Electric Vehicle C

andrewwski
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Re: Electric Vehicle C 2009

Post by andrewwski » March 2nd, 2009, 6:52 pm

Yes and no.

The batteries are constantly discharging as they're being used. That means they're constantly dropping in voltage, and thus the motors are constantly running slower. Assuming you're not using a voltage regulator.

I saw this firsthand with my EV. I used 3 v for my motors and then series'd in another 3v to power my timer. Each round of testing would show the speed to be slightly less than the previous round.

There is a discharge curve for the battery voltage. Search up one for alkalines. It will drop fairly quickly at first, then sort of level off, then drop of quickly at the end.

However, since you're using a mechanical EV, that may not be important. For me, it was, as I was using a timer to control my EV. However, I was able to get the proper tests in to get it to stop a few millimeters from the line...so it worked fine. With a mechanical braking system, however, I'd think it'd be based on distance regardless (especially if you're using wingnuts). In that case, as long as you know how long your predicted time will be (successively greater, but probably not measurable along most of the curve), the minor variations probably won't matter.

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Re: Electric Vehicle C 2009

Post by Untitled » March 2nd, 2009, 8:10 pm

Our ev is based on a distance-based braking system with the classic string scrambler brake. So I'm guessing there isn't much we can do about dropping voltage. I'm guessing we figure what the time is and then we compensate by estimation.
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Re: Electric Vehicle C 2009

Post by Untitled » March 2nd, 2009, 8:48 pm

Oh and sorry for the double post.

What's the difference between using heavy duty and alkaline batteries? We ran out of alkalines but we have heavy duties atm.
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Re: Electric Vehicle C 2009

Post by fleet130 » March 2nd, 2009, 10:32 pm

Untitled wrote:What's the difference between using heavy duty and alkaline batteries? We ran out of alkalines but we have heavy duties atm.
See if you can find a discharge curve for both types in the internet. You may also find RadioShack's On-line Battery Guidebook informative.Alkaline cells generally last longer and their voltage varies less than carbon-zinc (heavy duty) cells. They should produce more consistent results.

While you're at it check out the discharge curve for rechargeable cells (NiMh & Ni-Cd). While they have a lower initial voltage, their discharge curve is more linear and voltage is fairly constant. They also have a MUCH lower internal resistance and can therefore source much higher currents. In fact because of their extremely low internal resistance and negative temperature coefficient is is inadvisable to place a direct short across then. Doing so can cause burns and in extreme cases they can explode.
andrewwski wrote:The batteries are constantly discharging as they're being used. That means they're constantly dropping in voltage, and thus the motors are constantly running slower.
This is true, but the voltage only affects the top speed of the vehicle and the voltage change from one run to the next (and therefore the change in top speed) is fairly small.

Think of it this way: A vehicle that has barely enough power to make it go, will slow down considerably if it comes to a slight hill. If the vehicle has plenty of power, it will slow down much less. This is what happens when using 2 vs 4 cells.

Since Power = I^2 x R, the example above with 4 cells will have twice the power of the vehicle with 2 cells. (.857/.600)^2
andrewwski wrote:I saw this firsthand with my EV. I used 3 v for my motors and then series'd in another 3v to power my timer. Each round of testing would show the speed to be slightly less than the previous round.
This is precisely what I would expect to see; however, since you were using only 2 cells to power the motor, the time difference was more likely caused more by the difference in power due to decreasing current than by the decrease in the top speed of the vehicle caused by the voltage change.

Try 2 or 3 cells and see if the times are more consistent. If it doesn't work, you can always go back to 2 cells. If you choose not to try, you will have discovered nothing.

Note: Another solution might be to lower the internal resistance of the cells and motor used in the vehicle. Unfortunately this requires switching to a different type of cell chemistry and a different motor.
Information expressed here is solely the opinion of the author. Any similarity to that of the management or any official instrument is purely coincidental! Doing Science Olympiad since 1987!

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Re: Electric Vehicle C 2009

Post by dudeincolorado » March 2nd, 2009, 11:44 pm

I'm having issues finding a gearbox, I went to radio shack and the guy there said that they didn't have them. Also would it be possible to post a picture of a string break? Side note! something about string just appeals to me, it's just string, it didn't ask to be amazing, it just is.
SO stressed!

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Re: Electric Vehicle C 2009

Post by Flavorflav » March 3rd, 2009, 2:21 am

Usually the gearbox comes with the motor. Look online.

Check the Scrambler Wiki for the string brake: http://scioly.org/wiki/Scrambler#String_Type

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Re: Electric Vehicle C 2009

Post by andrewwski » March 3rd, 2009, 7:03 pm

Fleet, are you talking of adding batteries in series or parallel?

Adding them in series will increase the voltage, and thus the speed of the motor.

You also can't count the motors as having a single resistance. As their torque increases, they're going to draw more current.

And battery capacity is measured in charge. For example, a D-cell has about 15 amp-hours. A C-cell has about 7.8. You can put a load of 30 amps on either of these batteries. However, the C-cell will last half as long.

Thus, adding batteries in parallel will not alter your current drawn by the motors, but will alter how long it can be drawn for.

Now - you can't base the current the motors will draw on the internal resistance of the batteries. The current is proportional to the resistance of the single component (the motors).

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Re: Electric Vehicle C 2009

Post by fleet130 » March 3rd, 2009, 9:36 pm

andrewwski wrote:Fleet, are you talking of adding batteries in series or parallel?
Please accept my apology for being unclear on how the cells are connected. I assumed that stating 2 cells would produce 3 volts and 4 cells would produce 6 volts was enough to tell they were connected in series.
andrewwski wrote:You also can't count the motors as having a single resistance. As their torque increases, they're going to draw more current.
For practical purposes, the motor's resistance is constant (other factors produce the change in current due to the motor's speed). It doesn't matter so much whether the motor's resistance is constant, what matters is that it has some resistance that limits the current. The cells in the circuit also have resistance. The combined resistance of the cells and motor place an upper limit on the possible current in the circuit. Torque produced by the motor is dependent on current, not the other way around. When the torque is marginal, small variations in mechanical load can produce large changes in time required to travel a given distance.

It's important to understand that what happens at the instant current begins to flow has a significant effect on the time it takes the vehicle to travel the target distance. A small change in acceleration for this instant is compounded and accumulates throughout the run to produce a significant difference in overall travel time.

To simplify the problem, ignore the changes in current due to changes in the motor's speed. They are due to other factors, not the motor resistance. There are a few other factors involved in how/why the motor reaches a constant speed, but that's another story!


andrewwski wrote:Now - you can't base the current the motors will draw on the internal resistance of the batteries. The current is proportional to the resistance of the single component (the motors).
Regardless of the battery and motor motor characteristics, everything must obey Ohm's law. Current is determined by the voltage and resistance. Resistance anywhere in the circuit, even inside the cells themselves, does affect the current in the circuit. This WikipediA entry on internal resistance or this article on Resistance-Grouping of Cells may help explain it. Here's an MS Word document on Internal Resistance and another website on Source Resistance.

I made the mistake of assuming the resistance was the same in the previous 2-cell vs 4-cell example. Here is a corrected version.

The total resistance for a series circuit is equal to the sum of the resistances of all the resistors:
R(total) = R(1) + R(2) + R(3) ...+ R(n)

Example:
R(motor) = 3ohm
R(cell) = 1ohm

For the vehicle with 2 cells:
R(total) = R(motor) + R(cell1) + R(cell2) = 3ohms + 1ohm + 1ohm = 5ohm
E=I x R
Therefore I = E/R = 3V/5ohm = 0.600A
P = I x E
E = I x R
Therefore P = I^2 x R = 0.600A^2 x 5ohm = 1.8W

For the vehicle with 4 cells:
R(total) = R(motor) + R(cell1) + R(cell2)+ R(cell3) + R(cell4) = 3ohms + 1ohm + 1ohm + 1ohm + 1ohm = 7ohm
E=I x R
Therefore I = E/R = 6V/7ohm = 0.857A
P = I x E
E = I x R
Therefore P = I^2 x R = 0.857A^2 x 7ohm = 5.1W

Since the 4-cell vehicle has about 3 times the power of the 2-cell vehicle, It will accelerate to it's top speed sooner and less error will accumulate during the acceleration phase.

Here's an idea that might show where the variation lies. Make several runs over a 10 meter distance. Use 2 timers, 1 to measure the time for the entire 10 meters and another to measure the time for just the last 5 meters (from these 2 times you can determine the time for the first 5 meters). Compare the variation in time for the first 5 meters to the variation for the last 5 meters. It might be even better if you could determine the time for each meter of the run.
Information expressed here is solely the opinion of the author. Any similarity to that of the management or any official instrument is purely coincidental! Doing Science Olympiad since 1987!

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Re: Electric Vehicle C 2009

Post by Untitled » March 3rd, 2009, 10:59 pm

What are you guys using to aim your ev? Ours isn't really going straight.
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Re: Electric Vehicle C 2009

Post by dudeincolorado » March 4th, 2009, 5:08 am

Is it possible to build an EV with out gearboxes?
SO stressed!

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