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Does Voltage Equal Speed?

by Jacob Owens
(Sharon, TN)

Excellent lightning picture

Excellent lightning picture

I'm thinking about converting a five-speed Toyota to electric. If I put in a DC motor, how fast will it go? How many volts are the systems, usually, and can I just add more voltage if I want more speed?


Hi, Jacob!

I couldn't resist the lightning picture. It has nothing to do with your question, except for the voltage; )

Let's see:


  • Five speed Toyota, good...lightweight, manual transmission. I approve, hehe.


  • DC motor, how fast will it go? Freeway fast. Exactly how fast depends on your next question...


  • Voltage. How many volts do these typically have? Usually between 100 and 150 volt systems, in multiples of 6, 8, or 12 (because that's what size batteries you can get). 120 volts is really common, so is 144.


  • Do more volts mean more speed? Yes, pretty much. The voltage determines how many amps you can draw from the battery at a time, which determines your acceleration.


  • Regards,
    Lynne

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Voltage, Current, Power, Acceleration Relationship
by: Steve

I have to argue with some of what was said. Say you have 10 12-volt bateries arranged in series giving you a 120-volt system and your motor is rated for an input range of 100-150 volts, then adding two more batteries in series would allow you to now have more charge on hand, essentially giving you more power and range, but may actually reduce the current delivered to your motor for an identical speed (not necessarily a bad thing).

Lets remember the relationship of voltage, current, and power. Power is really what you want in order to go fast (accelerate). We measure most household appliances in Watts (1 Joule/second), while we measure cars in horsepower (746 Watts). Here's the relationship: Power=Current*Potential (1Watt=1Amp*1Volt). Raising the voltage with the same battery array just lowers current and keeps the power the same. It comes down to finding the best voltage for the motor your using to draw current most effectively.

Here's a scenario. You have two battery arrays, which can both hold the same amount of charge and run a 70HP (52220W) motor.

Amps=Watts/Volts

120 Volts=435 Amps
144 Volts=363 Amps

Also, amps are not drawn in a time period. They represent charge per unit time (Coulumb/sec). The following is the relationship between power and acceleration:

F=ma(of car as whole), E=Fd, P=F*d/t, so:

POWER=mad/t, and since d=.5at^2 (basic kinematics)

P=.5m(a^2)t

As t apporaches 1/infinity and mass is held constant, power is proportional to half of the square of acceleration. More power equals good.

-Steve

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