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Need help on an Electric Go-Cart Project

06/09/2007 5:44 PM

I'm a Physics teacher and just received a grant to buy an Electric Go-Cart. I've assembled it and it's ready to go but now I want to make some modifications and need help.

1. I want something that acts like a governor to control the speed when younger kids are driving. I want something simple to install (no building complicated circuits - but would rather just pay extra and have it shipped to me built). I've been looking at DC motor speed controllers but not sure what I need. The go cart runs on 4, 12 volt batteries (look like they came off a golf cart but maybe smaller). Also, I want to be able to adjust the controller while presenting a lesson on resistors so I don't want to have to remove anything but just turn a knob.

2. I want to be able to kill the go-cart from a slight distance. I want a remote kill switch in case a student gets in trouble or is breaking the rules. Any ideas on this would be great!

Thanks,

Deborah Carder

Lakeside Physics

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#1

Re: Need help on an Electric Go-Cart Project

06/09/2007 9:17 PM

In it's simplest form, speed limiting in a DC motor system can be obtained by voltage limiting, which can be done with a resistor in series. You could find a resistance value that works to accomplish your desired limits or better yet, buy a rheostat to make it adjustable, then cover the knob with something lockable and you have the only key. The "gas" pedal will already be a resistor of some sort if it is a variable speed cart, you just need another one in series. If the cart pedal control is just On-Off, you will need to find a resistor of the right power rating and resistance value to do it. Keep in mind, it will get hot though, so you will also need to be concerned about keeping the little hands and feet away from it.

A DC drive could also work, but it will be expensive. What is your budget?

For a remote kill switch, you will need a radio powered device, which will not likely be cheap either. Look for a remote door lock or remote start kit for a car, and use the contacts of the switch to power a contactor that supplies power from the batteries. If you hit the "clicker", the contactor opens and the cart motors are deprived of power. it will still coast however, so be aware of that issue.

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#6
In reply to #1

Re: Need help on an Electric Go-Cart Project

06/10/2007 10:52 AM

Thanks for the advice - very helpful! I've already found a remote kill switch that is suppose to be used for a Honda and I also found one they are selling on the Internet for 4 wheelers. Not sure which one is the best though. I suppose I'll check into how far each can reach.

I'll look into the rheostat. The gas pedal is hooked up to one now - I never thought of using another!

As for budget, we have $2000 to use for the entire project. Right now here's what the students have proposed:

1. Buy new lithium batteries - will run longer (go farther) and less weight

2. Use tarp or plexiglass to cover the bottom, sides, and front clip for aerodynamics

We take the go-cart to a state competition next May and compete in a drag race (overall speed) and compete in a distance (how far until the batteries die).

My only 2 requirements are the ones we're working on now (remote kill and governor).

Thanks again for all your help.

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#16
In reply to #6

Re: Need help on an Electric Go-Cart Project

06/11/2007 7:33 AM

On competing next May on how long the batteries run until the die you should replace the resistive speed controller with a electronic speed controller will give you better battery life. Curtis makes one that they use in golf carts and pallet trucks. It instead of controlling speed by placing a resistor in series with the motor pulse the power to the motor. Speed is controlled by varying the frequency.

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#21
In reply to #16

Re: Need help on an Electric Go-Cart Project

06/11/2007 10:24 AM

I checked out the Curtis electronic speed controller and that is the exact one I have on the cart. The cart is controlled through a pedal that moves a pot forward and a spring that brings it back. This adjusts the speed. What I'm trying to do is be able to turn a knob (or something simple) and even if the kid pushes the pedal all the way down, they will only go 1/4, 1/2 or 3/4 of the speed (whichever I choose). The other posts says I can attach a resistor to the controller but I'm not clear on that at all.

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#22
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Re: Need help on an Electric Go-Cart Project

06/11/2007 10:28 AM

The bottom line is either experiment or contact the manufacturer of the speed controller.

Don't rely on us for the fine detail!

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#31
In reply to #21

Re: Need help on an Electric Go-Cart Project

06/11/2007 1:32 PM

After looking at the wiring diagram the manufacture of the cart has already thought of that. You may find they all ready have a device available to reduce the speed.

In working on electromotive equipment the resistance of the potentiometer increases as the petal is depressed increasing speed. Resistance in series is additive. Resistance in parallel is an average. If the resistance of the limiting switch is too great the vehicle will jump as the petal is depressed. May cut out on safety features of the controller. Curtis may make a controller that functions with a decreasing resistance. That may be what you have. Need Controller model number.

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#34
In reply to #31

Re: Need help on an Electric Go-Cart Project

06/11/2007 2:02 PM

Resistance in parallel is an average.

Not to nit pick, but Deborah is already confused. Resistance in parallel is not an average. For example, if you parallel two 5000 ohm resistors the resistance is not the average (5000 ohms); it is 2500 ohms. You add the reciprocals of the resistances (1/5000 + 1/5000 = 2/5000) and then flip that fraction back over, and simplify it.

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#42
In reply to #6

Re: Need help on an Electric Go-Cart Project

06/18/2007 4:22 AM

2. Use tarp or plexiglass to cover the bottom, sides, and front clip for aerodynamics

Cor! How fast is this thing going?! It might be cheaper and easier to pump the tyres up a bit harder to reduce rolling resistance as at the sort of speeds a go-kart goes (usually!) aerodynamic advantages have to be balanced against the disadvantages of increased weight.

What about a lighter driver?

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#44
In reply to #42

Re: Need help on an Electric Go-Cart Project

06/18/2007 5:06 PM

Hi PW:

This kart will go about 55mph, apparently even without streamlining. At that speed, aero drag predominates over rolling friction, even for very clean shapes.

I'm prototyping a very small three wheeler, with a Cd of .15 (very low -- although not at in the solar racer range of .10 or .09) It sports both an engine and some hefty batteries, so it is relatively heavy, as compared to a go kart. Being rather heavy and very streamlined, then rolling resistance should be pretty significant. But as it happens, at 55 mph (on a perfectly flat, windless road) my vehicle consumes 2.2HP in overcoming wind resistance, and 1.7 in overcoming rolling resistance. For my heavy vehicle, the speed at which the two drags are about equal is 50 mph. In a lighter vehicle, such as this go kart, this speed would be lower.

If I change Cd in my spreadsheet to .7 (probably a little low for an open go kart) then the HP consumed in bashing air about goes up to 10.3 at 55 mph. With that Cd, the point at which rolling friction and aero drag are equal drops to 22 mph, even with my heavy vehicle.

I'm pretty sure that the reason for aero drag having such a pronounced effect is that the God of Land is less easily offended than the God of Weather.

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#8
In reply to #1

Re: Need help on an Electric Go-Cart Project

06/10/2007 9:45 PM

A rheosat would be perfect. What size would I need if I connected it in series with the accelerator potentiometer?

Thanks again for the help.

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#12
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Re: Need help on an Electric Go-Cart Project

06/11/2007 1:58 AM

Hi Deborah:

You would not want to do this, because with your controller, higher resistance means higher speed. You'd want the resistance in parallel with the accelerator pot, just as the speed limiting resistor shown in the schematic is wired. Paralleling resistors lowers the combined resistance.

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#2

Re: Need help on an Electric Go-Cart Project

06/10/2007 3:15 AM

Can I be young again and attend your school?

Are the batteries in serial or parallel?

Kids are clever, I would not go for a knob.

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#4
In reply to #2

Re: Need help on an Electric Go-Cart Project

06/10/2007 10:37 AM

I think I'm learning a lot more than they are! I've place the schematics for the go-cart above. Hopefully this can answer your questions better than I can. The governor will be on the back of the go-cart where the kids can't reach (while driving). My older students will be able to drive it full speed but I hold a Physics show once a year and my students set up booths and perform experiments and demonstrations for K through 9th grade. Last year a student build a 6 foot tall trebuchet and shot water balloons at the little kids! They loved it. Anyway, the go-cart will be a part of this show but I want to greatly limit the speed for say 2nd graders and maybe not so much for 9th graders.

Thanks for the help!

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#10
In reply to #4

Re: Need help on an Electric Go-Cart Project

06/11/2007 1:44 AM

It turns out that your controller is set up to do exactly what you want to do. If you notice, there is a jack (labeled only 'Jack"). Beside the jack are two plug drawings -- one with speed-limiting resistor, one without. I can just barely see, but it looks like the non-resistor plug is the equivalent of two straight through wires, simply connecting the two leads from the accelerator potentiometer to the controller. The plug with the resistor puts that resistor in parallel with the accelerator pot. Therefore, less resistance means less speed. So, the accelerator potentiometer must be wired so that 5000 ohms means high speed, and 0 ohms means zero speed. (0-5000 ohm pots for this purpose are very standard.) (Many controllers can be set to work either way: 5000 for fast, or 5000 for slow.)

I'd guess that the speed-limiting resistor is about 5000 ohms, making the equivalent resistance (of the pot plus resistor) 2500 ohms. You could certainly try that first, and see if it gives you a slow-enough speed. I'd be almost certain that 1/2 watt resistors would work fine, so you could go to radio shack and get several (10,000, 5000, 2000, 1000, 500).

I'd guess that the speed-limiting plug, (already equipped with the resistor) is probably tethered to the cable near the jack, or in a little box of spare parts, etc. These are pretty standard on electric go carts for the reason you mention. If you wanted you could have a set of three or four, for different max speeds.

If you look at the controller, you should be able to find the manufacturer's name (Curtis would not be a surprise). If you check on the web for the manufacturer, they could give you the resistor value for the speed you want.

As an alternative to two plugs, a single plug could be wired to a switch which would switch the resistor in and out of the circuit. The resistor could be a small pot (called a trim pot) so that you could set the low speed to whatever you wanted. (If you did not switch this in and out of the circuit, then the upper value would need to be fairly high -- 20,000 ohms or so -- to keep from slowing the cart too much for the fast-enabled kids.

The following is probably a bad idea, from the stand point of safety: If you wanted to be exotic, you could use a model airplane remote control to control a servo, which would turn a pot, paralleled with the accelerator pot. If the receiver, pot and servo were in a small plastic box, with just the antenna (usually about a foot or wire) sticking out then you would have remote control over speed limiting, and tampering would be difficult.

(You'd want to be careful, though: if your driver has his foot at full throttle, and you've turned the speed way down he may be going 5 mph. Suppose you almost drop the radio control, and as you regain control , you inadvertently set to speed back to full. Suddenly the kart zooms ahead, and the surprised student runs into something.) Such a system would enable you to set the max speed at zero, if need be, to stop a misbehaving rider.

BUT DO BE CAREFUL -- relying on a remote control, where safety is a concern, is probably unwise. Best bet is probably to rely on the speed limiting as designed in, lowering the value of the resistor, if necessary, to get a slower speed. Also, check that a 1/2 watt resistor is adequate -- if the resistor burns out, then the kart reverts to full-speed capability.

Incidentally, with the plug setup, if a kid tried to remove the plug, the kart would not operate -- a good thing.

Another thought: a relay, controlled by a model airplane transmitter, could be put in series with the key switch. Then you could stop the kart remotely. But you'd want to run this past the controller manufacturer.

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#43
In reply to #10

Re: Need help on an Electric Go-Cart Project

06/18/2007 4:26 AM

A thought on remote control:

If radio were to be used as a safety cut-out, it would be wiser to install it so that the presence of the correct radio signal at the receiver enabled the vehicle and its absence disabled it, perhaps?

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#26
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Re: Need help on an Electric Go-Cart Project

06/11/2007 11:10 AM

Hello Deborah! Keep it up! Wish I had that kind of budget when I was teaching physics!

Blink has given you good advice - he either has better eyes or a sharper image of the circuit than I do... It would help if you could enlarge the image before posting.

The one thing I haven't seen anyone address (although one post mentioned it) is Coast prevention. If the remote control Kill relay is a Double Pole Double Throw one (one pole for each motor), the Off position could place a low value high power resistance across the motor (eg. 1 Ohm 100 Watt, which could be as simple as a few turns of stainless steel or nichrome wire wrapped around a ceramic form). This causes the motor to become a generator, and the resistance load will stop it very quickly. You could experiment with different numbers of turns to avoid excessive deceleration. If you need more info on connecting the relay, just say so...

Good luck, and let us know how it turns out!

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#27
In reply to #26

Re: Need help on an Electric Go-Cart Project

06/11/2007 12:26 PM

To get that kind of budget I applied to over 20 different grants and finally got lucky and received 2 of them. One from the state department paid for the entire go-cart ($5200) and then the 2nd from a local group ($4000) to be used for upgrades, tools, and anything else we might need for the project. I am lucky to be at Lakeside Highh School in Hot Springs though. I've never had a community support education as much as this one does.

Back to the car though. You're idea sounds great. I'll save this posting for after we've accomplished the governor and the remote kill switch. I'll need more details later though!

I found a manual online for the Curtis controller I have. On top of it, there are 2 holes and the manual says I can use an allen wrench to adjust acceleration and braking with these. Can I use this as a governor? I've included a picture below - hopefully it's large enough to read.

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#32
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Re: Need help on an Electric Go-Cart Project

06/11/2007 1:36 PM

I Hadn't heard the term 'Plug Braking' before, but it sounds like it provides essentially what I was suggesting.

Since I don't see any large resistors in the controller, it may use the resistance of the motor windings, simply shorting them to stop, again apparently using PMW (Pulse Width Modulation to control the braking rate.

Dick

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#33
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Re: Need help on an Electric Go-Cart Project

06/11/2007 1:54 PM

Hi Deborah:

The Curtis controller you are using is a speed control type, meaning that when you set the "throttle" at a particular place, the kart accelerates to the commanded speed, and holds there. This is quite different than a car's throttle, where you use a lot if you want to accelerate quickly and a little if you want to accelerate leisurely.

The controller, given a half speed command will "floor" its internal "throttle" until the half speed is reached, and then back off to maintain that speed. So, to prevent the control from feeling too abrupt, such controllers are equipped with an acceleration rate pot. If you set this for less acceleration, then, when you ask for half speed, the kart gets there more gradually. Initially, it may be good to reduce this setting a little to make it easier to drive smoothly. Once your competition driver is trained, then he or she will probably want all the available acceleration, so you'd reset it. The acceleration control should not change the top speed, just how long it takes to reach it.

The controller sets both motor voltage and current. Voltage equates to speed, with permanent magnet motors having a linear voltage-speed curve. Current equates to torque (which equates to acceleration, or hill climbing ability). The controller will protect the motors, by limiting maximum current, and most controllers will also protect themselves by further limiting current if the controller overheats. In most controllers, this overheating protection comes on gradually, so that the vehicle gets sluggish as the controller starts to overheat. If the condition causing the overheat (going up a very long hill, for example) is "fixed" then the controller will re-enable full performance. In your competition, you may find that the motor, controller, or both overheat. If so, get back with us... but for now, make sure the controller is firmly mounted to a metal surface which can conduct heat away.

The plug braking control is probably not used for your application. In plug braking, the motors are "told" to apply reverse torque by the controller (and the brake pedal has a pot similar to the accelerator pot, or occasionally just a switch). Then, braking energy goes off as heat (in resistors), just as it would from mechanical brakes. In regenerative braking (which your controller can probably not accomplish), the braking energy goes back into the batteries. If your endurance trial includes going up and down hills and braking, then you might want to consider a controller which implements regenerative braking. (Controllers are about $400) If the endurance run is on a flat track, there would be no advantage in regen, unless one needs to brake for turns.

In your controller, it is probably possible to accomplish plug braking by first moving the direction control switch to reverse, and then pressing on the accelerator pedal. It would be good to find out if this is the case. If it is, kids will play with this feature, putting unnecessary loads on the drive train, and they might try to see if they can get the wheels to spin in reverse while stopping. Your kart kit manufacturer would probably consider this abuse. When I was a kid, I had an electric industrial cart, and I sheared the keys in the drive train several times by plug braking this way. The sheared keys also left the vehicle without any mechanical brakes.

BTW you mentioned use an allen wrench to adjust acceleration. You may already know this, but the allen wrench is only to remove the screw covering the adjustment pot, which should be turned with a non-conductive screwdriver. You can get one at Radio Shack, where they will call it an alignment screwdriver (or tool), probably. (I'm guessing when they say "insulated" this is what they mean, namely that the blade itself will not conduct. An ordinary insulated handle screwdriver could be found anywhere -- you would not need to get one from Curtis. But in any case, using an alignment tool will not hurt.)

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#3

Re: Need help on an Electric Go-Cart Project

06/10/2007 4:39 AM

A variable resistor in series with the existing 'gas pedal' resistor could be used to limit top speed, if the speed control circuit works that way.

On no account try to put a variable resistor in series with the motor! (Unless is is something extremely weird like 1 ohm 1kw !) These motors draw high currents and would burn a potentiometer very quickly....In my early youth I tried to dim lamps with pots and wasted a few!

Easiest way to reduce speed is probably to reduce the number of batteries, assuming they are in series to find a reasonable compromise.

The high current warning also applies to your cut out device. (any devices need to be rated at about 20Amps minimum I would guess. (At 48 volts the currents will not be as high as on a 12v cart...but the stall currents are still nasty).

Maybe the kids should be doing this themselves?

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#5
In reply to #3

Re: Need help on an Electric Go-Cart Project

06/10/2007 10:42 AM

Thanks for the advice. The kids are working on this and many other parts. My advanced classes are researching just like me trying to find the best ways to accomplish this. My lower Physics in Context (Physics for Poets!) are learning to solder, connect wires, read schematics, and so on.

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#7
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Re: Need help on an Electric Go-Cart Project

06/10/2007 4:02 PM

"Dell the Cat" fairly much covered it all. Additionally there may be some additional controls located on the controller (referring to your drawing supplied) that you can alter to adjust the maximum motor speed.

Best option I would suggest is placing a resistor in series with the accelerator potentiometer. The controller is powered directly off the 48V from the 4 series-connected 12V batteries and may not work correctly if you remove one of them.

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#11
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Re: Need help on an Electric Go-Cart Project

06/11/2007 1:53 AM

Jack: I think you might not have looked closely at the schematic. The speed limiting resistor is in parallel with the accelerator pot, and therefore reduces resistance. So she would not want a resistor in series with the accelerator pot -- that would increase speed for a given accelerator position. She'd want it in parallel -- like the designed-in speed limiting plugs.

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#9

Re: Need help on an Electric Go-Cart Project

06/10/2007 11:22 PM

Deborah; I am building an electric car. The controler I am useing uses a throttle pot to control the motor speed. By limiting the distance the pot can be moved form zero to one hundred per cent you limit how fast the motor will go. A simple adjustable stop for the arm on the throttle pot will do the trick for that part of your problem.Use a spring in the linkage between the foot pedal and the throttle pot arm, that way the kids can mash the pedal to the floor and not bend anything when you have them limited.

As for the remote kill; Are there any go cart tracks near you? The last few times I was to a GC track ,they have a remote kill for the ignition.If no go cart tracks, maybe a hobby shop could fix you up with some kind of a remote control. A radio operated switch could be set up to supply a ground to a solenoid (like an electric door lock solenoid from a car) connected to the main battery disconnect switch. Turn on the main switch by hand and off by radio for what ever reason. If I didn't describe things well enough ,let me know and I will try for more detail.

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#13

Re: Need help on an Electric Go-Cart Project

06/11/2007 4:00 AM

I am confused about the different opinions regarding serial / parallel and I hope Deborah is not.

I see the jack top right but I also see a controller top left (without detail)

The accelerator circuit may control a oscillator loop which in turn regulate the length of 48 volt pulses sent to the motors. This will eliminate wasting of energy to heat up the resistance.

The designer seem to have done a good job.

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#20
In reply to #13

Re: Need help on an Electric Go-Cart Project

06/11/2007 10:08 AM

I hope Deborah is not confused too.

The controller is almost certainly a PWM. It has been many many years since golf-cart-like vehicles were controlled by resistance in series with the motor. The speed limiting plug clearly has a resistor wired in parallel with the accelerator potentiometer, not in series as some seem to be advising. (This is not conjecture -- the drawing shows it clearly.) Therefore, the speed limiting resistor has the effect of lowering the resistance that the controller sees. Putting a resistor in series with the controller would have the opposite effect, which would not be good.

Speed limitation is designed into the control setup, as it usually is in electric go karts. It the supplied resistor plug is missing, or of the wrong value, then it should be simple to recreate one, or simply buy one from the controller manufacturer.

Deborah should have the documentation on the controller before she experiments with the control circuit willy-nilly. Using the designed-in speed limiting feature makes sense, especially since, if she ends up with an injured kid, she will have a difficult time in court, if she says she just stuck some resistor in there somewhere without consulting the controller manufacturer.

Our high school has an electric go Kart which can accelerate to 45 MPH in the blink of an eye. Deborah's might not be of such high performance (although 48 volts suggests that it probably is reasonably fast, as does the $2000 budget for mods.) So there is clear potential for injury: even a 10 mph go kart running into a kid's ankles could break them.

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#14

Re: Need help on an Electric Go-Cart Project

06/11/2007 4:10 AM

Resistor...series..parallel.

Just try it and see!

It's called an 'Experiment' it's how we learn!

I don't s'pose you can do any damage...and if you do...blame the designer!

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#15
In reply to #14

Re: Need help on an Electric Go-Cart Project

06/11/2007 5:29 AM

I Agree!

The little ones might appreciate a fast drive. Weeeeee!

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#24
In reply to #14

Re: Need help on an Electric Go-Cart Project

06/11/2007 10:48 AM

This is really bad advice.

Our high school's go kart can do 45 mph. If Deborah's is similar, it has the potential to put a kid into a wheelchair for life it it gets out of control, hits a curb, flips and causes a severed spinal cord. If a kid hits a tree at such speed, she would be considered lucky if she lived. (Consider that automotive crash tests are done at 35 mph, and that the dummies are buckled in with high quality seat belts, often with sophisticated pre-tensioners, and the the car is equipped with crumple zones, and airbags, and collapsing steering column, and far more crush distance -- and sometimes the dummy still "dies".)

If she mods the go kart, the designer is not the one to blame. Better to use the designed-in speed-limiting plug, or a more conservative replica.

Some controllers are sophisticated enough to detect that the throttle is not at the zero speed position (in this case, something other than near-zero resistance) at startup. Sensing that, they will not operate until the fault is cleared. But one would be unwise to count on that, without the documentation in hand. If you put a resistor in series with the accelerator potentiometer, and the pedal is not depressed, and the resistor you use is 2500 ohms, then you are telling the controller you want half of full speed*. If there is no fault detection, the go kart could take off and accelerate when the key switch is turned on.

With kids around, better to read the controller documentation first, call the manufacturer if necessary, and do it right.

I don't think Deborah should be blaming the designer when some kid ends up with serious injuries. (Kids have sustained life-altering injuries on even the tiny motorized plastic jeeps, etc that go somewhat more than walking speed.) Cavalier is OK sometimes, but not at other times, particularly when writing to a non-engineer.

Suppose Deborah is large and doesn't fit in the kart. Then the first test driver will likely be a kid. Best not to un-engineer a safety feature, in that case, right?

* Or, you are asking for half of full acceleration. Many of these controllers can be set to either speed or acceleration control.

We've got the dam schematic. It shows the resistor in parallel. Why tell someone to experiment and put it in series, given that injuring a kid is a real possibility?

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#25
In reply to #24

Re: Need help on an Electric Go-Cart Project

06/11/2007 11:04 AM

D'uh.

I don't mean experiment with kids in it!

Stck it on blocks or make a rolling road....we need to assume sort of level of common sense...

Yeh, tell me about spinal injury...my son is in a wheelchair!

If we really have to spell out the answer.

MOUNT THE CONTROLLER SOMEWHERE ACCESSIBLE. USE THE PLUG IN RESISTOR FACILITY PROVIDED BY THE MANUFACTURE. IF NECESSARY BINGING IT TO THE OUTSIDE VIA A SHORT CABLE WITH A SUITABLE CONNECTOR. WIRE SEVERAL DIFFERENT RESISTORS ONTO THE CORRESPONDING CONNECTOR TYPE. COLOUR CODE AND LABEL THEM SLOW, MEDIUM, FAST.

Every man is his own safety officer.

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#17

Re: Need help on an Electric Go-Cart Project

06/11/2007 7:37 AM

Lakeside:

The easiest way of limiting the motor would be to drill and tap the accelerator foot pedal and screw a spacer block onto the back of the foot pedal. This limits the travel of the foot pedal thereby governing the speed of the engine. We use to do this regularly at the go-kart track that I worked at as a teenager. No messy circuits or complicated resistors to fool with. Don't know why these electrical guys didn't pick up on this. If you need any help with this, IM me.

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#29
In reply to #17

Re: Need help on an Electric Go-Cart Project

06/11/2007 12:53 PM

The electrical guys did not pick up on this idea because some realized that the speed limiting function is already built into the design, as shown in the schematic. No need to undue the engineering already there. Swapping a resistor-equipped plug can be done in a second. You can have four of them set for different speeds. Doing the equivalent mechanically would be crude, subject to wear, and it would be time-consuming to build the various block heights, and time-consuming to switch from one to another.

Complicated resistors? The only component less complicated is a piece of wire (which is, after all, a resistor with a very low value).

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#45
In reply to #29

Re: Need help on an Electric Go-Cart Project

06/25/2007 8:28 AM

Ken:

First of all, speed limiting should never be done as Deborah wants (by remote control and electronically). Think about what this circuit is being used for. Children will be driving this cart and if your resistor fails or burns out, there is a distinct possibility that the child driving the cart may accelerate to quickly and crash. A mechanical means even as a backup, like I've suggested will have an almost infinitesimally small chance of failing.

I would hate to be the person telling a parent their child has been hurt. Just my two cents worth of common sense.

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#46
In reply to #45

Re: Need help on an Electric Go-Cart Project

06/25/2007 2:36 PM

Hi Unclefastguy:

I think Deborah was thinking of two separate issues, one being the governor, the other being the additional capability to disable the kart, entirely, remotely (which strikes me as a bit of overkill, but I can understand the motive).

I certainly agree with the intent of this and your original your post. (The exception being that your original post characterized resistors as being complicated, while they are anything but. They are also an unavoidable and essential part of speed control on this kart.) When working remotely, my inclination is to say, "let the original engineer handle the safety issues". Thus I've advocated for using the designed-in speed-limiting plugs.

But your points are well-taken, and there is a failure mode under which the designed-in plug might be dangerous. The controllers are built with several self checks, so if the speed pot has failed open, the kart won't start up and go, for example. But a failure during driving might not be detected: With the speed sensing set so that high speed means high resistance, then putting a resistor in parallel with the speed pot will decrease resistance, and decrease speed. So far, so good. Suppose you want a half speed maximum (and suppose the relationship between speed and resistance is linear -- which is pretty close to true). Then a 5000 ohm resistor, in parallel with the speed pot at full "throttle" (also 5000 ohms) yields an equivalent resistance of 2500 ohms -- half speed. But suppose the kart is in motion, the start up checks have been done, and the speed pot fails open. Then, the equivalent resistance is 5000 ohms -- full speed -- and it would remain that as long as the speed pot is open. (Suppose the wire at the speed pot breaks or falls off. I am not aware of anything in the controller that would detect that problem, (when the speed limiting resistor is in place) if it occurs while driving.

(Without the speed limiting resistor, the controller should detect an open circuit, and shut down the motors.)

A pedal limiter in combination with the speed-limiting resistor, would not help here: the kart is being commanded to full speed regardless of pedal position. I'd think the kart would be safer with a speed-limiting resistor in series with the speed pot (and with the controller set for the other sensing arrangement). Then, if either one fails open the kart stops. If either one fails closed, you at least don't go full speed. (Failing open would have to be many times more likely than failing closed, I'd think.)

But maybe I am missing something here. As I say, I am reluctant to second guess the original engineer. Nevertheless, I'll plan to email him, to be on the safe side.

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#18

Re: Need help on an Electric Go-Cart Project

06/11/2007 9:03 AM

America could use a few thousand teachers like you! Salutations.

I am a ME so I will leave the electronics to the very capable people that are replying to your query and stick to what I know.

On the subject of "streamlining" the cart.

Stay away from hard plastics!!!

If a crash would occur the hard plastics could shatter and it will shatter into very hard, sharp pieces that could cause injuries. (Most prison shivs/knifes are made from plastics.)

Since the cart will be low speed, under 15 MPH, foam construction board or even cardboard should work just fine. When I was designing Soap Box Racers I used the foam board with much success.

The best to you and your students and best wishes for success in the upcoming competition.

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#19

Re: Need help on an Electric Go-Cart Project

06/11/2007 9:29 AM

Re: aerodynamic streamlining. See if you can get some benevolent donor to give you some thin polycarbonate, aka Lexan. Shatterproof and slippery.

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#23

Re: Need help on an Electric Go-Cart Project

06/11/2007 10:41 AM

Yes I am very confused! As a Physics teacher, I majored in Newtonian Motion and not electronics - I was a little worried about undertaking this electric go-cart and now I'm getting even more worried but I'm still attempting to understand all this.

First, I looked up the Curtis controller and to my amazement that is the exact one I have. The original directions for assembling the cart included a resistor plug that you could put in the circuit or remove depending if you wanted to limit the speed. The only problem is that is would be wired in and not easy to remove or adjust as needed. So we chose to leave it out and deal with the governor idea after getting the cart running. So far, I understand that I need to buy something called a trim pot and connect it either in series or parallel (we'll experiment and try both). However, I do not know what that is and could use more detail on what size of trim pot, where I can get one, and so on. Right now I'm also considering the block under the pedal but my students are not happy about this (they want cool electric stuff!).

Second, the cart is suppose to race up to 55 mph with an 80 pound kid on it so I don't think cardboard will work for aerodynamics but the last post suggested a material that I'm not familiar with. I would love some more details on that as well.

Thanks for all the posts!

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#28
In reply to #23

Re: Need help on an Electric Go-Cart Project

06/11/2007 12:35 PM

Sorry, Deborah, for assuming a bit more electrical knowledge than you possess.

The schematic clearly shows the resistor in parallel with the accelerator potentiometer. You would not want to use a resistor in series. That would increase resistance, which for this controller, means more speed, not less. You should not experiment willy-nilly. A go kart can kill a kid.

You should use the designed-in safety features. Check with Curtis, and they can probably supply you with a set of several plugs, for different speed limitations. If I were you, I'd feel secure knowing that I have the all the plugs (other than the one in use) in my pocket, and that if the student tried to remove a plug, the kart will not go. The people at Curtis have been doing this stuff for zillions of years, and you'd be unwise to alter their (or the kart kit supplier's) safety arrangements. You might want to paint the various plugs different colors to make them easy to identify.

I note that the schematic does not show a seat switch. These are almost always used on fork lift trucks (for which Curtis also makes controllers) and make it so that the truck cannot operate without someone in the seat. I'd want a seat switch on a go kart too. (Curtis can advise.) If you are experimenting with the kart, there are all sorts of possibilities for injury with a kart zooming away pilot-less. Also, when anyone is working on the kart, be certain the main disconnect switch, in addition to the key switch, is open (turned off). An alternative to a seat switch is a set of rigid rules, which if violated, means that the violator is kicked off the team. Even highly-trained industrial mechanics make deadly mistakes when working on equipment. Kids working around something that will go 55 mph is a very risky situation.

Carefully check physical guarding of chains and other moving pieces. Some commercially available gas go karts have caused people to be severely injured (killed if I remember correctly) as a result of getting caught in chains or belt drives. (A long- haired girl was pulled into one such drive, I believe.)

Be very careful with the batteries. A set of pliers dropped across the terminals can cause the battery to explode, spewing sulfuric acid.

Your communication should be with Curtis from this point. They know the ins and outs of their controller. Clearly, some of the people here are advising you without even having read the schematic. Some are saying "I don't s'pose you can do any damage...and if you do...blame the designer!" This is completely irresponsible. You can do grave damage; you can kill a kid.

Avoid such advice. Kids get seriously injured and killed on go karts. 55mph is far beyond fast enough to kill. I witnessed a head-on collision with two full-sized cars going 45 mph. (This is the physics equivalent of one vehicle going 45 mph into a brick wall*, plenty of which exist around school yards). The head of the mother of two kids in the convertible flew easily 75 yards, being only slowed a little as it was severed by the windshield frame.

A good acceleration problem for your kids to work out: A kid hits a wall in a go kart going 55 mph (81 feet per second). The kid's breaking arms and legs help to slow his head to 70 FPS by the time it impacts the wall. Assuming the head flattens from 8 inches in length to 3 inches as a result of the impact, what is the rate of deceleration? What is the impact force on the head?

Another one for them to work out: In a fully crash-engineered car going only 35mph into a crash test barrier, parts of the dummy's body are subjected to accelerations of 55 G. How much would a 15 lb head weigh under the influence of 55G?

You may be getting my drift. Don't experiment willy-nilly. Talk to Curtis. Don't undo the safety features the manufacturer has built in.

Polycarbonate is the generic name for Lexan, the "bullet proof" plastic. If you came up with an efficient aero design these people could make it for you. They might even do it for a low price (or nothing) if you did enough sweet talking, letter writing, etc., and promised to put their decals all over your kart. If the kart will do 55 without a body, it will do substantially more than that with good aero. (A go kart has a drag coefficient of about .8 or even higher. A really efficient body [a la solar racers] can have a drag coefficient of .1 -- i.e., not much more than a tenth the drag (given equal frontal area). Also, you can blow-form (or vacuum form) Lexan yourself, if you build a large oven. Search the web for "making a blown canopy." I agree with the suggestion of whomever said to stay away from any plastic (such as Plexiglas or acrylic) that can crack, rather than deform, on impact.

*because the total crush distance in two cars is double that for one car running into an unyielding object. See this site for links and info about crash testing.

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#35
In reply to #28

Re: Need help on an Electric Go-Cart Project

06/11/2007 3:28 PM

Thanks! This has been the most helpful post yet! We've emailed Curtis and requested more info on speed limitation. You called them "plugs". Is this what Curtis would call them or do they have a technical name we should all learn.

As for safety, we're being overly careful do to my lack of knowledge! My students are complaining on a daily basis about my level of safety so that generally means I'm doing something right

As for the "bullet proof" plastic - you've made a very happy group of teenagers. It seems you use the words "bullet proof" and 16 year old boys get very excited! We'll look into cost and shapes after we've gotten the electronics how we want them.

Thanks again

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#36
In reply to #35

Re: Need help on an Electric Go-Cart Project

06/11/2007 4:00 PM

Plug would be fine -- that's the term from their schematic. ("Plug with speed-limiting resistor", "Plug without resistor") A jack, also the term from their schematic, is the thing that a plug goes into. (I am using an admirable amount of restraint not to suggest any analogy -- especially with 16 year old boys around!)

Don't confuse this use of "plug" with the plug braking which I mentioned. Plug braking got its name, I assume, because it's a little like plugging up a pipe -- your are actually pushing current through the motor in the reverse of the usual direction. It tends to be a little hard on everything: the motors, the controller, the drivetrain pieces -- especially if done excessively by a sixteen year old, when the teacher is not looking.

There is a trick you can do where you go fast in reverse in your car, put it in drive, (or first, if you have a manual and your synchros are ok) floor it, (don't touch the brakes, cuz it ruins the fun) and spin the wheels forward while still going backwards. Eventually, the car stops and starts going forward, after leaving distinctive tire marks on the road. Plug braking, taken to extremes, can have similar effects.

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#30
In reply to #23

Re: Need help on an Electric Go-Cart Project

06/11/2007 1:12 PM

Hi,

While we are onto safety warnings.

At the risk of stating the obvious.

When you power up, or even connect the batteries for the first time. Have the Kart securely mounted with the wheels off the ground!

A rolling road test rig can be very useful and informative (but still v dangerous)

Have fun.

Del

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#37

Re: Need help on an Electric Go-Cart Project

06/11/2007 8:45 PM

Lexan plastic is used as safety glass for doors & windows & is available at hardware stores & home centers. Talk to your schools industrial arts instructor. Industrial arts instructors should be able to give you good advice.

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#38

Re: Need help on an Electric Go-Cart Project

06/11/2007 11:51 PM

Deborah,

Here's a link that you and your students might find useful as the project unfolds. The 4QD people provide lots of information on how motors are controlled in vehicles like yours.

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#39
In reply to #38

Re: Need help on an Electric Go-Cart Project

06/12/2007 9:51 AM

Thanks - the sites looks really helpful. The remote kill switch came in the mail so we're working on connectig that. Also, the engineer that created the go-cart is working on making us a prototype for the governor and said he'd have it ready in a week along with schematics on connecting it (he said we'll be connecting it in parallel just for arguments sake). The last thing we'll be working on is aerodynamics and we found a website that sells sheets of polycarbonate in about any thickness we want so we're working on our design and measurements.

Thanks to all that posted on this site - my students and I learned a lot. We'll repost if we end up needing more advice

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#40
In reply to #39

Re: Need help on an Electric Go-Cart Project

06/17/2007 8:29 PM

Probably a bit late now... You mentioned the cart cost $5,200 US? thats about 5 (four) times what I'd pay in Australia (AUS) for a petrol (Gasoline) cart brand new with racing engine and real Disc brake. Electric motors sell for between $75.00 and $1000 AUS. depending on speed and power. Batteries are similar, even lithium isnt terribly expensive now. controllers and chargers range from $25.00 to are around $350.00.

my point is I think your cart may be a little overpriced.

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#41

Re: Need help on an Electric Go-Cart Project

06/17/2007 8:35 PM

Hi,

I have4 plans for a GO Cart at home. I was thinking there are many lessons in such a tool. the steering geometry is fascinating. Little things like the ackerman angle and the wheel turning circle. the History of these items is also rather interesting.

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