Since many of the visitors to this site have electric R/C vehicles, I thought it would be a good idea to go over the force that actually makes electric R/C vehicles go – electricity!

The whole topic of electricity is exceedingly complex. It’s full of strange terms, mathematical formulas, and abstract concepts. However, it can also be very interesting, so let’s get started!

First off, what is electricity?  When people think of electricity, they are actually thinking of two separate things: electric charge and electric energy.  When you hit the throttle on your R/C controller, two things are flowing out of your battery pack. First, electric charge flows out of your pack’s negative terminal, through wires, through your motor, through more wire, and into your pack’s positive terminal.  But something else is happening too.  Electrical energy is also flowing out of your battery pack. This energy flows out of your pack, along the wires and into your motor where it is converted into torque and heat.  The torque spins your vehicle’s wheels and makes it go and the heat is radiated into the environment.

Believe it or not, your wires and motor are full of electrical charge all the time.  However, this charge is balanced (positive and negative) so it doesn’t flow. Only when an imbalance is created (by adding a battery, for example) does it flow. Charge moves very, very slowly.  It moves through wire about as fast as the minute hand on a clock.  In a direct current (DC) circuit, the charge flows in one direction around the circuit. In an alternating current (AC) circuit, the charge just vibrates back and forth (in the U.S., AC circuits vibrate (oscillate) at the rate of 60 times a second (60 Hertz)).

Electrical energy (also known as electromagnetic or EM energy) is created by the flow of electrical charge. But unlike electrical charge, EM energy flows very fast, almost as fast as the speed of light (186,000 miles per second in a vacuum). EM energy is added to and lost from circuits. It’s what makes lights glow, motors turn, computers work, and more.

Here’s how electrical charge compares to EM energy:

If you’re having a little bit of trouble understanding how something that moves so slow (charge) can move so fast (EM energy), consider the following example.  You and a friend are standing 100 yards apart.  The air is still – there is no wind.  You shout “Hi”, which your friend hears almost immediately.  What just happened? By saying “Hi”, you created sound waves that travel at 720 mph through the standing air molecules to your friend’s ear.  Even though nothing physical moved from you to your friend, you caused your friend’s ear drum to vibrate.  The air molecules that carry the sound waves is like electric charge and the sound waves that move from you to your friend is like EM energy.

Now that you have a grasp, more or less, on what electricity is, let’s take a look at how it’s measured!

Voltage

Everyone has heard the term “voltage”, but what exactly is voltage?  Many times voltage will be described as electrical pressure, but that’s not really correct.  More accurately, voltage is electrical potential.  Electrical potential isn’t pressure even though a potential difference seems to push electrical charges through the circuit.  Voltage causes the electric current.

Here’s a good way to picture voltage.  Pretend that you carry a brick to the top of a sky scraper, 100 floors up.  By carrying the brick up there you have added potential kinetic energy into the brick.  This energy is stored in the brick for as long as it remains up there.  The energy can be released by dropping the brick down to the sidewalk below (heads up!!).  You can think of gravity as being the electrostatic field and the height of the tower is voltage.  Can you see it now?  The more floors we take the brick up, the more potential kinetic energy (or gravitational potential) we put into the brick.  The height of the building is no more like pressure than voltage is like pressure.  The height of the building is present whether or not a brick is on top of it.

A unit of voltage is called a volt (big surprise there!) and is represented by the letter “E”.

Amperage

An amp (short for ampere) is the measure of the flow rate of charge and is represented by the letter “I”.  Amps as a physical item do not exist, they are simply a measurement.  Consider a pipe with water flowing in it, you wouldn’t say that the pipe is full of 100 gallons per hour, would you? More correctly, you would say that 100 gallons of water are flowing through this point on the pipe per hour.  Likewise, amperage describes the number of electrons flowing past some point per second. Specifically, 1 Amp = 6.242x10 ¹⁸ electrons per second flowing past this imaginary point (that number means 6,242,000,000,000,000,000,000! ).

Here are a few things to think about.  When you have a battery sitting by itself, you have voltage with no current (there’s voltage between the positive and negative terminals but no flow of electrons).  You can also have current with no voltage (a charge injected into a superconducting circuit with no load will continue to move around the circuit forever).  Voltage is associated with electrostatic fields and current is associated with magnetic fields.

Watts

A watt, like an amp, is the measure of the flow rate of something. You already know that amps are the flow rate of charge, well watts are the flow rate of energy.  Watts are represented by the letter “P”. Watts are calculated by multiplying volts by amps (P=V*I). So, if you know two of these three things, you can calculate the third.  Here’s a neat fact, 746 watts equals 1 horsepower. If you have an E-Maxx, you already know it runs on a 14.4 volt system.  So to produce 1 horsepower, how many amps must the motors draw? The answer is 51.8 amps (746 watts = 14.4 volts * 51.8 amps). 

What’s the most power (watts or horsepower) a stock E-maxx can put out?  Let’s see.  At maximum efficiency the stock motors in an E-Maxx (Titans) draw 10.5 amps of current each (85 amps each at stall). Assume you are using 14 cells freshly charged at 1.2 volts each. At maximum efficiency the motors are pulling 21 amps at 16.8 volts (1.2 volts * 14 cells) for a total power output of  352.8 watts (0.47 horsepower) and just prior to stall the motors are pulling a little less than 170 amps for a total power output of 2,856 watts (3.8 horsepower).  So it’s safe to say the theoretical maximum power output that a stock E-Maxx is capable of is just under 2,856 watts or 3.8 horsepower.  Keep in mind that it could hit this maximum for probably less than a second before the EVX ‘thermals’ or something melts!

Resistance

The last thing we’ll look at is electrical resistance. Electrical resistance is defined as the resistance to the flow of an electric current through a material.  All materials (excluding super-conductors) offer resistance to the flow of current.  Electrical resistance is determined by how tightly a material holds onto its electrons. Conductors hold onto their electrons ‘loosely’ while insulators hold onto their electrons ‘tightly’.

A number of things can influence the resistance of a wire including its diameter, length, and the amount and type of impurities contained. A long and narrow wire will have more resistance than a short fat wire and a wire high in resistance causing impurities will have more resistance than one without them.

It takes energy to move a current through a material that offers resistance and this lost energy is dissipated in the form of heat.  If the heat becomes great enough, the material will not only become hot, it will start to glow and/or melt.  In fact, this is how an incandescent light bulb works. Actually, these bulbs radiate most of the energy they consume as heat and not light which makes them very inefficient!

Electrical resistance is measured in ohms and is represented by the Greek letter Omega (Ω).  However in practical use the letter “R” is commonly used.  Ohms come from Ohm’s law which defines the relationships between (P) power or watts, (E) voltage, (I) current, and (R) resistance. One ohm is the resistance value through which one volt will maintain a current of one ampere.

Ohm’s law is represented by the formula E=I*R.  By knowing the amps and resistance, we can derive the voltage.  Likewise, we can calculate the resistance by dividing voltage by amps (R=E/I). Voltage divided by resistance is equal to amps (I=E/R) and voltage multiplied by amps is equal to watts (P=E*I). Got it???

Here are some useful formulas:

Resistor Calculator

If you start working with electrical circuits, one thing that you’ll quickly realize is how often you’ll need to step down voltage in a circuit.  As you now know, this is done with a resistor.  But which resistor do you use??  What do you do if you want to put a 3 volt LED into a 5 volt circuit? That’s easy if you use this handy Excel spreadsheet!  Once you’ve downloaded the spreadsheet, all you’ll need to enter is the supply’s volts and the maximum volts and amps for the item in question. The spreadsheet will then calculate the voltage drop, the resistor ohms, and resistor watts that you need. To download the spreadsheet, click here.

Incredible Examples

Click on a photo below to see the movie. The files are large (1.5 MB) so it may take a while to download depending on your connection speed.

Drop me a line at webmaster@e-maxximumrc.com!