JayTee

Coming from 72mHz and slope soaring in the 70's and 80's, I'm still getting my feet wet with electric. Just when I think I'm starting to get it, something surprises me. Thought I was starting to figure out some basics on brushless motors, i.e., watts/ounce for basic, sport and 3d flying. Just bought a motor and paid more attention to the wattage, that the size. This thing is micro compared to some of the other motors I'm dealing with. It's a GARRT F2204-1800KV. Here are the specs they list for it:

Motor KV 1800RPMV
Idle Current (Io/10V) 0.45A
LiPo Cell 2-3S
Weight 21Gm
Max Cont. Current 18A
Max Cont. Power 220W
Configuration 12N/14P
Motor Resistance .1495Ohm
Stator Dia. 22mm
Stator Thickness 4mm
Motor Dia. 27.6mm
Motor Body Length 19.8mm
Bolt Hole Spacing 31.5mm
Bolt Thread M2
Propeller 7"/8?


8V 8038 Prop
Throttle Amps Watts Thrust Efficiency g/W
50% 2.83 22.6 173G 7.64
60% 4.86 38.9 262G 6.74
70% 6.89 55.1 337G 6.11
80% 10 80.1 412 5.14
90% 12.1 96.8 461 4.76
100% 12.4 99.4 466 4.69

12V 8038 Prop
Throttle Amps Watts Thrust Efficiency g/W
50% 4.61 55.5 330 5.97
60% 7.57 90.8 449 4.94
70% 11.9 142.7 555 3.88
80% 16.1 193.2 634 3.28
90% 18.6 222.8 655 2.94
100% 19.6 234.6 675 2.87

I honestly did not pay attention to the size when I bought it; it's about the size of a slim men's watch! Are these power numbers realistic? Or is it just going to use a lot of juice delivering the wattage?


I can use it on another project that needs a smaller motor, so it's not a loss if it's just not that capable.

Thanks!

John

Wildflyer

I highly recommend getting a wattmeter and a temp gun, then you can test a motor set-up and know what it is doing.
I believe the important place to measure the temp is the motor windings, not the outer case.

We used to figure a motor could safely put out about 3 times it's weight in grams, in watts out. But I have several motors that far exceed that.
Your motor would only be 60-65 watts if that was true. I think newer designs have let motors produce more power without so much heat.

JayTee

Hi Wildflyer,

Thanks for your response. A wattmeter is on the Christmas Wish List, so we'll see how that goes. <grin> Harbor Freight is pretty reasonable on temp guns. Will be interesting to see how it goes. My brother is going through the same thing out in SoCal, coming from the same place as me. We've both retired and gotten back into 'old' hobbies. He just got a 1600mm Volantex and wants to put a 3S in for power. Has a guy at his club that is going to help him figure out batteries, props and whether a larger motor/lower kV motor than the stock Volantex 2212-1400 is going to be needed.

I'll do some searching in the forum for max temps for motors and ESC's.

Thanks again!

John

solentlife

Another good item yo have : Register with "eCalc" ..... then you can put combo's of motor / ESC / LiPo / Props in to get calculated results .. thrust, Power to Weight, Amps max, flight time etc. etc.

https://ecalc.ch/

Its only a small registration fee and pays for itself first use .. literally.

quorneng

JayTee
Heat is a limiting factor for both motors and ESCs or rather the actual limit is set by the rate at which heat can be dissipated to the cooling medium.
A motor has a significant mass in the heat generating component, the winding, thus it can tolerate an over load of 100% or more for a short period provided it is then allowed to dissipate the excess heat. The relatively open construction of a brushless 'out runner' motor means its maximum continuous power can significantly effected by the cooling arrangements provided.
An ESC is a bit different in that most of the heat is generated by the individual MOSFETs. The tiny internal component is embedded in a support medium which limits the rate at which heat can be dissipated which in turn sets the maximum power the ESC can handle. The fact the MOSFET has virtually no internal thermal inertia means it cannot handle anything like the over load capability of a motor about 10% overload for 10 seconds.

JayTee

Morning solentlife,

I've seen eCalc and considered registering. Are the motors and ESC's that are 'ghosted' the extent of what's in the database, or does registering make more visible? I was not seeing much of the motors or ESC's I have.

John

JayTee

Thanks quorneng! I've been searching the forums and trying to find if there is there a recommended 'Do Not Exceed' temperature for motors (What would that be? "TNE - Temperature Never Exceed? <grin>)? I found one reference to using the back of your hand with the general guideline of something like, 'If it's too hot to touch, it's too hot' . . .


John

solentlife

Registering expands all sections / parts hugely .....

The Author is also contactable direct through eCalc or the RCGroups thread : https://www.rcgroups.com/forums/show...lc-xcopterCalc

It really is my GoTo when setting up models.

If it only saves you one ESC or Motor ... its paid for itself already.

ron_van_sommeren

A Nobel Prize in the making.

Conservative rule of thumb for max.continuous power: 4watt per gram motormass.
Efficiency is Power_out/Power_in. However, here they are talking effectivity.

ron_van_sommeren

Keep in mind that the plane does not care about systemvoltage. It's a practical consideration, not an electrical consideration.
3s in combination with velocity konstant Kv=1000rpm/volt will result in same (no-load) rpm as 6s combined with a 500rpm/volt.

Power, in watt, is what makes it fly, not voltage, not current.

But if your friend already has (standardized on) 3s packs he should use them, no problem.

Without a watt/volt/current/multi-meter you are in the dark.
Until something starts to glow

• To trust is good
  
• To calculate is better
  Will get you to say within 10%, if input ok.
  e-flight calculators & propdata (compilation)
  
• To measure is a must
  A watt-meter will more than pay for itself, several times over, your battery, ESC and motor will love you for it.
  After calculating, always check/measure current when you have a new/changed setup.
  Will also help you find optimal setup.
  And it's a great tool for debugging your power train.
  
• About watt-meters and how (not) to use them, by RCG user vollrathd:
  www.wattflyer.com/forums/attachment.php?attachmentid=117297
  
• A very long clearance sale, originally 200+$, high quality, now literally peanuts:
  Hyperion Emeter II wattmeter with local&remote logging, optical&electrical tach, servo tester - RCG
  Contents
  • Closing out at
  • Reviews by Ken Myers
  • Tech info & help threads
  • Increasing max.current, double, triple, simple and cheap
  • Software and manuals downloads
• Careful!
  (Always!) keep battery-/watt-/multi-meter wires short:
  Too long wires batteryside will kill ESC over time: precautions, solutions & workarounds - RCG

Wildflyer

I had one experience with overheating that was enlightening. A small motor in a glider, ran only 30 seconds to reach altitude.
Testing with my hand didn't indicate any problems, a bit warm after 30 seconds but not alarming.

One day the motor quit. When I took the spinner off I was shocked! The windings were burned black! When I unscrewed it off the firewall the mount had burned a mark like branding into the plywood. If I had flown the plane around like a sport plane, it may have caught fire.

Apparently, this motor ( I have no idea what it was now) could cool off the outer case enough to hide that I was severely overloaded by the folding prop I had on it. Now I take a temp reading on the windings. Sometimes not so easy but that is the way I do it now.

ron_van_sommeren

Another possible scenario.

1. Overheating magnets causes de-magnetizing (irreversible! ), which in turn results in higher velocity konstant Kv. Motor wants to run faster, thus drawing more current and more power.
2. Current and -power drawn are proportional to Kv³ , resistance losses in the copperwire are proportional to current² Therefore resistance losses are proportional to Kv⁶
3. Higher current → hotter motor → weaker magnets → higher Kv → higher current → hotter motor → weaker magnets → higher Kv → higher current ⟲⟲⟲ etcetera etcetera.

Determining whether magnets still have original strength.
By determining whether velocity konstant Kv (in rpm/volt) is still the same, three simple straightforward methods.

1. www.bavaria-direct.co.za → motor constants
2. Generator test, only using a power drill and a voltmeter
   (Re)winding and building motors - RCG (sticky) → opening post → #40 Generator test.
3. If you don't trust your ESC: use another brushless motor as a generator:
   Three Phase Alternator - Three Phase Motor? - RCG

ron_van_sommeren

Costs less than a broken prop per year

List of free and paid calculators
e-flight calculators (compilation) - RCG (sticky)

Full eCalc version, free until Jan.4th.
From the eCalc FAQ, suggestions and help thread at
eCalc: Q&A for propCalc, fanCalc, heliCalc, xcopterCalc - RCGl


JayTee

Thank you Ron and Solentlife. I'll be getting signed up. Also have a 'late' Christmas present of a Turnigy Watt Meter coming in the near future.

Happy Holidays!

John