Brushless motor minimum speed ?

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TOURNEUR
Posts: 11
Joined: Jan 5th, 2013, 12:37 pm

Brushless motor minimum speed ?

Post by TOURNEUR »

Hi everybodys

I think will employe brushless motor on my futur rov
But in my some system test before ,see couldn't run under a minimal speed rpm with any esc controler .

Do it possible run from null to few rpm speed with a brushless motor like 0 to 60 rpm in my exemple ...


Or does are minimum speed rpm for a rov thruster ?

How can we do the relation between speed, size and rov weight to have a correct choice ?

Thank for who can answer
rossrov
Posts: 383
Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: Brushless motor minimum speed ?

Post by rossrov »

Hi Tourneur. I purchased some Turnigy Trackstar 18A ESCs. Not quite ready to totally condemn them, (want to re-adjust some of the ESC settings, again...), but at present they do not want to start quick reliably at even moderately low RPMs. If you do not already know, the motors need to be moving a bit before the ESC can sense the motor's position and know what phase and when to energise next. To get the motor moving, the ESC randomly energises a set of coils in the motor and looks for what comes out. Some ESCs will do this better than others at lower RPMs. BikerBones posted a video of his ESCs running the motors very smoothly, but certainly not in the low range you describe. There is mention on the forum of what ESCs he is using.
Unless there are some really neat ESCs around that I do not know about, the only way to get 60RPM and lower (?) would be to either use a gearbox or make your own ESC, or re-write the code of an existing ESC. This is what I'd like to do with the Trackstars. Not enough ROV time at the moment though!
martinw, another member of the forum, has some pictures of ESCs he built himself on a separate blog. Do not know where he is at with them, or how they are designed to function...
martinw
Posts: 91
Joined: Sep 20th, 2011, 11:02 am
Location: Aberdeenshire

Re: Brushless motor minimum speed ?

Post by martinw »

I'm guessing the brushless ESCs that are generally available use a sensorless control philosophy, which would explain why they randomly energise coils initially until they know where they are. The motors I use and the philosophy is slightly different as they have hall effect sensors in them so the controller knows exactly where the motor is at start up and the low rpm control is very good.

I don't know how easy they are to get in the "RC World" as they're not that interested in low speed ccontrol, just the power output, simplicity and cost but I'd suggest trying to find a motor with sensors in it and a controller that uses the sensors.

I'm still testing my controller at the moment. Once I'm happy with the design I may release it to the wider world but I need to thoroughly test it first for my own peace of mind :)

Here's a very good animation showing the halls H1-H3, which you can single step through too ....

http://www.townbiz.com/animations/4-pole_bldc.html

Hope this helps.

Martin
rossrov
Posts: 383
Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: Brushless motor minimum speed ?

Post by rossrov »

Thanks for post Martin. Yes, the vast majority of hobby ESCs are "sensorless". Personally I haven't checked out the sensored thinking that they are more expensive, and have more bits to go wrong etc. Did look around for non-hobby motor driver chips, a few about but the easier-to-use ones require the Halls. Brushless drills are becoming more available now and will soon replace brushed I guess. From what I can tell, these, as one would expect, are sensored too...
Hope you go further with your ESCs. Power electronics is less forgiving and way more complex than logic, so understandable your being thorough
martinw
Posts: 91
Joined: Sep 20th, 2011, 11:02 am
Location: Aberdeenshire

Re: Brushless motor minimum speed ?

Post by martinw »

You've obviously been there before, exploding capacitors do make you jump :lol:

I think I'm getting pretty close with the design, final tweak to the PCB, which I think will make this the fourth, maybe the fifth revision and it should be good.

I'm doing some extended testing as I type, in the garage ....

http://martin-wareham.blogspot.co.uk/

It's actually up at 73 degrees now (after about two hours) but it seems to be stabilising and is well off the 150 degree maximum of the FET :shock:

In air at full load they do cook (damage themselves) though, as the PCB is very small.

Martin
scubersteve
Posts: 251
Joined: Jan 28th, 2013, 10:29 pm
Location: Milton, Florida

Re: Brushless motor minimum speed ?

Post by scubersteve »

Wouldn't just running a smaller prop at a slightly higher speed give you the same thrust as a bigger prop at a lower speed?
So use conventional rc brushless setup with the lowest kv rating you can find and size your prop accordingly.
Just my thoughts on the matter.
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TOURNEUR
Posts: 11
Joined: Jan 5th, 2013, 12:37 pm

Re: Brushless motor minimum speed ?

Post by TOURNEUR »

Thanks for all reply !

Ok ,sensored can be a solution for hight efficient, and can be a solution .
But I'couldnt think modify one esc soft or hard for a beginner like me .

I read your post ,and effectualy after test a less Kv motor can be a solution ,but big motor ,need hight voltage for run !
i see different planetar brushless gearbox ,it's can be a good solution

At this time I thought about a waterflow tunnel with force gauge to can mesure more appropriate motor

And use less size motor can be a good solution ...


Thank for your help !
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