculated pretty easily from this. The formula is:

amps * volts = watts

So if a motor chart states that it will pull 18 amps using a 3 cell battery on a given prop, we can figure this out.  Lithium Polymer (LiPo) batteries are just about a given these days, so we'll plan for those.  A Lipo battery is generally figured at 3.7 volts/cell.  The motor data stated a 3 cell pack, so 3.7 * 3 = 11.1 volts for the nominal pack voltage.  Using the "amps * volts = watts" formula, we find:

Easy right?

This would be a good motor if the airplane were say a 2 lb (32 oz) trainer type airplane, or any airplane where just basic aerobatics were desired. Loops, rolls should be possible, but not unlimited vertical climbs.  If you wanted that I would look for a 250 watt or more motor.

I need to mention one other motor parameter here - The KV rating.  This is the RPM that the motor will attempt to achieve.  A motor rated at, say, 950 KV will try to spin at 950 rpm for every volt of electricity given to it.  Earlier we mentioned using 3 cell packs, so the voltage here would be 11.1 volts.  So in this case:   11.1 * 950 = 10545 rpm.  The RPM won't quite get that high (drag, motor inefficiencies, etc) but it's close enough for our purposes.   This is just like a glow engine… and you are already used to rpm's and such there.  The part that is different is more relevant to section 2 coming later where I will get into things like varying the cell counts on the battery to adjust amp draw and how that inter-relates to the KV choice you should go for on the motor.  But for the purposes of this "beginner" section we are going to assume that you will stick to the very popular 3 cell lipo battery sizes that are so prevalent in the market today.  For these you are looking for a motor in the 900-1100 kv range. Prop the motor according to the manufacturer suggestions!  Electric motors do not work like glow engines. If you overprop the motor you can significantly damage the motor, battery and ESC!.  More on this in section 2.

Battery Choice:

Now that you know how to choose a motor, let's choose a battery.  Batteries have three major things to choose from when picking one:

• Cell count
• "C" rating
• Capacity 

Cell count:

This one is easy! We stated that in this beginner section we will stick to 3 cells.  Three cell packs will work for the majority of airplanes that are ARF's and available out there today.  Everything from a 10 ounce airplane to a 60 ounce airplane can be made to fly perfectly on a 3 cell setup.  When you would want to go to a larger battery is usually in the larger airplanes (40 size, 60 size, and even giant scale!) or in special applications like an electric ducted fan or hotliner where speed is usually the goal. More on these in section 2 later on.

"C" rating:

This is pretty simple too. It is basically how many amps the battery is capable of supplying.   So to choose the right battery you need to know how many amps you need.  In our earlier example we had a motor that would draw 18 amps continuous.  So let's say that we were using a 3 cell pack.  If I had a 3 cell, 1500 mah pack rated at 10C, would that work?

Here is how you calculate it:

To determine the max draw [in Amps] you take the capacity (1500) and divide it by 1000.  This will give you the 1C rate.  The battery is stated as a 10C pack, so then multiply the 1C number times the pack rating to get the max continuous draw.

1500 / 1000 = 1.5
1.5 * 10C = a 15 amp rating

Not enough. So we would need to vary something here… either we need a larger capacity, maybe 2000 mah (2000/1000=2C 2*10C=20 amps) OR a higher "C" rating on the pack. Going back to the 1500 battery, maybe rated now at 15C, 1500/1000=1.5 1.5*15=22.5 amp draw.

Either one works!  The battery capacity will determine your flight time, at the expense of weight.   The "C" rating doesn't usually change the weight of the pack much, but does frequently change the price.   Higher "C" rating packs tend to cost more than lower "C" rating packs.  These days 20C packs are almost a standard, with 25C and 30C packs becoming more popular.