The limits of torque production for a given weight of motor

[Miles]

What would be any technical drawbacks or benefits in making and running a low enough kV motor to equal 1 motor rpm equalling 4rpms of a 24" wheel, such as on my bike (example)?

I can't see any benefits... We're struggling to generate enough torque to run the motor at wheel rpm and you're suggesting running it at 1/4 of wheel rpm. That means the motor has to make 4 times as much torque! On top of that you also have the losses from the overdrive .....

 

[nutnspecial]

Ok thanks, yes I thought it would be minimally applicable here if at all. Was just wondering.

How about just simply: how low is it physically (and conceptually) possible to make kV in any given/various motor(s), and what would be the implications?

edit// we're always pushing boundaries on lams,
but what about smaller wire toward nano and kV of <2 ?

Would battery/controller tech be adaptable to the subsequent possibilities of higher volts and less amps, and what kind of numbers would they be: like 900v and 7a instead of 90v x 70a? What would be a reason and what conceptually is the limit in that direction of thinking?
This applies somewhat I think http://www.rcgroups.com/forums/showthread.php?t=1096719

 

[Miles]

I can think of two practical considerations. Firstly, you need a higher number of turns in each coil which makes it tedious to wind. Secondly, as the wire diameter decreases, the insulation becomes a greater proportion of the gross cross-sectional area - the fill factor goes down.

 

[nutnspecial]

Thankyou. Yes that should be obvious I guess and I'm sure there'd be more. I was willing to overlook winding difficulty and insulation as I'm imagining the implications of theoretical super high turns on motor application and how controllers/power sources would be considered, as an amateur.

I imagined as the wire got thinner so could the insulation. . . and yes there's still impact of fill and resistance is a consideration. It's just a dumb question/consideration I'm not knowlegable enough to answer/ follow to definite conclusion. What is even the lowest kV motors and where are they used? G searching didn't turn up anything initially, but I do know I like my 11lb 30kV bht.

 

[sn0wchyld]

Thankyou. Yes that should be obvious I guess and I'm sure there'd be more. I was willing to overlook winding difficulty and insulation as I'm imagining the implications of theoretical super high turns on motor application and how controllers/power sources would be considered, as an amateur.

I imagined as the wire got thinner so could the insulation. . . and yes there's still impact of fill and resistance is a consideration. It's just a dumb question/consideration I'm not knowlegable enough to answer/ follow to definite conclusion. What is even the lowest kV motors and where are they used? G searching didn't turn up anything initially, but I do know I like my 11lb 30kV bht.

as voltage goes up, so does insulation thickness

 

[liveforphysics]

Would battery/controller tech be adaptable to the subsequent possibilities of higher volts and less amps, and what kind of numbers would they be: like 900v and 7a instead of 90v x 70a? What would be a reason and what conceptually is the limit in that direction of thinking?

I have a few for you:

1. You would die before you finished building it.
2. If you were resuscitated successfully, you wouldn't be again after you rode it in the rain. (unless you decided to dedicate at least 10x the engineering time and resources as the pack costs to make something that can actually function in a condensing humidity environment at 900v and do more than burn down in a week).
3. Corona just naturally streams off any sharp geometric points once field strength gets high enough. Corona + time breaks down most things known to man.
4. You would already be dead by this point, but your BMS management harness and cell stack chips would be close to the weight of your pack, and much more cost than the cells if you used something that's not just a fire-waiting-on-a-pcb-at-900v BMS.
5. Your controller would require SiCFETs (to be efficient) that would cost more for 6 of them than most ebike hubmotors+ controllers cost, or run IGBT's and eat a series diode drop loss of ~1.4-1.7v x 2 per phase (roughly like throwing one of your series cells away in heating the controller pointlessly).
6. Your motor would stream corona for a bit before arc flashing through your winding enamel in some invisibly defected area from winding it. If you re-wound it with the right rating of wire and didn't so much as chaffe through it anywhere, it would still be a very meaningful copper-fill and ultimately continuous torque and continous power penalty from such a greater percentage of the slots area being varnish rather than copper.
7. At least a few of us would miss your very open minded posts here, due to reason #1, and best case #2.

ATB,
-Luke

 

[Punx0r]

A quick look at general purpose industrial IEC induction motors suggests you're looking at a 10% weigh penalty going from a <690V motor to 3300V and another 10% to go to 6600V. However, at ~10kg/kW continuous these aren't exactly high-power density motors. I'd also guess an induction winding is much easier to make and fit - no wrapping it around the sharp edges of teeth.

Maybe a bit OT, but It'd puzzled me for a while why I've seen 11KV motors in use when a 380/440V one would do the job. Available supply on site I guess as 11KV is local transmission voltage here.

 

[nutnspecial]

Lol, nah I wasn't thinking of dedicating my life to anything other than perhaps the pursuit of similar thinking and values of tesla/ einstein- not the subject matter they dwelled on and remained unfinished past death.

Anyway, thanks for the info guys!