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Hello. Recently I’ve gotten the idea to make a electric car, a small one. Since I have no experience with curvy shapes I decided to do a 1980’s Lamborghini style car, you know the really angular ones with sharp edges? With the flip-up lights? Anyway, as this idea got stuck in my head a few questions passed my mind.

1. How do I increase an electrical motors rpm?
I have a relatively old electric toy car which uses a 12V motor, the other specifications I am unsure of. However, the car is really slow, it can reach a maximum speed of 5mph. So my question is, how do increase the rpm of this engine to make me come up in at least 20mph? (It’s a wooden car so it’s relatively light).

2. How large should my wheels be?
My car is roughly 1 meter wide and 2 meters long (sorry for non-imperial), I’m unsure of what type of and how large wheels to use. The current old electric kids car uses plastic wheels which obviously is nothing I’m fond of, I remember that they were really annoying to drive with.

I’m fifteen and this is a project with my dad so I decided to ask you guys since this seems like the perfect forum. Thanks in advance.


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The "Help Desk" section is for help with the forum; the rest of the forum is about building DIY EVs. If you post this is a more suitable section, you'll probably get more help - I would suggest "All EV Conversions and Builds" or perhaps "Non Road Going Vehicles".
 

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Hi Spruce
I have moved your thread

How to increase a motor's rpm? - increase the voltage!

BUT you will need to do a bit of dismantling - wiring
Your motor will have a controller that will feed the motor - the controller is normally the limiting factor for how much voltage

You CAN just try feeding it 24v - but there is a good chance that will simply let the magic smoke out
 

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1. How do I increase an electrical motors rpm?
I have a relatively old electric toy car which uses a 12V motor, the other specifications I am unsure of. However, the car is really slow, it can reach a maximum speed of 5mph. So my question is, how do increase the rpm of this engine to make me come up in at least 20mph? (It’s a wooden car so it’s relatively light).
I agree with Duncan: in general, for all types of electric motor, operating speed is limited by available voltage. But, in addition to needing a higher-voltage battery (easy when you're starting with only 12V), and a controller that can handle that voltage, there will be a limit to what the motor can stand... both how much voltage is can handle electrically, and how fast it can spin without coming apart.

There are a lot mobility devices (riding scooters for disabled or elderly people, power wheelchairs, and so on) which run on 24 volts. That might mean you can find a used 24 volt controller, and maybe even a more powerful 24 volt motor, as used parts salvaged from old equipment.

If you end up with the stock motor and it can't handle 24 volts but can handle something lower (for example 20 volts), you may - depending on the controller - be able to use a 24 volt battery and set or program the controller to limit to that lower voltage to protect the motor.
 

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2. How large should my wheels be?
My car is roughly 1 meter wide and 2 meters long (sorry for non-imperial), I’m unsure of what type of and how large wheels to use. The current old electric kids car uses plastic wheels which obviously is nothing I’m fond of, I remember that they were really annoying to drive with.
At the higher speed, you'll definitely want pneumatic (inflated with air) tires. Depending on what size, there might be something like scooter tires, but I can't think of what might be car-style, rather than motorcycle style (rounded to lean in corners) which is that small. Maybe quad (ATV) pavement tires? You'll presumably want larger than the sizes used for racing carts.

If you can change the ratio of the gearing, you can make any diameter of tire work. If you can't change gearing (by changing gears, or sprockets if it has a chain drive), then you can work out how overall tire diameter affects motor speed:
The circumference is pi * diameter.
For each turn of the wheel, the car moves by a distance equal to the circumference.
So if you convert the road speed to units of measure that you can work with, you just have some simple math. 20 miles per hour is 32 km per hour, or 32000 m/hr, or 32000/60=530 m/min.
If the tire is (for example) 20 cm in diameter, then it's 0.63 m in diameter... so 530 m/min would be 530/0,63=840 revolutions per minute (rpm).
Multiply that by the gear ratio and you have the motor speed.​
 

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Discussion Starter #7
First of all, thank you all for replying!

All of this information will really help me, I will begin to construct the car in a few days, so don’t expect this topic to be too quiet ;)

I’ll get back at you if I have any more questions, and thank you Duncan for calling me “Spruce” :)


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Howdy,

Most (small) DC motors will rotate at ~70 - 100 rpms (revolutions per minute) per volt of input.

so, I'll use 85 (kinda in the middle) for this example
a 12V motor will rotate at ~1,020 RPM's (12V x 85 = 1,020 RPM's)
If you supply it
...24V it should rotate at ~ 2,040 RPM's
...36V = 3,060 RPM's
...& @ 48V = 4,080 RPM's

Once you have an idea of the motors RPM"s, there are (3) main variables to figuring out how fast a (given) motor will propel your car/kart

1.) Motor RPM's

2.) Gear Ratio

3.) Tire Circumference


By inputting your

...motor RPM's

...gear ratio

...& your tire circumference

in to this equation, you can figure out what your, potential, MPH would/could be


MS/GR=ASxTC=IM/FT=FMxHR=FHxMM= MPH


Motor Speed/Gear Ratio=Axle Speed x Tire Circumference = Inches per Minute traveled/Foot(12) = Feet per Minute traveled x Hour(60) = Feet per Hour traveled x MPH Multiplier(.000189) = Miles Per Hour

* Using e-Lemon-aid, as an example
[email protected] 24V (@ 2,000 RPM's) (Rated at 2,500 RPM's max)
...an 8:1 gear ratio
...& tires with ~38" circumference


2,000/8 = 250
250x38" = 9,500"
9,500"/12 = 791.67'
791.67x60 = 47,500
47,500x.000189 = 8.98 MPH


Motor speed is (2,000 rpm's) divided by the gear ratio (8) = axle speed (250)

Axle speed (250) times the wheel circumference (38") = inches per minute traveled (9,500")

Inches per minute traveled (9,500") divided by inches in a foot (12") = Feet per minute traveled (791.67)

Feet per minute traveled (791.67) times minutes in an hour (60) = feet per hour traveled (47,500)

Feet per hour traveled times miles per hour multiplier (.000189) = 8.98 MPH

Yup, according to the GPS she topped out at ~9 MPH

https://www.youtube.com/watch?v=uBT9B0sqNe4

Using it helps me to determine the potential speed of a vehicle (motor gear ratio & wheel size)
...it does NOT take into account wind resistance, rolling resistance, total weight & many other factors
...but, It'll at least gives me a rough idea or put's me "in the ball park".
 
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