It seems that a lot of people have trouble understanding electric power systems. If you are confused about how electric motors work then follow along this series and I am sure by the end of this article you would have a better understanding of the electric power system. I am presenting this article assuming that you have no prior experience or knowledge in this field and are complete newbie. I will start with a simplified system and then go onto more complex ones including real time factors like resistances, back emf of motors etc. What I am going to present was learned by me over the years through internet & other sources etc. I had a soft copy but unfortunately lost it…now all is left a hard copy with me.
Lets imagine a simple electric motor which we call the ideal motor. We call it ideal because it has extremely simple characteristic and is 100% efficient in operation. Our motor has a single quality: for every volt of energy applied to it, it turns exactly 1000 rpm. On 5 volt the motor will turn 5000 rpm, on 25 volts the motor will turn 25000 rpm. The rpm will always be equal to the amount of volt applied to the motor times 1000.
Now we need a power source for the motor. Let’s imagine that we have an ideal cell to match our ideal motor. For now, our ideal cell can be defined by a single characteristic: the amount of voltage it produces. Lets assume for simplicity a cell produces exactly 1 volt of electricity.
So now we can make up a pack of ideal batteries and attach to our motor. Lets see what the results would look like:
Cell count to RPM Relationships for our Ideal Motor
|No of Ideal Cells||RPM|
Our ideal motor and ideal cell make life very easy for us. We can almost use the words cell and volts interchangeably because of our contrived values. The real world isn’t that simple, but its not entirely unlike our ideal world either. Keep that in mind as we go along. The relationship between volts (cell) and rpm for our power system can work backward as well as it does forward. So if I measure that my ideal motor is spinning at 4000 rpm I can bet that the input voltage is 4 volts. That in turn means that 4 cells are being used.
But you may notice that our motor is just happily spinning away and doing no work. We need to add something to output shaft so that it can twirl around and move lots of air. So lets put a propeller on our motor and watch what happens to the RPM. In fact, lets compare two different propellers with the same motor and cell count.
If you are really new to RC then you might need to know how props are specified. Each prop has a diameter and a pitch. I assume you know what diameter means, but the word pitch can be a little confusing. The pitch is defined as the distance the prop would travel forward in one revolution in a perfect medium. The higher the pitch the more angled the blades of the prop are and the farther it would travel in single revolution. A high pitch is usually used on a fast plane while a low pitch prop is usually used on a slow plane. Out of the two props we will use one will have a 5 inch dia and 5 inch pitch, designated as 5×5. The other will have a 12 inch dia and 8 inch pitch, designated as 12×8.
Cell count to RPM relationship for our ideal motor and two different props
|No of ideal cells||RPM (5x5)||RPM (12x8)|
Nothing in the above table should surprise you because our ideal motor always turns 1000 rpm for every volt regardless of what kind of load is placed on the output shaft. This is a crucial point and its real world analogy is one of the hang-ups that keep many beginners from understanding electric power.
Of course we must also realize that it takes far more energy to spin a 12×8 prop at 4000 rpm than it does to turn a 5×5 prop at the same rate in the real world. There must be something missing from our simplified motor model or all those Speed 400 pylon racers out there would be flying with 18×18 props.
Indeed, the thing we are missing is called current. Current is the other half of the energy equation. Sadly, we can no longer go on further without introducing some kind of formula into the discussion.
Power (Watts) = Volts x Amps
The formula relates power (energy/time) as a product of Volts and Amps. You see a volt is just a part of the energy equation. Without amps, you don’t have any energy at all. The formula relates power (energy/time) as a product of Volts and Amps. You see a volt is just a part of the energy equation. Without amps, you don’t have any energy at all.
The word watt is just a unit of power. Lets put watts to work right now. Remember the last chart that showed our ideal motor mated with a variable number of ideal cells to turn two different props?? Remember how the rpm was always dependent solely on how many volts were applied to the motor, even though it takes a lot more effort to turn a big prop than it does to a small one? Lets use watts to show just how much effort is involved :
|RPM||Prop (5x5)||Prop (12x8)|
|note these are mythical values|
|1000||1 Watt||10 Watts|
|2000||4 Watts||40 Watts|
|3000||10 Watts||100 Watts|
|4000||25 Watts||250 Watts|
Please note that these values are not actual values. I am just using simple values (that mirror realty in a few key ways) to demonstrate a point. And that point is that it takes a lot more power (energy) to turn a 12×8 prop at 4000 rpm than it does to spin a 5×5 prop. Also it takes more than twice as much power to spin a prop at twice as much rpm.
Since we now know how to express energy (watt= volts x amps), we can pull an example out of the table above and see what is going on with our ideal motor. Lets concentrate on the table entry which shows that it takes 100 watts to spin a 12×8 prop at 3000 rpm. Since we know that watts is the product of volts and amps this means that we would need a combination like the following examples :
Cell count to RPM relationships for our ideal motor and two different props
So here’s what we have learned so far:
- Our motor turns exactly 1000 rpm for every volt regardless of load.
- Our motor draws the current necessary to make the watts of electrical energy equal to the watts of power it takes to turn the prop at the rate demanded by the voltage.
- Watts is the product of volts and amps.
- A big prop requires a lot more power to spin at a specific RPM than a small prop does.
- Given a fixed prop and motor, increasing voltage will increase current at exponential rate
Based on what we have learned in this part, you can try and answer the following questions:
- At what RPM will an ideal motor run if I apply 17 volts?
- How much current will an ideal motor draw if a 12×8 prop is attached to it and run on an ideal cell? 2 cells? 3 cells?
- Suppose I had an ideal airplane that was lacking I power but I didn’t want to add cells, what would you suggest I do?
You can answer in the comments section. That’s it for part one. Next part will have the answers to the above question and we will add a little more real world effects until finally we have a complete understanding of the power system. To stay updated with this series please register with the website.