# Help calculating max speed of my electric car



## juliangliniecki (Feb 16, 2021)

What would be the max speed of my electric vehicle? I need a systemic way to calculate this given BLDC ratings: Voltage, Rated Power, Rated Speed, Rated Torque, Rated Current, No load Current, No Load Speed. Also using these constants: Weight and coefficient of friction. There are two motors. We are ignoring any resistance from the shape of the vehicle. The car is in a vacuum.


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## MattsAwesomeStuff (Aug 10, 2017)

juliangliniecki said:


> What would be the max speed of my electric vehicle? [...] *We are ignoring any resistance from the shape of the vehicle*. The car is in a vacuum.


Mhm. Good good.

_does some math_

Looks like your top speed will be just slightly under 670,616,629 mph.


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## kennybobby (Aug 10, 2012)

you will need the gear ratio and the size of the tires, and pick a max rpm point for the motor. You will need the torque-speed curve of the motor for whatever voltage you have.


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## ishiwgao (May 5, 2011)

juliangliniecki said:


> What would be the max speed of my electric vehicle? I need a systemic way to calculate this given BLDC ratings: Voltage, Rated Power, Rated Speed, Rated Torque, Rated Current, No load Current, No Load Speed. Also using these constants: Weight and coefficient of friction. There are two motors. We are ignoring any resistance from the shape of the vehicle. The car is in a vacuum.


sounds like a university homework...


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## remy_martian (Feb 4, 2019)

Systematic, not systemic

Work your problem in pieces....nice easy steps.

Calculate the rolling resistance based on weight and Cf

Torque at the radius of the tire is the force has to equal that resistance

Power determines your speed as a function of torque and rpm. Once you get the rpm of the tire, compute its speed by using its circumference

The rest you need to do yourself. Remember that two motors doubles everything.


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## remy_martian (Feb 4, 2019)

MattsAwesomeStuff said:


> Mhm. Good good.
> 
> _does some math_
> 
> Looks like your top speed will be just slightly under 670,616,629 mph.


This is what happens when you don't do it systematically and by failing to recognize your constraints and degrees of freedom.

Matt gets an F


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## Duncan (Dec 8, 2008)

remy_martian said:


> This is what happens when you don't do it systematically and by failing to recognize your constraints and degrees of freedom.
> 
> Matt gets an F


Nope Matt gets an A - the problem was a car in a vacuum - no wind resistance - and the only place with lots of vacuum is space so no rolling resistance either


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## MattsAwesomeStuff (Aug 10, 2017)

Duncan said:


> and the only place with lots of vacuum is space so no rolling resistance either


That's not necessarily true. You could also pull a vacuum in some kind of sealed test chamber here on Earth, if we're going to play the pedantic game. But why assume Earth?

I'm mostly just trolling and saying the funniest thing possible because it's obviously someone's homework question, not a person who actually wanted to know and needed help with the question.


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## juliangliniecki (Feb 16, 2021)

Thank you all for your insight. It's not for homework. I am on a team designing a scale model hyperloop pod and my partner and I are stuck on finding the best way to calculate the speed of our pod based on the BLDCs we are choosing. That is why we are ignoring air drag and I said vacuum.


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## MattsAwesomeStuff (Aug 10, 2017)

juliangliniecki said:


> It's not for homework. I am on a team designing a scale model hyperloop pod


Wait, so this isn't for a class, you're a professional legitimately designing a hyperloop vehicle, and yet you need help doing basic calculations from a DIY forum?

I'm all for ambitious projects, but, do you think perhaps you're too far out of your wheelhouse to engineer this solution?

This would be the equivalent of saying you're designing an F1 racecar, and then asking on an internet forum "How do you add air to the tires? What does PSI mean?"

Everyone starts somewhere, but, I can't see how far you could even take this project without everyone literally doing it for you, if you need help at step 1. The knowledge is - at least to some degree - cumulative. You'd by definition need even more help at steps 2-100 too, right?

Maybe start with something less challenging while you take the time to build your knowledge to a point where you can apply it to your project?


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## ikellymo (Dec 3, 2020)

I'll bite. There are two primary methods I've used:

1) Power limited. Consider your losses. Even the hyperloop project will have some losses (going up hills?). Every motor will have a continuous rating, which is what you should size for. At some point, as power(vel), you will reach the max power that can be output by a given motor. 

2) Gearing limited. Every motor has a max RPM. You need to decide what type of transmission you can use, then your torque needs. Torque and max RPM are inversely related if you only have a single ratio. This assumes that you have an acceleration requirement, and know the mass of your vehicle. It's also important to know all your losses here, because a very low acceleration requirement may be dominated by the actual losses. 

If your losses are truly negligeable and acceleration is the most significant, then your max velocity may alternately come from safety requirements or time-to-accelerate between stops. 

I'm currently building software to do these calculations, and have quite a bit of analysis code producing visualizations already. 










PM me if you're interested in engaging a consultant.


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## remy_martian (Feb 4, 2019)

juliangliniecki said:


> Thank you all for your insight. It's not for homework. I am on a team designing a scale model hyperloop pod and my partner and I are stuck on finding the best way to calculate the speed of our pod based on the BLDCs we are choosing. That is why we are ignoring air drag and I said vacuum.


Your pod doesn't need a propulsion motor. The little "pod" you put your deposit slip and money into at the bank drive-thru doesn't have one...

FWIW, pumping the tube down to partial vacuum will take more power than it's worth, not to mention the crush loads on the "pipe" you're traveling in putting materials costs/km(mile) through the roof.

I can see it now...after Musk got done reading his 1950's comic books, someone pulled him aside, showed him the math, and all of a sudden he abandoned the HL and had cars running in tunnels without the vacuum nonsense....which arguably is still nonsense.

I'm hoping this is a university project/competition and none of you quit your day jobs for Elon's hype-loop.


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## kennybobby (Aug 10, 2012)

And the likely corona discharge problems with the HVDC at partial pressures under vacuum.


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## sadcar (Mar 18, 2013)

juliangliniecki said:


> What would be the max speed of my electric vehicle? I need a systemic way to calculate this given BLDC ratings: Voltage, Rated Power, Rated Speed, Rated Torque, Rated Current, No load Current, No Load Speed. Also using these constants: Weight and coefficient of friction. There are two motors. We are ignoring any resistance from the shape of the vehicle. The car is in a vacuum.


question: are you using gears?, if not then its 1:1, that means motor rev per minute times tyre(wheel) circumference gives meters done per minute * 60 / 1000 = kmh, you may have a ratio motor to drive shaft if the motor is not directly driving the wheels, my mitsubishi express has a fix ratio of 4.625 (motor to gears) and then the ratio depending of which gear i'm driving, result if i have 2500rpm & 5th gear in my van moves at 100kmh (144v 24kw dc motor, max 5000rpm)


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