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Old 24th July 2006, 11:44 AM   #1
Hayden is offline Hayden  Australia
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Default watts vs watts

what Im talking about is electrical watts vs mechanical watts.
e.g. I have a brushless motor and its rated at 1200watts now would that be the electrical power it draws volts x amps? Or output mechanical torque?

this is the 1930/8 turn motor:
http://www.finedesignrc.com/lehnerspecs.htm

It says "Power(Watts) @ Amps" so they tested the 1930 on 40v?
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Old 24th July 2006, 11:51 AM   #2
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The max power the motor takes up is when is loaded at max mechanically, just before it stalls. But there are also mechanical losses. If the motor has an efficiency (and I'll just pluck a number out of thin air now) of 75%, the mechanical output at that 1200W electrical input is 900W, just over one HP (which IIRC is 750W or thereabouts).

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Old 24th July 2006, 12:30 PM   #3
BWRX is offline BWRX  United States
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Quote:
Originally posted by janneman
If the motor has an efficiency of 75%, the mechanical output at that 1200W electrical input is 900W, just over one HP (which IIRC is 750W or thereabouts).
1 hp = 745.7 W

C'mon Jan, you were way off

Quote:
Originally posted by Hayden
It says "Power(Watts) @ Amps" so they tested the 1930 on 40v?
They didn't test it with 40V, otherwise it would have been spinning at 2613rpm/V * 40V = 104520 rpm which I doubt it was...

The torque of the motor is dependent upon the thickness of the windings and the number of windings used (ok and the magnet strength, and geometry, etc...). The thicker the wire and less number of turns in the winding the more torque the motor will develop. Seeing that the windings are just copper wire their impedance is very low and doesn't require a whole lot of voltage for massive amounts of current to flow. That's why the lower turn motors have higher rpm/V ratings than the higher turn motors.

I've been into electric RC cars and helis for a while now and this brushless stuff is way better than the brushed motors of the past! I love 10 minute flight times with a 7 cell NimH pack in my heli
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Old 24th July 2006, 12:37 PM   #4
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Quote:
Originally posted by BWRX


1 hp = 745.7 W

C'mon Jan, you were way off
[snip]


... but closer than most goverment work..!

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Old 25th July 2006, 09:04 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
to get 1200W @ 30A, they must have tested at 40V.
That's some battery pack 40V & 30A.

The spec sheet says rpm/volt. I think they are saying what the rpm is when used at the specified voltage (mis-use of terms?). The problem is they don't actually spell out the voltage, anywhere.

Most motors operate at maximum efficiency when driven at about half their no load maximum RPM.
Does this rule apply to brushless?
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Old 25th July 2006, 09:13 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
just looked up Wikipedia for confirmation.
Quote:
0.74569987158227022 kW (33,000 ftlbf per minute)
is the traditional "horse power".
But there are two otherdefinitions in common use.
Quote:
The electrical horsepower is used by the electrical industry for electric motors and is defined to be exactly 746 W
and finally the wimpy metric version used on the Continent and in Japan
Quote:
1 PS = 75 kpm/s = 0.73549875 kW = 0.9863201652997627 hp (SAE)
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Old 26th July 2006, 01:46 PM   #7
Hayden is offline Hayden  Australia
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It is rpm/volts meaning if the kv rating is 5000kv if you put 2 volts on the brushless motor you x it by 2 = 10000rpm 6v on the motor = 30000rpm I have not seen yet a maximum volt or rpm rating on a brushless motor there will be a limit so what is it?


Btw the world record r/c boat used a 38.4v pack on a lmt 2250 3000watt @ about 41,318 rpm up to 100amps
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Old 30th July 2006, 12:57 AM   #8
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janneman: " ... The max power the motor takes up is when is loaded at max mechanically, just before it stalls. ..."

Well, I would have said just before the motor begins to slow down toward a stall ... or if motor RPM is just below its best power point = increasing current draw and thermal runaway ...

Interesting conversion tool is on the newer Apple Mac Dashboard ... "Unit Converter" ... converting horsepower to watts = ~ 1 HP to 746 Watts (as noted by BWRX above). The Wikipedia conversions carry out further, decimal point wise, but are a litle combersome to find and use.

The original British "horse power" rating was the ability of a healthy horse to pull and lift 550 pound weight one foot in one second = no mean feet / feat. The more modern (and less satisfactory Society of Automotive Engineers (SAE) have (variably) compared "horsepower" to all manner of standards, eventually landing on fluid power ~= 1 HP = ~ 1 gallon per minute (GPM) @ 1460 pounds per square inch (PSI) ... the French equivelient (meteric) being seldom used for anything except classroom excercises and term papers ...

AndrewT: " ... Most motors operate at maximum efficiency when driven at about half their no load maximum RPM. Does this rule apply to brushless? ..."

More like 66% of maximum RPM, brushless or not ...

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