This is the typical winding used in CD-ROM motors and it’s usually terminated Wye (Star).
It is the first winding scheme that many motor builders and rewinders will encounter. It is also the easiest winding to do, since all teeth are wound the same direction, i.e. ABCABCABC.
dLRK [12N10P & 12N14P]
Here is the most common winding scheme of all, since it is used in the majority of model outrunner motors that are currently being manufactured.
The dLRK scheme is a derivative of the LRK winding scheme, which put outrunner brushless motors on the map.
Connect together: Start A — End C, Start B — End A , Start C — End B.
dLRK Evolution [12N10P & 12N14P]
Here is an interesting mutation of the dLRK scheme, no doubt aimed at making termination of the windings easier and neater. It is my favourite for winding 12N14P motors.
Note how the starts and ends of the phases that need to be connected together, exit from the same slots, making the termination process totally fool-proof.
dLRK Evolution (Delta)
LRK [12N10P & 12N14P]
This is where the ‘brushless revolution’ started. The LRK winding scheme will always be remembered as being the ‘grandfather’ of outrunner brushless motors.
‘LRK’ is an acronym that comes from the names of the three men who popularized the outrunner motor: Lucas, Retzbach and Kьfuss. In 2001 they documented the use of this winding scheme for model airplane motors. And as they say in the classics, ‘the rest is history’.
Connect together: Start A — End C, Start B — End A , Start C — End B.
This 12 volt, 3-phase generator is one of my latest projects.
The client needed to power high-intensity LED spotlights and other airborne equipment in model airplane drones that are being used for air reconnaissance in wildlife conservation. Typical mission duration can be up to 6 hours long, thus in-flight charging is a necessity.
The shaft of the generator is turned via a timing belt by the same 60cc petrol engine that powers the aircraft.
The generator had to be mounted in close proximity to the engine, therefore temperature was an important design concern. Potential heat-related problems were averted by using a Scorpion motor, because it employs magnets with a temperature rating of 200°C.
The 150 Watt, 12 volt generator is based on a Scorpion S-3020 brushless motor kit. The stator was wound so that charging voltage is obtained at just above idle speed.
Scorpion S-3020 brushless motor wound as 12 volt generator
Scorpion S-3020 3-phase generator next to DLE-60cc petrol engine
Three of the drones used in wildlife conservation
Over the course of the last 15 years, I’ve been fortunate to have been involved in the design and contstruction of a few wind generators, ranging in size from 500 Watt to 4 KiloWatt.
With my first generator project, I was thrown in the deep end to do the stator and winding design of a 4 KiloWatt "monster", relatively speaking. It was a joint effort with my flying buddy who was an armature winder. Up until then, my motor building experience was limited to rewinding some CD-ROM motors and building a couple of small outrunner motors for model aeroplanes.
With the generous help of the very knowledgeable members of the LRK Torquemax forum, many stumbling blocks were overcome or prevented.
To be able to start turning in light wind conditions, low cogging is an important requirement for a successful wind generator.
Because this was a permanent magnet generator of the "outrunner" type, the main challenge was undoubtedly to minimize cogging, which we succeeded to do very well. There was hardly any cogging noticable.
Roughly machined flux ring and some stator plates stacked
Flux ring with 50x22x8mm curved Neo magnets
The parts before winding the stator and final assembly
The stator wound and terminated
This 500 Watt wind generator is also a recent project. With the experienced gained in previous generator projects, designing this unit was rather straight-forward. Because the generator was intended for vertical-axis installion, the exterior design was styled to be weather resistant and therefore doesn’t require an enclosure.
The very low cogging torque that was achieved with this design, is a direct result of an optimum slot/pole combination and good stator design. Observant viewers will notice that the aluminium parts are not anodized. We tried to keeps costs to a minimum, because this is a prototype unit.
Trial fitting of the parts of the generator
Have you ever wondered if it’s possible to measure the Kv of an electric motor yourself? The answer is "yes", and it is actually quite easy to do at home with relatively inexpensive equipment that you may already have. All that is required is a way to measure volts accurately (multimeter), a tacho to measure RPM and a drill press or another method to spin the motor at a constant speed.
Once you know how to measure Kv, you’d want to do it whenever you buy or rewind a motor, to confirm the Kv value.
On commercial motors, the stated Kv is usually a nominal value, because no two electric motors are completely identical.
Therefore, a manufacturer may round the Kv value to the nearest 50 or 100 according to what they measured on a test sample. In some extreme cases, manufacturers even publish inflated Kv values, hoping to take advantage of the notion that bigger is better. Some manufacturers don’t even bother to give the basic specs, presumably because they don’t know how to determine the motor constants.
And sometimes manufacturers make mistakes. E.g. when O.S. (who is well known and trusted for their IC engines), brought out their first electric outrunners, the specs for the OMA-3825-750 motor were horribly incorrect.
Three Important Motor Constants
The three motor constants which are very handy to know about any electric motor are:
The speed constant or
motor winding resistance
(Rm), also abbreviated Ri
Using these 3 constants, the performance of any electric motor can be predicted fairly accurately with simulation software.
Obviously, the more accurately the constants are measured, the
more accurate the simulations will be for any given motor.
This means that using nominal values or inaccurate values will yield less than desirable simulation results. It is a strong argument for measuring your own motor constants.
Some software will also predict motor temperature using a fourth value, the weight of the motor.
Examples of really great, free motor simulation software are listed at the bottom of this page.
Kv (Velocity Constant)
It is surprisingly easy to measure the Kv of an electric motor.
The unit for Kv is RPM/volt, and in effect Kv is exactly that — the RPM that a motor will turn when one volt is applied. For brushed motors, this is straightforward: Kv = RPM / Volts.
With brushless motors however, the formula becomes slightly
more complex because power is not applied directly to the motor leads, but goes through an electronic commutator, in the form of a speed controller (ESC) first. The ESC sends power via PWM pulses to the three phases of the motor.
Below are three methods for determining the Kv of a brushless motor, not necessarily in order of preference.
Drill Press Method
is probably the most accurate, as well as quickest and easiest method for measuring Kv, as the motor doesn’t have to be connected to the ESC and battery.
Spin the motor in a drill press at a known and constant speed.
Measure the AC Volts (RMS) with a multimeter. To do this, take a voltage measurement on one of the three phases of the motor (any two of the three motor leads).
Apply the formula to get the Kv of the motor alone — without the effects that ESC settings will introduce.
For a more accurate result (true Kv of the motor), measure all three phases and average the values.
I suggest measuring the speed of the drill press beforehand. Keep in mind that the accuracy of the final result depends on the accuracy of the testing equipment and readings. I usually try to get at least two decimals when taking voltage readings, because it makes a difference when the calculated Kv is going to be used in simulation software.
Another method to determine Kv involves running the motor without load at full speed, and then measuring Volts, Amps & RPM, as well as specifying the motor resistance constant (Rm).
This method provides a Kv constant that includes the effects of the ESC, which may be desirable in some cases. The result may also be called the intrinsic Kv constant of the motor and ESC.
Hobby King sells a product called the "K1 KV/RPM Meter" which was designed to measure the Kv of brushless motors. I strongly suspect that this meter uses the "Drill Press" formula described previously. But unfortunately, in order to measure Kv, the user has to enter the number of magnet pairs into the unit (presumably to calculate the RPM). This may pose a problem if the number of poles (magnets) is not known to the user.
-zero or No-load Current)
is the no-load current of a motor and in my opinion is best measured at intended operating speed. Usually, a reading is taken at 10 Volts, however I prefer to measure
at the RPM at which the motor will most likely be going to work. There are other methods, i.e. the ‘four constant model’ which is outside the scope of this article.
With outrunners, it’s quite easy to measure RPM when two marks or lines are placed on opposite sides of the outside of the rotor. Then a tachometer set to 2-blade setting can be used to measure RPM.
It’s best to use a variable power supply and varying the RPM by adjusting the voltage until the desired operating RPM is reached with the ESC at
, and not by varying the speed with the ESC.
If a variable power supply is not available, try battery packs of different voltages till you find one that gives the closest speed to what is required.
Record the current and if possible, the RPM of the motor shaft.
With no load on the motor shaft:
More accurate modeling of
is possible by employing advanced curve fitting equations, but then multiple readings at various voltages and speeds will be required.
Rm (Winding Resistance)
Rm (or Ri) is the resistance of one phase of a motor (after termination). In other words, if one measures the resistance between any two motor leads of a brushless motor, that value would be the Rm. Unfortunately, this measurement is a little more tricky to perform than meets the eye, because a multimeter is just not sensitive enough for this purpose.
But fear not. There is a method to measure Rm which is within the capabilities of the average DIY person, yet it’s very accurate and only requires a few pieces of inexpensive equipment.
It’s a variation of the Kelvin (4-wire) method and has served me well for many years.
Equipment needed: Ammeter, voltmeter, 10-20 Ohm (10 Watt) resistor and a high capacity battery of about 12volt. The exact values are not important, because the accuracy gets taken care of by the calculation. A 2000mAh 3S or 4S LiPo battery is ideal.
The ammeter and DVM (digital volt meter) can be cheap multimeters, as long as the one used for amps can measure current to at least 5A.
Being an electric model flyer, I already had an accurate ammeter in the form of a Hyperion e-Meter. Although the standard 100A shunt will work, I chose to use the 20A shunt because then Amps is displayed with an extra decimal.
4-Wire method for
Typical setup using
Now that we know how to measure the three important motor constants, Kv,
and Rm, lets see how we can apply this knowledge.
One possibility is that the efficiency of the motor, as well as the efficiency of the complete system (motor, ESC and battery) can be calculated. More importantly, it’s possible to establish what exactly robs a system of efficiency.
Efficiency changes as the load on the motor varies. In other words, with a certain size or pitch propeller, a motor will have a certain efficiency at WOT. If a different size or pitch propeller is used, the efficiency will change. If the voltage goes up (by adding a cell or two), the efficiency will also change. If the motor is run at part-throttle, the efficiency will also change.
Some manufacturers give a ‘Maximum Efficiency’ rating, but don’t be fooled. That number is calculated and is far from what the efficiency is when the motor has to work hard, like when powering a model airplane, multicopter, or helicopter.
In order to calculate efficiency, we have to run the motor with a load (propeller) attached while taking Volt and Amp measurements using a reliable Watt meter.
This calculator is based on these formulas:
Copper Loss = Amps
Iron Loss = Volts
Motor Losses = Copper loss + Iron Loss
Power In = Volts
Power Out = Power In
Efficiency = Power Out / Power In
Kt = 1/Kv
(SI units: with RPM in rad/sec)
Torque = Kt × Amps
Km = Kt /
Hopefully, with the help of this efficiency calculator, you’ll be able to make informed decisions about choice of motors, ESC’s and batteries.
Motor Simulation Software
No respectable electric flier should be without a decent motor simulation tool. The two applications listed below are at the top of my list. Both are very realistic and powerful stand-alone applications that are free to download and use.
Drive Calc (D-Calc) is available for Windows, Mac and Linux. Databases are open and are updated regularly by the author.
Motor Calc Software
Distributed freely and customized for a variety of commercial hobby motors, e.g. Scorpion Motors (Scorpion_Calc) & HiMax Motors (HiMax Calculator).
Apart from supporting the respective branded motors, any electric motor (even custom-wound) can be simulated. A large library of propellers enables the user to do performance predictions for a wide variety of applications. Another important feature is that it works from minimal data input, since no real-world data is required, except when simulating custom motors.