Servo - November 2012

comparison of power. For example, a . 15 brushless motor will have the same power as a gas engine with a . 15 displacement. Also be aware that many times a . 15 gas or electric motor will be referred to as a 15.

3. Stator Size. On an outrunner brushless motor, the rotor is the outer part of the motor. It has permanent magnets attached to it. The rotor is the part that turns. This is what you connect your propeller to. The rotor is wrapped around the stator. The stator contains the coils which — when energized in a particular order — cause the rotor to turn. Many manufacturers now use this when describing their motors. For instance, a 2826 motor will have a stator that is 28 mm in diameter and 26 mm long. Along with the stator size, a kv rating is given. The kv rating is the rpm per volt of the motor. So, a motor with a rating of 2,826-2,900 kv will turn

21,460 rpm with 7. 4 volts.

With these ratings, you can start to compare motor sizes. Even motors that are rated using the can or glow engine size will have kv ratings. Some will also give you the stator size, as well.

Some manufacturers will provide the rotor size and not the stator size. Take the Turnigy 2211 motor shown in Figure 5, for example. The stator is actually 1809. It’s not that big of a deal as long as you know that they are doing this.

A lower kv rating means lower speed and more torque. These are good for driving large props. A higher kv rating means less torque and more speed. These are better at driving small props. A motor will also have a cell rating and max amperage rating. If you exceed either of these,

FIGURE 5.

you could burn up the motor. The amp rating will give you an idea of how much power the motor will draw. If a motor draws 40 amps and you have four of them running, you are going to drain your battery quickly unless you have a very large battery.

ESCs

While there are many Electronic Speed Controllers available, it is very easy to choose one. First, since we are using brushless motors, we need a brushless ESC. You choose an ESC based on its amperage rating. Two ESCs are shown in Figure 8. The larger one is 30 amps; the smaller one is 12 amps.

If your motor’s maximum amperage is 25 amps, you will need the 30 amp ESC. If you have a small craft that only pulls 10 amps, then the 30 amp ESC would be overkill and add unnecessary weight to the craft. In that case, you would choose

FIGURE 8.

the 12 amp ESC.

Various ESCs have features that you may want. For instance, on multi-rotors we use small computers for control. We want an ESC that is capable of responding quick enough to keep up with the controller. The type of connectors that come with the ESC may dictate which one you choose for a given project.

Batteries

In order for your quadcopter to get off the ground, it needs power. We will be using Li-Po batteries as our power source. A Li-Po's power-to-weight ratio is the best choice at this point in time. They have more capacity than their predecessors, so for the same storage capacity you have much less weight.

Li-Po batteries (like the ones shown in Figure 9) come in many different sizes, shapes, and configurations. They use a rating system that shows the number of cells in the pack. A battery marked 3S1P means that it is three cells connected in series to make the battery. A 2S1P will have two cells connected in series.

Li-Po batteries also have discharge ratings. This will be marked as an XXC number. A 25C means that it can be discharged at a rate of 25 times its capacity. So, in the case of a 2,200 mAh battery, you could discharge it at a rate of 55,000 mAh or 55 amps, without the battery being damaged.