November - 2004
MOTORS, BATTERIES AND CURRENT DRAW
This month’s Technical Article was prompted by a question from one of our Canadian Subscribers. He asked about the selection of Motor Voltage and Battery Voltage/Capacity when purchasing a power package for a model. The selection of the proper Motor and Battery for your ship can appear to be very complicated as there are so many factors to consider. It almost seems like Black Magic at times. A complete discussion of this topic would be very involved and is best left for the book on “Electronics” that we are currently developing. I will, however, briefly overview a number of factors to be considered. First, in most cases, a 12 V Motor will only draw ½ the current of a 6 V Motor. This will allow the use of a lower power speed control which is usually smaller and less expensive. The lower current drain will also allow you to get the most power possible from your battery. This is due to the fact that the higher the current draw the lower the effective capacity of the battery. This is not to say that 12 V Motors should be the only ones considered. Many times a Motor of the needed size or power level can only be found at a 6 V rating. In this case, there is no reason not to use such a Motor. Many of my boats have 6 V Motors and run great. I have to remember to deep the current draw in mind when selecting a battery and speed control. In many cases, a 6 V Motor can actually be an advantage. A good example of this would be: assume you are working on your newest project, and the only Motor that you can find that will physically (size and/or weight), fit in the Hull happens to be a 6 V Motor. You purchase the Motor, Install it in the Hull and do a quick “In the water” test. The boat moves out ok but could really use a little extra power. Instantly, you start thinking, “Time to start searching for a new Motor.” Not so, there is another choice. As long as you are using a good quality name brand Motor, (Pittman, Clifton, Buehler etc.), then you can usually run the Motor at a voltage higher that what it is rated for without any serious detriment. So in this case if your ship needs just a little extra power, you could consider using a 7.2 V or 8.4 V pack. This will up the power output of the Motor bit will also increase the current draw. The additional current draw is usually not a significant amount and if you achieved the added power you needed it will be worth it. I would not, however, recommend more than a 50% increase in voltage unless absolutely needed. Of course, the same could be done with a 12 V Motor to get more power, but then your choices of battery packs and speed controls would be limited by these higher voltages. As a word of warning, any time the voltage is increased over the rated voltage, you must be extra careful to make sure that the Motor is not overloaded as this will lead to a rapidly burned out Motor. Sure signs of an overloaded Motor are: Excessive Current Draw and Heating of the Motor Case. If a Motor is running hot to the touch, then it is overloaded and the voltage should be reduced or a smaller prop selected.
Next month I will discuss the selection of the proper Battery Size and Chemistry.