Power needed to keep a 200lb object in motion

[Jeet Minhas]

Hi guys,

So I'm building a small electric skateboard, and I'm having trouble understanding how much power I need to keep the board in motion. The total weight of the board is 30Lb and I am about 170Lb. I want to maintain a speed of 15Mph when not kicking my leg. This is to be assumed on a 1 degree incline. I have two motors that are 300 watts each and have a free speed of 18700rpm on a 12v 30A circuit. The torque before gearing is .71 Nm. I am hoping to do a 16:1 gear reduction connected to wheels 80-110mm. What size wheel will work best, and will this gearing be enough?

 

[mfb]

You'll need the rolling resistance of the skateboard. I guess this will be more important than the 1 degree incline.
Then you can just compare torque from the motor with torque from rolling resistance, the incline and maybe other losses to friction.

 

[sophiecentaur]

There is scope for experiment here. If you get hold of a force meter and a gps app for a smart phone, you can tow the board with a bicycle and measure the force needed and the speed. That will tell you the power needed and whether the motor / battery combination is likely to be enough on the surface you do the test on. You could do it with a car but that sounds potentially too dangerous.

 

[CWatters]

I have two motors that are 300 watts each and have a free speed of 18700rpm on a 12v 30A circuit. The torque before gearing is .71 Nm.

Is that the stall torque? You really need a bit more data for the motors like a power curve. If that's not available you could assume that max power occurs at roughly half the no-load rpm and half the stall torque...

18700 rpm = 1960 rads/s
half that is 980 rads/s

Stall Torque = 0.71Nm
half that is 0.35 Nm

As a check...

Power = Torque * angular velocity
= 0.35 * 980 = 343 W

..which is close enough to 300W

So let's say at max power your motor is doing 980 rads/s = 9300 rpm and the torque is 0.35Nm and do some more sums..

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First I think you have a problem. With a 16:1 gearbox you will struggle to do 15mph because it will be limited by the no-load speed of the motor..

Lets calculate the top speed using 100mm wheels assuming there is no rolling resistance..

No load speed is 18700rpm
18700 rpm / 16 = 1169rpm at the wheels
100mm diameter => 0.314m circumference
Speed = circumference * rpm
= 0.314 * 1169 = 367 meters/min or 6m/s

That's about 13 mph but remember at 18700 rpm the motor is delivering no power so in practice it won't go that fast.

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Lets suppose that 300W is enough power to go 15mph with the right gearbox (you need to confirm the power required to do 15mph by measuring it as Sophiecentaur suggested). Then we can work out the gearing needed...

15mph = 6.7 m/s = 402 meters/min
100mm diameter => 0.314m circumference
wheel rpm = speed/circumference = 402/0.314 = 1280 rpm

From above we estimated max power occurs at about 9300rpm so the ratio needed is around

9300/1280 = 7.2 call it 7.

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So in summary:

Ideally you need a power curve for the motor.
You need to measure the power required to achieve 15mph
I think you are likely to need a gearbox ratio nearer 7 or 8 than 16 (assuming 300W is enough to do 15mph). I suspect that with a 16:1 ratio it might only go 7-8mph.
Some experimenting with the gear ratio will be required so perhaps design it so it can be adjusted?

Is the motor rated at 300W max for brief periods or continuously?

 

[Jeet Minhas]

Is that the stall torque? You really need a bit more data for the motors like a power curve. If that's not available you could assume that max power occurs at roughly half the no-load rpm and half the stall torque...

18700 rpm = 1960 rads/s
half that is 980 rads/s

Stall Torque = 0.71Nm
half that is 0.35 Nm

As a check...

Power = Torque * angular velocity
= 0.35 * 980 = 343 W

..which is close enough to 300W

So let's say at max power your motor is doing 980 rads/s = 9300 rpm and the torque is 0.35Nm and do some more sums..

----------------------------------------------------------------------------------

First I think you have a problem. With a 16:1 gearbox you will struggle to do 15mph because it will be limited by the no-load speed of the motor..

Lets calculate the top speed using 100mm wheels assuming there is no rolling resistance..

No load speed is 18700rpm
18700 rpm / 16 = 1169rpm at the wheels
100mm diameter => 0.314m circumference
Speed = circumference * rpm
= 0.314 * 1169 = 367 meters/min or 6m/s

That's about 13 mph but remember at 18700 rpm the motor is delivering no power so in practice it won't go that fast.

----------------------------------------------------------------------------------

Lets suppose that 300W is enough power to go 15mph with the right gearbox (you need to confirm the power required to do 15mph by measuring it as Sophiecentaur suggested). Then we can work out the gearing needed...

15mph = 6.7 m/s = 402 meters/min
100mm diameter => 0.314m circumference
wheel rpm = speed/circumference = 402/0.314 = 1280 rpm

From above we estimated max power occurs at about 9300rpm so the ratio needed is around

9300/1280 = 7.2 call it 7.

----------------------------------------------------------------------------------

So in summary:

Ideally you need a power curve for the motor.
You need to measure the power required to achieve 15mph
I think you are likely to need a gearbox ratio nearer 7 or 8 than 16 (assuming 300W is enough to do 15mph). I suspect that with a 16:1 ratio it might only go 7-8mph.
Some experimenting with the gear ratio will be required so perhaps design it so it can be adjusted?

Is the motor rated at 300W max for brief periods or continuously?

the motors runs 300W continuously at 30A. and I have 2 motors totaling to 600W +/-5% (the 5% as per manufacture specs). My code is made to run the motors in sequence. ie- one runs at max power and .5 seconds the second powers on and the other coasts. so for a .5 of a second the total output is 600W.

 

[CWatters]

the motors runs 300W continuously at 30A. and I have 2 motors totaling to 600W +/-5% (the 5% as per manufacture specs). My code is made to run the motors in sequence. ie- one runs at max power and .5 seconds the second powers on and the other coasts. so for a .5 of a second the total output is 600W.

In general it's the load that determines how much power is drawn by a motor so I'm not quite sure what effect your code will have. I think it would be better to just have both motors powered all the time and use mark space modulation to control the speed. Add a current limit to control the max power?

 

[Khashishi]

The power is plenty as long as you have the gears right. For comparison, a fit bicyclist puts out about 200W.

 

[CWatters]

Your might find you need your software to limit the acceleration?

 

[Jeet Minhas]

Your might find you need your software to limit the acceleration?

I plan to install torque limiters so there is no counter rotation or burnout of the motors.

 

[CWatters]

I was thinking the board might accelerate too quickly to stay on it?

 

[gleem]

And how are you powering the motors?

 

[Jeet Minhas]

And how are you powering the motors?

i have a 14.7 LiPoFe battery.
i plan to take care of the acceleration by using a PID with encoder feed back, so that the voltage supplied controls the output of the motors.

 

[gleem]

i have a 14.7 LiPoFe battery

Capacity, Amp-Hours?

 

[Jeet Minhas]

40Ah running with a constant discharge of 30A

 

[gleem]

40Ah running with a constant discharge of 30A

And it is spec'd to handle this discharge rate?

 

[Jeet Minhas]

yes

 

[arydberg]

Sounds like you are well on your way to doing it so do it. In theory with a perfect skateboard (no losses) it takes zero power to maintain any speed. It does of course take power to accelerate you to any speed. The power consumption comes from air resistance, the losses in the wheels and in the transmission (on a level surface). My best guess is that the bigger the wheels are the less loss. and the harder they are the less loss. (That is why bicycle tires are high pressure) The bicycle idea is good in that you can measure the force required to maintain a given velocity.

 

[rons99]

I think need the wheel diameter to get the torque n speed of the output motor after gearing/rpm on the wheel shaft for max speed 15mph, how many kg/lbs for the 40Ah Li-ion batteries?. Thanks

 

[rons99]

Interesting project for me, so I'm thinking how if we using Li-Ion combine with ultra/super capacitor, offcourse using fast switching mosfet between them, let's say if possible the 2 motors works alternatively left/right during straight run, must be researching the duration so the passanger not feeling slated movements at any speed, if the L motors On, capacitor bank R charging, next R motor On and capacitor bank L charging, so the motors always draw current from ultra capacitors bank not from the batteries diirectlly, or make a bit conducting to the battery by mosfet controller driver n software, is it works?.thanks.