Of Current, Voltages, Resistance

There are three parameters in an electrical circuit. The CURRENT (I) is like the amount of water flowing. The unit measure is AMPERE. Next we have the EMF(E), or electromagnetic force. The unit measure is VOLTAGE. This is the equivalent of the pressure in a pipe pushing water. The third parameter is RESISTANCE (R) measured in OHMS. This is akin to the resistance of the pipe used. the smaller pipe has more resistance. So, we have E,I, R
. Like any measure, MAN, has to define a unit. The definition agreed upon is what is known as the OHMS LAW and the relatioonship is :

E=IR

How did they come up with this? Well, they said that that One Volt is pushing One Amp through a resistance of One Ohm. It means that if you have a One Volt battery lighting up a bulb, the definition is that the current is One Amp and the resistance is One Ohm. You see, they are not independent. The lower the resistance, the higher the current draw. This is the fundamental relationship. We want to mention an offshoot of these three parameters. The product of V and A is the power used in the circuit. It is called VA or WATTS.


With the above general introduction, we can perhaps now go into making motors or rewinding them.

Let us start with the POWER you want, in WATTS. You have to consider that the battery you want to use. Say you want to use 2S. That is 8.4 volts at peak. If you want 84 watts, you have to draw 10 amps from your battery. To do this, you have to compute for the resistance that you coils should have. Every size of copper wire has a resistance per foot or meter. At 8.4 volts and 10 amperes, R is 8.4/10 which is .84 OHMS. Then compute for the length of wires you need in your motor. You may put different size wires into your motor but the length will differ. The thicker wires has less resistance and thus will need to be more to get .84 OHMS. WE will end up choosing the sizes that will fit into the size motor we built. So much for this fundamental and let us go into rewinding motors.

Let's talk of brushed motors for the RC car. You have a stock motor running on six cells. At peak the voltage per cell is 1.4 x 6 or 8.4volts. Now let us say that this motor is drawing 10 amps. It is an 84 watts motor. If you want to double the power, double the voltage but you have to double the number of turns if you want to retain the 10 amps draw(which means 10 amps running through the wires). Now you have 16.8 volts x 10 amps=168 watts. What happens if you had just doubled the voltage? the R in there remained the same and your AMP draw will double! you will have double the V and double the A but your wires will burn cuz it cannot carry 10 amps. Say, you merely increased the V by 1.4V by adding one more cell, the motor wires might tolerate it but will be hot.

to be continued.........