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Thread: Brushless verses Brushed Motors

  1. #1
    Hi all,
    Being old school I still use brushed motors for my FW but am looking at the possibility of going brushless.
    Brushless motors from what I have read across the web are a totally different science and information on their performance seems very limited.
    I understand how to find wattage (v x a = w) not a problem, but the figures you are giving to work with are non loaded. Same for rpm (kv x v = rpm) again unloaded - this is based on a 100% efficiency so would suggest loosing 10% of the total eg kv x v x 90% = rpm.
    Reading one article written by a gentleman in Australia based on swapping out brushed for brushless in a boat, he suggests you half the rpm total with the 90% efficiency to get a true loaded rpm. He also stated that unless you are interested in prop thrust for an aeroplane then any other parameters are very hard to find. This I find very frustrating as I am used to referencing the wattage and looking at the torque under load or stall.
    Can anyone shed anymore light onto this subject please? or give examples / ideas to help, also pro's and con's for using a brushless against a brushed.
    Please use this as an open discussion base so we can all benefit.
    Many thanks
    Cliff

  2. #2
    Shakey's Avatar
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    The one thing missed by the guy saying to derate RPM by half is that, in FW's especially, vastly overspeccing a motors power is the norm.

    The common NTM Propdrive 50-60 or 50-50 used in FW drives are motors hitting into ~1-2.5kw per motor, well over what's really needed for a feather. The advantage of this though is that such larger brushless motors come with the bonus of rotating slower. May seem an odd advantage but when you are making your own gearboxes which is often the case slower motors can let you get away with simpler single stage spur gears to do the reduction. So in this case you can reasonably assume the rated kv is not far off the kv you actually get. With brushless motors the 3 main considerations are power (You want to over spec enough that startup torque is not much of an issue), kv/RPM (Ease of making gear reduction), Build of motor (Are they enough motor to cope with the heat generated, plenty of small motors would work... just briefly).

    For a bit of inspiration these are the setups I made for my feather. Very robust, fast and powerful. Have even taken a couple of direct hits from Nuts 2 and carried on (albeit with a slight twist to one now!).



    Of course there are other paths to brushless drive, such as more capable ESC's paired with smaller motors and gearboxes. I just tend to this one for feathers for reliability and cost.

  3. #3
    Hi Alex,
    Thanks for coming back with the explanation. By the way the drive looks really neat.
    With so much power on a FW how do you stop wheel spin? Thinking of control issues. I seem to remember that the HW were generally being powered via 800w to 1.2kw motors.
    Can you confirm what battery/s you are running on and speed controller/s you are using please?

  4. #4
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    You don't stop wheel spin. Being able to spin your own wheels is a desirable property even with brushed. If you don't have the power to break traction you are far more prone to putting your motors into stall, which can be pretty fatal to brushless motors and ESC's due to their higher power tendencies. Plus it's kinda fun to drift around the arena! You do just have to be a bit less stick mashing when driving more powerful bots.

    To add a bit more you don't actually get all that power out of a brushless motor unless you are going fancy with stuff likes VESCs. You overate them a lot to get a good level of power to use out of it despite this.

    For batteries I run 6S for 50-60's with redbrick 200A esc's flashed with Simonk.

  5. #5
    Many thanks Alex,

    Interesting points, so basically brushed are more forgiving when put into stall mode?
    I've had a look at your set up on the web, seems quite a nice system there. Do you need a separate power source for the receiver or does the Rebrick have the power supply?
    Once again being old school I am not up to speed with flashing esc's, is this a kind of rewriting the program within or changing parameters? In my days when rc racing I would use the transmitter and using a button on the esc watching flashing lights...
    I presume 6s, 5s and 4s Lipo use on say your 5060 motor is not linear as far as Amps drawn, not sure how to find out that information. My FW doesn't have the space for a 6s, a 5s will be tight.
    Also I am trying to understand power to weight ratio, from my days of racing slipping wheel is loss of power and in some cases reduces acceleration, hence the good old clutch slip to keep max torque to min slippage. Wonder if we could use centrifugal clutches...? lol.
    Finally, for both brushed and brushless, do you know how much over voltage is safe e.g. 18v motor running on 22.2v (6s)?

  6. #6
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    The ESC and its firmware used with Brushless make so much of a difference, that its very hard to give standard rules or values to how much power you will get out of a system. The reason the ESC's are reflashed is that the standard firmware is pretty dumb in the way it tries to move the motor. Normally these ESC's are used in drones or planes with set max weight and prop values so its all setup for that, but that's no good to us. SimonK and BL-Heli are smarter and can have their settings tweaked to suit your setup.

    Both Binky II and Conker 3 run NTM 3536's on Flashed TZ85A's at 4S, on 75mm Wheels, 16:1 gearboxes, and have more than enough drive power for Spinners. I believe we are currently running the default SimonK Values with no issues. I'd also say that Brushless motors are much more forgiving when stalled as you aren't in danger of burning out a single phase. We've stalled 3.7Kw Scorpions on 12S and they've been fine during a 3 minute fight, but we use VESC's, so they can detect a stall and restart. The one time we have blown one up was on an Castle Creations ESC, and every phase burnt out simultaneously due to the power it was dumping into the motor; not a very smart ESC.

    I personally can't see the point of over-volting a brushless as they are so powerful anyway. But if you get your setup right they can sip power. To give an example, Binky II ran the most recent FW Gladiator for 5 minutes with a 4S 1.3Ah drive battery and two 2.7Ah 4S lipos for its weapon (Wired to give 8S) In the 5 minutes we used 1000mAh of drive and 1.8Ah on the weapon. We used to run 550 motors on a 2.7Ah 5S, and had to recharge it every fight; wasn't paying attention to the figures at that point so I cant recall how much it used but it was enough that I felt it should be fully recharged.

    Take from that what you will. Everyone has slightly different preferences and experiences with Brushless, but hopefully it helps.

  7. #7
    Thanks Gents,
    Getting the general idea, looking at costs and what set up has been described above I get the feeling that brushed is a cheaper route but compromises the performance when considering size and weight. I also feel that brushed is a simpler route with less variations involved - am I right?
    I have a couple of brushed 150w, 12 to 24v dc motors with a rpm of 3000 to 6000 (the speed increases as the voltage increases), therefore I was considering using them with a 4s or 5s Lipo. My thought was to cap the throttle control so that max rpm is not reached and concentrate on pulling max power out of the motors for short bursts.
    Any thoughts or comments please?

  8. #8
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    The throttle is not controlling the RPM though in brushed. It is controlling the effective voltage fed to the controller, capping the throttle means you won't feed it full voltage and hence will lower max power. "the speed increases as the voltage increases" That's normal operation for any brushed DC motor (And the brushless DC motors we use). Motors all have a 'kv' or 'rpm/v' which is a measure of how fast it turns per volt that is fed into the motor.

    Brushed is often the simpler route due to the number of already paired motors and gearboxes out there and ESC's made for them being much closer to what we actually want. But brushless, especially these days, easily out performs on a weight for weight basis.

  9. #9
    Thanks again for the confirmation, starting to get the idea of the brushless. I am at Robodojo this Sunday so hopefully will catch up there if you are all about. Will also do some more digging whilst speaking with others.
    Regards

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