Electric Motor Numbers: What Do They Mean?

When you get started with Electric Power Systems, there is a lot you need to learn to fully understand how these systems work.  In addition to learning all the ins and outs of the components themselves, there is an entirely new language of numbers and terms that need to be learned and understood to be able to fully understand electric power systems.  In this post, I am going to explain everything you need to know about electric power systems, and what all of the numbers actually mean.

Motor Constants

There are specific motor constants that are used to describe various parameters regarding the motors.  Understanding these numbers will allow you to speak intelligently to other modelers about motors, without sounding like a complete noob!

Kv – Motor Velocity Constant

Kv does not stand for Kilovolts, as many people will commonly say!  It is the Velocity Constant of the motor, and is typically expressed in the units of RPM per Volt.  The Kv value of a motor tells you how fast the motor will spin as a function of the applied voltage, and nothing more.  It has absolutely nothing to do with the size of the motor or the power output of the motor.  Telling someone that you need a 1000 Kv motor means about as much as telling them you need a blue motor!

What Size Motor Do I Need?

When you build a model that is designed to be powered by an electric motor, most of the time the kit manufacturer already gives recommended motor sizes.  This is especially true if the kit manufacturer is large company like Horizon or Great Planes, who also sells electric power systems.  For other kit manufacturers, or for models that were originally designed for glow engines, picking the right motor can be a bit challenging.  In today’s blog post, I am going to go through the steps of selecting the correct Motor, ESC, Battery and prop for an aircraft.

To begin this process, you need to know a few things about the model.  Most important are the weight of the model, the type of model, the desired flight speed and the desired flight duration.  With these specifications, a power system can easily be chosen which will provide the required power.

There are a few rules of thumb that I like to go by when selecting a power system for a model.  The first ones are for Glow to Electric conversions.  For a decent, ball-bearing ABC type 2-stroke glow engine, each cubic inch of displacement is roughly equal to 2000 watts of electrical input power.  To get the required wattage motor for a model, you simply multiply the engine displacement by 2000, and you get the required number of watts.

Linear BEC’s versus Switching BEC’s

Quite often in emails from customers I get asked the question, “What is the difference between a Linear BEC and a Switching BEC?”  This is actually a pretty good questions, and knowing the difference between the two can really help in selecting the right speed controller for a model.

Well, the simple answer is that a Linear BEC throws away excess voltage by converting it to heat, while a Switching BEC does it by turning on and off really fast as needed to pass through the required voltage.  Unfortunately, that does not tell you very much!  Since I like to pass on educational information to fellow hobbyists whenever possible, the complete answer to this question is explained below.

In all BEC’s, the object is to step down the voltage level of the motor battery to a lower value, typically between 5 and 6 volts, so you can power the radio receiver and servos directly from the motor battery.  This eliminates the need to carry a separate 4-cell or 5-cell, Ni-Cad or Ni-MH, battery pack in the plane to run the receiver and servos.  This is where the BEC gets its name, since BEC stands for “Battery Eliminator Circuit”.  The BEC eliminates the need to have a separate battery on board to power the Receiver and Servos.

The batteries used to power RC aircraft are normally Li-Po type, and can be anywhere from 2 cells (7.4 volts) up to 12 cells (44.4 volts).  The receiver and servos are typically designed to be operated from 5 to 6 volts, so the battery voltage from the motor needs to be dropped down to a lower level, so it can safely be used to power the receiver and servos.  There are two different ways to drop down the voltage level from the battery pack to the correct level, Linear BEC circuits and Switching BEC circuits.

Prop Selection and Care for FPV Quads

Unless you have been living under a rock, it is pretty hard not to notice that FPV Quad Racing  has become the biggest thing to hit RC this decade!  These tiny little craft, often weighing less than a pound, sport thrust to weight ratios in excess of 8 to 1, and can accelerate from 0 to 100 MPH faster than any exotic sports car, and turn faster than a cheetah trying to catch its next meal!

With props spinning in excess of 40,000 RPM on some of these craft, propeller selection, as well as proper maintenance of the props and power system, becomes more important than ever.  Unfortunately, many pilots have no idea what is happening in their power systems, and the demands that are being put on them often cause in-flight failures that can be easily avoided.

The typical power system for the smaller 200mm FPV quads consists of a set of four 2204 or 2205 motors, four matching ESC’s, four 5 inch props and a 4-cell Li-Po battery with a capacity of around 1300 to 1500 mah.  At full throttle, each motor can pull 20 amps of current, so a set of four motors is trying to pull 80 amps of current from a little 1500 mah battery pack.  This represents a discharge rate of 53C, and really pushes the poor little batteries to their breaking point.

The biggest problem that I see is that pilots slap props on their quads, without ever looking up how much current the prop actually pulls from the motor.  In most cases, the prop pulls more than the recommended full throttle current from the motor, which will cause the motor to fail eventually.  It may take 10 flights, 20 flights or even more, depending on how hard the motor is being pushed, and this gives the pilot a false sense of security.  Because the motor does not fail withing 20 seconds of the first flight, the pilot thinks that there is nothing wrong, and then is surprised when the “Magic smoke” comes pouring out of the motor 20 flights later.

Lucien will be speaking at the InterDrone Conference in Las Vegas on September 9th and 10th.

It is September 8th, 2015 as I write this, and this day is important for 2 reasons.  First, It is my grandson Elijah’s 3rd birthday!  Time really flies, it seems like yesterday I saw him for the first time at the hospital and he is now 3 already!

Second reason, later today I will be heading up to Las Vegas to participate at the 2015 InterDrone Convention, which is hosted at the Rio Hotel September 9-11, 2015.  I look forward to meeting a bunch of new people, and also catching up with old friends from across the country.

This year I am one of the speakers at the event, and will be presenting 2 different workshops at the conference.  The first one, which is Wednesday September 9th, from 11:15 am to 12:15 pm, is titled Intro to Electric Power Systems.  In this workshop I will cover all the basics of electric power systems, as they are used in multirotors, and explain how each part of the system works.  I will also cover the special tools that are recommended to have to make testing and servicing a power system as easy as possible.

The second workshop, which will be held on Thursday September 10th, from 1:15 pm to 2:15 pm, is titled Advanced Power System Selection for Multirotors.  In this workshop, I cover a brief look at each of the components in a power system, and then show the step-by-step procedure for selecting a power system to maximize power, efficiency and run time.

I have made PDF versions of the PowerPoint slides used in each of these presentations for those that would like to have a copy after the event, or for those that cannot make it to InterDrone, and would like to get this information.

The Intro to Electric Power Systems slides can be downloaded at the following link.

http://www.innov8tivedesigns.com/interdrone/power-intro.pdf

The Advanced Power System Selection for Multirotor slides can be downloaded at this link.

http://www.innov8tivedesigns.com/interdrone/power.pdf

Once I get back from the show, I will post an update with a few photos from the event.