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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Outboard motor powered by a lithium battery. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors On the waters, it is increasingly possible to meet anglers using outboard electric motors on their boats. Compared to gasoline engines, they have a number of advantages: their design is very simple, switching on and off is instantaneous, there is almost no noise and no emissions of by-products into the water. The biggest disadvantage of electric motors is the need to have a power source - a battery, the mass of which (usually lead-acid) is several times greater than the mass of the motor itself. This is especially inconvenient when fishing from a rubber boat, in which a heavy battery needs to be placed somewhere. Do not create amenities and connecting wires. At the same time, there are batteries that, with a comparable energy intensity, have a significantly lower mass than lead-acid ones. These include various types of lithium batteries. The following describes a small-sized outboard electric motor powered by a lithium source placed directly on it. A diagram of the necessary for the operation of the electric motor and the power source of the electronic device is given. Various parameters of the outboard motor as a whole were also measured. For this purpose, a 12-volt two-speed Sevylor SBM18 outboard motor was used, the weight of which (without battery) is about 2,5 kg (the same motor is also produced under the Aqua Marina T 18 brand). At higher speeds, battery power is supplied directly to the motor, and at lower speeds, through an additional resistor. In the latter case, as measurements have shown, a third of the energy consumed from the battery goes to heat the resistor, i.e., is wasted. According to the motor passport, the current consumed at a higher speed reaches 14,7 A, and at a lower speed - 7 A. In this case, in the first case, the motor must develop a mooring force of 8,1 kgf, and in the second - 3,6 kgf. According to the measurements, at a supply voltage of 11 V, the motor developed a force of about 2,5 kgf at a lower speed, which is close to the declared value. In the original version, the power source was a battery composed of three batteries connected in series, each of which, in turn, consisted of two IMR 26650 (KeepPower) lithium-ion batteries connected in parallel with a capacity of 5200 mAh. As you know, lithium batteries are very "gentle" power sources: for each type, the maximum voltage is set to which the battery can be charged, and the minimum voltage when it is discharged. For lithium-ion batteries, these values per cell are 4,1 and 2,9 ... 3,2 V, respectively. In addition, you need to ensure that the battery temperature does not exceed 50 ... 60 оC. All batteries were placed in a cassette, the mass of the outboard motor with such a power source installed in its body was 3,9 kg. When testing, the electric motor was installed in a bath filled with water and worked with the original propeller at a lower speed. To automatically disconnect the battery when the voltage drops to the lower permissible level, an electronic device was used, assembled according to the diagram below. Before shutdown, the motor worked continuously for an hour and a half. At the same time, the current consumption decreased from 7,5 to 5,3 A. Tests revealed the following problem. Measurements showed that the internal resistance of the entire charged power supply is 210 mΩ. At 7,5A, the power dissipated in the battery is approximately 12W. Being in a closed case, it heats up quite strongly: after about an hour of continuous operation, its temperature reaches 50 оC and continues to rise. Two miniature one-watt fans were used to cool it, which eliminated this problem. At the same time, the design of the case had to be somewhat complicated so that the battery and electronic device were protected from water ingress, but at the same time air flow was ensured. The final version uses a lithium polymer battery. It has an order of magnitude lower internal resistance, so even with long-term continuous operation in a sealed case, forced cooling is not required. In addition, to eliminate the energy losses that occur in the original motor when operating at a lower speed, a pulsed power supply mode was used. Among other things, this allows you to smoothly adjust the power of the outboard motor and, accordingly, the speed of the boat. The power source was a Turnigy Multistar 14,8V battery. Its capacity is 16 Ah, weight - 1,3 kg. For a lithium polymer battery, the maximum voltage when charging is 4,2 V per cell and the minimum voltage when discharging is 3...3,3 V. The battery consists of four cells and the total voltage in the charged state is 16,8 V. Measurements showed that the internal resistance in this case is 8 mΩ, so that even at a current of 10 A, the power dissipated in the batteries will be less than one watt. The outboard motor control circuit is shown in fig. 1. Automatic shutdown of the battery when its voltage drops to the minimum allowable level is carried out by a Schmitt trigger assembled on transistors VT1 and VT2. This level (in our case it is equal to 13 V) is set with a trimmer resistor R2. Note that for accurate setting it is desirable to use the so-called multi-turn tuning resistor (with a worm gear).
When the SB2 button is pressed briefly, the transistor VT1 opens, and VT2 closes. This leads to the opening of the transistor VT3. As a result, relay K1, included in the collector circuit of this transistor, is activated. Its contacts operate on a short circuit and allow DC switching with a power of up to 16 A at a voltage of up to 24 V. On the generator of rectangular pulses, assembled on the elements of the DD1 microcircuit, the supply voltage is supplied with a slight delay due to the presence of a relatively large capacitor C2 and resistor R14. The delay allows you to use a low-power button to start the motor. Voltage pulses from the output of the element DD1.3 periodically open the transistor V74, in the drain circuit of which the motor M1 is connected. Its reverse is made by switch SA1. When the supply voltage drops to the lower set level (as the battery discharges), transistor W1 closes and the entire device operates in the opposite direction: the relay contacts open and the power source is disconnected. To turn off the engine running at a higher voltage, use the SB1 button. With the ratings of the parts indicated in the diagram, the pulse repetition rate is about 50 Hz. The duration of the voltage pulses applied to the motor is regulated by a variable resistor R6. The values of the resistors R8 and R9 are chosen so that with a fully charged battery, the average current flowing through the electric motor can be smoothly changed from about 5 to 9 A. The details of the device are mounted on a fiberglass board with dimensions of 138x47 mm. The V74 transistor is mounted on a small heat sink. The power dissipated by it does not exceed one watt. On fig. 2 shows the dependence of the voltage of the storage battery and its temperature on the time of continuous operation at the maximum power of the electric motor. A graph of the dependence of the current consumed by it on time is also given. The motor itself with the original propeller was fixed in a bath of water. It can be seen from the figure that the battery voltage, as it is discharged, decreases rather smoothly to a value of approximately 14,3 V, after which it decreases sharply. A rapid drop in voltage as it approaches the lower acceptable level is typical for lithium polymer batteries. The maximum temperature of the power supply in a closed case after two hours of continuous operation did not exceed 45...46 оC. At the same time, as studies have shown, a significant contribution to heating is made by the heat sink of the transistor VT4 and relay K1 located next to the battery.
The battery, together with the engine control board, is located in a sealed duralumin box on the outboard motor. The lid of the box is made to open, and the battery can be easily removed. The general view of the motor is shown in fig. 3 (it can also be used to judge its size). The mass of the motor together with the battery is approximately 4,4 kg.
Tests of the motor on the boat were carried out on the lake in the absence of excitement. The total load of the double rubber boat JAM 220 T was approximately 100 kg. Its speed with a fully charged battery and maximum engine power was 4,5 km / h. The motor worked continuously for 2 hours and 20 minutes before stopping. At minimum power, these figures were 3,6 km/h and 3 h 45 min, respectively. Thus, from the above data, it can be seen that a lithium-polymer battery can be successfully used to create easy-to-use and low-weight outboard electric motors with a power source placed directly on the motor. Author: A. Gavrilov
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