Unexpected Watts from new motor

[Tony H]

Hi everyone,

 

I just bought a little brushless motor from GC for a 400-500g low wing plane I plan to build. However after selecting a prop I thought would suit the motor (7X5) the motor is drawing way more amps than recomended and/or expected 23-25amps (motor rated at 16amps max), the motor had a slight burning smell after just a couple of seconds. Here is the link to the motor I have.

 

http://www.giantcod.co.uk/a221210-140w-1400kv-brushless-outrunner-motor-p-405611.html

 

Could anyone recomend a prop which might give me 130-140w at around 13amps? I was thinking of trying a 7X3 or 6X4. I wonder if the motor I have really is a 1400KV motor?

 

Any thoughts would be good. Thanks

 

Tony

Edited By Tony H on 26/01/2011 00:17:23

[Bravedan]

Tony, you don't say what prop make/model, which might have helped! Different make props give VERY different readings!

 

I have had exactly that motor with an APC Slowfly 8x3.8 (NOT the Speed Type! ) on a GWS Corsair for nearly two years. MORE than sufficent power and usually flown near silent on half throttle, but I have flown around like a loony on full throttle (when it's far too ballistic to be a "warbird", but great fun!!).

 

Here's the data:-

 

Time: 1:08
Rate: 3 SPS
Charge: 75 mAh
Battery Efficiency: 84%

Max Current: 12.75 A
Avg Current: 3.97 A

Max Voltage: 12.12 V
Avg Voltage: 11.26 V
Min Voltage: 10.01 V

Max Power: 129.31 W

Avg Power: 42.28 W

 

Ergo, I do NOT suggest you go smaller !!??

 

 

 

[ken anderson.]

hello tony also as well as the advice from bravedan......dont run the motor again on the set up you've tried as the 'burning' smell will equal heat which will loosen the magnet's and kill the motor......... ..

 

 

ken anderson.......ne....1.

[martyn sharp]

A lot will depend on the batterys you are using as well I have some small motor that run nicely on a 2s lipo and run fast and hot on a 3s so as well as the prop size look at the power suplly

hope this helps a bit

 

Matyn

Edited By martyn sharp on 26/01/2011 08:40:25

[Tony H]

Thanks Guys,

 

I'm even more confused now. I would have thought making the prop bigger would draw more amps. The 7X5E Prop i'm running is a EMP sports prop although it says XYH on the prop hub. When would you use a Sloe fly prop I always thought they where for vintage type planes used indoors and with a very low KV like 890.

 

I won't run the motor again until I know of an appropriate prop to use. Do slow fly props work ok outside? What would make you decide to use a slow fly prop?

 

Tony

[Tony H]

Oh and Bravedan, as the plane you have is similar to what I am looking to build could you tell me your exact setup, wingspan of plane, weight of plane, ESC, Lipos etc... Thanks that would be a big help.

 

Tony

[ken anderson.]

tony - try the GWS...7*4.....8*4....check the motor with a watt meter to keep it in the req amp's etc........you need to lighten the load on the motor....

 

 

ken anderson ne...1.

[Steve Hargreaves - Moderator]

A shameless plug for the electric flight database just here......all contributions greatfully received......!!!

 

Tony, this seems very odd....I have a TowerPro motor with a kv of around 1600 & this pulls 23 amps on an APC 7 x 5 or 25amps on an 8x4 so I'm not sure why a 1400kv motor would pull the same current!!!!

 

Worth noting that kv values are always approximate but they shouldn't vary by that much....

 

One thing is for sure however.....at 25 amps you are going to kill it very rapidly. Bravedans set up seems the way to go.....try using a GWS or APC slowfly prop

 

 

[PatMc]

Tony, I would guess that you've been sent this version of the motor in error. I don't think there will be any way to recognise which one you have other than by the performance.

If I'm right then it will need either a 5.25 x 4.75 on 3s or 7x5 on 2s.

Using a Slowfly prop won't help.

[Tony H]

To be honest PatMc That was my first thought too, its not the first time GC has sent something similar but not what I ordered. although in their defence the plastic packet it came in does have a little price tag sticker on it stating 2212/10 1400KV. I don't now if it actual has that in the box it doesn't seem like it. What difference would it make to my plane flying such a small prop and high KV it was mean't to be a 29" wingspan 400-500g sports plane?

 

 

Edited By Tony H on 26/01/2011 10:24:53

[Tony H]

Ken Do you mean the GWS slow fly props?

[PatMc]

A small prop on such a small model would probably be OK. In fact it would be less prone to landing breakage.

Some of the lads in our club use that size prop on Pushycats with highish Kv motors. Ken Anderson might be one.

 

[ken anderson.]

tony - i would def try the GWS prop's i mentioned to keep the load down on your model...500g isnt heavy as such so i would say that's the way to go........another lad in our club use's the small prop's mentioned but they are on inrunner's(d/f) motor's......not watt you have.......

 

ken anderson........ne...1.

[PatMc]

Ken, it doesn't matter whether they're "out" or "in" runners it's the Kv & max current that are important.

A GWS 7x4 isn't going to be that much less load than a 7x5 (APC ?) & a GWS 8x4 will almost certainly be a greater load but the definitive way is to check with an ammeter .

Edited By PatMc on 26/01/2011 11:08:59

[Tony H]

Do you think I should try a 6x3E prop first? what would you expect from that?

[PatMc]

If you have a 6x3 I suggest you try it first on the bench, check what current it draws & if it's within the motor rating try it on the model.

I used to fly an OD model that weighed 750g with a Graupner 6x3 folder using a brushed 400 can motor & 8 nimhs. It drew 9A on that combo & whilst the performance wasn't sparkling it was acceptable. Using a Master 6x3 ic type prop was about the same.

By using a much more efficient brushless & 3s lipo I wouldn't be surprised if you got near double the power output.

IMO it's certainly worth giving it a try.

 

[Tony H]

I've just purchased a couple of different props and a Tachometer and here are my results (they seem more like it)

 

EMP            6x3E 96W 8.5Amps 14614RPM

                   7x3E 179W 15Amps ?RPM

 

GWS (Style)       7x4E 140W 11.5-12Amps 13600RPM

 

GWS (Style) Slow Fly       7x3.5E 101W 8.5Amps 14700RPM

 

The EMP 7X3E is much thicker bladed prop like IC or larger EP Props with 11mm hub. however the others are tiny thin bladed props very weak and flimsy looking with around a 7mm hub.

 

Do you think my motor is a 1400KV judging by the results? Which prop would you go with for the 29" wingspan (400-500g) low wing plane I wanted to use it for?

 

Props vary a lot more than I thought!!

 

Tony

Edited By Tony H on 28/01/2011 22:18:27

Edited By Tony H on 28/01/2011 22:42:48

[PatMc]

With the new info it looks as if you do have a 1400Kv motor.

As far as which prop to use I think it's "suck it & see" time Probably best to start with the smallest though & work through them until your happy with the performance.

 

But I'd be very cautious about using these props at such high rpm & definitely would not use the prop at all. Remember the prop tips will be moving at circa 250 - 300mph at these rpm. Don't stand in front or to the side of them in case they shed a blade.

Edited By PatMc on 29/01/2011 10:36:03

[Tony H]

Just checked another few props at 11.7v with the following results.

 

EMP 6X5E - 113W 10Amps 14150RPM

 

AIRTEK Slowfly 7X4E - 182W 16Amps

 

AIRTEK Slowfly 8X3.8E - 195W 18Amps

 

 

I couldn't get hold of any GWS props in the right size all sold out. Judging by my data which prop would you go with for a low wing 500g aircraft? The EMP props seem more robust than the GWS Style ones but the 6X5e is toping out at 14150RPM, would you say this is to high?

 

 

Tony

Edited By Tony H on 01/02/2011 16:09:01

Edited By Tony H on 01/02/2011 16:09:15

[Tim Mackey]

Revs are not gonna change hugely with prop changes - so to keep the current and power sensible ( longer flights too) I'd use the 6 x 5. I wouldn't be too worried about the fairly high revs TBH - I dont know the prop material or quality, but cheap little EDfs are running WAY more revs than that, and Ive yet to see one shed a blade.

IC 2 strokes also rev way higher than that and the props have to contend with far more vibration too . Just be sure to balance it well, and check for nicks etc.

Edited By Tim Mackey - Administrator on 01/02/2011 16:43:27

[flytilbroke]

Hi Tony, The EMP 6X5E by your figures, is giving you near enough 100W per pound, = good

 

The Airtec 7x4e looks to be borderline on the high power draw.

 

You might get away with the high Amps in flight, not my choice though.

 

The above from a confirmed IC chap.

[Tony H]

Thanks Tim and Flytilbroke I think the 6X5e suits it best and is one of the more robust looking props for it's size.

 

Tony