continuation MOTORS REWINDING

We talked of rewinding the RC car motor. Now, lets see what we usually did to customize our motor. We read of 27 turns stock wind, then we hear of 24 turns.
Everytime the number of turns is lowered, the RESISTANCE of the coil is lowered. Thus , if we were using the same battery pack or 8.4 volts, the OHMS law says this:

8.4V/lower resistance=Higher current draw

No wonder the run time got shorter. And there is a limit to how many turns you want to remove from your motor. The higher current will make it hotter and hotter. What should have been done. Well, lower the turns but fatten the wire. And the following discussion will explain it.

You have a 12 volts motor and say you are running it on 10 nicads of 1000 mah capacity. You series an ammeter and see that you are drawing 0.5amp at the load you have on it. You want it running on 6 volts. if you bring it to me, I will unwind the wires on each pole, halve it and put the two halves in parallel. Wind it back. That is now a 6 volts motor. You can run it on 5 cells but your running time will be halved cuz you doubled the wires and doubled the current draw. What is the solution? parallel the other 5 cells to your pack. You will end up with a 5S,2P battery pack. Did you lighten the pack in your airplane model for example. No! you simply reconfigured your battery! Of course you have now some flexibility cuz you can fly on 5S, land, switch to the another 5S. You did not have to carry a 10S. OBSERVE: You halved the V but you doubled the A. the power in VA or watts remained the same. How did you double the AMP draw? Well, you halved the turns right?thus you halved the R. But why did we have to double the wires. Answer:to carry the doubled amps draw.

If you bring to a rewinder a 110 volts charger, he unwinds the primary and puts in a wire half the size and double the length to make it 220v. How does he know how much thinner wire to buy. Just weight the original wire and buy half the size at the same weight and wind it back. He did not have to count the turns really. Now, if you were going back to the US and brings a 220 charger to be converted to 110V, he does not have to buy new wires. He unwinds the primary, halve the wire then parallel them. Wind it back, that is now 110v. your 110 charger draws double the current than the original 220v charger but don't worry. You are paying in watts which is VA. So the the VA for both remains the same. (110v is not cheaper to run).


This will take us to a relevant topic to our RC: WHICH IS BETTER, HIGHER VOLTAGE LOWER CURRENT OR LOWER VOLTAGE HIGHER CURRENT?

Answer: Utility companies(NAPOCOR ,MERALCO) transmit electricity to as much as 30,000 volts on those bare stranded cables high above. Why? So they can use smaller cables since for the same power(watts), they will be transmitting lower current.

During the vacuum tube era, the tube was a high voltage, low current device. For those of you afficionados and old enough, remember the 500volt power supply? An output transformer had to lower the voltage but with increased current to drive the speakers. The transistor, of which the MOSFET is one type, is a CURRENT device. However, this should nto be confused when designing an ESC. The muscle on an ESC are the MOSFETS which are rated in terms of V, A, and resistance when ON. Look at you ESC. We usually look for the AMP capability. Did you look for the Voltage it can handle? Why? If you want more power, get a higher voltage motor and use 4S or 6S. Power is VA and you can raise V or A to get a higher VA or watts. but , ah, you will say, you will need more cells. True but for the same run time, you need less capacity on the battery since the AMP draw will be less. Remember the above discussion in making a 12V motor into a 6V motor? If you are beyond park flyers and into giant electrics, the voltage used are high, 32 cells in series is usual. That's about 50 volts! So, next time you have a 12 volt motor, just used more cells but don't rewind it to six volts!


Now, how come, you might ask, your Brushless for 3S burned. It wasn't overvoltage. It was over current draw. A stalled motor draws more current than a spinning one. You were "overprop" and your motor was spinning slow. Now, you will see why gearing is good for your motor. You are like in first gear with your real car. The motor spins 4 times for every one for the prop. You have 4 times the torque(speed and torque are inversely proportional). You can put a bigger prop or a higher pitch prop or both without taxing your motor). (The lite Ace is a highly geared small engine. Do you hear its higher RPM?). But can we not use a smaller prop to let the motor spin and since its spins faster, don't we get the same trust? You are darn right if you follow the computation. But why is a bigger prop better? the best answer so far is that it is covering a wider arc of space!

to be continued again....