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Old Apr 08, 2015, 03:40 PM
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overwatting but not overamping, what happens?

What happens when you overwatt a brushless motor but are well below the max amperage specs?

I am planning on using 6s and propping the plane to draw 40 amps on a motor rated for 500watts and 50 amps. In other words I am supplying 880watts to a 500 watt motor.
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Old Apr 08, 2015, 03:43 PM
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You can't get there from here.

If you limit the current, the voltage will drop, and the power will drop as well.
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Old Apr 08, 2015, 03:53 PM
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Its my understanding that larger gauge copper can carry more current. If the copper is large enough to handle the current (or shall I say amperage) that I am supplying why would extra voltage be a problem? Does high voltage heat things up somehow too?
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Old Apr 08, 2015, 04:06 PM
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Maybe to put it more simply, why do manufacturers label their motors with both an amperage and a wattage limit? Why don't they just put max amperage in the specs?
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Old Apr 08, 2015, 04:36 PM
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Quote:
Originally Posted by parajared View Post
Its my understanding that larger gauge copper can carry more current. If the copper is large enough to handle the current (or shall I say amperage) that I am supplying why would extra voltage be a problem? Does high voltage heat things up somehow too?
Voltage and current go hand in hand. More voltage results in more current.

Quote:
Originally Posted by parajared View Post
Maybe to put it more simply, why do manufacturers label their motors with both an amperage and a wattage limit? Why don't they just put max amperage in the specs?
I think they specify a maximum current.
And specify a rated power(wattage) that you can expect.
And specify a recommended voltage (1S, 2S, 3S etc.)

It's all just information, there are things beyond their control. If you change prop, weight, wing area, etc. it affects motor load.
Don't exceed the max. current and you won't burn out the motor(probably).

If you get it all right, you extract the most efficiency out of the system.
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Old Apr 08, 2015, 04:46 PM
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Without knowing WHAT motor you're talking about and WHAT you're using it in here are a few guesses:

Basically you're planning on using twice the voltage that's specified. So you're turning the motor twice as fast as the maximum rpm it's designed for and using a very small prop to keep the current down. So because of the small prop you won't have very much static thrust but you will have a ridiculously high pitch speed.

It's possible the plane will still fly in some fashion. And the motor might still hang together even if you are abusing it but then again the excessive rpms might cause it to throw the magnets or the bearings might seize up or it might just get very hot and melt the windings.

Good luck testing it .

Steve
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Old Apr 08, 2015, 05:20 PM
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Quote:
Originally Posted by parajared View Post
What happens when you overwatt a brushless motor but are well below the max amperage specs?

I am planning on using 6s and propping the plane to draw 40 amps on a motor rated for 500watts and 50 amps. In other words I am supplying 880watts to a 500 watt motor.
It's quite simple. The amount of heat generated by the motor has to be taken into consideration. Let's be conservative and say the motor is operating at 80% efficiency. At the max power rating of 500 watts the motor will have to dissipate 100 watts of heat. When running the motor at 880 watts, even though the current is under the max current rating, the motor will still have to dissipate 176 watts of heat. Are you willing to take the chance?

Larry
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Old Apr 08, 2015, 05:26 PM
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Quote:
the motor will still have to dissipate 176 watts of heat
ah, ok that makes sense. I knew there was a reason
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Old Apr 08, 2015, 06:24 PM
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My question is .....WHY ?

What result do you want , and why not acquire and use what you need to get that result ?
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Old Apr 08, 2015, 07:18 PM
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I want to build a non-edf high speed jet. I am thinking either skyfun or funjet. Instead of buying a new motor I am considering using a motor that I have lying around from a previous attempt to make a Tek Sumo go really fast, a NTM propdrive 1400kv 500watt.

I think 800+ watts could scoot a funjet along at a respectable speed.
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Old Apr 08, 2015, 08:30 PM
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You haven't mentioned prop selection - for more speed , use a prop with higher pitch .

And instead of jumping to 6S right away , you might want to work your way up to 6S ( first 4S , then 5S , then 6S ) , checking motor/ESC temp each time .

My personal experience with trying something similar to this was a toasted motor , and the specs for the motor indicated it could handle 5S . In the video , you can hear the motor self destruct at about 2:40 .

Simple Dollar Tree Jet on 5s battery (4 min 13 sec)
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Old Apr 08, 2015, 10:05 PM
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Motor quality will have a lot to do with it too.
I used to fly some stupid fast Strykers, running 50+ amps through a 23 amp rated motor.
But then I was using a Mega motor. High quality motors and you paid for it.
NEVER had a motor electrical failure.

With something like what you are planning on using, I don't see the same happening.
But you never know till you try.

And if it wasn't for folks pushing the limits, we'd never know what something will do.
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Old Apr 08, 2015, 11:36 PM
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Part of a motors voltage rating has to do with bearings. If the mfg specs bearings good for 20k rpm, and you run it at 30krpm, bad things are bound to happen....
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Old Apr 08, 2015, 11:58 PM
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Quote:
Originally Posted by Lnagel View Post
It's quite simple. The amount of heat generated by the motor has to be taken into consideration. Let's be conservative and say the motor is operating at 80% efficiency. At the max power rating of 500 watts the motor will have to dissipate 100 watts of heat. When running the motor at 880 watts, even though the current is under the max current rating, the motor will still have to dissipate 176 watts of heat. Are you willing to take the chance?

Larry
Ah but inefficiency from ohmic heating is P=RI^2, so heating is proportional to current squared (as resistance is constant). So the limit to burning your motor out is to do with current, not wattage. The ohmic heating melts the insulation on your coils and the motor shorts out. You get more efficiency by increasing the voltage and hence decreasing the current (V is inversely proportional to current) because inefficiency is dependent on current. That's why high voltage power lines are used to shift electricity from the generator to the substation. Over that long distance, so much energy is saved by minimising current.

I think slipstick may have it right in that the power limit given by the manufacturer may be effectively an rpm limit, simply in an easily measurable form.
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Old Apr 09, 2015, 12:28 PM
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Quote:
Originally Posted by parajared View Post
What happens when you overwatt a brushless motor but are well below the max amperage specs?

I am planning on using 6s and propping the plane to draw 40 amps on a motor rated for 500watts and 50 amps. In other words I am supplying 880watts to a 500 watt motor.
It's your money.

TP
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