Electric Flight Wiring

September 1, 2002  for  Sailplane & Electric Modeler Magazine

Recently, someone e-mailed to ask if I had information on wiring an electric power system for a model airplane. It seems that it isn’t all written down in one place. For those with electronics experience, wiring may seem obvious, but electric flight is reaching more and more modelers, many of whom do not have an electrical or electronic background.

Circuit Basics

In any electrical system, current must flow in order to get any work done (like turning a motor). In order for current to flow, there must be a circuit, which is simply a path that starts at one terminal of the power source, passes at some point through the device doing the work (the motor in our case), and eventually returns to the other terminal of the power source.

Figure 1 illustrates the simplest possible circuit that could power an electric model. It consists of only the battery and the motor. Such a simple circuit would not be practical, since the motor would run all the time until the battery went dead, and there would then be no way to recharge the battery.

The simplest practical electric flight circuit is shown in figure 2, which adds a switch and a pair of connectors to the previous circuit. The switch turns the power to the motor on and off, and should have a current rating as high as the expected operating current. The switches shown in the diagrams are single-pole, double-throw. When switched to the left, the switch connects the center and right terminals to each other; when switched to the right, the center and left terminals are connected. If the switch is hand-operated, then the plane has to fly until the battery is dead. Alternatively, the switch can be operated by a servo, in which case the power can be switched on and off at will by radio control.

The connectors allow the battery to be disconnected for recharging, and allow another battery to be used while the first is recharging.

The next two improvements we can make are to add a second switch to be used as an arming switch, and to include a fuse for safety. Figure 3 is such a circuit. Electric motors have the characteristic that they will draw more current if they are restricted from turning. This is why a motor with a propeller attached requires more current than one with nothing on the output shaft, and why a motor whose propeller is stuck (in the ground for example) will draw more current than one in flight. A stuck propeller can often draw enough current that the battery, motor, or wires get hot enough to start a fire. With the right fuse, the fuse will blow before this can happen.

Notice that the arming switch is just another switch, possibly the same as the power switch. In fact, it doesn’t matter which switch is the arming switch and which is the power switch. As long as either switch is turned off, the motor will not run. By turning off the arming switch, you are protecting against a stray radio signal accidentally starting your motor.

Braking

When the power is disconnected from an electric motor, it spins relatively freely, especially if a propeller is attached to the shaft and the prop is moving through the air. With electric sailplane models, it is desirable to stop the propeller from turning, so it can fold and thus greatly reduce drag during the glide (even a non-folding propeller will produce far less drag if prevented from free-wheeling).

Fortunately, another characteristic of permanent magnet DC motors is that they stop spinning freely if the motor terminals are electrically connected to one another. When a motor is being driven by its shaft (instead of the motor driving the shaft), it acts as a generator. If there’s nothing connected to the motor terminals, no current will flow, because there’s no circuit. Since the generator isn’t doing any work, it is not hard to turn, so it spins freely. If however we create a circuit, the generator becomes harder to turn. A short-circuit makes it the hardest to turn (a short-circuit is merely a circuit with very low resistance, and hence very high current). So, by connecting the motor terminals to one another by a length of wire, we make it hard for the propeller to free-wheel, causing it to stop.

By using a double-throw switch, we can modify the circuit from figure 3 to make such a connection to the motor terminals whenever the circuit to the battery is broken. This is illustrated by figure 4. When the switch is to the left, everything is the same as in figure 3, but when the switch is to the right, power is disconnected from the motor (as in figure 3), and the positive motor terminal is connected to the negative one, providing the braking action.

Electronic Speed Control

Although a simple on/off control circuit as we’ve seen so far is suitable for an electric sailplane, and for some low powered models, it’s not very practical for day-to-day sport models with glow-like performance. Full power is not needed during the whole flight, and power-off is not usually used either until landing.

An electronic speed control (ESC) gives smooth zero to full power throttle control, just like the throttle arm does on a glow engine (actually better, because one can’t throttle a glow engine all the way down to a full stop and then back up again).

The way the ESC works is just like our power switch, except that it switches the motor on and off very rapidly, generally about 1,500 to 3,000 times per second, which is many times during a single revolution of the motor. The ESC uses MOSFETs (metal-oxide semiconductor field-effect transistors) to do its switching, instead of a mechanical switch. The ratio of the amount of on-time to off-time determines the amount of power reaching the motor. For further details, see An Electronic Speed Control Primer from the Winter 1997 issue.

Figure 5 is a typical electric power circuit using an ESC. Notice that the ESC appears to be connected to both the positive and negative sides of the circuit, unlike our switch which was connected only on the positive side. In actual fact, in most ESCs, the positive wire actually passes straight through the ESC, with just a little power being tapped off to operate the ESC itself. The rapid switching action of the ESC is done on the negative side of the circuit.

(In our switch-based circuits, we could just as easily have put our switches on the negative side, but by convention they are usually on the positive side. ESC MOSFET switches are on the negative side only because the nature of semiconductors makes negative-side electronic switches more efficient.)

In addition to being connected to the motor power circuit, an ESC has to be connected to the model’s radio receiver, from which it receives the signals telling it which throttle setting to run the motor at. This receiver connection can be thought of as a separate circuit, unrelated to the motor power circuit, just as it would be if the power were controlled by a servo-operated switch. As a matter of fact, in some ESCs (those with optical isolation), the connection to the receiver is part of a completely separate circuit. With many ESCs however, the circuits’ negative sides are connected, but not in any way that they should affect one another.

Figure 5 also shows an arming switch connected to the ESC. Not all ESCs have arming switches. Those that don’t should be used with a separate arming switch, connected between the fuse and the ESC on the positive input wire. If the ESC does have an arming switch, it is usually a very small, low-current switch, because it does not directly switch the motor current; it only tells the ESC whether or not is can turn on the motor.

Battery Eliminators

Many smaller electric models and even some larger ones use an ESC that incorporates a Battery Eliminator Circuit (BEC). As its name implies, this eliminates the receiver battery (too bad it can’t eliminate the heavy motor battery as well). The way it works is by tapping some power from the positive motor wire, reducing it to a voltage appropriate for the receiver (usually 5 Volts), and then supplying it to the receiver through the receiver connection. Most receivers get their power through a dedicated battery input connector, and then supply power to each servo through the servo output connectors, but they can also get their power through a servo connector. Since the ESC is plugged into the receiver’s throttle servo connector, it can supply the power to the receiver and other servos.

Figure 6 shows the wiring for a power system using a BEC-equipped ESC. Notice that the fuse has moved, and is now between the ESC and the motor. Because the ESC is supplying the power to the radio receiver and servos, we can’t allow power to the ESC to be cut off, even if the fuse blows. If the ESC did lose power, the radio system would stop working too, and the plane would crash.

Electrical Noise Reduction

One thing that has been omitted from all the power system diagrams shown so far is filtering of electrical noise. All brushed motors produce some level of electrical noise. If this is not suppressed, it can interfere with the model’s radio system. I wrote a detailed description of noise suppression in the May 2000 issue, but figure 7 shows a typical noise reduction filter. There is one capacitor connected between each motor terminal and the case, and one between the two terminals. The capacitors are often supplied with motors intended for radio control applications. This arrangement can be used with any of the power systems in figures 1 through 6.

Some ESCs also require the modeler to install a diode across the motor terminals. When this is the case, the instructions accompanying the ESC will describe how to do this. The diode serves to increase motor/ESC efficiency, and also to reduce electrical noise.

Brushless Power Systems

Power systems using brushless motors, such as those from Aveox, Astroflight, MaxCim, are wired the same way as regular brushed-motor systems from the battery up to the ESC. The difference lies in the connection between the motor and the ESC. Just like a regular ESC replaces the mechanical switch of a simple on/off power system with a high-speed electronic switch, a brushless ESC also replaces the mechanical commutator found inside a brushed motor with a set of six electronic switches. Effectively, part of the motor is integrated into the ESC.

The connection between a brushless ESC and motor usually consists of three wires, connecting to the coils within the motor. Some older motors and ESCs also have another set of five thinner wires which carry information about the rotation of the motor back to the ESC. The newer sensorless ESCs don’t have these extra wires. Figure 8 illustrates a typical brushless hook-up. Notice that the fuse is between the ESC and the battery connector, but recall that the fuse cannot go there if the ESC provides a BEC. Unfortunately, there’s no place to put a fuse in a BEC-equipped brushless power system.

When connecting a brushless motor and ESC, follow the manufacturer’s directions as to which wires to connect, maximum wire lengths, using a fuse, and so on. Because brushless motors and ESCs are not yet as common as brushed systems, the instructions furnished with them are usually fairly comprehensive.

Wire Sizes and Connectors

Electric flight power systems operate at relatively high currents, so it is important to use sufficiently large wire, and good quality connectors. As a rule of thumb, Speed 400 power systems should use at least 18 gauge wire, and preferably 16 gauge (the smaller the gauge number, the thicker the wire). Speed 600 or 05 sized power systems should use at least 14 gauge wire, and preferably 13 or 12 gauge. The same goes for larger power systems operating in the 20 to 30 Amp range. Higher current systems as used in some high performance models should use even heavier wire, such as 10 or 8 gauge.

The wire used should be flexible, consisting of many fine strands, and covered in a heat resistant insulation. Most electric flight suppliers sell silicone insulated high-flex wire especially for this purpose. Automotive wire, although available in the desired gauges, is far too stiff and will quickly wear out from repeated flexing when changing batteries. Because of its stiffness, it can also put strain on motor terminals and solder joints.

Even more important than good wires are good connectors. Many battery packs and some ESCs come with poor quality connectors pre-installed. These connectors work fine for a while, but after a very few insertion/removal cycles, they develop very high resistance, robbing power and generating very high heat (often enough to melt the connector housings). Use a good quality, low resistance, high current connector, such as the Astroflight Zero-Loss, Anderson Powerpole, or Deans Ultra Plug. There’s a detailed analysis of different connectors in my January 2000 column.

                    

Related Articles

If you've found this article useful, you may also be interested in:

• What About the Wires?
• The Battery Eliminator Circuit
• On Fuses in Electric Flight
• NiCd and NiMH Battery Care
• To BEC or Not To BEC
• Electric Flight Power Connectors
• Using Sanyo 1100AAU Cells for Speed 400
• An Electronic Speed Control Primer