# Mythbusters: Pack voltage is limiting factor in car speed



## Sunking (Aug 10, 2009)

It takes both voltage and current to get power. Cannot have one without the other. So the answer is the higher the voltage and current, the higher the power. 

However it is a bit more complicated than that. Time for you to do some homework. Here is a hint. You can have a 200 HP motor at 48 volts DC, or 500 volts 3-phase AC. What is the difference?


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## surfislands (Jan 26, 2010)

i have been wodering that my self a transmission should help with torque maltiplication there are electric transmissions out there hybrids... transmission and power are the last of your problem b/c those are farely easy things to acomplish but self generating your motor is the real goal... its simple the lighter (getting rid of the huge battery) the faster


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## Georgia Tech (Dec 5, 2008)

The reason is it take ALOT of Physical size to carry and produce VAst amounts of current...The Battery would be 5000 lbs the controller would be the size of you back seat Wire and cables the size of your arm....Just not Practical to go over 2000 amps....


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## surfislands (Jan 26, 2010)

well think modern not what man is now capable of doing... but what will we be doing in the future

there has to be an easier way for instance instead of wires the size of your arm why not small wires going to coils mounted on te curtain part your trying to induce mass amount of electricity onto(volts then step up coil) (amps step down coil)

you can use all computer controlled circuit thats multiplexed the make
actuators acually do work and this circuit can be small

also why not find a way to eliminate the giant battery no one said thats what we are limited to ik there is going to be wieght involved but what about a large generator on a pully system (different gear ratio to maximize efficiancy of motor) attached to the motor so when you get it started it becomes self generating???


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## meanderingthemaze (Jan 25, 2010)

Sunking said:


> It takes both voltage and current to get power. Cannot have one without the other. So the answer is the higher the voltage and current, the higher the power.
> 
> However it is a bit more complicated than that. Time for you to do some homework. Here is a hint. You can have a 200 HP motor at 48 volts DC, or 500 volts 3-phase AC. What is the difference?


Ah, I sense a teacher in the midst. I'll try my best to answer the Socratic questions.

So, you are referring to P=IV
ie:
I=200HP/48V ~ 4
vs.
I=200HP/500V ~ 0.4

So, are you saying that you would fry a low voltage motor pressing it beyond the max amperage limits and that is why voltage is the "limiting" factor. 

I still don't have the real life information to tell how much power a car actually needs to move at different speeds and conditions. If I knew that, then I suppose I could calculate how much current I would need to run a car at whatever given voltage the motor was rated for. If I needed more amps than the motor could take, then I'd have to either find a motor that could take that amount of amps, or get one with a higher voltage requiring less amps.

So I guess this is the real question I should have asked...
Is voltage the practical limit for speed in electric car conversions rather than the theoretical limit, based on typical power requirements and motor designs?


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## Duncan (Dec 8, 2008)

Hi Meandering

Just to calibrate you

1 horsepower = 750 watts (745.7 actually)

200 Hp = 150,000 watts

150,000 watts = 48 Volts x 3,125 Amps

150,000 watts = 500 Volts x 300 Amps

So even at a very high voltage you still need lots of Amps to produce a lot of power
This is why we need large battery packs and big liquid cooled controllers

This is about what the Tesla has in the way of power

This is how big your engine is

The next question is how big is your "fuel" tank

It takes about 200 Watt - hours to go a mile

The Tesla has a 54 Kilo-watt-hour battery pack and can go about 250 miles


I am aiming at about 100Hp (75KWatts) and a 100 mile range (20 Kwhr Battery)


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## meanderingthemaze (Jan 25, 2010)

Well, I always like chatting, but I'm not sure if the original question was ever answered. Anybody?


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## piotrsko (Dec 9, 2007)

all things considered (and very simply put), more volts will tend to give you more watts, which is what you need to move down the road. This compounds until you get either a thermal meltdown or a commutator "zorch" from running too much through the motor, {or controller, or wires, or......}


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## Sunking (Aug 10, 2009)

meanderingthemaze said:


> Well, I always like chatting, but I'm not sure if the original question was ever answered. Anybody?


The question was answered. Higher voltage means more speed in general. Now with that said there are limits to how high of voltage and current you can run.

In your 48 volt example you observed the current needed to get to 200 HP is beyond reasonable expectation of any wire or cable. So the only way to get the current down is via higher voltage. But even voltage has a limit. I deal with NEC realm of things and one limit in the NEC is 600 volts. Above that and special precautions and methods have to be used. I suspect 600 volt limit would be a reasonable limit for passenger EV's 

Someone noted my numbers looked like Tesla, in fact they were. For any commercial application I think you will see 3-phase induction motors operating at 400 to 600 volt range. Just need a battery to make it workable and appealing to the mass public.


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## Georgia Tech (Dec 5, 2008)

IN theory it IS possible to have 48 volts and go fast. But It is Just not pratical! You have to pull to much amps...
There is a project called poor man EV he has a FAST low voltage Car..
do a google on "Poor man's" ev...He uses a 13 inch GE motor to do this. He is pulling like 2g's worth of amps..

Yes its possible but man it will be TOUGH!


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## neanderthal (Jul 24, 2008)

Low voltage motors tend to be larger and heavier than high voltage motors of the same output (meaning hp, watts, whatever you wanna measure it in)

I have a 9'' diameter series wound motor from a company called advanced dc in my electric car. The amperage it can use before getting hot is determined largely by the windings inside the motor, (thickness, number of turns etc). It can take about 900 amps of current in short bursts, and about 144 volts.

I can use a 72 volt battery pack and controller, and draw 600 amps from the batteries and have 43200 watts (or 57.9 horsepower) of output, and it will warm up the motor pretty good (but not enough to cause any trouble). Over simplified I know but I'm just making a point.

Now if I decided to use a 144 volt bettery pack, things would be different. (I would need either a bigger, heavier battery pack, or I'd need to stress the batteries more to make this happen, but lets forget this and just look at the motor for now.)

I could 144 volts and draw 600 amps and make 86400 watts or 115hp and it wouldn't heat up the motor much more than the first scenario. If I wanted 115 hp from the 72 volt system I'd have to use 1200 amps, it would make the motor way too hot.

I have a transmission on my car, so I can change gearing and get the same top speed either way, as long as i'm getting those 115 hp

I could up the system to 180 volts and get more power, but I would endanger the motor in a new way. Not from heat, but from arcing internally. We call it zorching (zap and torch put together)

Anyhow, I made this example, not to show how to run voltages to a motor, but to show how motor size and weight work.

To have a powerful low voltage motor it generally has to be large and very heavy, lots of copper. To have a powerful high voltage motor it has to withstand arcing inside from high voltages, but it can still be smaller and lighter. 

So basically you are right power is power, and the right gearing will give you the same top speed regardless. But a high voltage motor is lighter than a low voltage motor (than makes the same power) most of the time


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## meanderingthemaze (Jan 25, 2010)

Thanks neaderthal, and to everybody.

the main thrust of the question was practical not theoretical. I wanted to understand why voltage was a limiting factor in most situations, as it was not explained well before. Just stated as fact without any rationale or real world examples as neanderthal illustrated very well.

if showrooms existed, where you could test drive these motors, all of this talk would be somewhat unnecessary, because there are so many factors, it's hard to generalize when speaking of real world applications. 

Overall, I think things are becoming clearer in general, as more conversions happen and there is more data to pull from.

Thanks again everyone!

EDIT: BTW, I'm about 99% sure this is the motor for me. Was able to test drive it in a 2009 Ford Ranger conversion and while not a race car, it was substantial and did what it needed to do. The conversion will look something like this...
1969 Datsun L521 pickup
curb weight...2150lbs
96V AC motor...http://hpevs.com/drive-systems/ac-50
(and Curtis controller that is bundled)
16 x 6V lead acids

Hoping to achieve...
60 mile range
65-70 top speed
0-60 in 10 secs.


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## green caveman (Oct 2, 2009)

meanderingthemaze said:


> the main thrust of the question was practical not theoretical. I wanted to understand why voltage was a limiting factor in most situations, as it was not explained well before. Just stated as fact without any rationale or real world examples as neanderthal illustrated very well.


Having been down this path just recently, there may be more to it than just power. I think that even if you could put enough amps into the motor at, say 36V, you'd still have a speed problem because of the "back emf". This is the best explanation I've seen (from Electric motors and generators):

"_Every motor is a generator_. This is true, in a sense, even when it functions as a motor. The emf that a motor generates is called the *back emf*. The back emf increases with the speed, because of Faraday's law. So, if the motor has no load, it turns very quickly and speeds up until the back emf, plus the voltage drop due to losses, equal the supply voltage. The back emf can be thought of as a 'regulator': it stops the motor turning infinitely quickly (thereby saving physicists some embarrassment). When the motor is loaded, then the phase of the voltage becomes closer to that of the current (it starts to look resistive) and this apparent resistance gives a voltage. So the back emf required is smaller, and the motor turns more slowly. (To add the back emf, which is inductive, to the resistive component, you need to add voltages that are out of phase. See AC circuits.) "

I'm not entirely sure though, because you'll hear a lot her "don't test the motor with no load with more than 12V because it'll spin so fast it'll fall apart" which suggests that at, say 24V you could get the speed required, if you could get the current into the motor.

Since I'm not really a motor expert, I found the site above and one of the referenced sites How real electric motors work quite an interesting read (every little helps!).


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## Georgia Tech (Dec 5, 2008)

green caveman said:


> Having been down this path just recently, there may be more to it than just power. I think that even if you could put enough amps into the motor at, say 36V, you'd still have a speed problem because of the "back emf". This is the best explanation I've seen (from Electric motors and generators):


Yeah we know that, he was saying if he had REALLY tall gears, like 750 RPM makes 50 miles per hour. That way the motor would turn real slow and the car still goes fast


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## green caveman (Oct 2, 2009)

Georgia Tech said:


> Yeah we know that, he was saying if he had REALLY tall gears, like 750 RPM makes 50 miles per hour. That way the motor would turn real slow and the car still goes fast


Aw, now you just made me feel bad - I didn't know it until recently. On the plus side my ignorance of all thing automotive and EV related is so vast and expansive that I can easily become happy again learning even the smallest things.

As for gearing, I think that you could make a fairly decent vehicle at 48V and, say 400A peak with the right gearing. Pretty good acceleration, you're using the motor at a lower speed which is a happier spot for the motor. You could easily feed it through a gearbox - when I did the research a nice PTO ~4:1 box is about $500 or so, less if you hunt one down used/eBay, etc.. I was thinking that this was a good way to go until I realized that these boxes are only 70% efficient. Best you could do is about 90% (big money) or so. Quick (and obvious translation) you'll loose range - 30% of the range for the cheaper boxes.

Started to loose it's attraction at that point. Powerwise I think it works.


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