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Old Oct 31, 2003, 09:37 AM
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Why brushless motors are more efficient than brushed ones?



JL
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Old Oct 31, 2003, 09:41 AM
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Marietta, GA
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No brushes!
..a
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Old Oct 31, 2003, 09:41 AM
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Stockbridge, Georgia, United States
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No brushes to transfer the power, this equals NO drag from the brushes. The only moving parts with any kind of friction are the bearings.

Glenn
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Old Oct 31, 2003, 09:53 AM
Arp
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Brushes are also inefficient at transfering electrical power, because of sparking.

To me, the friction of brushes appears the main source of friction in a brushed motor. Bearings and cooling air cause much less, and are present in both motor types.
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Old Oct 31, 2003, 10:32 AM
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It is not safe to be too general in saying brushed motors are not efficient. Just because it says brushless does not make it better. In fact many of the low cost brushless motors are LESS efficient than quality brushed motors. For both the devil is in the details.

Steve
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Old Oct 31, 2003, 11:13 AM
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I've heard exactly two people put forth that purported statement, neither have backed it up.

Please provide data which "quality" brushed motors are more efficient than "many of the low cost brushless" ones.

Even the low cost brushless motors are in the 80's. Which brushed motors are that high?

Data please.
Darren
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Old Oct 31, 2003, 11:51 AM
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Many coreless brushed motors can reach over 90% efficiency. Even cobalt or Neo brush motors can approach 90%.

Efficiency is designed into the electrical characteristics a motor. The mechanical design is just part of the motor.

Greg
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Old Oct 31, 2003, 11:54 AM
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Steve, you got to it before me! Thanks.
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Old Oct 31, 2003, 11:59 AM
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Again, I'm requesting data. The Wattage Cobalts aren't running at 90% efficiency. Neither are any of the coreless motors I'm familiar with.

Sorry but the Mechanical parts of the motor are part of the losses when looking at power in vs. power out in an airplane motor application. You don't get to pick and chose which losses to count and not to count to make a point.

I want someone to show me a data sheet of a brushed electrical airplane motor outperforming a brushless in efficiency with both motors in their max efficiency ranges.

Nothing in my years of experience says you can find one.

I'd be happily proved wrong... with data from RC aircraft motors in RC aircraft applications.

Darren
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Old Oct 31, 2003, 12:21 PM
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Wattage Cobalts don't exactly qualify as "quality" motors in my opinion.

You need to compare a Plettenburg or Astro brushed motor to some of the cheaper chinese or cz made brushless.

Steve Neu has run and published dyno tests of many motors, much useful info than the data sheet. However I don't have the articles here to make a specific comparison.

For other sources of info look at
http://www.wemotec.com/
look at the performance figures for the Midi Fan and HW 609, the brushed motors have overall efficiency just as good as the brushless.
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Old Oct 31, 2003, 12:25 PM
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I used to work for a company that made actuators for the aircraft industry. The actuators had to survive stall conditions without burning up motors. The result was Cobalt motors that had peak efficiencies below 50% in some cases. They also started venturing into brushless motors and had the same requirements, and ended up with similar results. In this case efficiency was not the requirement and while mechanically the motors were capable of much higher efficiency, the electrical design limited the power output and efficiency.


Try this link for very efficient coreles brush motors.
www.thingap.com

Greg
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Old Oct 31, 2003, 12:50 PM
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I will try to change the focus on the debate:

What determines efficiency in a motor? (mechanicals & electricals, material quality )

And then at the same conditions and don't thinking about the mechanical losses due to brushes, are any differences in efficiency between brushed & brusless? Is a design issue?

Can you design for a target efficiency?


JL
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Old Oct 31, 2003, 01:09 PM
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When you are designing for peak efficiency there are limitations you have to try to cheat to extract as much efficiency as possible from the design. If you don't need so much effiicency and cost is more important you can choose what parts give you the best efficiency verses cost.

Greg
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Old Oct 31, 2003, 01:19 PM
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Its really simple. Effeciency is a ratio of power in to power out. More accuratly though it is a ratio of power in to usfull power out since in the end all power going in has to actully come out someplace since these things are not nuclear.

In the case of electric motors we input electricity and we want to get out kenetic energy in the form of a rotating shaft. Not all the electricity gets converted to kenetic energy however. Some of it is instead converted into heat. This is true in any moter wether it is brushed or brushless. Both brushed and brushless motors can be tuned to minmize this loss and will be nearly equal in efficiency with respect to heat loss; however, brushed motors have some extra losses. First they have brushes that cause friction and as such heat which accounts for more heat loss. The brushes also cause a great deal of arching and as such some of the electricty is converted into radio waves, again more loss. Due to these extra sources of loss a brushed motor is inherantly less efficient. This however does not mean that you can not have a brushed motor which is more optimized for the given application and as such is more efficient than a non-optimzed brushless motor.
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Old Oct 31, 2003, 01:22 PM
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St. Mary, Maryland, United States
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Quote:
Originally posted by Darren Hintze
I've heard exactly two people put forth that purported statement, neither have backed it up.

Please provide data which "quality" brushed motors are more efficient than "many of the low cost brushless" ones.

Even the low cost brushless motors are in the 80's. Which brushed motors are that high?

Data please.
Darren
Review the dyno tests Steve runs in S+E now Quiet Flyer each month. The Graupner Ultra 3300 series are big brushed motors (expensive) that have efficiencies in the high 80% area at quite high current levels. You asked for a specific example, that one comes to mind. The smaller HP 290 series are probably not quite as good but still in the low to mid 80's.

There are hardly ANY brushless motors that exceed 80% on less than 10 volts. Between 10 and 20 volts most but by no means all brushless motors will exceed 80% by a small amount for some range of current draws. The brushless that falls in the 87-93% efficent range is a rare bird. It's usually the most expensive in the class.

One of the new cheapie Chinese brushless motors was tested this past month and it looked pretty poor against the more expensive Kontronik / Plettenberg / Aveox counterparts. OTOH, it is cheap. Similarly the Mega's test out at lower than high class efficiency but the $ per watt measure makes them good deals.
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