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 by Bob Aberle |
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The ESC is probably the heart of the electric power system. The ESC in electric-powered flight takes the place of the throttle (or engine) servo used on a fueled model. The ESC in this instance weighs roughly 3Ú4 ounce and has two pairs of wires: a servo-type cable and a cable with a switch on the end, all exiting the case. Two wires, with APP connectors already attached, will plug into the motor wire connectors. Polarity is important, so it is red to red and black to black. Two more wires will have APP connectors attached, and they will plug into the battery pack. The third cable has a servo cable connector on the end. That cable is plugged into the throttle port (usually the number-three position) on your RC receiver. The switch will be mounted on the side of the fuselage and must be manually turned on to activate the entire electric power system. There are all kinds and sizes of ESCs on the hobby market. The one chosen for this project is the Jeti 110. The ESC will be rated for current; in this case it is 11 amps continuous operation. For our application we need 8-9 amps, so there is a margin of safety. If the rating wasn't high enough, you could overheat and possibly burn out the ESC. The rating must also take into account the number of cells in the battery pack. The Jeti 110 can handle six to 10 cells. We will be using an eight-cell battery. Many small ESCs (such as the Jeti 110) will contain what is called a "Battery Eliminator Circuit" (BEC). It will permit the main motor battery pack to also power the onboard RC system (receiver and servos) on a shared basis. This saves the weight of an extra airborne battery pack. It is also a convenience because this one battery is recharged for every flight. In actual practice, the BEC has a special circuit that provides a regulated 5 volts to the RC system via that cable that is plugged into the receiver throttle port. When the battery gets down near 5 volts the circuit cuts off the motor, but it still provides the necessary power to operate the RC system so that you can safely land the model. Most ESCs with this BEC feature (the Jeti 110 included) will allow you to briefly restart the motor after the first shutdown. You do this by moving the throttle stick on the transmitter all the way to idle, then back up. That resets the ESC and will allow a few more seconds of power so that you can line up on the runway for a safe landing. Most modern ESCs (including the Jeti 110) employ "smart" circuitry via a microchip, which can add considerably to the safety of electric motor operation. Remember what I wrote earlier: when the battery is attached to the motor, it could start instantly. If you have the throttle stick at full or partial with the entire system plugged in, the motor and propeller could start turning. The microchip in the ESC will sense anything other than a dead idle position and prevent the motor from starting. To start the motor you must physically move the transmitter throttle stick down to idle then go back up. The motor will then start, and its speed will be proportional to the control-stick position. The Battery: This is also an important part of the electric power system. Batteries come in all types, sizes, weights, and capacities. The choices are critical to the model's performance. For this project the choice of battery has been made for you; it is a Nickel Metal Hydride type (NiMH) consisting of eight cells made up as a pack. Each cell has a capacity rating of 1100 milliampere-hours (mAh). The nature of these ratings will be explained in later articles. APP connectors have thoughtfully been attached to the two wire cables. As in the case of the motor connections, the polarity is critical to the system's correct operation. It is always positive to positive and negative to negative. If the color-code convention is followed, it is usually the usual red to red and black to black. However, not everyone uses that color convention. Do not mix up connections between the motor and battery. The connectors going from the ESC to the motor are generally connected once at the time of initial installation and not touched thereafter. Placing a piece of masking tape on each connector can remind you not to touch them until such a time as you transfer the equipment to another aircraft. Estimating that this direct-drive Speed 400 motor will have a current of 8-9 amps, this particular battery pack should be capable of providing six to seven minutes of electric power at full throttle. In reality you will have much more power than you need; therefore, you will be able to throttle back during a normal training flight. Throttling back reduces the motor current, and, as a result, increases the flight time. With average throttling back it will be possible to extend your flying times to 10 minutes or so. Owning more than one battery pack will allow you to fly on one while the other is on charge. That will provide you with more flying time and less waiting time.
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