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  1. #1
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    Efficiency curve of brushless versus brushed motors

    I sometimes see the optimal efficiency mentioned of electric motors for R/C cars. These are often in the range of 60-80%. Now Iíve seen that brushless motors may reach higher efficiency scores around 90%.

    This may not seem too great of an improvement over the best brushed motors (10%). From this you would expect an increase in runtime of 9/8 = 1.125 times (12,5%). But I was thinking that the increase in efficiency from using brushless motors must be higher when considered through a greater range of speed (RPM). So the difference in average efficiency would greater than the difference in maximum efficiency.

    The highest eff. of a motor is usually found at a medium RPM. When going to a higher RPM, brushed motor will experience drag from the brushes. This drag absorbs a lot of energy and will reduce efficiency. (My GM purple bull 15T motor uses 4 Amps unloaded!)

    Also, I assume a brushed motor usually can attain a higher RPM. So if you were to gear your brushless motor for the same top speed as you were running your brushed motor, your gearing can be lower. This will decrease the current draw at low running speeds (accelerating). This may not improve actual motor efficiency but I does increase driving efficiency (correct me if Iím wrong).

    I figure that R/C cars must benefit most from brushless motors because the range of RPMís in which the motors are used is greatest for this type of R/C application.

    Does anyone know a website where I can find data sheets for brushed versus brushless motors. (power output, efficiency, rpm)

    Has anyone here geared a brushless motor to stock speed on his car and compared running times. What is the increase in runtime?

  2. #2
    RC Champion HiAmplidude's Avatar
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    I'm not sure if this will help in your question for comparitive data, but if you are quite familiar with non-standard (for cars) brushed motors, you may be able to run some comparisons here:
    http://brantuas.com/ezcalc/dma.asp

    You are definitely on the right track, and it's refreshing to see someone else thinking about the big picture.

    Even in comparing a brushed motor to a brushless system, it's almost impossible to find a good way to compare them. You may be able to find a good comparison for one aspect, but to be fair to the motors, would have to change your subjects to test a different characteristic. Kinda' confusing, I know.

    There may be several interesting characteristics to test against, like:
    * run-time
    * torque
    * power curve
    * temperature
    * rpm/V
    * and so on...

    For each one of those, generally speaking, brushless is dominant. But, there are so many different BL motors, it would be easy to find a challenge where the brushed motor was clearly the winner.

    Even matching RPM/V of a brushed motor to the BL motor is not a decent way to match motor to motor. Additionally, there may be slight differences from one type of BL motor to another, even within the same class. For example, one type of BL motor may be designed to offer greater efficiency at top RPM, where another has a higher efficiency at mid-range, and still others offer a slightly flatter efficiency curve.

    So many factors it boggles the mind. For hard numbers though, it would be extremely interesting (to me) to actually see some numbers or graphs comparing at least similar sized motors to each other.

    Nice topic!
    rcNuts.com

  3. #3
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    Thanks for that great link HiAmplitude! Even my brushless MEGA motor is listed! This is interesting stuff to do some calculations on my new plane. And maybe I can figure some thing sout about motors for the E-maxx.

    I would like to study this right now, but I promised my wife not to be sitting behind the computer all day!

  4. #4
    RC Racer
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    Efficiency is, as you said, not a songle number, but it depends very much on the rpm and current.

    Here are two graphs. One is from the Mabuchi homepage and the motor type I found is pretty much the stock motor in the E-MAXX. The max efficiency is 75%, but look at the graph. It has this efficiency only at a small range and drops down a lot before and beyond that.

    Titan Motor

    The next is an image from the Lehner Homepage. I picked a motor that is commonly used in the E-MAXX, the 1930.

    The max efficiency is 91% and the curve is much flatter. That means the efficiency ouside the optimum is far better compared to a brushed motor.



    Ok these two graphs have different scale but you get the idea.

    Another thing to keep in mind is that the efficiency on Lehner's homepage includes the cotroller while it is not included on Mabuchi's graph.

    A brushlessmotor should be able to rotate faster than a brushed motor. I talked to Mr. Lehner about this myself. The limiting facter is usually the bearings, not the motor itself. He said if I wanted to I can get any motor with special bearngs that work safe up to 100 k rpm or higher. That is way faster than brushed motors.

    In general brushless motors roughly spin twice as fast as the Titans. If you keep gearing the same you are comparing apples an oranges.

    If you want to see how runtime differs you would have to drive a BL MAXX at the exact same speed and output power as the stock setup. But nobody does that since the idea behind getting a BL motor is better performance.

    Another advantage, although small, is the fact the a BL motor works as a generator during braking and a small amount of that engery is fed back to the batteries.

    Orca

  5. #5
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    Those are interesting graphs Orca! I'll use the mabuchi graph to roughly calculate the efficiency at which the maxx is riding in first gear and in second gear (stock gearing). I'm curious if it's near max efficiency with its rather narrow eff. curve.

    At the end of your reply you mention the recharging of the battery when braking. I've once read about a regular brushed motor controller that does the same (i believe it was a Tekin controller). Isn't it true that when you spin a brushed motor you will generate current just like with a brushless.

    By the way, isn't recharging the battery every time you brake (even with a tiny amount) bad for the battery? Wouldn't it shorten the lifespan of the battery? You can only charge a battery a limited amount of times. (Or maybe you can think of it as inversed pulse charging = pulsed discharging )

  6. #6
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    The max efficiency of a brushless motor is at the top of the RPM range. Running nonstop in the midrange will drain the batts just as fast or faster than on full throttle. I ran my truck with the same packs, same Mah input and same voltage, ran a race using as little throttle as possible and never hitting full throttle. I kept a fast pace up, and compared it to a full throttle as much as possible race. I got more runtime running full throttle, and more laps completed with less heat on the motors and controllers.

  7. #7
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    Yes a brushed motor can also work as a generator, but not as effective and it is hard to find a controller that supports it. Hacker, BK (warrior) and Schulze support this "recuperative braking".
    It doesn't harm the batteries at all.

    Is it not true that a BL motor always uses the same energy no matter how much you are on the throttle. If that would be the case it would dissipate half of its maximum power into heat at half throttle. This much heat would kill the motor or controller instantly!

    Orca

  8. #8
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    Is it not true that a BL motor always uses the same energy no matter how much you are on the throttle. If that would be the case it would dissipate half of its maximum power into heat at half throttle. This much heat would kill the motor or controller instantly!
    The heat of a half throttle run is far higher than a full throttle run, and runtime is VERY similar. 6.5 minutes on average, and half throttle will sometimes dump before then. The controllers heat up very quick at half throttle, and at full throttle they stay cool.

    This much heat would kill the motor or controller instantly!
    yea, and you never see a burnt brushless controller.....


    (1/2 throttle on 16 cells........)

  9. #9
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    When I bought my Mega motor for my plane I asked teh guy at the store about flying at half throttle. (We want to have the power to take off and then fly with less power to save energy for longer flight).
    He said that half throttle doesn't work well on brushless motors. He advised me to fly at 3/4 throttle instead of 1/2

    So maybe this will not increase flight times at all

    The plane will probably be finished on tuesday so haven't had time to test it yet. And maybe it isn't even a good idea to test it at all

  10. #10
    RC Turnbuckle Jr. EMAXX1333's Avatar
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    i know one thing, with my aveox setup, running slow to medium results in instant thermals (or at least it did until i put a fan and heat sinks on), but running WOT i can go all day without a single thermal.
    You never get something for nothing.

  11. #11
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    exactly, the controller bleeds off the excess juice when at partial throttle, creating heat. Thermals are very common at 1/4-1/2 throttle. The only tim they really thermal at full throttle is when they are really overgeared and under volted.

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