View Single Post
Old 08-15-2019, 09:16 PM
itsid's Avatar
itsid itsid is online now
Join Date: Jul 2013
Location: Ruhrpott [Germany]
Posts: 10,700
Thanks: 1,447
Thanked 4,470 Times in 3,370 Posts

Don't gt me started again on what I think about "Manta" ...
I mean I don't need to remind you about what they suggested you to controll the motor with, do I?
So their claim of 72 RPV is taken with an ounce of salt (or say the heaviest grain you find )

There is no such thing as RPV on any motor really..
not on brushed and not on brushless motors..
it's an indicator value
(a value you might expect under controlled ideal circumstances; however those might need to look like; AT BEST)
at worst it's a blunt lie (I mean Manta motors... *cough*)

I think we've come to the conclusion yours is at least a sibling to the Motenergy ME0709, right?

So, let's go with that well documented "non fictionally improved" motor
as a comparision, shall we?

it runs slightly slower (2400 rpm'ish in both directions at a fixed 48V) completely unloaded
so in theory 50 RPV

btw at 24V it runs 1150 RPM unloaded ~ 47 RPV
and at 36V it runs 1700 RPM unloaded ~ 47 RPV

it itself maxes out at about 4.5k rpm at 72Volts as far as I know
So in theory 62 RPV (cannot find the diagram for a 72V setup atm)

And there you go... different volts yield different RPVs on the exact same motor

Talking of it..
Click image for larger version

Name:	me0709_48V_CW.jpg
Views:	1
Size:	47.0 KB
ID:	110910
Click image for larger version

Name:	me0709_48V_CCW.jpg
Views:	1
Size:	47.0 KB
ID:	110911
And the ripples (most apeparant in the efficiency curve)
tell you it preferes clockwise rotation,
drawing less amps and running smoother..
BUT the difference in rotational speed is not noteworthy

IIRC the programmable kelleys come limited to 90% max power.
but they're current limiting, so the motor is limited to draw no more than what 180 Amps (can't recall the controller peak amperage you bought tbh)
And w/o load I doubt it draws more than maybe 30-50 Amps even if you haven't removed the rear wheel (which in fact is already "load")

think about it this way: if you can easily move the 5.x:1 ratio driven rear wheel with two fingers back and forth in rapid succession by touching nothing but the chain then it's no load..
else it is

BUT you should never run a PMDC w/o load at full speed,
it'll not like it, so an attached rear wheel is the minimum I'd consider safe to floor
Motenergy for example asks you to never run any of their brushed motors above 5k rpm.. ever

Anyways back to RPV..
You more often than not read KV ratings for BLDC motors (especially on RC stuff)
And the way THAT works is the exact opposite of what most think it is..
DRIVEN at a specific speed, it can generate a specific voltage
(say a 200kv motor needs 500 rpm to generate 2.5 Volts)
And that is in fact more or less a constant.
But it does not mean that if you apply 25Volts it'll automatically spin 2000 rpm
since the commutation of a BLDC is purely electronical,
the controller can limit the speed to whatever value he finds plausible, he just cannot exceed the 2k rpm limitation.
(in fact due to losses he might have troubles hitting 1900 rpm)

And a similar thing happens with mechanical commutation;
it takes a moment (some Ás) for the magnetic field of a coil to collapse
when it's disengaged,
so the faster a motor spins, the bigger the residual magnetic field in the disengaged coil at the time it passes it's magnet increasing the attraction between the two,
and the bigger the attraction the more the motor gets slowed down.

So it's essentially self braking at it's max mechanically possible speed.

And the reason you should NOT go there is,
that magnetic friction can go nowhere and causes a ton of heat,
so a tiny bit of load to the rotor is in fact a good thing
Much better than cooking your magnets off, burning the insulation of the coil wires, or melting heat cracking the epoxy fixing it in place

Jokes about german sausage are the wurst.
Reply With Quote
The Following User Says Thank You to itsid For This Useful Post:
Functional Artist (08-16-2019)