Motors - high voltage/low voltage??
 

Hi - I'm not sure if this question belongs in this forum, "Beginners", "Power Systems" or all three??
I know the kv value of a motor indicates its rpm (dependent on voltage applied) and some motors can utilise a 2S or 3S battery - is the motor always turning at this speed (V x kv), regardless of how much power is supplied? And if the same power is supplied by each battery, am I right in thinking that with the lower voltage (lower rpm) battery, the compensatory factor is more thrust/ torque?? If weight is not an issue, is it always preferable to use the higher voltage battery (to give extended rpm/power) - provided the permissible current draw of the motor is not exceeded??
Regards.

The motor always ATTEMPTS to turn at a speed of Kv x Volts...it never actually makes it because of losses. The bigger the prop and/or higher the current the more the losses and the lower the actual speed attained.

If you use the same POWER (i.e. watts, which = volts x amps) then you'll find that with a higher voltage you need to use a much smaller prop to keep the current down. So whether 100W turning a 6" prop at say 13000 rpm on 3S or an 9" prop at maybe 7500rpm on 2S works best on a particular plane involves so many variables that I'm not even going to try to sort them out.

Steve

Thanks for that information, Steve.

I was specifically thinking of my Easy Star where a 2200kv inrunner seems a popular upgrade choice. Some guys seem to run it on 2S and some on 3S, but in both cases with a 6x4 prop. Invariably they fly about half-throttle with power to spare.

Returning to the original question - with same motor/ same prop and 2S or 3S batteries supplying the same power, what is the difference with the 2S where the prop is (presumably) turning slower?

Can I ask an additional question - some inrunners are an exact fit in the EZ. Do they run any hotter than the standard brushed motor or is some additional cooling needed?

Regards.

The two situations => two different power levels.
With the 3S, the motor will be drawing/providing much more power.
To draw the "same power" with the 3S (as compared to the 2S), the propeller pitch and/or diameter would need to be reduced.
You would need to measure the actual currents being drawn by the motor to determine the actual power difference. A "Wattmeter" is a very desirable piece of equipment if you want to avoid burning up motors and/or ESCs.

With the 2S (vs 3S) and same prop, less power is being drawn by the motor and provided to the prop. => less thrust is available at full-throttle; available top speed is reduced; available climb-rate is reduced.

As I said earlier but obviously didn't explain well enough for you to understand, it doesn't work like that. With the same motor and same prop the 3S battery will be trying to turn the prop a lot faster and because of that will draw a lot more current. Overall the 3S setup would be running at more than DOUBLE the power of the 2S setup.

Steve

Actually if you went from 2s to 3s with the same motor and prop then you would pull around twice the Amps and three times the power:eek:

Steve

With regard to cooling....depends.
Some motors are less efficient than others (can you say hobbyking).
I spent a BUNCH of time looking over every single suitable outrunner for my Funjet and finally rejected them all.
Cheap, yes, but they make more heat for the same output.
OTOH and ES isn't a Funjet.

Some inrunners have cooling holes...others do not.
So....there are LOTS of variables in addition to the basic question.

If you run a given prop on 2S the power it will pull will be a lot less than 3S > so, heat output will be lower.
OTOH, if you are flying around at 1/4-1/2 throttle all the time it may not matter much.

As indicated, you don't PUSH power. Power is determined by the load on the motor which is a function of the prop and voltage. Since you really can't change the prop much in this instance, your biggest variable is the voltage.

In applications where the prop is not a limitation you can prop up or down to make a 2S (larger prop) and a 3S (smaller prop) pull approximately the same. The 2S will pull more AMPS as the voltage is lower-simple electronic math. That's why if you are PUSHING the motor envelope higher voltage and smaller props are more desirable, within limits.

You can go for inrunners with different KV. A stock Stryker C is around 1900kv and is happy on 3S with that prop size.
Something like the eFlite Six-Series Brushless 2700Kv Motor would be loafing on 2S and screaming on 3S.

There are a number of computer software programs that will give a lot of information on the type of questions you have raised.

Check out www.motocalc.com, free for 30 days, then $39. With this program, you can put in a range of propeller sizes, and, a range of difference number of cells on your proposed setup. Then just run the program and see what happens.

If you put in as an example, a propeller size from 10-6 to 14-12, the program will spit out results on every combination. And, if you also add as an example, 2 to 6 series LiPos, that will multiply the above results by four times. Just studying the results, and looking at motocalcs opinions can be a nice learning experience.

The program will give approximate RPM, thrust, amps, volts, and percent efficiency of the motor. A high quality $$$$ motor will usually have efficiencies on the order of 85 to 90%, where a cheap motor might only be in the 60% range. If you've got 60% efficiency, the 40% difference is going to be heat in the motor and its windings. When you get to some of the higher powered motors, that efficiency gets important, when you've got a 1 kilowatt motor, and 400 watts of that 1KW is heat.

This program is only as good as the motor specifications put into it, and unfortunately some of the cheap import motor's specifications are not even in the ballpark. But, motocalc will still give ratios on different prop and cell count setups.

There are also other programs that are free, and provide a lot of good information. Motocalc is the one I use, and have had it for some 4-5 years.

Hi – a thank you to everyone for their help, advice and information (and patience!). I understand design fundamentals but readily concede I’m just not grasping the electrical basics.

I realise that power required is related to plane weight/ performance required. And a motor/ battery combo is then chosen to deliver that power. And the chosen propeller causes the motor to draw that power.

A 2200kv motor with a 10V battery would turn at 22,000 rpm with no load (ignoring losses) and minimum current.
When the motor is hooked up to a propeller and an ESC, am I correct in thinking the ESC controls the current/amps delivered to the motor? And that at design maximum current/amps with a well chosen motor/ battery combo, the motor will approach the theoretical maximum speed (22,000 rpm) and deliver its maximum power?

Returning to the EZ upgrade, I’ve seen articles that show a 28mm diameter inrunner (ie same diameter as stock motor) fitted into the nacelle – no mention is made of any additional cooling requirement so I have assumed it’s not an issue. With outrunners on an EZ, I’ve assumed exposure to natural air cooling in flight is sufficient.
Regards.

PS After the 5th attempt to edit the micro-type in my last reply and receiving the message "Internet Explorer cannot display the webpage", I've concluded I also don't understand Wattflyer's editing system!!

[QUOTE=RONALDO_UK;853906]Hi – a thank you to everyone for their help, advice and information (and patience!). I understand design fundamentals but readily concede I’m just not grasping the electrical basics.

I realise that power required is related to plane weight/ performance required. And a motor/ battery combo is then chosen to deliver that power. And the chosen propeller causes the motor to draw that power.

A 2200kv motor with a 10V battery would turn at 22,000 rpm with no load (ignoring losses) and minimum current.
When the motor is hooked up to a propeller and an ESC, am I correct in thinking the ESC controls the current/amps delivered to the motor? And that at design maximum current/amps with a well chosen motor/ battery combo, the motor will approach the theoretical maximum speed (22,000 rpm) and deliver its maximum power?

Returning to the EZ upgrade, I’ve seen articles that show a 28mm diameter inrunner (ie same diameter as stock motor) fitted into the nacelle – no mention is made of any additional cooling requirement so I have assumed it’s not an issue. With outrunners on an EZ, I’ve assumed exposure to natural air cooling in flight is sufficient.
Regards.[/QUOTE]

You're making progress :D The maximum RPM any brushless motor can reach is the no load condition with no prop (Or a terminal dive straight down!:eek:) Since the motor is not turning a propeller, the actual power output on the motor shaft is zero.

Or, if you take that motor and really load it down with far to large of a propeller, and have perhaps 500 RPM, that motor is going to pull a LOT of amps, and get hot, real quick, like 5 or 10 seconds before the magic smoke gets out. As for power output, the motor is going to have a lot of torque, but because of the low RPM, again the power output on the prop will be pretty low.

So, somewhere in the middle, between no load, and smoke from the motor windings is the best power output of the motor. Only it's not quite that simple. The highest power output is something like where 1/2 of the watts into the motor is turning the shaft, and 1/2 of the watts into the motor is turning into heat. Problem is, that motor running under this condition is likely not going to last very long.

So, the best "Loading Point" for that motor will be something where the motor shaft efficiency is somewhere around 80-90%. Some of the cheap motors might never reach 80% efficiency under any propeller load. And the $$$$ motors typically reach the high 80% area with the proper prop selection.

Now, you've got to balance that motor/ESC/battery/prop selection against the type of model you are going to fly. One extreme is the "Piper Cub" slow flyer type of model. That would not be a good design to put a motor turning a tiny prop at 20,000 RPM on this slow flying Cub. Might work, but over all efficiency would be awful. So, a big diameter prop with a slow turning motor would be a good choice for the Piper Cub.

On the other hand, if you're going for a real FAST 100 MPH plus "Foamie" model, you would not want that slow turning motor on the Foamie. You'd want a motor that runs 20,000 RPM turning a little toothpick prop.

That is the big difference between glow power and electric power. For the most part, the glow engines are restricted to a certain RPM range, say between 8000 and 12,000 RPM for a mid sized model.

On the same mid sized model, you can select electric motors that can turn over anywhere from say 4000 RPM to 30,000 RPM, just by varying the brand of the motor, prop diameter, and cell count with the KV of the motor.

All of this is confusing to say the least. That is where software programs such as www.motocalc.com have been designed by people with a lot more knowledge in this stuff than 99% of us (Including me) can save the day.

Motocalc will let you know if you are trying to put a 20K RPM motor on a Piper Cub, or some other combination that is not optimum.

Great explanation DennyV and thank you.

But HOW did you know I had a Piper Cub??! Several in fact and I'm counting my Champ as one too. They're mainly off the shelf but one, which I believe was built by an experienced modeller, is tissue and balsa, about 36" wingspan. It's fitted with a 1700kv 55W brushless motor, a 10A ESC and a GWS 8x6 hyper drive prop. He recommended a 2S 500mah lipo - I've yet to fly it (inexperience and cowardice!).

By the way, I presume I was at least right in saying the ESC controls the current (amps) delivered to the motor (must do, it certainly doesn't control the voltage)?

I dread to think how much time you spend on this site? - you've certainly helped me on a few occasions and I'm sure many others. That's what I call passion for your hobby!

Regards.

No. the ESC merely passes through that what is demanded by the motor based on the voltage and the load (prop).
If you have too small an ESC it will limit the throughput before it burns up.
Too large does not improve things, it just weighs more and costs more but is less likely to burn up if you screw up. It may run cooler, but if you've sized right about 20% larger than the maximum output of your prop/motor combo in the voltage range required will do fine.

LOL, as for the Piper Cub, I didn't :D

And, the maximum current and maximum watts that can be pulled by the motor is controlled by the motor specifications, the propeller size, and the battery and its voltage. That is the power output you will get when the ESC gets a "Full throttle" command from the receiver.

The receiver sends a train of voltage pulses to the ESC (and all servos for that matter) where the pulse width varies from one millisecond at low throttle to two milliseconds at full throttle. The ESC takes that one millisecond pulse, and translates it to zero power to the motor, and takes that two ms, and translates it to full power.

At pulse widths between the one and two millisecond wide pulses, the ESC "Chops" the power to the motor at a very high rate, like 10 or 20 thousand times a second. If you've got good hearing, you might be able to hear this switching take place from the motor noise. This is what is called PWM, or pulse width modulation. The ESC controls all this stuff with a little computer chip microcontroller. Those microcontrollers are found everywhere, like a bunch of them in your automobile, digital control for the microwave, digital watches and clocks, cell phones, dishwashers, you name it. You can buy those little chips nowdays for under a dollar.

Times have really changed over the years.

DennyV

A watt-meter might help?
 

Hi DennyV - I'm not your star pupil by any stretch of the imagination and I've come back to re-read this thread and marvel at your infinite patience!
I'm also investing in a watt-meter (not yet received) which may give me a better insight into the mysteries of electrickery. :)

Regards.

Don't forget, every one of us was a pupil at one time or another! :D :D

Actually that's pretty much what an ESC does.. it controls voltage on it's output side (voltage on the input remains at battery voltage).
It does this by by turning the power on and off very fast, so if it's supplied with 12v and it outputs on-off pulses that is off 50% of the time (zero volts) and on 50% of the time (12v) then the average voltage output is 6v. Because the rate that the power is turned on and off is so high the motor responds exactly like it would if fed with smooth 6v.

E-flight 101 by RCG member Ken Myers, will at least save you a ruined LiPo (or worse!), a burnt motor and a fried ESC:
http://www.theampeer.org/framesefo.html
-> Electric Power Basics
and this e-book
-> Everything youw wanted to know about e-flight (Ed Anderson)

Thanks for all of the above advice/info - that link looks promising. Re. batteries/ESC, I bow to your greater knowledge (JPF). But certainly the full battery voltage is used in calculating the energy (watts) drawn by the motor??

Regards.

Not quite the average voltage. All answers in this thread by seasoned EE's (I'm an EE gone over to the dark side, embedded software), Bruce Abbott designed a few diy controllers and Greg Kamysz was involved in the first brushless RC motors/controllers:
http://www.rcgroups.com/forums/showthread.php?t=1664402

Picture from
http://www.consult-g2.com/
-> course
-> chapter 9: electronic controllers

http://www.consult-g2.com/course/chapter9/f9.2r.jpg


Picture from
http://www.aerodesign.de/peter/2001/...index_eng.html
-> Why does the Torquemax rotate so slowly and so forcefully
and
-> Speedy-BL diy brushless controller

Voltages at partial throttle
http://www.aerodesign.de/peter/2001/...last_Motor.gif

Prettig weekend ;) Ron
• diy motor tips • Drive Calculator •
• diy motor group • Cumulus MFC •
• Get a life ... get a 3$ Wattmeter •

Why not average?

The picture you posted calls it 'mean voltage' but mean and average are the same thing in this context.

The ESC reduces average voltage, so average amps must increase for the power out to be the same as the power in.

The ESC absorbs a few watts in the form of heat but if you ignore those losses power out of the ESC always equals power in, only the volts and amp 'balance' changes.

I didn't want it to get TOO involved!
Looking at the excellent link Ron VS supplied (bedankt) which is like RC For Dummies (just what I wanted!) - it said the ESC is like a very fast switch so voltage changes from either zero to max/ max to zero with throttle inputs, which I suppose is kind of same/different to what you said.

Regards.

Here are two more links for what's involved in these brushless motors, (and why its far better to buy one, rather than design your own!) Even if a person had the ability to design one of these ESC's and the ability to program the microcontroller for it, building one of them from scratch, or even a few dozen from scratch would far exceed the cost of buying one ready to go. Buying components in very large quanities really reduces the cost of the product.

Microchip ESC Info
The first few pages of this PDF file shows basic information on what is involved to drive a brushless motor. Take a look at page 3 of this file, which explains in very simple terms, what happens to your brushless motor during one revolution of the motor.
http://ww1.microchip.com/downloads/en/appnotes/01083a.pdf,

This PDF file covers some of the mathematics on what is involved in these brushless ESCs.
http://ww1.microchip.com/downloads/en/AppNotes/01292A.pdf,

As can be seen from these files, there is a lot of stuff going on "under the hood" of these brushless ESC's. Its all being done by the microcontroller that is controlling the power drivers of the ESC. Circuitry wise, compared to a cellphone, these ESC's are pretty simple. But, the computer programming to actually run these ESC's is NOT simple!

Answered somewhere in the RCG thread.

Prettig weekend ;) Ron
www.modelbouwforum.nl