MagLev C

[blakinator8]

So how would you make a motor that would run with rc run at max speed when a switch is turned on?

Depends on your setup. If you're using a speed controller, I would advise to use something else(I assume they have IC's in them), like just wiring a switch to the motor and connecting it directly to the battery.

[iwonder]

Yes, speed controllers will have motor drivers and MOSFETs(transistors, basically). However if you're just going to run a DC motor full blast, it's actually more efficient to not use an ESC, even if you had the choice.

[chalker]

I agree with this entirely. It isn't hard to find ceramic magnets with 6-9 pounds of holding force, 8-10 of which could be placed on a single vehicle. It would basically be like trying to lift a 60 pound weight.

Which illustrates part of the reason for having this test in the first place. What if your finger accidentally gets caught between the vehicle and a metal object? The prohibition on rare earth magnets is for 2 reasons: first is general safety, they are just too dangerous due to the significant strength they can exert, particularly when coupled together. the second reason is that when used improperly, they can demagnetize a maglev track very quickly, effectively ruining that track. If you can't easily remove the vehicle from the test plate, and you are sure you aren't using rare-earth magnets, you are going to have a lot of explaining to the event supervisor as to why your vehicle isn't in violation of general rule #5 which prohibits unsafe items.

Since the event rules only mention not using rare-earth magnets then wouldn't a receipt or original packaging be enough to show that they are non-rare-earth magnets. Would this be enough to support that they are not the rare-earth magnet type?

While I agree that general rule #5 prohibits the use of unsafe items (spinning fan blade ), don't forget General Rule #2: Unless otherwise stated, if items are not excluded, they are permitted unless they violate the spirit of the problem. This is just a case of the teams designing something within the boundaries of the rules they were given and has nothing to do with their misintrepretation.

It would have been different if the rules had said something about this steel plate test instead of just saying "rare-earth magnets".

Receipts or packaging should be enough to prove they aren't rare-earth magnets. Note I didn't say you'd be DQ'd nor that you were not within the spirit of the problem, just that you'd have to convince the event supervisor your device wasn't unsafe. Regardless of of whether or not the supervisor uses a metal plate test (which if they do, you should still be able to slide the vehicle off laterally instead of perpendicularly to the plate), there is bound to be other metal objects around the competition area, thus if you really have magnets totaling 60 lbs of pull force on your vehicle, you'll need to be very careful with them. General rule #5 overrides general rule #2. Also note the rules don't specific how the event supervisor will check your vehicle to ensure it meets construction specs, so they are well within their right to utilize the 'metal plate test' if they want to.

[JTMess]

Will Ohm's law (V=IR) necessarily apply to maglev vehicles in real life? For example, we are using a 7.4 volt battery (actual 8.4 voltage when charged fully) connected to a motor with 0.4 ohms of resistance. The wiring has 0.09 ohms of resistance for a total resistance of 0.49 ohms. 8.4=I(0.49) yields the current equalling 17.14 A. When measured, the actual current does in fact equal about 17 amps. We're wondering if we can increase this amount of current by purchasing a battery with a higher C rating or if the current will always remain the same due to Ohm's Law. Any help is greatly appreciated!

[joeyjoejoe]

Whoooa! You measured 17 amps!? What kind of moter are you using, one from a Tesla sports car?? With the motor dimension restriction that we have for this event, I would be surprised if anybody's motors were pulling much more than 4 amps per motor. WIth that said, you need to be very carefull with current measurements. Most low-end meters are not capable of measuring amperage above around 10a and many will actually burn up. As far as a larger battery, you could put 100 of your 8.4v batteries in parallel (for an increase in current available not voltage) and your motor would behave just like it does on one battery; just for a lot longer.

[JTMess]

Our motor behaves very sporadically. Occasionally our motors melt or simply have wires snap and our batteries seem to discharge insanely quickly. I certainly wouldn't advise trying any super low resistance setups; one of ours caused several wires to start smoking and suddenly melt all the insulation off.

[mrsteven]

Whoooa! You measured 17 amps!? What kind of moter are you using, one from a Tesla sports car?? With the motor dimension restriction that we have for this event, I would be surprised if anybody's motors were pulling much more than 4 amps per motor. WIth that said, you need to be very carefull with current measurements. Most low-end meters are not capable of measuring amperage above around 10a and many will actually burn up. As far as a larger battery, you could put 100 of your 8.4v batteries in parallel (for an increase in current available not voltage) and your motor would behave just like it does on one battery; just for a lot longer.

Mine pulls 10A

[iwonder]

Not quite, actually... Sorry, theory vs reality is a complicated beast...

17A total is not entirely insane, my motors pull 10A total and they're just the cheapest things the local hobbystore had on the shelf, just watch out for your wiring getting hot after long run times(hey, look I was right ).

And yes, ohm's law does apply in real life, however, putting 2 batteries in parallel would slightly increase the current drawn from them. Here's why...

Your 7.4v batteries change under voltage under load, if they start at 8.4v they probably drop to 7.8v or so under load, maybe more since it's such a large load. However that drop is related to the percent of total capacity you're drawing from the battery. So if I try to draw 17A from a 1Ah battery, the voltage will drop a whole lot and you'd drain the battery fast. However if I try to draw 17A from a 200Ah battery, it probably wouldn't show a very measurable voltage drop. So basically you'd see a little increase only because the battery voltage wouldn't drop as much, but it's likely on the order of a few tenths of a volt and not something you should worry about for the added mass.

As for the wire issue(I typed this before seeing your other post), use larger gauge wire, for 17A I'd actually go up to at least 12ga, or minimum whatever the motors have on them. What rating are your batteries?

joeyjoejoe, to put it all in perspective, have you ever been standing next to a motor/generator pair that's testing motor controllers rated for 2000A at 600v AC? 17A isn't a whole lot in that realm, so it all depends on your setup.

[JTMess]

We are using a 350 mAh 25C lipo battery

[joeyjoejoe]

Our motors have a DC resistance of around .8 ohms. Since you mentioned melted wires and motors, I'd expect you have a motor problem. What kind of motors are you using? Even the cheapest hobby store dc motors under 3.5cm in diameter shouldn't pull more than around 10 A (or at least not for very long). Also, you are breaking the circuit to measure current, right? I still stand by my original statement that 10+ DC amps is too high for the small motors we are allowed to use for this competition.
Iwonder: Not sure that setup would be legal for the competition .

*edit* just saw your post about your battery. Suprised the wires melted at all with that battery. You need more power than that. I'd say a battery that supplied your motor's requirements (around 8.4v) and at least 800mah should do. I've gotten my batteries and battery holders off of ebay for around $10 for a complete setup.