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!

The Kv value of a motor, multiplied by the battery voltage, will give you the No-Load speed of the motor.  The No-Load speed is the condition where the motor is running without a prop, or any other load attached, and is the fastest that the motor can possibly spin, given a specific voltage.  For example, if you have a motor with a Kv value of 950, and you power it with a 3-cell Li-Po battery that produces 11.1 volts, the motor will spin at 950 x 11.1 or 10,545 RPM at full throttle in a no-load condition.

The Kv value of a motor is a theoretical value, and will vary depending on input voltage.  The Kv value stated for a motor is typically given at a specific voltage.  A motor may have a Kv value of 950 at 10 volts, but due to drag and internal losses in the motor, the measured value will drop at higher voltages.  If you measure the Kv value at 12 volts, it may drop to 946 and then at 14 volts it could be 942 and so on.

The Kv value of a motor will also vary a small amount from motor to motor, due to variations in manufacturing tolerances, usually within +/- 3% of the stated value.  If you purchase two motors with Kv values of 1000 and actually measure them, you may find that one has a Kv value of 985 and the other has a Kv value of 1018.  This is perfectly normal, and within normal manufacturing tolerances.  When the motors are loaded down with a prop, they will spin a prop at virtually the same speed.

The timing setting in a speed controller will also change the measured Kv value of a motor.  Typically, the measured Kv is done with the ESC in neutral timing, with no advance.  As you increase the timing advance, the measured Kv value of the motor will increase.

Kt – Motor Torque Constant

This parameter measures the torque output of a motor, and derived by taking the motor torque and dividing it by the motor current.  Kt is typically expressed in units such as Inch-Ounces per Amp or Newton-Meters per Amp.  For any given size motor Kv multiplied by Kt is a constant, so if the Kv goes up, the torque goes down, and if the Kv goes down, the torque goes up.  If the Kv value is doubled, the Kt value will be cut in half.  Likewise, it Kv is cut in half, the Kt value will be doubled.

Kt is normally not given for brushless R/C motors, because it really cannot be used by the end consumer to determine motor performance.

Io – No-Load Current

As the name implies, Io is how much current the motor pulls when run in an unloaded condition.  In a perfect world, when there is no load applied, an electric motor would not pull any current.  Unfortunately, electric motors are not perfect machines, and they do have losses.  These losses come in several different forms including electrical inefficiencies, friction and drag.  The windings of a motor have a small but measurable resistance.  Because of this, there will always be some heat loss in the motor, and this requires a specific amount of current to overcome.

There are also losses in the stator core of the motor which are caused every time the magnetic field changes directions.  Ideally, the core would not retain any magnetism when power is shut off, but in actual practice, a very small residual magnetic field is left in the stator when the power is turned off.  During the next power cycle, when the magnetic field is reversed, this small amount of residual magnetism must be overcome to allow the pole to be charged with the opposite polarity.  This type of magnetic loss is called hysteresis loss, and also takes a small amount of current to overcome.

Finally, there are the frictional losses caused by bearing drag and airflow through and around the motor.  The ball bearings used in most brushless motors do an excellent job of removing most of the rotational friction in the motor.  However, because of the grease or oil used in the bearings, there is always a small amount of drag that must be overcome.  Likewise, most motors are designed so that the front housing acts like a small cooling fan, to help pull cooling air through the core of the motor to pull heat away from the magnets and the stator assembly.  The outer can of the motor also drags across the air, and these air drag forces combine to draw even more current.  This is especially true in helicopter motors which typically have a fairly efficient cooling fan designed into the end bell of the motor.  These types of motors are designed to pull a lot of air through the motor and as a result, produce a considerable amount of air drag.

The No-Load current also varies as a function of RPM.  Motors that spin faster usually have higher Io values, because the air drag is much higher on these motors.  The Io value of a motor is always given at a specific voltage, and will vary with higher and lower voltages.  For example, a motor may have an Io value of 1.55 amps at 10 volts.  At 8 volts, the Io value could be 1.42 amps, and at 12 volts, it may go up to 1.73 amps.  Io typically increases in a linear manner with respect to increasing battery voltage.

Rm – Motor Winding Resistance

Virtually every brushless motor is made by winding multiple turns of insulated coper wire around the slots in a stator.  Even though copper is an excellent conductor, it does have a finite, measurable resistance.  This resistance is referred to as the Rm value of the motor, and is typically given in either Ohms or Milli-Ohms.  There are 1000 Milli-Ohms per Ohm, so a motor that has an Rm value of 0.018 Ohms can also be given as 18 Milli-Ohms.

The Rm value is typically expressed on a per-phase basis, and can only be measured as such before the motor is terminated during manufacturing.  Once the motor is terminated, typically in a Delta Configuration or a Wye configuration, the individual phase resistance can no longer be measured at the motor lead wires.

When a motor is terminated in a Delta configuration, if you use a meter to measure the resistance between two motor leads, what you are actually measuring is the resistance of one motor phase, in parallel with the sum total of the two other phases.  This can be seen in the diagram below.  When measured this way, the value you get will be 2/3 of the actual phase resistance.


When a motor is terminated in a Wye configuration, if you use a meter to measure the resistance between two motor leads what you actually measure is two phase coils in series with one another.  In this case, the measured value is actually double the resistance value of each individual phase coil.  This is shown in the next diagram below.


When trying to measure the Rm value of a motor, you cannot use a standard Ohm-Meter.  On most Ohm meters, the lowest scale is 200 ohms, and is only accurate to +/- 2 to 3 Ohms.  When you are trying to measure a value as small as 1/10th of an ohm or less, you simply cannot do it.

The best way to measure the Rm value of a motor is to run a specific current through the motor, which is small enough for the motor to handle on a continuous basis, and then measure the voltage drop across the motor windings.  If you run exactly 1.00 amps of current through the motor windings, it will drop 1 milli-volt for each milli-ohm of resistance.  So if you run 1.00 amps of current through a motor, and measure a voltage drop of 0.038 volts, or 38.0 millivolts across the winding, then you know that the phase to phase Rm value of the motor is equal to 38 milli-ohms.  If this motor is a Delta wind, you then take this value and divide it by 2/3 (or multiply it by 1.5) and get an individual phase Rm value of 57 mΩ.  If this was a Wye terminated motor, the measured value is double the individual phase value, so you would divide the measured value by 2 and get an individual phase Rm value of 19 mΩ.

Motor Size and Part Numbers

This is where things can get extremely confusing.  There is at least a half-dozen different ways that motor manufacturers use to describe the “Size” or “Model Number” of their motors.  This can create a lot of confusion when going back and forth between different brands of motors, and trying to maintain consistency of power.  Some manufacturers use the stator size of the motor to determine the model number.  Others use the external dimensions of the motor, typically in millimeters, to determine motor size.  In some cases, there is a mixture of stator size used with terms like S, M, L and XL to mean Small, Medium, Large and Extra-Large stator size, and in some cases, manufacturers will use a term like “Power 15” to signify that the motor makes about the same power as a .15 glow engine.

The main thing to look for when comparing one motor against another is the weight of the motor.  If one company has a 2217 size motor that weighs 65 grams, and another company has a 2836 size motor that weighs 66 grams, then you can safely assume that they are the same “size” motor, and will produce similar power.

Trying to maneuver through all of these different numbering conventions can be difficult for those educated about motors, and a complete mystery for those that are new to electrics.  Below is a listing of these different numbering conventions, what they mean, and which major motor brands use them.

Stator Size

Since stator size in electric motors is similar to cubic inches of displacement in an internal combustion engine, this is the most accurate way of giving the size of a brushless RC motor.  In glow engines, the bigger the piston, the more power you can get, and in electric motors, the bigger the stator, the more power the motor produces.  The size of the stator is generally a 4-digit number, with the first two digits giving the diameter of the stator, measured in millimeters, and the second two numbers giving the length of the stator, again in millimeters.  For small motors, with a stator length less than 10mm, a leading zero will be used to maintain the 4-digit format.  Motors sizes such as a 2205, which is 22mm in diameter by 5mm long, or a 2808, which is 28mm in diameter by 8mm long, are examples of this extra zero in the part number.

These motors tend to have a “dash number” at the end of the stator size such as a 2820-12 or 2820-850.  This signifies the Kv value of the motor either directly, or in a round-about way by giving the number of turns or wire wrapped around each stator pole.  If the dash number is a small single digit or 2-digit number, then it is most likely the number of turns used to wind the stator.  If this is done, you need to look at the specification chart for the motor to get the actual Kv value.  If the dash number is a 3 or 4-digit number such as -850 or -1130, then that is the actual Kv value, given as RPM per volt.

When motors are measured by stator size, it is easy to tell the approximate power increase by going from one size to another.  In an electric motor, the amount of power produced is based on the volume of the stator.  A stator shape can be looked at as a cylinder, and if you remember from high school geometry, the volume of a cylinder is equal to the area of the round end multiplied by the length.  The area of a circle is equal to πr², so the volume of the cylinder is equal to πr²xL.

From this you can see that if a stator is twice as long as another, then that motor will put out twice the power.  Because of this, a 2216 size motor will put out twice the power of a 2208 motor.  Looking at changing the diameter, you could determine the power difference between a 2208 motor and a 2808 motor by looking at the ratio of the volume.

The cross sectional area of a 22mm stator, which has a radius of 11mm would be calculated by taking π (3.14159) x 11 x 11 which gives you 380.13 square millimeters.  Multiplying this by 8mm gives a volume of 3041 cubic millimeters.  Doing the same math for the 28mm stator 3.14159 x 14 x 14 = 615.75 square millimeters for the area of the face, and multiplying that by 8 gives a volume of 4926 cubic millimeters.

Now if you take 4926 ÷ 3041, you get an increase in stator size of 161.98%, so a 2808 motor will put out approximately 62% more power than a 2208 size motor will.  When you know the math for calculating power on motors based on stator size, this method gives you the most information. Some of the manufacturers that uses this motor designation are AXI, Cobra, T-Motor and Scorpion.

Outside Dimensions

Most of the lower priced Chinese Import motors use the outside diameter of their motor, measured in millimeters, to determine the model number.  This can lead to confusion about the actual size of the motor because a kit may recommend a 3542 size motor, based on outside dimensions, but this is actually a 2820 size motor, based on stator size.  This can create a situation where a customer buys a motor that is way larger than they need, because of the difference in outside dimensions versus stator dimensions.

In some cases, but not many, the manufacturer may list the stator size of their motor in the description or specifications.  If they do, it makes it much easier to compare one motor size against another.  If this information is not available, the next best thing to do is to compare the weight of the motors.  Since most motors are made with the same materials, the weight of the completed motor will be similar from one manufacturer to another.  Because of this, if one company, using outside dimensions, makes a 3542 size motor that weighs 6.3 ounces, and another company, using stator size, makes a 2820 motor that weighs 6.4 ounces, you can assume that internally they are the same size motor.

Most of the time, these types of motors will have a 3 or 4-digit “dash number” at the end of the part number, such as 3542-1190 or 4350-580.  In this case, the dash number indicates the Kv value of the motor, with the units of RPM per Volt.

The other problem with this type of measuring system, is that the motors can be designed slightly different, or the exact place on the motor where they take the measurements may be different from one manufacturer to another.  Because of this, one company’s 3542 motor may be the same size as another company’s 3540 motor or the same as another company’s 3544 motor.  Some of the manufacturers that use this type of motor sizing are Dualsky, EMP, Himax, Leopard, NTM, RimFire and Turnigy.

Glow Equivalent Size

Some companies, in an effort to make it easier for modelers to convert from Glow powered models to Electric power, will name their motors based on the equivalent power output of a glow engine.  On the surface, this looks like a great way to do things, but it can lead to some serious problems.  With all electric motors, the prop is what “pulls” the power out of the motor.  Electric motors tend to spin at a constant RPM, and simply pull more current, and thus make more power, as the load increases.  If you have a motor that is called a Power .40 or a Glow .40, to signify that it puts out the same power as a .40 size glow engine, it will only make that amount of power with a specific prop at a specific voltage.

If you look at a typical 2-stroke .40 glow engine, a 10×6 prop is a very common size to use.  In a lot of cases, electric motors tend to spin larger props at slower speeds to make their power, to take advantage of the higher torque that electric motors offer.  A “.40 size” electric motor may need to spin an 11×8 or 12×7 prop to make the power level of a .40 glow engine when running on a 4-cell Li-Po battery pack.  If someone new to electrics gets a “.40 glow” size electric motor, and puts the 10×6 prop on it that they would normally run on their .40 glow engine, they will be extremely disappointed with the performance.  With that small of a prop, the electric motor will most likely make the power of a .20 or .25 size glow engine, and the modeler will think that there is something wrong with the motor.

There is no problem with using this type of naming convention for a motor, but to be accurate the manufacturer needs to include additional information such as, “Our Glow .40 motor will put out power equal to a .40 size glow engine when powered by a 4-cell Li-Po battery and using an APC 11×8-E prop.”  Without a battery size and prop specified, calling a motor a “Glow .40 equivalent” can create problems for the modeler.  Some of the manufacturers that use this type of naming convention include E-flite, RimFire, Thrust and Turnigy EasyMatch.

Stator Size Hybrid

There are a few motor manufacturers that will have their own type of motor numbering scheme that is partially based on the stator size, and partially on their own proprietary numbering system.  For example, Hacker motors use a pat number such as A30-16M or A40-14S to name their motors.  The letter A at the beginning signifies that it is an airplane motor.  The number 30 is the diameter of the motor stator in millimeters.  The -16M signifies that the motor uses a 16-turn wind, and that the stator length is the Medium length of the 30mm family.

This works fine, but the problem with it is that there is no consistency from one stator diameter to the next on exactly what the S, M, L and XL actually mean.  For example, for a 20mm stator, the S, M, L and XL could be 5mm, 8mm, 13mm and 20mm, while in the 30mm size it could be 8mm, 14mm, 20mm and 26mm.  Likewise, in the 40mm stators “S” could be 20mm and “L” could be 30mm.  In order to do a direct comparison from one of these motors to a motor based on stator size, you would need to match up external dimensions and weight to make sure that you were looking at equivalent size motors.

Torque motors also use their own hybrid numbering scheme for determining motor size.  A typical Torque motor will have a part number such as 2814T/820 or 2814T/605.  In these motors the 28 is the diameter of the stator in millimeters, and the 14T means that it is a 14-turn wind in the motor.  The /820 at the end is the Kv value of the motor.  The confusion with this numbering scheme can be seen in the two part numbers shown above.  At first glance, it looks like you have 2 different Kv values of a motor with a 2814 size stator.  Unfortunately, this is not the case.  The 2814T/820 motor actually has a 28x20mm stator with a 14-turn wind, giving a Kv value of 820 RPM per Volt.  The 2814T/605 motor actually has a 28x26mm stator with a 14-turn wind, giving a Kv value of 605 RPM per Volt.

If you look at the specs of the Torque 2814T/820 motor, it weighs 143 grams.  This is very close to a Cobra 2820/14 motor, which has a Kv value of 840 and weighs 140 grams.  Likewise, the Torque 2814T/605 motor weighs 177 grams, which compares very closely to the Cobra 2826 size motor, which weighs 171 grams.

Wrapping it all up

As you can see, there is a wide range of numbers that are used to describe brushless electric motors, as well as several different naming conventions to give the sizes of the motors.  While all of this can be a bit confusing at first, once you understand the numbers, and what they mean, picking out an electric motor can be a fairly straight forward affair.

To try and take the confusion out of cross-referencing all of the different motors that are available, I took the time to create a massive cross-reference chart that compares 13 different major brands of motors, and matches them to other similar size, Kv and power output motors.

A 2-page printable PDF version of this chart can be downloaded from my website at the following link.  http://innov8tivedesigns.com/docs/Airplane-Motor-Chart.pdf

There is also an online HTML version of this chart available for viewing directly at this link.  http://innov8tivedesigns.com/docs/Airplane-Motor-Chart.htm

I know that there was a TON of information covered here, but hopefully it will help you understand the various numbers and parameters that are associated with electric motors, and enable you to make an intelligent decision when it comes time to purchase your next motor.

Thanks for reading!


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