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Old 12-05-2010, 03:51 AM   #1
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Default What does a motor's watt rating represent?

I have a 450 size, 1800 kv motor that says 165W on it. I'm assuming that the 165W means 165 watts. What does that mean though?

Does it mean that it needs 165W? Does it mean that it was only designed to draw 165W? Does that mean that's all it should be pulling from the battery? Does that mean it's all it can safely pull? What does it mean?

The reason I ask is that I put a watt meter on it, and with a 2200 mAh 3S 20C battery, and a 7x6 APC Prop, it was pulling about 24 amps, and about 250 watts at WOT.

Thanks
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Old 12-05-2010, 04:10 AM   #2
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That means assuming adequate air flow you do not want to pull more than 165 watts through the motor. There is a good chance you will burn up the motor. Less airflow and you will want even less watts through the motor.

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Old 12-05-2010, 05:15 AM   #3
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Originally Posted by starfire73 View Post
I have a 450 size, 1800 kv motor that says 165W on it. I'm assuming that the 165W means 165 watts. What does that mean though?

Does it mean that it needs 165W? Does it mean that it was only designed to draw 165W? Does that mean that's all it should be pulling from the battery? Does that mean it's all it can safely pull? What does it mean?

The reason I ask is that I put a watt meter on it, and with a 2200 mAh 3S 20C battery, and a 7x6 APC Prop, it was pulling about 24 amps, and about 250 watts at WOT.

Thanks
FYI, also, watts is a direct measure of horsepower. And it takes 746 watts to make one horsepower, so your 165 watts divided by 746 watts/HP results in an input of 0.22 horsepower INTO the motor. Multiply that times the motors efficiency, you get the horsepower put into the motors prop. Typical motors run about 80-85% efficiency, with a really good motor hitting perhaps 90% or so.

But, that lost 15 or 20% of efficiency results in heat directly winding up in the motor. So, if your battery is putting 165 watts into the ESC and motor, 80% = 132 watts into the motor shaft, and 20% equals 33 watts into the motor, making it hot.

Thirty Three watts into the motor itself may not sound like much, but find yourself a 40 watt incandescent light bulb, and hold your hand on it after its been on for awhile. Same for the motor, only it has less surface area to get rid of the heat. And, they can not run as hot as that light bulb, so air cooling is important on these motors.

Bottom line, if you can hold your finger tips on that motor for 15 seconds (without branding the motor nameplate into your fingertips!) after a hard flight, your motor is running OK.
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Old 12-05-2010, 08:41 AM   #4
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Watts is not actually a measure of energy.. it's a measure or power. Power is energy divided by time, or 'the rate of use' of energy if you like.
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Old 12-05-2010, 08:47 AM   #5
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Originally Posted by starfire73 View Post
I have a 450 size, 1800 kv motor that says 165W on it. I'm assuming that the 165W means 165 watts. What does that mean though?

Does it mean that it needs 165W? Does it mean that it was only designed to draw 165W? Does that mean that's all it should be pulling from the battery? Does that mean it's all it can safely pull? What does it mean?

The reason I ask is that I put a watt meter on it, and with a 2200 mAh 3S 20C battery, and a 7x6 APC Prop, it was pulling about 24 amps, and about 250 watts at WOT.

Thanks
Avoid constant wide open throttle, and you should be ok, with that setup.

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Old 12-05-2010, 05:02 PM   #6
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Good info
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Old 12-05-2010, 07:26 PM   #7
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Yes, helpful info.

Well, here's my first test flight results.

I flew for about 6 minutes most of the time at about 80% throttle, but varied between 50% and 100%.

Temperature outside was 32 F. As measured with a Turnigy Infrared thermometer:
- 30A ESC = 48 F
- Outside of motor casing = 78 F
- Inside/Coils motor reading = 138 F

I'm curious what the readings will be in the summer, when it's 80 F - 90 F.

So, as long as I'm getting readings at around 150 F or less, I should be ok pulling more than 165W into the motor? Right?

I could set my top end adjustment limit for the throttle down to where it only pulls 165W into the motor if I have to, but I would like to have that extra power if I need to climb aggressively.

I like Chellie's advice of just not running WOT all the time. I guess I'll keep an eye on the temps, and if I see I'm getting much above 150 F, I'll either limit the top end, or fly slower.
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Old 12-05-2010, 11:19 PM   #8
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you can also drop down to a 7x5 prop, for the summer time, that should reduce the Draw/Watts with out reducing your performance to much

http://flyairhobbyrc.com/electric-prop-p-702.html

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Old 12-06-2010, 04:53 PM   #9
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Watts= volts times amps. It is a measure of the power your motor produces. Changing prop size and/or pitch changes the amount of watts consumed. Get a wattmeter. All electric flyers should have one.
The 165w is a general statement telling you what performance you may expect. Like a base 4 cylinder motor when purchasing a car. You may wish to get the optional 6 cylinder.
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Old 12-06-2010, 05:41 PM   #10
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Originally Posted by CHELLIE View Post
Avoid constant wide open throttle, and you should be ok, with that setup.
This is a bad way to do it. Lucien Miller @ Innov8tive Designs advises that limiting throttle doesn't necessarily limit the current (amps) going to the motor.

In short, if you are using lower throttle hoping to stay under a specific amp/watt level, you'll probably burn the motor or ESC.

Check out these episodes of ATTF:

ATTF#71 at 55:00 AMP ratings of ESC/system at partial throttle?
ATTF#84 at 53:39 Can you use endpoints to control/limit full throttle?
ATTF#148 at 1:07:30 Limiting throttle end point to limit amps?

You can find the episodes at http://allthingsthatfly.com/

The second number in there is the time-code where Lucien answers the question about your specific issue.



Now, all that being said, I know a *lot* of people who (successfully) use the throttle to stay under a specific current. So, this may be a case of where theory and practice produce different results. YMMV.
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Old 12-07-2010, 08:30 AM   #11
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a) I'm amazed at the readings you have obtained ....
b) If they are true - I for one would be looking to bring that wattmeter reading down ....... it's way too high for the motor and will shorten it's life seriously or if you start flying at WOT for long periods - burn it out.

Your readings say : 11.1V Lipo nominal X 24A = 266.4W .... so appears that meter is reading similar to calc ... but low.

Reason I am amazed ... I have a 135W motor on 3S 30C LiPo ... 8 x 6 prop and would expect to draw high amps / watts ... but in fact I draw a lot less than the rating of the motor ... in fact at WOT I get 110W highest reading I ever obtained.

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Old 12-07-2010, 05:51 PM   #12
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Originally Posted by solentlife View Post
a) I'm amazed at the readings you have obtained ....
b) If they are true - I for one would be looking to bring that wattmeter reading down ....... it's way too high for the motor and will shorten it's life seriously or if you start flying at WOT for long periods - burn it out.

Your readings say : 11.1V Lipo nominal X 24A = 266.4W .... so appears that meter is reading similar to calc ... but low.

Reason I am amazed ... I have a 135W motor on 3S 30C LiPo ... 8 x 6 prop and would expect to draw high amps / watts ... but in fact I draw a lot less than the rating of the motor ... in fact at WOT I get 110W highest reading I ever obtained.

It could be the fact that he's running a 1800kv motor on a 7*6 prop. If your motor has a lower KV rating, then your prop would be spinning at a lower RPM and pulling fewer amps.



Another thing to keep in mind is that if you are running a static test on a high-pitch prop, then you'll see more amps than you would once it's flying.
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Old 12-07-2010, 07:07 PM   #13
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Duplicated, deleted by kyleservicetech

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Old 12-07-2010, 07:09 PM   #14
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Originally Posted by JetPlaneFlyer View Post
Watts is not actually a measure of energy.. it's a measure or power. Power is energy divided by time, or 'the rate of use' of energy if you like.
Agreed, Changed it to horsepower.
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Old 12-07-2010, 08:06 PM   #15
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Originally Posted by kyleservicetech View Post
Duplicated, deleted by kyleservicetech

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Old 12-07-2010, 10:28 PM   #16
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I will probably try a different prop - smaller pitch or smaller diameter. I don't know though... maybe the static test vs. actual flight, will drop the watts enough to get it in the ballpark. From what I understand - ultimately if I'm not overheating the motor, it should be ok. Maybe I'll leave the 7x6 prop on there and keep an eye on the temps.
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Old 12-08-2010, 11:17 AM   #17
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Originally Posted by Xptical View Post
It could be the fact that he's running a 1800kv motor on a 7*6 prop. If your motor has a lower KV rating, then your prop would be spinning at a lower RPM and pulling fewer amps.



Another thing to keep in mind is that if you are running a static test on a high-pitch prop, then you'll see more amps than you would once it's flying.
mmmmmm I think I may disagree with you on both counts .....

KV as far as I know has no relation to amps pulled - it decides rpm per volts. The load on the motor is the factor that has direct affect on amps.
The rpm is set by the switching of the esc and volts available ....

I think the load on static vs dynamic is debatable as it depends on pitch and rpm at the time. At high rpm and pitch - the prop would likely cavitate and in doing so lose load ... speed up ... but amps pulled reduced. In flight - dynamic - the prop is unlikely to unload fully or cavitate in flight even at full throttle ... meaning that load remains on prop.

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Old 12-08-2010, 01:37 PM   #18
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Originally Posted by solentlife View Post
mmmmmm I think I may disagree with you on both counts .....

KV as far as I know has no relation to amps pulled - it decides rpm per volts. The load on the motor is the factor that has direct affect on amps.
The rpm is set by the switching of the esc and volts available ....


But if you take two motors, one a 1000kV motor and the other a 1800kV motor, and put the same prop (load ) and the same battery on both, the 1800 kV motor will draw more current trying to move that load faster.

I think the load on static vs dynamic is debatable as it depends on pitch and rpm at the time. At high rpm and pitch - the prop would likely cavitate and in doing so lose load ... speed up ... but amps pulled reduced. In flight - dynamic - the prop is unlikely to unload fully or cavitate in flight even at full throttle ... meaning that load remains on prop.
I have always heard, and accepted the idea that in flight the motor will unload slightly vs static testing due to the airflow moving into the prop. I have never been able to verify this myself. As I have flown alongside the airplane trying to look at the wattmeter my cape keeps getting tangled up in the tail surfaces
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Old 12-08-2010, 01:45 PM   #19
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So apart from the inflight bit - we actually agree ... as the KV example is saying load vs motor.



On the prop static bit ... I can only refer to my boating experience. If I start up my boat engines ( I have 4 different full size pleasure boats ), I can open throttle and props cavitate, RPM goes up ... fuel feed drops. Do same when boat can move and prop now stays biting and fuel feed goes up and stays up.
I assume air-props would do similar.

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Old 12-08-2010, 02:02 PM   #20
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If the weather is nice today, I'll test it out. I have a Hyperion e-meter that I use to test most new setups.

I'll run the motor for 10 seconds or so with me holding the plane and then do some forward flight at WOT and see which gives the bigger numbers.
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Old 12-08-2010, 02:05 PM   #21
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Originally Posted by solentlife View Post
KV as far as I know has no relation to amps pulled - it decides rpm per volts. The load on the motor is the factor that has direct affect on amps.
And what do you think the "load on the motor" is ? It's the prop size TIMES the rpm.

So higher Kv, same voltage = faster rpm = more load = higher current .

Steve
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Old 12-08-2010, 02:57 PM   #22
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Originally Posted by slipstick View Post
And what do you think the "load on the motor" is ? It's the prop size TIMES the rpm.

So higher Kv, same voltage = faster rpm = more load = higher current .

Steve
I'm only saying that just because a KV is higher does not mean higher amps unless load is same across the two examples ... which is similar to what other guy finally said.

I have various motors ranging from 900 up to 3700Kv and amps drawn depends on load put on them.
If I put the same 8x6 prop on the 900KV and the later motor I replaced it with a 1150KV ... then I get an increase in amps drawn - accepted. But that is NOT because of KV alone ... it is because motor is trying to get to its RPM .... If I prop down I can get my higher KV motor to draw less amps than the lower KV motor quite easily.

My initial comment was based on a general comment made by another without qualified detail. It can easily be mis-interpreted by others ... my purpose was to get the detail in to reduce that possibility.

We all near agree on the basics - it's the route to the same point that is differing ..

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Old 12-11-2010, 04:40 AM   #23
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Sorry... I was wrong. That was a 7x5 prop I was using, not a 7x6.

I tried a 6x4 on it today, and it brought the amps drawn down to about 13, and the watts down to about 162. I guess that's the right prop for it then. I like the way she flew on that 7x5 though... I may try to run it with the 7x5 in the winter and just keep an eye on the motor temp.
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Old 12-20-2010, 11:07 PM   #24
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I tried it with a 6x4 prop today. That sucked. It totally changed the motor performance.

It went down to drawing 13 amps, and 160 watts at WOT. Which is right were I wanted it... however, the plane flew like a dog! Hardly any power at all. Terrible vertical climb.

I think I'm going to put the 7x5 back on and run it a little hotter.
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Old 12-21-2010, 06:28 AM   #25
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Originally Posted by solentlife View Post
On the prop static bit ... I can only refer to my boating experience. If I start up my boat engines ( I have 4 different full size pleasure boats ), I can open throttle and props cavitate, RPM goes up ... fuel feed drops. Do same when boat can move and prop now stays biting and fuel feed goes up and stays up.
I assume air-props would do similar.
You cant really take the boat example over to an airplane prop. The key difference is the boat prop is in water and the plane prop in air. When a boat prop 'cavitates' it boils the water due to the low pressure created on the front side of the prop. The liquid turns to a gas so not surprisingly the prop unloads and speeds up because it's easier to move through a gas than it is through a liquid.

An airplane prop on the other hand is working in a gas all the time (air).. Gasses cant cavitate because they are already gas. The nearest thing that can happen to a prop in air is a stall. A stall is when the airflow over the prop separates from the prop surface, this causes a large drag increase so puts more load on the prop, not less.

Generally then a prop will unload as the plane gains forward speed up to the point where forward speed matched pitch speed. How much the load decreases is dependant greatly on pitch and rpm, where the prop is quite coarse and the blades are stalling in the static condition then the unloading at speed will be very significant. If the prop pitch is less coarse and there is no blade stalling then the unloading will be slight, but there will always be some.

MotoCalc can predict blade stall.

Steve
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