Using RC motors on E-bikes [Archive]

[dirty_d]

Kv already takes into account all the physical aspects in the motor, you can just use it directly to calculate torque without worrying about the motors construction. Kv is actually the same numerical value as Kt when the units used are V/radian per second and Nm/A, your AXI motor produces 0.042V for every radian per second of rotational speed when spun as a generator, and it produces 0.042Nm per amp, to convert rpm/V to V/rads or Nm/A simply do 1.0 / (225rpm/V / 60.0 * 2.0 * 3.1416).

i don't want to bring any bad news, but the no-load current for the terminator is 6.0A at 11V while the AXI is 2.6A, if the AXI was measured at the same voltage the peak efficiency of the terminator is likely less. also if im correct in my assumption of 80mOhms for the winding resistance which i hope its not, it will produce less power at the same voltage as the AXI, but it probably wont be noticeable as you're not running it near the peak power anyhow.

 

[recumpence]

The rated efficiency of the two motors shows a loss of efficiency. So, I am expecting that. Also, bear in mind, the AXI no load efficiency is given without a fan. The Plettenberg has an integral fan. Also, the physical size increase has to have some affect on the no-load current.

My WH per mile is already very good. If I lose a touch, I am not too concerned. I just want the darn EMF resonance to go away along with greater top speed potential without over using the motor. The AXI is awesome except for the EMF issue and it is sensitive to gearing.

Matt

 

[recumpence]

Kv already takes into account all the physical aspects in the motor, you can just use it directly to calculate torque without worrying about the motors construction. Kv is actually the same numerical value as Kt when the units used are V/radian per second and Nm/A, your AXI motor produces 0.042V for every radian per second of rotational speed when spun as a generator, and it produces 0.042Nm per amp, to convert rpm/V to V/rads or Nm/A simply do 1.0 / (225rpm/V / 60.0 * 2.0 * 3.1416).

i don't want to bring any bad news, but the no-load current for the terminator is 6.0A at 11V while the AXI is 2.6A, if the AXI was measured at the same voltage the peak efficiency of the terminator is likely less. also if im correct in my assumption of 80mOhms for the winding resistance which i hope its not, it will produce less power at the same voltage as the AXI, but it probably wont be noticeable as you're not running it near the peak power anyhow.

How can it produce less power? It is rated at 50% more power than the AXI?

Matt

 

[eP]

How can it produce less power? It is rated at 50% more power than the AXI?

For the same amount of power_in is will be able to produce less power out than AXI one at load below AXI's power peak efficiency point.

Despite its wider power producing capabilities. Simpli AXI motor is more competitive for light loads (below peak power point ) - more efficient for that range for the sake of less idlle loss.

It was the reason since my early days here - double motors drive idea:
lighter motor more efficient for light loads is better at flat for moderate speed, larger motor used only for much higher loads - massive torque or torque and power for top speeds and uphills.

But until you don't need monster power than only one (lighter motor - lighter than monster like terminator) should be sufficient for enough power and decent efficiency.

 

[recumpence]

A two motor system would not be impossible. Maybe that would do everything, high efficiency and high power. Each motor would have its own free or one-way so each could run independently.

Matt

 

[dirty_d]

recumpence, i think the reason for the higher rated power is because it has a built in fan and maybe is built to be able to dissipate more heat and withstand higher tempurature that the AXI one. both motors can produce much more than 6500W if you allow them more current, but heat is the limiting factor.

 

[John in CR]

A two motor system would not be impossible. Maybe that would do everything, high efficiency and high power. Each motor would have its own free or one-way so each could run independently.

Matt

This is exactly what I've been considering, except the big 6kw RC motor would be my low power / high eff motor. It's for my electric motorcycle project that is well under way. Gear the R/C motor to hit it's power/efficiency band in the 40-50mph range and use it for cruising after the acceleration blast from my big motor.

John

 

[recumpence]

My Terminator arrived today!

The build quality of this thing is phenominal! It has a large bearing around the skirt of the can (as all there small motors do). This thing spins very smooth. The magnet gap is less than the AXI. It also uses thinner magnets around the can. The front mounting plate is a removeable part. It uses different screw spacing than my AXI. I may machine a new front plate for it to use the same mounting holes as my AXI.

This is, by far, the most advanced motor I have ever owned.

I will post pics soon.

Matt

 

[Kraeuterbutter]

i saw once one of this Plettis in an very big F3A-like plane..
when looking on the machine i did see some wires, but no motor...
???
were is the motor to power this big and heavy plane ?

it was hidden INSIDE the spinner !!!
like on this picture: http://www.plettenberg-motoren.com/Motoren/zusatz/Predator_37_Spinner.gif

i own two plettis, like them also very much (but mine are considerable smaller (Orbit 15-12 and 20-12)

 

[recumpence]

I have had numerous Plettis myself. I have an Orbit 10-18 on a heli right now.

I cannot wait untill I install and run thins thing.

Matt

 

[Kraeuterbutter]

what efficience do you expect from the Predator ?

i mean, i was looking at this diagramm from Plettenberg for the XTra25:
http://www.plettenberg-motoren.com/german/Motoren/aussen/xtra25/daten.htm

this is only a 450g light motor
and it shows: at 500Watt and 36V only ~81% efficience...
it needs around 900Watt to reach the 86%+ region
and tops out at 87,7% with 1250Watt

your motor is a lot bigger,
constructed for higher powers..
so it will run even worse at that low powers

nevertheless i think that most time you will be in the below 500Watt region ?!?
so most time the motor will probably run with less than 80% efficience..

what do you think?

 

[The7]

i hope im wrong though, if someone can tell how to make two motors the same size and weight that output the same power except one being slow and torquey and the other fast and little torque without gears let us know!

This is 100% right.
No one could change the laws of physics.

These RC motors are of very high rpm.
So their relative power output to weight (mass) ratios are always higher than that of e-bike motors.
However, they will require a higher reduction gear ratio for operation in e-bikes.

 

[eP]

i hope im wrong though, if someone can tell how to make two motors the same size and weight that output the same power except one being slow and torquey and the other fast and little torque without gears let us know!

This is 100% right.
No one could change the laws of physics.

These RC motors are of very high rpm.

What is 100% right ???
Tell us what physics law we need to change to make two the same size and weight motors but different poles count ? ( 10 poles and 36 poles )

Some of them are able to work at very high rpm but som eof them works still at moderate rpm not higher than brushes e-bike geared motors.

 

[Miles]

How do you get more torque by having more poles, Piotrek? It's a genuine question...

 

[eP]

How do you get more torque by having more poles, Piotrek? It's a genuine question...

More poles at the same rpm means more power (higher voltage per tooth/coil) as magnetic flux raising and falling faster.
But more power at the same rpm means more torque.
So more poles as a consequence means more torque. Or i'm still wrong ?

 

[The7]

i hope im wrong though, if someone can tell how to make two motors the same size and weight that output the same power except one being slow and torquey and the other fast and little torque without gears let us know!

This is 100% right.
No one could change the laws of physics.

These RC motors are of very high rpm.

What is 100% right ???

Dirty_d's saying is 100% right.

Let me use an illusatration.
a) A certain RC motor has 500 W output at 10000 rpm.
b) A motor is to have 500 W output at 500 rpm for a certain e-bike.

Is it possible to build (b) which has the same size and same weight as (a)?

 

[The7]

More poles at the same rpm means more power (higher voltage per tooth/coil) as magnetic flux raising and falling faster.
But more power at the same rpm means more torque.
So more poles as a consequence means more torque. Or i'm still wrong ?

In this case:
Ares the physical size of the motor with more poles the same as that with less poles?

 

[eP]

Dirty_d's saying is 100% right.

Let me use an illusatration.
a) A certain RC motor has 500 W output at 10000 rpm.
b) A motor is to have 500 W output at 500 rpm for a certain e-bike.

Is it possible to build (b) which has the same size and same weight as (a)?

It is quite different question !!!
We don't know how advanced is motor A, how many poles has, or how strong magnets, how wide air gap etc.
20 times lower rpm is a really big difference. I think 4-5 times difference is possible under certain conditions.
If motor A is ultra high efficient (96%+), so chances raising, but if A if very ineficient at 10000 rpm and 500W output for the sake of small dimensions so chances sharply falling.

But you didn't aswered my question yet. What physics law ... ?

In this case:
Ares the physical size of the motor with more poles the same as that with less poles?

Yes of course

 

[Miles]

I found this in my Bookmarks:
http://groups.yahoo.com/group/lrk-torquemax/message/9133

 

[dirty_d]

bingo.

 

[The7]

20 times lower rpm is a really big difference. I think 4-5 times difference is possible under certain conditions.

Sorry that your thinking is wrong.

 

[eP]

I found this in my Bookmarks:
http://groups.yahoo.com/group/lrk-torquemax/message/9133

#9131 From: "Stefan Nilsson" <nilton_61@...>
Date: Tue Aug 23, 2005 10:27 pm
Subject: Re: [lrk] Re: Low RPM, maximum torque 50-100rpm nilton_61
Offline
Send Email
Actually torque dpends on number of flux which depends on airgap area and
flux denisity which is limited by statormaterial to about 1,2T max. Other
factors like geometry and bell thickness will lower this further. This means
that you can only increase MMK (AmpereTurns) up to a certain limit.

So a low rpm/high torque motor should have many poles.

If i'm wrong, if the torque depends on flux on airgap area and don't depend on poles count, so why 14 poles motor are builded ???

2 poles equivalents should work much better ( the same air gap area, the same flux density, but less poles means less iron loss - lower motor freq.)

Could anyone explain that ?

Best regards

 

[dirty_d]

it would be the same, notice how he said you could use the same number of poles with the same turns of thicker wire and more amperage, or more turns of the same size wire with the same current, or use twice as many poles with the same wire and number of turns, all would be the same. the 2-pole 3 stator motor would have the same torque, but its undesirable because of the high torque ripple.

as far as comparing motors with each other based on how much torque they make for their speed, we shouldn't be using Kv because you can have a motor wound for a different voltage that produces the same power torque and speed, but has a different Kv constant. we should be using the speed regulation constant, the symbol is Rm and to find it, Rm = R / (Kt * Kv) where R is the winding resistance. Rm will be in units if radians per second per newton meter, if Kt and Kv are in units of Nm/A and V/rads/s. you can use units of rpm/V also thought like this Rm = R / (Kt * (1.0 / Kv)).

for the 225Kv AXI motor guessing at 0.054Ohms while running hot: 0.054 / (0.042 * (1.0 / 225.0)) = 289.29

for the terminator: 0.080 / (0.044 * (1.0 / 215.0)) = 390.91

higher numbers indicate less torque for every percent under no-load speed, it represents the slope of the speed/torque curve.

heres how to use it

if you want to find out at what rpm a certain torque is produced you do this:
10320.0rpm - (390.91 / 10.0Nm) = 6410.9rpm, 10320 is the no load speed for the voltage you are solving for 10320rpm for the terminator at 48V, so the terminator produces 10Nm of torque at 6410.9rpm

to find out what torque is produced at a certain rpm you do this:
(10320rpm - 5000rpm) / 390.91 = 13.6Nm, at 5000rpm, 13.6Nm of torque is produced.

of course you could still calculate torque at speed with Kt and Kv alone, but this just shows that Rm works, Rm shows you the slope of the speed/torque curve regardless of the voltage or power of the motor.

 

[eP]

I give up - i agree with You for RC motors a least.
But ...
lets look for the transverse motors for a moment.
Are their torque, poles count independent too ?
For these babies things are quite different IMHO.
All poles pairs magnetic paths are around the same (single phase) coil.
So more pairs means faster flux slope for the same rpm.
Faster slope means higher voltage for coil. RMS current is still the same.
So power is higher. Higher power (for higher poles pair count) means - higher torque for the same rpm.

Am i wrong here ?
I know it is a little OT here, but it is very interesting case IMO.

many thanks once again for all explanations

Best regards

 

[Miles]

Interesting comparison of two motors by Colin Dedman:
http://tech.groups.yahoo.com/group/power-assist/message/2542