Using RC motors on E-bikes [Archive]

[dirty_d]

Miles, i guess "speed regulation constant, Rm" and "motor constant, Km" are the same thing they both use the same formula.

eP, i don't really understand how those motors work, are there any more links that have better pictures or something to help explain how they work?

 

[Miles]

dirty_d,

Isn't it confusing to use Rm? That's more frequently used as a term for the motor resistance, no?

I'm used to seeing motor constant as: Km = Kt / SQRT (Rm)

 

[eP]

eP, i don't really understand how those motors work, are there any more links that have better pictures or something to help explain how they work?

Did you seen this document ?
http://www.eocycle.com/EN/technology/ICEM2006 PAPER412 DUBOIS.pdf

At fig 16 you can see how such 3 phase machine looks like.
Each phase has own stator torus like. Each torus is segmented for teeth. Inside all of these teeth is copper coil.
Only one coil for each phase/"torus" structure.
So lets analyse how one phase segment work.

If you still have no idea how this machine work i will try to explain it for you, or maybe Miles or other guy could do that better than me.
Very important: you should keep in mind each segment works like one phase synchonous machine.

Next one doc (maybe better to understand):
http://www.ansoft.com/news/articles/design_of_tranverse_flux_machine.pdf

Best regards

 

[Kraeuterbutter]

speaking here 26pages long about rc-motors in ebike-use i forgot to mention:
there are already 2 sets sold with "RC-Motors" (or lets say: with motors which are also used in RC-hobbie)

1.)
Akkubike: http://www.akkubike.com/D-Home.htm

its a cheap set for do-it-and-convert-your-bike yourself
the motor is something like a Speed700 used in RC-hobbies (sold for 15 Euro)

2.)
SRam Sparc

there are two version:
# the old one with gears (i have)
# and a newer version with belts (quieter)and higher voltage (i think still the same motors)

pictures of my version:
at lot more pictures: http://www.kraeuterbutter.at/Bilder2/SRam%20Sparc/

its redicoulouse when you see the price for this set: 700Euro and more !!!!
with "high-tec-motors"....
haha.. as said, this motors are sold for 15 Euro in the rc-hobby-shop..
they are Mabuchi 550

just for comparing:

left: its a Titan (mabuchi 550) from my Traxxas E-Maxx
(video of that: http://www.kraeuterbutter.at/Videos2/E_Maxx_WOT_highqual_and_music.wmv )
two motors of this power that 4,5kg monstertruck up to 45km/h
in the sram-sparc two motors of this are used to go 25km/h (with an 75kg heavy biker + 20kg bike)

the cool thing on the sparc:
it has an included 5gear-sram-hub !
so: the whole thing is 2,5kg heavy which makes it VERY light compared to normal hub-motors (~1-1,5kg is alone the 5gear-hub)
the motor itselfs: 440g both together

another point: the electronic also looks not very complicated or even expensive..

all this brought me to a idea:
replacing the two brushed Mabuchi motors by one Lehner brushless-motor (~5 times as powerfull)

then it would also be nesseccary to replace the 12V 20A electronics by an brushless-controller:
from space absolutely no problem !

25V controller, 80A rating

just to compare the efficiences:
Mabuchi:

hey.. with 71% not that bad..
the Lehner on the pictures has 88% but that does not sound THAT far away..

BUT: look WERE the brushed motor has its 70%: at 6A current, thats only 50Watt output !
maximum Power of the brushed is 110Watt.. but look at the efficience... down under 50% !!!

now compare this (Efficience curve) with the lehner brushless:

http://www.lehner-motoren.de/diagramm/1930-18.11v_liste.txt

i think here you can see best, why just comparing peak efficience is not usefull...
its the efficience-band over variouse power-levels that makes a brushlessmotor so superior over (cheap) brushed onces

 

[eP]

1.)
Akkubike: http://www.akkubike.com/D-Home.htm

its a cheap set for do-it-and-convert-your-bike yourself
the motor is something like a Speed700 used in RC-hobbies (sold for 15 Euro)

Nice kit

How much is this thin chain and big thin wheel ?

 

[dirty_d]

700 euro, that is absolutely ridiculous.

 

[Miles]

How much is this thin chain and big thin wheel ?

You can get #25 HD chain and sprockets from 9t up to 125t. It's what the small scooters use.

 

[dirty_d]

where can you get a 125 tooth one? the largest ive ever seen was 90

 

[Miles]

where can you get a 125 tooth one? the largest ive ever seen was 90

In the US, SDP have 125t. https://sdp-si.com/eStore/

9t to 125t will give you almost 14:1

 

[dirty_d]

$57 each, how do they justify that price, its not in the material, and a cnc machine could cut that piece out in no time.

 

[Miles]

Maybe these people would give us something closer to a finished product?
http://www.ablproducts.com/

 

[John in CR]

Maybe these people would give us something closer to a finished product?
http://www.ablproducts.com/

Shouldn't we think smaller and lighter in the form of planetary gears? Then we keep the weight down and get rid of that pesky chain or belt issues. While I've mentioned the dual motor friction drive with my wheelchair motors, I haven't talked about the aluminum housed gearboxes that came with them. They're a 24:1 reduction and I attached a 15lb 5hp powerpack motor that does almost 20krpms. That was before I heard about these RC motors, and it's just waiting for an E-moped platform to go on (too many projects, too little time). While the silent worm drive gearbox probably won't hold up at 5-6 times the design rpm and power, if we accept a bit more sound then planetary gears will work. If someone comes up with a group buy for appropriate gears, count me in.

John

 

[Miles]

John,

How are you proposing to link to the rear wheel, then....?

24:1 in a single stage would be a worm gear, no? So, pretty inefficient...

 

[johnrobholmes]

I just wanted to poke my head in here. Too much to read all at once, but I am using an RC motor with a mini bike tranny into final belt reduction. It works well, but the chain in the tranny is very noisy. I have a friend that can cut gears, maybe I could have him fab up something? What kind of output shaft would be best used in the electric bike world? We could certainly do a small production run if others wanted one too. The gears would all be hardened steel.

 

[johnrobholmes]

To add to the discussion about motor construction and power vs speed vs torque- just keep it simple in your head and don't over think it.

For example, we have a motor that weighs 6 ounces that can produce 500w continuous. The RPM and torque of the motor is irrelevant, and consider all other design aspects to be constant (including the switching frequency that it is driven). It can produce 500w continuous because of the efficiency and ability to dissipate heat. If it is constructed two magnetic pole/ 3 slot design it will be best suited for high speed use because of the large angular shaft rotation per phase switch (generally 60 degrees per switch, which makes 6 switches per revolution). If the construction is changed to a 14 mag pole/12 slot design the shaft rotation slows to 8.57 degrees per switch and takes 42 switches per revolution. The motor is now 7 times slower but can still produce the same power.

The only difference between the two motors is the final geardown. To operate at the same power and wheel speed, the two pole motor will need 7 times the geardown that the 14 pole motor needs. The switching frequency will be the same. The fast motor will be capable of 7x the shaft speed. The slow motor will be capable of 7x the shaft torque. Both still make the same power.

 

[Miles]

If the construction is changed to a 14 mag pole/12 slot design the shaft rotation slows to 8.57 degrees per switch and takes 42 switches per revolution. The motor is now 7 times slower but can still produce the same power.

To do that it would need to produce 7 times the original torque, no?

I can't see how, in this respect, changing the number of poles is any different to changing the number of turns in the winding.

Did you miss that we were keeping the mass as a constant, John?

 

[johnrobholmes]

To do that it would need to produce 7 times the torque, no?

I can't see how, in this respect, changing the number of poles is any different to changing the number of turns in the winding.

Correct. Since the shaft is moving 7x less distance per phase switch, the 14pole/12slot motor will produce 7x the torque at 1/7 the speed as compared to the 2pole/3slot motor. Basic physics of a torque arm really. Use a 1 foot wrench to twist a bolt vs a 7 foot wrench. You get more torque with the 7 foot lever, and with a constant hand speed the bolt turns 7x slower with the same net work done overall.




Changing wind is a different deal, but can accomplish the same task within boundaries of a motor design. You still have the same power and torque (and max rpm) for a 6 ounce motor, but the voltage and amperage change to get said 500w of power. While a higher turn motor (slower KV) will have a higher kT (torque per amp), the amp draw also goes down because of increased phase resistance when voltage and gearing is held constant . To increase power back said 500w you have to gear the motor taller (more torque) OR volt back up to desired final speed before the kv was changed. Again, motor power has not changed- only the voltage and amperage. Torque will be relational to the shaft speed: wheelspeed ratio at a fixed voltage.

Another way to state this is that if motor design and vehicle rollout is held constant (two pole/three slot etc.) you will need to hit a certain RPM for a power level. As example we have a vehicle that goes 10mph and takes 500w and a motor shaft speed of 5,000rpm. Our 100kv motor takes 50 volts to get there (10 amps) but our rewound 50kv motor would take 100 volts (5 amps) to get there. Both motors still produce the same power and torque, just at different voltages and amperages.

Clear as mud?

edit: I am assuming fixed mass of motor as well. Fixed copper density and fill, fixed magnetic power, flux gap, lamination thickness, etc.

 

[Miles]

Correct. Since the shaft is moving 7x less distance per phase switch, the 14pole/12slot motor will produce 7x the torque at 1/7 the speed as compared to the 2pole/3slot motor. Basic physics of a torque arm really. Use a 1 foot wrench to twist a bolt vs a 7 foot wrench. You get more torque with the 7 foot lever, and with a constant hand speed the bolt turns 7x slower with the same net work done overall.

If you were correct, it would be the electrical equivalent of mechanical gearing......

How are you getting your longer lever? Surely it's the same lever moving more slowly?

 

[johnrobholmes]

It is the equivalent of mechanical gearing, but don't say that to an engineer.

Really it isn't a longer lever, it is a shorter distance traveled for the same work if we take into account the motor speed and final gearing differences. The 14 pole travels 7x slower and needs 1/7 the geardown of the 2 pole motor. Therefore to produce the same power it must produce 7x the torque to keep final wheelspeed the same.


Basically a motor can be built for torque or speed that is dependent on the shaft rotation per phase switch. A shorter movement per switch yields more torque at a sacrifice of top speed without further design changes, as a motor can only be switched so fast before switching losses begin to produce more heat than the motor can dissipate.

 

[eP]

The 14 pole travels 7x slower and needs 1/7 the geardown of the 2 pole motor. Therefore to produce the same power it must produce 7x the torque to keep final wheelspeed the same.

Hi johnrobholmes

As a devil's advocate i'm asking some questions:
Who (or what law of physics) told you it must produce the same power ?
Maybe 14 poles motor needs 7 times faster coils energizing to reach the same power ?
Which law of physics telling us the latter cannot be the truth ?

Best regards

 

[dirty_d]

i don't think more poles automatically increases torque, torque is proportional to current * turns, or surface area of the stator poles, so you can either have a lot of turns of high resistance wire and low current for X torque, or small number of low resistance wire with high current for X torque, the first one will produce more backemf since there are more turns and lower the rpms the power output will be low, the second one will produce less backemf so higher rpms and more power. if you want more torque and the same power as the second motor, then you need more turns of even thicker wire to keep the resistance the same but increase torque and increase backemf for lower rpms.

for the 2 pole 3 slot motor, you can put lots of turns of thick wire on the 3 teeth. because there is a lot of room between each tooth.

for the 14 pole 12 slot motor, you put fewer turns of thick wire on each tooth, the number of turns add up with all the poles in the phase and the backemf adds up since they are in series. the surface area of the poles is still the same just spread out more evenly around the circumference.

so it seems to me more poles doesn't equal more torque, what it does equal is less torque ripple and faster commutation.

 

[John in CR]

It is the equivalent of mechanical gearing, but don't say that to an engineer.

Really it isn't a longer lever, it is a shorter distance traveled for the same work if we take into account the motor speed and final gearing differences. The 14 pole travels 7x slower and needs 1/7 the geardown of the 2 pole motor. Therefore to produce the same power it must produce 7x the torque to keep final wheelspeed the same.


Basically a motor can be built for torque or speed that is dependent on the shaft rotation per phase switch. A shorter movement per switch yields more torque at a sacrifice of top speed without further design changes, as a motor can only be switched so fast before switching losses begin to produce more heat than the motor can dissipate.

Looks like 7 times the levers to me with 1/7 the distance to travel with the same power input so 7 times the torque.

Now just so I can apply this thought process to my Erato motor, the pole count changes from 18 to 6 with the mechanical switch in the hub going to high gear, and obviously the magnet count is constant. Is the speed and torque relationship a simple 3:1 ?

 

[dirty_d]

john, it seems that only using 1/3rd of the stator windings will lower the resistance by 3, so you would get 3X the current, but since there is 1/3rd the active stator poles, you get the same torque, the backemf would also be 1/3rd its original value, so the rpms can increase much higher, the motor seems to actually be more powerful in this arrangement. power is proportional to resistance for a given voltage.

 

[johnrobholmes]

As a devil's advocate i'm asking some questions:
Who (or what physics law) told you it must produce the same power ?
Maybe 14 poles motor need 7 times faster coils energizing to reach the same power ?
Which physics law telling us the latter cannot be the truth ?

Best regards

Great discussion here.

We will assume fixed motor mass, final wheelspeed, vehicle wattage, and voltage.

It must produce the same power based on the formula (torque * RPM * 1/5250 = hp) (one source http://www.engineersedge.com/motors/rotating_horsepower_equation.htm) I am assuming a specified final wheel speed (500w of power), so if the RPM of the motor changes the torque produced must also change with the final gearing to keep power and wheel speed constant. If gearing is not changed then the slower motor will make the vehicle run slower, and our total wattage will decrease- thus we must change vehicle gearing to keep wattage the same. This is also true for changing the wind of a motor when voltage is not changed to suit.

If a 14 pole motor is running at the same shaft speed as the 2 pole motor and producing said 500w at a fixed vehicle speed, it will be switching 7x faster than the 2 pole motor because of design. For both motors to run the same shaft speed both motors will have the same KV, produce the same torque, and produce the same power. Now we would need to decide what the shaft speed needs to be, and with that information one motor may run much more efficiently because of switching losses, copper losses, or iron losses. Generally a motor designed to run with 7x the switching rate per revolution will run more efficiently at 1/7 the speed if all else is unchanged, mostly because of switching and iron losses.

 

[John in CR]

john, it seems that only using 1/3rd of the stator windings will lower the resistance by 3, so you would get 3X the current, but since there is 1/3rd the active stator poles, you get the same torque, the backemf would also be 1/3rd its original value, so the rpms can increase much higher, the motor seems to actually be more powerful in this arrangement. power is proportional to resistance for a given voltage.

I'm pretty sure the poles get added in parallel, so they can handle more current in low gear. The ding with their original 2kw motor (the one I got) was that people would forget to switch gears and take off in high gear which burned the windings.