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Jul 17, 2014, 02:21 AM
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trying to find my dream motor for 1:1 gear ratio on skateboard


not a typical application.

Lots of different low kv outrunner motors in the 50 or60mm size are used for electric skateboards with 120 or 150amp esc.
They are typically in the 300kv range, and then a 2:1 gear ratio is used to the wheel to give it enough torque I'm trying to put the motor in the wheel, so there'll be a 1:1 gear ratio (dont want to add gearing since it seems a complication and takes space) so I need an even lower kv and higher torque

Seemingly I'll need a Y wound motor with lots of arms. Gimbal motors I imagine will be too slow or not enough power. I've got an 8s controller and a couple tacon160 motors I was planning to try with but think their 245kv will give me a big top speed and not enough torque to do hills or maybe even get up to speed. Maybe the motor would burn out?
I'm no guru...I'm a plug n play wana-be really and would like to find such a motor ready to go instead of rewinding. I'm seeing epoxy on my motor wires that I don't want to mess with.
So looking for the lowest kv outrunner I guess
Last edited by Hummina; Jul 17, 2014 at 02:55 AM.
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Jul 17, 2014, 04:14 AM
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Fourdan's Avatar
Hi Hummina

Dualsky XM6360EA-12 Kv 187 rpm/V , 615g
Hacker A60-L-18 Kv 149 rpm/V but 910g

Another solution is to wind a naked Scorpion kit S5330
look here
https://www.rcgroups.com/forums/show....php?t=1207034
Are you interested by the rewound S5330 ? (PM if yes)
Louis
Latest blog entry: Scorpion Calc
Jul 17, 2014, 04:26 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
See also
www.endless-sphere.com/forums
-> Electric Scooters and Motorcycles

Vriendelijke groeten Ron
Jul 17, 2014, 12:00 PM
Registered User
A typical motor has about 2 N of force for each cm² of air gap area. Assuming your motor has an air gap that is 10cm diameter and 2 cm wide, that would be 62 cm² of air gap. The maximum air gap force would be about 120N. Assuming the wheel is 15cm the forward force of the wheel would be 120/1.5 or 80N (17 pounds).

A motor with a stator (or rotor) that is 10cm diameter and 2 cm wide is a big motor and a 15 cm wheel is large for a skateboard.

The number of 2 N/cm² is the amount of force generated by the magnetic field of normal magnets.

You can rewind motors till your fingers bleed and never beat 2N/cm².
Jul 18, 2014, 07:46 AM
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Thread OP
Humm. Mjas I don't understand what ur saying but u feel there is a motor out there that will give me the power I need (possibly two) and be slim enough to to not be rolling on the ground.

Fourdan's motor sounds like a possibility although likely expensive.
I'd like to find something I could wind a a apposed to rewind...but as I said I'd rather have it already done since there's more to it than I know.
Jul 19, 2014, 04:21 PM
Registered User
The drawing should help understanding how the force was calculated.

On one side of the air gap are the permanent magnets, good magnets have a field of about 1 tesla. On the other side is the stator windings. The strength of the stator winding's field (H field) is the number of amp-turns. If you use thin wire you get more turns but fewer amps, use thick wire and the result is more amps but fewer turns. In the end it always results in the same number of amp-turns. The field strength cannot be changed by rewinding the stator. (assuming a good winding job)

Air cooled electro magnets have a field strength of about 1 tesla so the result is an air gap force of 2 N/cm².

It is possible to get higher air gap forces but not with your intended design.
Jul 19, 2014, 04:37 PM
Registered User
The ceramic materials used to make permanent magnets have much lower permeability then iron. Therefore motors which use electro magnets for both the stator and rotor have stronger magnetic fields.

The Tesla electric cars uses two - 3 phase induction motors. These have an all iron magnetic circuit and have enough torque so a single speed transmission is used. They do not need a lower gear to start from stop.

I have considered making a 3 phase induction motor by replacing the rotor in an automotive alternator with a squirrel cage rotor. Ceiling fans have large diameter squirrel cages and it might be possible to find a rotor that would fit.

Also many ceiling fans are outrunners with an outrunner, squirrel cage rotor. In this case the motor would make a good wheel motor.

Induction motors are not synchronous motors so the ESC would be simple and the motor would have very good starting torque.
Jul 19, 2014, 05:20 PM
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Fourdan's Avatar
Quote:
Originally Posted by mjsas
The drawing should help understanding how the force was calculated.

On one side of the air gap are the permanent magnets, good magnets have a field of about 1 tesla. On the other side is the stator windings. The strength of the stator winding's field (H field) is the number of amp-turns. If you use thin wire you get more turns but fewer amps, use thick wire and the result is more amps but fewer turns. In the end it always results in the same number of amp-turns. The field strength cannot be changed by rewinding the stator. (assuming a good winding job)

Air cooled electro magnets have a field strength of about 1 tesla so the result is an air gap force of 2 N/cm².

It is possible to get higher air gap forces but not with your intended design.
Hi Mjsas
Maybe the thickness of neodyme magnets and the gap 0.4 ... 0.5 mm (of airgap)
are also acting on the force 2 N/cm2
Louis
Latest blog entry: Scorpion Calc
Jul 20, 2014, 01:36 AM
Registered User
Hi,
Mjsas,i calculate for normal motors wit 2,5 N per square cm and 4 N for good cooled . Our old trollybus motors do this numbers . The speedmodelairplane motors reach up to 8 N . Our best motors can do peak force between 14-16 N .
Liquide cooling required ,direct of the coil.
Jul 20, 2014, 09:23 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Christian, I take it "... Our best motors can do peak force between 14-16N ... " refers to www.magnetmotor.com, not to the motors we use in RC?

Vriendelijke groeten Ron
Jul 21, 2014, 12:56 AM
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Thread OP
Quote:
Originally Posted by mjsas
A typical motor has about 2 N of force for each cm² of air gap area. Assuming your motor has an air gap that is 10cm diameter and 2 cm wide, that would be 62 cm² of air gap. The maximum air gap force would be about 120N. Assuming the wheel is 15cm the forward force of the wheel would be 120/1.5 or 80N (17 pounds).

A motor with a stator (or rotor) that is 10cm diameter and 2 cm wide is a big motor and a 15 cm wheel is large for a skateboard.

The number of 2 N/cm² is the amount of force generated by the magnetic field of normal magnets.

You can rewind motors till your fingers bleed and never beat 2N/cm².

On an electric skateboard people get more than enough power from two 50mm motors or one 60mm motor like the Tacon 160 motor I have. I haven't done it yet because I've been stuck on one thing or another (the cad cnc machine at techshop here in california is harder to get doing what I want than I'd like and time on it is rare). Anyway, to do it they add roughly a 2:1 gear to give it more torque and with 22volts and a controller than can handle up to 120 or 150amps they're flying up hills and hitting ridiculous speeds on flats (with less gearing) and many of them say motors and things aren't even getting hot.

hopefully my pic will show and you can imagine how I plan to make this. the tube in the center will have bearings and bolt onto the outside of the motor can, and slide onto the skateboard truck axle. And the stator is fastened on the skateboard truck hanger. the bearings tolerances hopefully will be tight enough to hold the can in the right relation to the stator and also the piece sits inside the hub spokes of the somewhat large skateboard wheel. So no gearing. What's going to happen?
I'm making lots of assumptions probably but figure with two of them and the right low kv (dont know what would be ideal) I'll be able to avoid too much heat.
What other obstacles do you think I'll encounter or how better could I make this efficient. I've been looking for low kv 50mm outrunners and found some on alibaba.com if I buy 100. I could rewind it Y I guess but for all I see in motor descriptions it might be Y already. I should shoot for more poles maybe. Gimbal motors seem ideal in theory but then I've never actually seen one and info and them is very small.
I'm going to look up the induction motors and squirrel cage rotors. In a sentence how are the esc different and wouldn't I then need to convert ac to dc?
Jul 21, 2014, 01:54 AM
I am a nice guy! Really!
An adjustable speed induction motor drive will typically have a rectifier on the front end to create a DC bus off of the AC main lines (usually three phase except in the case of motors of about 1/2HP or less). This DC voltage is then converted to ac three phase using IGBT's. There is a controller that pulses the transistors to adjust the output frequency of the drive. The pulses are controlled such that they create a close approximation of a sinusoidal waveform. You will have a hard time finding an induction motor that is small enough to use on a skateboard that will be able to develop the power required, but don't let that stop you from looking.
Jul 21, 2014, 12:59 PM
Registered User
Quote:
Originally Posted by Mike Dubovsky
An adjustable speed induction motor drive will typically have a rectifier on the front end to create a DC bus off of the AC main lines (deleted to save space)

You will have a hard time finding an induction motor that is small enough to use on a skateboard that will be able to develop the power required, but don't let that stop you from looking.
I copied a set of torque curves from Wikipedia for typical induction motors. The caption for the picture was:

"Speed-torque curves for four induction motor types: A) Single-phase, B) Polyphase cage, C) Polyphase cage deep bar, D) Polyphase double cage"

The torque curve for motor D is more or less flat from stall to full speed. For an EV you would only need a fixed frequency ESC with variable voltage which would be a simple PWM. In fact a variable frequency controller would not work very well as the throttle on an EV should control torque, not RPM.

I don't know if a small induction motor could power a skateboard but the following video shows that an induction motor can kick butt. Note that the Tesla has a single speed transmission and pulled the Corvette off the line with awesome torque.

http://www.dragtimes.com/blog/2013-t...orvette-c7-z51

The rule-of-thumb number of 2N/cm² I used is for cheap, BLDC motors. Of course if you liquid cool and micro machine to a very small air gap the number can be very large. In any case the sample calculation shows how to size the motor needed for a direct drive wheel motor. If you don't like 2N/cm², use whatever number you can pay for. Even a crappy BLDC motor made from a junk auto alternator can make 4N/cm².
Jul 21, 2014, 01:52 PM
Registered User
Hi ,
@Ron,
up to 8 N in the speedplanes are kit motors from Scorpion ,no secret and no spezial micro maschine tooling . Plane Neo magnets . For higher force flux wedge are helpfull and for the highest force simpli use a Halbach magnet system . But the most needed thing is a sensored esc with inteligent program .
@Hummina,
Have you ever thought about a singel wheel at the back . I have build a scatbord this way as you can use a directdrive train from bicycles.
Jul 21, 2014, 02:12 PM
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Thread OP
Have you ever thought about a singel wheel at the back . I have build a scatbord this way as you can use a directdrive train from bicycles.[/QUOTE]

I've seen something like that and it doesn't appeal to me. I'm into this projects for two reasons: the power to weight of lithium batteries and outrunners coupled with the efficiency (or at least light weight and small size) of a skateboard gives the vehicle a practicality you can't get with a bigger vehicle. And my other goal is to try to make its electronics n motor as hidden as possible. And I also like the mechanical simplicity of the on-axle motor...seems at least mechanically more efficient and quiet. And people think it's super cool!!! Me too.

While I'm really interested, and getting more interested, in the electronics and details you guys have given me I'm way over my head. The only waves I was thinking about were sine because they supposedly produce a quieter motor and that's a big goal for me.
I'm going to get bearings in an hour and then haunt the welding station at techshop here in San Fran...looking for someone to weld a plate on the hanger too into the stator. I'd really like to do a keyway with a broaching tool ( a slot in the stator and another on the skate truck hanger and line them up n bang in a soft metal...but am second guessing it thinking cutting into the center of the stator'a iron will lower the motors ability. But it wouldn't need to lose much material and I think it could handle a lot of twisting force ( similar to a bicycle free hub spline but those are only made of aluminum and pretty thin)

I'll be looking up all the big words you guys threw me. I appreciate it and will be getting up to speed soon.


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