Brushless motor Current rating.

[Djay Ahluwalia]

 

 

Wat does the current rating on a brushless motor mean?I need a brushless outrunner that can run on 5A at 5V, is that possible?Any help greatly appreciated.Thanks! 

 

 

[birdy]

The current rating is the maximum current a motor can take before bad things start happening (eg. The varnish on the winds start melting) and is generally over rated by the manufacturer.

 

If you need a brushless out runner that will run at 5A at 5V, then yes, that is easily possible; The amount of current a motor pulls is dependent on three things. First the KV. This is the rpm it will acheive with no load per volt - eg. an 2000KV motor at 10V should run at 20000RPM.

Second is the voltage; The higher the voltage the faster the motor will try to spin. Spinning faster means it will work harder and pull more amps.

Last, the prop. A larger or higher pitch prop will provide more resistance the a smaller one, but the motor will be trying to acheive the same RPM, so it will use more power to compensate.  

 

I'm not really the person to ask which motor two, but someone here will be able to help. 

Can I ask what its being used for (that often affects the KV / Prop choice - but no need to go into that right now)? What batteries are you using to be getting 5V?

 

Hope I've been at least abit helpful ,

 

Birdy 

[Bob Moore]

I see you posted on the forum I e mailed you Djay 

 

I'm sure you'll get some good responses here. These guys know what they are talking about!  Good luck again with the project.

Edited By Bob Moore on 12/05/2010 18:17:46

[Tim Mackey]

As Birdy says...we need more info to give any more advice.

[Djay Ahluwalia]

Bob, yes thanks again, the site looks like a great idea for new starters like me.

 

Birdy,

 

Thanks for the reply!  

 

I am trying to build a solar powered aircraft. I have 10 mono-crystallin 15.5% efficient solar cells. I am currently testing them and logging the data. Looks like on a decent day the cells can generate 5A and 5volts.

 

I will either use this to run the motor/prop. Alternatively, i will use the energy generated by the solar cells to charge a battery (LiPo) and use the battery to run the motor/prop. 

 

Hence why, I need a motor/prop combo that can run on such low current/voltage.

 

Djay 

 

[Djay Ahluwalia]

Also, the above mentioned plane is under construction, it is a lightweight glider, that I intend to modify into a prop driven RC controlled aircraft.

 

I have'nt weighed it yet but the total all up weight should come to 450-500 gms (including the solar cells, which weight 100 grams)

 

Djay 

Edited By Djay Ahluwalia on 12/05/2010 19:02:54

[Tim Mackey]

5A from small solar cells is pretty good  - the issue you may have is the low voltage. most modern brushless motors are designed around 7V minimum, and up to round 30+V

What size and weight will the finished aircraft be... maybe you could consider the very small motor and ESC units from the indoor stuff?

 

[Djay Ahluwalia]

Timbo, 

 

Thanks for the post!

 

Yes, I am struggling to find a lightweight motor to run at such low current and voltage. The local hobby shop pointed me to a 1500 KV (12 A continuous, 16 A max) motor, but the guy in charge seemed to think that the motor would 'not operate' under 12 A. And obviously, the current I get from my solar cells is a lot lower than that.

 

Djay 

 

[Hamish]

How big are your solar cells as that appears to be a good output?

[Djay Ahluwalia]

 Hamish,

 

The solar cells I have are a 125mm X 125mm square.The aircraft is 1.5 m wingspan, made using balsa wood.

 

[Tim Mackey]

Well at over a pound AUW, then the earlier suggested small motor esc combo is no good.

How about something from micron indoor flyer

[Myron Beaumont]

I'm intrigued ! What make are your solar cells that are 15.5% efficient ? Compared with what ? I have fitted solar cells (long time ago) to a boat & we were talking milliamps output ie trickle charging .Things must have moved on at a tremendous rate since then ?

[Djay Ahluwalia]

Timbo

 

Wow! Thanks, I think the 18mm 10g Micro Brushless Outrunner , might just be what I need. Also, I am not familiar with the way Thrust is expressed on this website, what does a thrust of 140 gm mean? 

 

Myron,

 

I got these cells from Singapore, but they might have been made in China. Nifty little things, altho recent reports suggest cells touching 30 % efficiency might be hitting the market soon, at a very high price. 

[Hamish]

Djay

 

I am a bit amazed about 5amps from a 5" square solar panel.  At 5 volts that's 25 watts which is great. Quick look  on ebay for a 25 watt panel was 532mm x 418mm x 23mm at 12 volts and  a cost of £90 and this output would be in the Sahara desert.

 

Are you sure about the output?

[John Privett]

They're quoting the thrust in grams.  This is equivalent to a force of approx. 1.4 Newtons.  Note that this is assuming power input of 40W (6A, at approx. 7v) whereas you'll have a power input of only 25W so the thrust will be less.

[John Privett]

Hamish - I think the 25W is from all 10 of the 125mm square panels - not per panel.

[Djay Ahluwalia]

Hamish, 

 

Indeed, John beat me to it. The (potential) 25 watt power output is from all the 10 cells connected in series. I got 0.55 v from each cell at 5 A. Total cell area is 10 X 125mm X 125 mm = 0.15625 m2.

 

John/Timbo,

 

Thanks ever so much for clearing that up. So with 25 watts in , I get 87.5 gm thrust which translates to 0.86 N forward Thrust .

 

For a (nearly) 500 gm aircraft seems a bit low(?)

 

Mind you i am going for a slow and steady aircraft, no high speed acrobatics intended!

 

Edited By Djay Ahluwalia on 12/05/2010 23:46:56

[Tim Mackey]

I still think 5A is pretty remarkable...despite the low voltage. You cant get owt for nowt as they  say oop norf, and for higher power you need higher input, and thats where youre stuck. I imagine your best way forward is to do absolutely everything you can to reduce weight to a minimum.

You certainly wont get anything near sprightly performance. 

[Djay Ahluwalia]

Timbo,

 

Expectedly, the current generated depends on the solar insolation received, which depends on the time of the year and geographical location. I am told, the same cells could potentially produce close to 10 A in somewhere like Singapore, Australia! 

 

Although, still apprehensive about the model weight and the Thrust generated by said motor, I am going to take the hit and may be order the 1800Kv version tomorrow. Run it using a 10A ESC, sound ok?

 

I already have a 7 X 4 prop....but if need be , I might buy a 8 X 4.3 , as the webpage suggests. 

[Chris Card]

Hi Djay,

 

An interesting project you've taken on and I'm sorry to sound like a sceptic, but I'm not sure it's going to work.  When planning power systems for electrically powered flying models, the power to weight factor is a key metric.  Expressed in watts per pound, it's generally accepted that at least 30 W/lb is needed just to keep an electric plane in the air (with very sedate performance).  To take off from the ground you would need between 40 - 50 W/lb.

 

In your case you have 25W available (presumably when all your panels are perpendicular to the sun, which in flight they won't always be, but let's ignore that).  So call it 25W.  This means your aircraft all up weight must not exceed  380g.  That's everything, motor, airframe, speed controller, servos, radio receiver, wiring, linkages, hinges, glue, covering material,  and, of course, solar panels.

 

Now tell us how much the solar panels weigh before we go any further with this, because I'm wondering whether that will kill it straight away.  

Also, I think you're going to need some kind of voltage stabilisation, either a big capacitor or a battery, because with the panels directly connected to the ESC you will suffer from fluctuations as the angle of bank changes the amount of sunlight falling on the panels.  Bank into a turn away from the sun (so the wings are in line with the sun) and the solar cell output could fall to nothing until you level out again.

 Sorry, not trying to knock the idea, but there are problems here! 

Edited By Chris Card - Moderator on 13/05/2010 06:43:05

[Djay Ahluwalia]

Dear Chris,

 

Thanks for the post!

 

I have been thinking about the whole Thrust/Weight ratio, but have struggled to find a motor with a high enough Thrust at such low power input (low current).

 

My mass inventory shows that the total aircraft weight should be around 350 gms (including 100 gms for the solar panels), as you can see below. But you are correct, I am sceptical at this point in time. The total weight of the finished product may be higher.

 

Edited By Chris Card - Moderator on 13/05/2010 10:51:29

[Djay Ahluwalia]

After 2 tries, for some reason I cant post my whole reply!

 

Anyways the mass inventory above, as you can imagine is incomplete, may be theres a word limit for each post.

 

Continuing with above Chris,

 

I do need to add a MPPT to deal with the power fluctuations due to time of the day, aircraft inclination, cloud cover etc. However, as of yet I have not found anything suitable ( read cheap and inexpensive).

 

This is my first attempt at such a project and my inexperience is showing as I was completely stumped by the whole motor/prop selection issue before Timbo here pointed me towards the micron flyer which 'may' be suitable for what I am intending.

 

Opinions? 

 

 

[Chris Card]

Hi Djay,  I had to remove the table from your penultimate post as it had completely messed up the page format.  Would you mind re-adding it in plain text?

 

With 100g of solar panel to carry your weight will be very marginal but I've changed my mind a little, I was hasty to say it would never work.  I'll let the scratch-build experts advise you further! 

[Djay Ahluwalia]

Chris,

 

I guessed something was up with the formatting, thanks for the heads up. Heres the inventory again,

 

Solar Cells     100 gms

Main Wing      105 gms 

Motor/ESC        30 gms

Fuselage           45 gms

Tail Plane          15 gms

Battery                30 gms

Servos                10 gms

Prop                      5 gms 

Receivers            10 gms

 

 

Total                    350 gms

 

Lets say I am hoping it will be NO more than 400 gms. 

 

[Tim Mackey]

I have already offered mine....a brave and challenging project as Chris has said, but frankly I doubt it will "ever get off the ground" excuse the pun.

Your expected AUW seems to have dropped a LOT, and if you can really get to 350 grams, then it might fly, but is very unlikely to ever  ROG ( rise off ground ).

You didnt stipulate that a rolling take off was required, so a decent hand launch into a fair breeze may work, and you may get a short flight of sorts.

I hadnt realised - sorry - that you wer planning on direct drive of the powertrain from the solar panels - and again as Chris has stated, this wont work IMO.

FWIW my advice would be to try a redesign of the whole craft, using super light materials ( Micron site will help here again ) and use one or a multiple of of the purpose built micro flier type power units such as from the "Vapor".

These are powered by a single very small LiPo cell.

Of course the only real remaining "project" then will be to see if the solar cells can be made to effectively charge the cell(s) which will of course require some sort of electronics to perform, as well as a reverse block to prevent the cells discharging into the panels.

Plus of course, this is then far from a solar powered flying machine in honesty