Advise for a project involving a motor and a scooter.

[GabYoung92]

The daily walk from my car, to uni, and back again takes over an hour walking.  I've been using a kick scooter to get there and back which cuts my travel time into a quarter of what it is walking.  Most of the time (70-80%) i'm traveling along a level footpath.

My main goal for it, is to be light and portable so that once I'm at uni I can easily fold it up and carry it, and also powerful enough to possible drive up slight inclines.  The batteries and circuitry will hopefully be installed under the deck in the tubing and the motor ontop of the deck close to the rear wheel.

I'm mainly wanting the motor to overcome friction and drag on the long flat run so that I don't have to keep 'kicking' it back up to speed. It would also be nice to have the ability to 'drive' up a slight incline. I will buying a new scooter to modify.   


My main question: Should I investing upon a Brushless motor or a DC motor?  (I'll probably be using 11.1V Li-Po batteries)

Thanks!

[Simon]

First you need to work out what power you need based on your weight and desired speed, then see what battery power you need and if it is feasible to fit that much battery on.

[T4P]

First of all, DC motors aren't going to cut it, definitely not.

[David_AVD]

Don't some commercial scooters and bikes use DC motors?  What makes a DC motor so unsuitable?

[Rerouter]

equally i have seen them used, in fact they have the added bonus of being able to act like a generator / brake down a hill, but i can only assume its as you can get a bit more power per KG out of a brushless,

[T4P]

They use DC motors because it's cheap at the expense of poor torque and low efficiency plus maintanance cost is higher per km

What he needs here over commercial scooters is torque, brushless can give him what he wants, maybe a 300-500W RC motor?

[G7PSK]

Dc motors will give as much torque as you want, with a series or compound wound motor the the more you try to stall the motor the greater the torque output brush less motors are basically AC motors which will give less torque the nearer  you get to stall.

[GabYoung92]

Yea I don't want to get a pre-made one.  Rather build one that suits me, and doesn't have the cheapest possible setup on it to get it on the shelf's as quick as possible.

Been thinking about brushelss more than DC. I've got a small 3650KV motor, 11V with a 20A controller (~200W) (No idea on brand, its in a hovercraft, Jaycar's GT3740).  Doesn't seem like it would be powerful enough from testing it.  Reading up, I've found that I should be looking for a lower KV which increases the torque...

I've calculated that I need the wheel (d=12cm) to spin at 1000rpm Max (650-700rpm is going to be the usual speed).  So my thought was to gear the motor down to increase the torque, which in turn lets the motor spin faster than the wheel (As under load brushless should be spinning fast I believe).  Max Acceleration force should be ~100N.  So if I gear it at 1:10, torque of the motor needs to be ~150N/m??.  Total weight of the scooter + myself should only be around 80kg

Problem is that I don't know how to calculate the torque as I don't have enough information about the motors.    Which leads to the reason for me creating the thread.  Wanted others opinions on this.  Thanks

[Rerouter]

newton meters is the unit of torque, and at 100 newtons of acceleration that is 1.25m/s/s of acceleration, which in my opinion is more than just cruising 
(Newtons = Mass{Kg} * Acceleration{Meters per second per second})
(Torque = Newtons * meters)

to make it a bit easier to process, 100N of acceleration would be accelerating by 5Km/h every second,
(m/s * 3.6 = Km/h)

and now for your gearing, you will have some issues there, as RPM gets scales along with torque, so at a gearing of 10:1 with your wheel whizzing along at 700RPM, would require your motor to be running at 7000RPM, tad high....

lets assume a more practical 3:1, as most motors happily run into the 2-3000 range, this is where that torque equation once again matters, to get 100N to a point 0.06m (radius) from the axel of the wheel, requires 6N/m of torque, if direct driven,  (newtons * meters)

hmm, looks like you wouldnt really even need the gearing, , ((KW = torque * 2 * pi* wheel rpm)/60000),
as that comes fairly nice value of 500W, (440W actually) and this value scales linearly with your acceleration so if you want to use a 200W motor instead, thats only 0.5m/s/s or 1.8Km/h/s
but just to make it convinient for you, no matter what your gearing ratio the effective power must remain the same, only the torque required of the motor is lowered, so 3:1 would only require 2N/m of force and the motor would spin along at 2100RPM

note that this is assuming power at the rotor of the motor, not power fed into it, as no motor is 100% efficient,

[GabYoung92]

Rerouter, Yea that sort of torque sounds more reasonable.  I think I know how i stuffed calculations up.

I was looking at having somewhere around 1m/s2 acceleration to possibility get up hills and to speed up along flats.  I want to design something just over what i'm looking for so its not under full load.  Isn't 7000rpm for brushless reasonable?  I was going to gear it so I could get a cheaper motor and still provide the torque.  Yep all assuming 100% efficiency, which is also why I'm designing higher to compensate for this. 

Ok, now I need an opinion from someone who has a good idea with brushless motors, as to what I should be looking for. 

 

[Rerouter]

well if you really wanted to cover all bases, we could always work out what slope of incline will the motor stall out at,
which is very simple, you have 100N of force being applied at the wheel, and while stalled your motor is only fighting gravity,

so 100N / gravity (9.806) comes to about 10Kg of mass, so that means you motor can only handle a slope that applies 1/8th of its mass vertically, which means we use Sin^-1(1/8)
this comes to roughly an incline of 7.2 degrees,

edit: ok big mistake, its tan (opposite over adjacent) so 10/80, which comes to an angle of 12.5 degrees,
(tan^-1(1/8) to solve for the angle)
edit2: ok now i am just showing how badly i get messed up, a tan of 7.2 degrees seems to line up, so i'm calling it a night for math,
so hope that suits you perfectly

[GabYoung92]

haha long day?

7 degree incline doesn't sound too bad.  now to get a motor of that strength and do some testing to see what batteries & controller. 

[Smokey]

One of the big reasons to decide between AC or DC motors is voltage vs current.  DC motors can't take the high voltages because the brushes start arcing.  Because of that, they need to run at higher currents in order to have the same wattage rating. 
AC motors on the other hand are just coils of wire so they can run at much higher voltages and require less current for a given wattage rating.  There is a reason electric cars run battery banks in the 300V range.

For 11V, go DC motor.  Even better, go DC motor at like 36V.  And the drive electronics are WAY easier to do for DC.

[GabYoung92]

Not sure if I will be able to fit 36V of batteries under the deck... 

The electronics don't worry me at all
I've got a controller here already which i'm thinking about modding to drive some external FETs to cope with the higher current of a larger brushles motor.  And creating a circuit to drive the controller is no issues either

[kathy45]

I'm not sure, but I have the impression that project involving a motor is larger than a lot of those scooter motors.
One company I'd have about a project involving a motor would be determining a replacement if I needed one. It would be kind of nice to be able to get 4 of the same motor and to be able to repair one if needed.

[cs.dk]

Maybe tatus1969 project could inspire you? https://www.kicksurfer.de/index.php/kicksurfer/