plasticbaldy

I'm designing an electric start for a 40cc gas (petrol) engine and a brushed Mabuci RS550 is used to start slightly smaller engines so it's my starting point.
This is the standard RS550 spec ;

http://www.trossenrobotics.com/store...Motor-12V.aspx

And another similar RS550. I'm not sure what PC/VC stands for ? ;

http://www.mabuchi-motor.co.jp/cgi-b..._ID=rs_550pcvc

I'm trying to find out what would be a similar Power & torque in a Brushless inrunner. I'll adjust my gear ratio to match the RPM of the brushless.
Am I correct in saying that Brushless motors have lots more power & torque for the same physical size & weight ?
So is there a (say) 25mm or 28mm brushless inrunner with similar power/torque to RS550 please even if the Kv is very different ?

Dr Kiwi

This is about as close as I can find... much the same weight (205g) and about the same Kv (1600Kv)... this should be good for 250W or so which is probably more than you could safely put through an RS550.

http://www.hobbyking.com/hobbyking/s...otor_FIN_.html

There are any number of 28mm brushless inrunners which are far lighter than, and can do way better than, an RS550. As you say... almost any Kv will do, since you can adjust gearing accordingly.

plasticbaldy

Thanks Doc. How many watt do you think the Mabuci RS550 is good for at it's max 12V ? And is the torque a consideration here ? How does torque compare generally on a Brushless to a Brushed motor of equivalent KV and A ?
You can probably tell from my questions that I don't understand this technology.

Dr Kiwi

I would have guessed about 20A max. and indeed I found a Banebots site which has a spreadsheet for various brushed motors and for the RS550 it says "Maximum wattage = 253W"

plasticbaldy

Cool. Thanks. I was hoping that 350 - 400W would be enough.

Dr Kiwi

That KB 28-57 I linked to probably won't get you anywhere near that.. it is rated for 23A max and even if it can survive on 4s you are still only just over 300W.

plasticbaldy

It's a pity that Hobby King doesn't keep good stock levels of brushless inrunners because there's lots in their list for around $20.
Maybe I can use an outrunner. Do outrunners generally have more power/torque for their size & weight ?

Dr Kiwi

If you want a 400W outrunner, look for something weighing around 130g.

Maybe this?: http://www.hobbyking.com/hobbyking/s...ner_Motor.html

plasticbaldy

Thanks Doc. Reading the reviews I see that guys are getting 1000W on 4s out of this. Overkill maybe for my application.

Dr Kiwi

I wonder how long their motors last! 1000W into a 130g motor is 7.7W/g... that may well be "overkill" for the motor!.... "rule of thumb" is ~3W/g

plasticbaldy

Now you're talking critical specs that are useful for me.
Quoting you "rule of thumb" is ~3W/g "
Can you give me a W/g figure for Brushed 12V motors please Doc ?

Dr Kiwi

I really have no clue since no distributor seems able to post max. amp ratings for any brushed motor! I know Speed 400's are about 65g and rated for 75W-100W.. Speed 480's (103g) can do 20A/200W....big 12v 600's (220g) are probably good for no more than 20A-30A... so we seem to be in the order of 1W-2W/g.