voltage, ohms and amps.

Hi guys, sorry there has been a long silence.

Ok, I promised to discuss VOA. These are the unit measures for emf,resistance, and current. EMF is electromagnetic force. They are related and my favorite analogy is your plumbing. Voltage is the water pressure you have. Resistance is the pipe you are using, and the current in amperes is akin to gallons per minute, your volume. If you increase the resistance of the water flow by making the pipe smaller, you will increase pressure and lower current. If you make the pipe bigger, you will bring in more water per time but at a lower presure. OHMS LAW is :

E=IR

Let's apply to brushless motors(same as brushed motors anyway).


People bring me their motors and tell me to make it stronger. But you see stronger refers to power and power is a combination of speed and torque. Well, I can it make run faster or at a higher torque or both to make it "stronger". You see, by the use of gears, speed and torque are inversely proportional and can conveted from one to the other. The quickest way to make it run faster is to lower the number of wire turns. In RC cars we hear of 27T as stock and 24T as souped up. It will heat up tho, assuming all else the same (prop size, weight of model). It is because the lower number of turns means less resistance and the current draw will go up. (V/R) Now, since power in watts is VA, power goes up. To reduce heat, use a thicker wire. Something is interesting here. The turns you had removed should be placed back into the motor in terms of thickness. That is why rewinders use weight. It should be the same weight of copper wires for a given wattage motor. If you bring me a 12V motor and wants it converted to 6V, I will half the turns and double the thickness. (and vice versa to convert a 6V to 12V motors). You see something there, the same watts but different combination of V and Amps. Yes, but I thought we were changing the resistance. Sure and it changes the amps in turn.

Why do motors burn? Well, you can burn it even if you had used the correct battery voltage if you had used a wrong prop(overprop). A loaded motor will draw excessive current. The more common cause I suspect is using hihger voltage. Many many modellers burn their brushed motors by using li-poly, especially 3S. A seven cell mH pack is 1.4x7=9.8V max and it drops more rapidly than a li-poly pack. A 2S is 8.4V and a 3s is 12.6V peaked. Now, even a 2S can burn up a brushed motors even if the V is lower. That is because the 2s can maintain delivery of strong current longer than an MH. It is wise to throttle back once you shot that plane up. So, if you want more speed , you can lower the turns or raise the V.

I think I have said this before. If you have a 12V motor and uses 2 packs of 6V each, if we rewind it to 6V, to get the same duration , we have to parallel the batteries anyway. We do not really save on battery weight. We just reconfigured the VA combination. The only real advantage is that we can carry one pack first, land then switch to the second pack. We didn't have to carry 2 packs. What is the conclusion anyway? Well, say you have a 12V motor. Do not rewind. Find a battery pack of 6V each at half the capacity, presumably at half the weight. Series 2 of them to get 12V. Yyou will have the same weight as using a 6V motor with a 6V pack but at twice the capacity.


THE RADIO SYSTEM
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Two Americans invented the proportional system when transistors were already available(post 1950). The heart of it all was the servo. The servo developed is the one still widely used today. The circuit makes the motor turns on pulses smoothed out by capacitors. An incoming pulse from the receiver is added to a locally generated pulse with an opposite polarity. If they are the same, they cancell each other and no differential pulse appears. But if one is longer than the order(duration they are on), a residual pulse appears of either polarity. This residual pulse keeps coming on at about 60 times per second. The circuit senses the polarity and turns on the appropriate drivers to turn the motor CW or CCW. But as the motor turns gears to multiply its torque, a pot is also turned adjusting the local pulse to match the incoming pulse. Once matched, the motors stops but the motor output shaft is now at a different position. The inventors decided on 1.5MS as the centering pulse and 1MS for full left and 2MS for full right. One MS is one millisecond. The size of the pulse limits the repetition rate per second and it cannot be made too small either due to bandwith problems in transmission. Imagine a single servo system. The TX can be turned on between 1MS to 2MS repetitively for the RX to interpret as the pulse. Another appraoch to save power is to momentarily turn the TX ON. At the RX end, the On switches a Flip-Flop ON, then OFF at the second ON of the TX. The OFF time will be the TX at rest and will save on power. But it was recognized that the OFF time will make the RX more prone to pick up an interference. The reverse was adopted. Keep the TX carrier ON. Switch it OFF momentarily twice, the gap representing the ON time of the pulse created by the Flip-Flop at the RX.(the Off is momentary). To have multi channels, we add more FFs at the receiver for the next servo. the TX then simply have more OFFS in the chain of commands.(number of servos + 1 for the starting OFF). Today a number of FFs are in one IC called the shift register. The RX converts the OFF time of the TX into a positive short pulses called clocks. First clock turns ch 1 ON, clock two OFF but CH 2 ON, etc.

to be continued...