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RC Motors

Preamble

Are you baffled by the numbers used to describe the plethora of RC motors which are available for multirotor fliers? So was I. So here is a brief description of what I have discovered whilst trying to make sense of it all.

See also the closely related blog, Propellers – Aero.

Guides

This guide, Brushless motors – how they work and what the numbers mean, is very informative.

What do the numbers mean?

As you shop around for some brushless motors you will notice that there is often a trend in motor naming with a series of 4 numbers and might wonder what those numbers mean. It’s simple enough; most motors have a few letters followed by 4 number. the letters hold no significance (usualy a model or series name, such as MT series motors, or Q series for quadcopter), whereas the numbers tell you the essential measurements of the motor.

Generally, the first two numbers indicate the diameter of the entire can of the motor OR the diameter of the stator (the inner part of the motor) only. Please see the picture at the below for clarification.

Similarly, the second pair of numbers can indicate either the height of the entire motor, or only the height of the stator. In this case of the MT2204 motors254 that unmanned tech, these numbers indicate the stator diameter and height, and so we have an 22mm stator diameter, and a 4mm stator height.

The size of the motor can give you an idea about what size drone you will use with the motor. Typical FPV racing miniquads will use 1806 or 2204 motors, whereas larger quadcopters that are designed to carry a gopro camera will typically be around the 2212 size.

Brushless ESCs

How does a brushless ESC work?

The controller generates the signal to the MOSFETs depending on the signal it receives from the back EMF generated by the motor. This EMF signal “tells” the controller what position the rotor is in so it can decide which MOSFETs to switch on.

The rpm is varied by changing the pulse width of the signal to each phase (PWM, Pulse Width Modulation). Just like a brushed controller the frequency is constant and speed is controlled by varying the length of time the fets are ON. EG. half throttle the MOSFETs to each phase are on for only half a cycle – full throttle they are on constantly (but phase still cycled +, – depending on rotor position)

Two MOSFETs are used per phase.

Sensorless “Delta” type BL motors have to be driven by 6 sets of MOSFETs. There are 3 N-Channel sets & 3 P-channel sets to conduct power from the windings to Vcc and ground. On “Star” wound motors there are only 3 sets of MOSFETs required, with the center point of the motor tied to Vcc and N-channel MOSFETs to conduct each lead to ground.

More info

More info on the workings of a brushless controller:
http://www.torcman.de/peterslrk/SPEEDY-BL_eng.html
-> functional principle

You can see that the three leads to the motor are either at Vcc or zero, thus creating a ‘sine’ wave (trapezoid actually). No need for a full bridge, a half bridge does the trick.

 

DC brushless motors vary their RPM depending on the value of input voltage. The ESC senses the armature position by detecting the voltage on the unused phase and switches to another phase at the proper time.

RPM = Input Voltage X Kv

Vary the input voltage by varying the pulse width and it varies the motor’s RPM.

The above explanation has been deliberately simplified by not differentiating between RMS and Average input voltages.

Motor and Propeller pairing

This thread goes into some detail about various propeller motor combinations, Multistar 2213-935Kv:

CF blades

The CF blades will almost always out-perform the APC SF props. Particularly so if you are running the APC SF at close to their recommended maximum RPM and have some throttle left.

Checking for overheating

you can test the larger prop by simply mounting it and (with the multi tied down) running run it up to the throttle setting that you can normally fly at with the other prop.

Do it in 15 seconds or so stages and pause after each one and put a finger tip on the base plate of the motor (the fixed part, not the magnet housing). When that area gets too hot to keep a finger tip on it it is around 130F/54C that is a good stopping point. The windings will be a little warmer than the outside of the backplate of course.

It is OK to run a motor that is that hot but you want to make sure that the temperature is stabilized and not still rising to get any duration out of it. When you find you can run it for 2-3 minutes like that in a static test and still keep a finger on the backplate it should be safe to fly it.

When you fly it, at the end of a flight, get your finger on all four motors as quick as possible and as long as they will pass the finger heat test you are OK. It is when you cannot keep a finger on it and don’t know how much hotter the motor actually is that it starts to get dangerous.

In my testing I have run the temperatures on the outer layer of the winding (I have a small sensor that fits into the back plate area) up as high as 160-170 degrees without damaging a motor. But that was by accident more than by intent. I would not intentionally try to run a motor at more than 130F/54C or so just out an interest in keeping it alive and well.

As I always say, you cannot burn a motor up without it getting too hot first. So if you work up to it checking for heat you would normally be OK.

For props in that size range I would be testing the 10 x 4.5, 10 x 4.7, and 11 x 4.7 CF prop I have on hand as they almost always out perform plastic props of the same size.

And kicking the cell count up will increase the RPM and that increases the thrust. Increase in cell counts need to be tested about the same way as when you increase the prop size. Work up to full throttle loads and full duration while checking for heat. If nothing gets too hot (you should be checking the ESC too of course) then you’re good to go.

Ignore motor weight

A good quality motor, that is more efficient will improve the performance.

More Kv, less weight motor, more voltage, sheap motors, less tooth stator, will reduce efficiency.

Respect less weight motor.
If we take 2 motors, the same brand, series, Kv, Propeller, Voltage.
The lightest model perform less rpm and less efficiently.

If the weight difference is huge, then efficiency gains may be lost.

In the case of a frame 450, a change of a 2212 to 2216 motor, will give us a tremendous benefits.

Especially if the change is accompanied by a good selection of the propeller.
A good selection the propeller depends on the weight of the motor, and Kv.
A maximum acceleration, the motor should not run below 70% efficiency.

Over-propping

Over propping (fitting a prop that is too large for sustained use at higher throttle) can have it’s benefits. If you are using high throttle or full throttle a lot, you have to be wary of the heat. But for more staid flight (FPV, camera work, etc.) where the flying is done mostly at lower throttle setting in the interest of duration an over-propped motor loafing along at half throttle or so can be the most efficient choice for that purpose.

Manufacturers over rate motors

they test on 2S so they can list a larger prop and then they go to 3S and 4S with smaller props. Then they say the motor is good with 2S to 4S and props from the smallest to the largest. But they never tell you that to use the highest cell count you must use the smallest prop.

NLS

It is said that an motor that will be used at 100% throttle, the propeller must turn at least 70% NLS ( % Not Load Speed) sometimes called % of Kv* Volt or voltage efficiency.

Heavier motor is more efficient

Between two equal motor, same brand, same features, same construction, the heavier will be more efficient motor.
More rpm same current,

Carbon Fiber versus Carbon filled

One thing you have to watch out for on props that are sold as “CF” props is that some brands of props use the description “CF” for carbon filled props. A carbon filled prop is a plastic prop with some ground carbon added to the plastic to gain a little strength. And a carbon filled prop will not have the same performance or strength as a prop that is made with a woven carbon fiber cloth. You can see the fabric in a “real” carbon fiber prop where a carbon filled prop will not have any fabric visible.

General thrust required

To get a multi that lifts cleanly and hovers and has enough power to deal with typical winds and maneuvering needs requires you to have an amount of thrust that is at least 1.2 to 1.4 times the ready to fly weight. So for your 1980g multi you need for props that will generate 2376g (good) to 2772g (better) of thrust at around one half to three quarters throttle (no one wants to need full throttle to hover). So you need to have four props that will generate about 600g to 700g of thrust each (total thrust = 2400g to 2800g).

The process is to choose a prop that will generate the thust and that will give you the input power and RPM that is needed to do that. And those numbers, and the battery voltage you choose to, use will determine the approximate weight of the motor (typical motors will handle 3W per gram continuous) and the Kv it needs to have.

As an example, the image is a motor being static tested with a 14 x 4.7 CF prop. And, as you can see, that motor at prop would generate the thrust you need at between 1/2 and 3/4 throttle.

And the input power for the combo is about 130W at the power level you need and it is at almost 200W at full throttle. And that is a 154g motor that was being used on 4S in that test.

You can read more about a similar version of that motor in the review I did on it here:

iPower MultiMate MT4114-320KV Motor Review – www.rcgroups.com/forums/showthread.php?t=1980619

That motor in the image has a 400 Kv, the review is on a motor with a 320 Kv. All of this gives you a rough idea as to what props you might use and what you need for motors for your multi.

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