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Old 01-30-2014, 09:40 AM   #1
MADMAC
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Default amp draw

At the same power (approx.) does a brushed motor draw more or less amps than brushless?
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Old 01-30-2014, 10:40 AM   #2
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All else being as equal as possible.... (not very possible) the brushless would draw marginally less power.

But (in model size motors) the brushless motors all use rare earth magnets which are more powerful than the magnets used in all except a very few brushed motors. The stronger magnets make the motors more efficient and thus you get more useful power for the same watts input.

There's also the better efficiency of the brushless ESCs helping the brushless power systems be more efficient.

So we generally see a significant improvement in efficiency with brushless.
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Old 02-12-2014, 04:56 AM   #3
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Originally Posted by MADMAC View Post
At the same power (approx.) does a brushed motor draw more or less amps than brushless?
By definition, at the same power, watts, a brushed and brushless MUST draw the same amps at the same voltage. That is how power is defined.

Now, on the brushless motor, more of that power will make it to the prop and less is lost as heat. That is to say that brushless motors tend to be more efficient than brushed motors.

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Old 02-12-2014, 06:54 AM   #4
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Originally Posted by MADMAC View Post
At the same power (approx.) does a brushed motor draw more or less amps than brushless?
Yeah, at the same power OUTPUT on the motor shaft, a quality brushless motor will draw far less input Amps than a quality brush type motor. The problem with the brush type motors is the brush timing issue. The ideal timing on the brushes versus the magnets varies with the amps being pulled by the motor. So, if you time the brush type motor to ideal at full power, that timing will be way off at 50% throttle. The result is at 50% power, the motor efficiency is absolutely terrible. Or if you set the timing at 50% power, its way off at full power. I did a lot of investigation on this with an oscilloscope and my various Astroflight Geared 25, 40, and 90 sized Cobalt motors.

When you get into motors around 1KW or larger, the brushless motor efficiency approaches 90%.

Years ago, I built a 150% version of that Electrostreak that was popular in the late 1990's. The front end of that model was designed to allow easy swap out of the motor mounting plate. It was bolted to the front of the model.

That model started off with a $$$$ Astroflight geared 40 motor, with a 13X10 prop, and 22 Nicad cells with 2400 mah rating. Power was around 650 Watts. Flights were on the order of 3-4 minutes. On landing, everything was so hot I put together a blower to cool things off while recharging those Nicad batteries. You could not touch that Astro motor after a flight without burning your self.

Moving on to the present, that same model now has a Hacker A50-12S motor, and a 6S2P A123 battery pack. The prop was bumped up to a 15X10, watts is around 1200. FYI, that Astro motor turned the 13X10 prop at about 7000 RPM, the Hacker turns the 15X10 at 7600 RPM.

The motor is putting double the watts on the propeller shaft, and at the same time, flying time with the 4000 Mah A123 pack doubled to 6-8 minutes. And, at the same time, the overall weight of the model was reduced by a full POUND! The model will fly straight up, out of sight on that Hacker motor. And, the Hacker motor is only lukewarm after a hard flight.

So, in my case, with the same model, brushless motors far outperform the old geared brush type motors.

More reading info
Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

Hacker 6S2P A123 powered Models
http://www.wattflyer.com/forums/showthread.php?t=44686

Hangar 9 Kantana Model
http://www.wattflyer.com/forums/showthread.php?t=68844

Hanger 9 Twist 40 Model
http://www.wattflyer.com/forums/showthread.php?t=70548

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Old 02-12-2014, 09:43 AM   #5
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Its not good to compare results using 2400 mah NiCd vs 4000 mah LiPo and make an inference about the difference in brushed vs brushless.... The change in battery is a huge part of that change.

Lower weight (typically half the weight for the same size pack) plus higher (166%) capacity will do a lot toward that doubled flight time per charge.
The lower weight needs less power to overcome the drag produced to lift it....

Just comparing swapping the motors without having changed battery types you would have seen about 5% increased duration. Enough to notice, but the battery made a lot more difference than the motor change.

LiPos coming out at the same time as brushless got a lot of people confused about which change made what part of the difference.
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Old 02-12-2014, 02:25 PM   #6
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Just to be clear, the motor can't tell what kind of battery is connected.

What Fhhuber is talking about is weight reduction leads to the ability to fly at lower throttle settings so you fly longer on a battery of the same capacity.

If you take a plane with a brushed motor and take out a 16 ounce 2100 MAh NiCd pack and replace it with 4 ounce 2100 mah lithium pack you will get more mixed flying time because you can fly at a lower throttle setting due to the lighter weight. If you run at full power all the time both battery packs will last about the same.

The motor only sees amps and volts and resistance in the wires. It has no "knowledge" of what kind of battery is attached.

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Old 02-12-2014, 06:00 PM   #7
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Originally Posted by fhhuber View Post
Its not good to compare results using 2400 mah NiCd vs 4000 mah LiPo and make an inference about the difference in brushed vs brushless.... The change in battery is a huge part of that change.

Lower weight (typically half the weight for the same size pack) plus higher (166%) capacity will do a lot toward that doubled flight time per charge.
The lower weight needs less power to overcome the drag produced to lift it....

Just comparing swapping the motors without having changed battery types you would have seen about 5% increased duration. Enough to notice, but the battery made a lot more difference than the motor change.

LiPos coming out at the same time as brushless got a lot of people confused about which change made what part of the difference.
Naw, I have to disagree a bit on this.

That Astroflight Geared 40 motor is actually two ounces more weight than the Hacker A50-12S motor.

The Astro motor was pulling 660 Watts out of the battery pack, and putting 430 Watts onto the prop shaft. Compare that to the Hacker's 1200 watt input, and 1050 Watts on the prop shaft. If you put a smaller prop on the Hacker and drop it down to 660 watts input, you'll get some 600 Watts on the prop shaft, compared to 430 of the brush motor.

Even though the Astro motor was running at 50% of the Watts input of the Hacker, the Astro motor got so hot you could not touch it after a landing. Motocalc suggests the Astro Geared 40 motor runs at about 55% efficiency, compared to the 87 % efficiency of the Hacker. The Hacker motor in the same airplane is only a few degrees above ambient after a hard flight. A big part of this is the motor winding resistance. The Astroflight motor has 9 times the winding resistance of the Hacker. And, add to that the resistance of the brushes.

So, replacing the 22 Nicad Cells With A123's on that Astroflight motor setup would have improved the flying weight by a pound. But you've still got the lousy efficiency of the brush type motors as compared to the brushless type.

Back in the mid 1980's I actually fitted an Astroflight geared brush type 05 motor with aluminum heat sink fins to keep the motor from overheating when running on a 10 Cell Nicad pack. Yeah, that motor was being pushed big time, but it was later replaced with a much more powerful brushless motor that ran only a few degrees above ambient temperature.

Then, add to all of this the price. Those brush type motors cost three or four times that of a brushless Hacker motor of the same size. Add to that another $45 for a required gearbox. Not to mention the many other brands of brushless motors that are a fraction of the cost of the $$$$ Hacker motors.
http://www.astroflight.com/motors/co...otors/640.html

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