R/C Naval Combat

Ok, so here's the dilema: I need to lower 6VDC from my primary battery to around 3VDC or less for my drive motors.

Yes, I am running an ESC, but this is still too much voltage for my poor little drive motors. The LST only runs at whatever 12 kts is in scale speed, so this is pretty slow.

I forgot alot of my electronics know how from years ago, but wouldn't simply adding a resistor or two in the main drive power line lower my voltage? I tried this will a 100 ohm resisitor and it just killed power to the motor.

http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm

If the motors are rated for 3V, run them in series, which'll drop the per-motor voltage to 3V.

Get two 3VDC Batteries, one large amp capacity, one small. Link them in series. That would be large batt '+' to small batt '-'.

Run the motors & esc from large battery - to + (3VDC)
& the 6VDC electronics from large batter - to small battery +. (6VDC)

OR replace the motors with higher rated ones.

Or, if possible, cut down the propellers and run the motors faster.

Or, get some 2 cell nicad/mimh packs to get your voltage down there in the 2-3V range.

quote:

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Originally posted by Tugboat

If the motors are rated for 3V, run them in series, which'll drop the per-motor voltage to 3V.

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I think we had this discussion years back? :) Boy, memories!

I'm telling you that since the resistance of the motor changes based on the load (water, twigs, muck) your props will run off balanced in anything but a 'perfect' situation. --> What that means to him...

if one of his motors muck up, it will naturally lower its resitance (trying to get more current) which will lower the voltage drop on it. This means less power is provided to the motor that needs it the most! If one motor is kinda-sort mucked up, and the other is completely free... the free motor will spin over twice as fast (more current total as less resistance on the system overall) it will absorb all 6V and the mucked up motor will stop (NOTE: NOT STALL, just stop trying).

Yeah, running motors in series isn't a good idea. When I first joined the club, I bought a used Scharnhorst. Its motors were wired in series, and it couldn't steer straight. I re-wired them in parallel, and I could sail straight. That also increased my speed to about 42 knots, so I flattened out my prop blades to get me down to speed.

If you can, flatten the pitch on your props or get props with fewer blades. Or, you can gear down the motors, or you can dial down your speed with an ESC. There's nothing wrong with dialing it down to 20% throttle if that's what you need to get it slow enough.

Then again, I notice that you're aiming for 12kts scale speed. Every Big Gun club has a minimum speed that, if your ship historically went slower than that, you can up your speed to the minimum. In the WWCC, that's 25 knots. Several other clubs use 24 knots. Check with your local club to see what the minimum speed is. You may find that you don't even need to reduce your speed at all!

quote:

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Originally posted by JustinScott

Get two 3VDC Batteries, one large amp capacity, one small. Link them in series. That would be large batt '+' to small batt '-'.

Run the motors & esc from large battery - to + (3VDC)
& the 6VDC electronics from large batter - to small battery +. (6VDC)

OR replace the motors with higher rated ones.

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No no no no no! Do not mix batteries of different capacities like that :( Is very bad.

Carl, if the motors are rated for the same speed in both directions, then they should be spinning the same speed. I agree that series has issues, but in parallel there are other sets of issues, too.

Running them in series is the wrong thing to do. The internal resistance is not the same in each motor, and you will have one running fast, and the other running slow. So you will never get on speed. Using resistors are a large current drain in themselves, plus the heat given off is a bad thing.

Mike had to dial down his new boat to 21% to get it on speed using the ESC, what is your voltage deleivered to the motors with it down to 20%?

Dialing down my speed control did not change the voltage to the motors. All it did was change how often the voltage was sent to the motors (esc switching rate). :)

I think the best option is to make a 3v battery pack instead of adding weight and/or complexity with some sort of voltage step down device. Any sort of resister large enough to step voltage down by 3 volts is going to give off lots of heat.

An esc should work too.

Another option is to use a higher voltage motor. For example, the Mogador I'm building has the option to use any of the speed 400 series of motors (4.8v, 6v, or 7.2v). If the boat is too slow with the 6v motor, I'll put in a 4.8 motor. If it is too fast on the 6v motor, then it'll get a 7.2v motor. And of course, the ship will have a esc for precise speed adjustments.

quote:

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Originally posted by Tugboat


No no no no no! Do not mix batteries of different capacities like that :( Is very bad.

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Why? You can't charge them like that, you would need to charge them separately. Otherwise, I don't see the problem?

you should also make certain your esc will run at a 3V input. Not all will.

The charge/discharge characteristics of the 2 batteries will not be the same. This is not a huge problem when charging because the battery will reach a certain point, and further charging beyond that will cause minor battery life degradation. When discharging, the lower (usually) capacity battery will reach its minimum voltage first, and you run the risk of reversing cells within the battery (bad when in combat, v.bad for life of battery). Normally, when you have reversed cells, they will revert to normal when a)the discharge is removed, or at least b)when you put them on charge. If you reverse cells and keep using the battery (like if the little one gets reversed while running from warships), then you can seriously cut your battery life, or destroy it entirely. Not going to burn down your ship or anything, but it's a silly way to kill batteries. If it needs 3V, then make a 3.6V by using sets of 3 rechargeables in series (1.2V x 3).

David, without getting into a theory discussion (just woke up), there is nothing inherently wrong with running the motors in series. If you want to run them both at 6 volts, absolutely run them in parallel because that will give a higher shaft rpm. BUT, the resistance of the motors in parallel has a much higher effect on differential motor rpm than it does in series. Remember, in parallel the lower resistance gets the lion's share of the current, and gets more torque, and therefore pushes harder. In series, both motors get the same current, AND as the resistance of the motors while running is largely due to counterelectromotive force (not internal resistance), keeping in mind that CEMF for two largely identical motors is pretty much identical, the speed of the motors will be close to the same. Where it's a problem is if you are using a pair of motors that are NOT rated for the same speed forward and reverse, and are running them counterrotating. Then, one of them will be slower, causing a pull to one side. This is not as big of an issue in parallel because the motor running backwards will have a lower resitance and therefore less CEMF, therefore drawing more current, making more torque, therefore bringing it up to a speed more closely resembling the other motor's speed.

But IF you moss a prop with a set of parallel-wired motors, and one prop stops... the CEMF in that motor goes to ZERO, and current shoots UP, robbing your one good prop of most driving power (since in parallel the lower resistance gets most of the current). More importantly for people who dislike shipboard fires, the motor with no CEMF (therefore high high current) will be getting HOT, as will the wires, and possibly your ESC or MAG switches. I wonder how many people have burned out ESCs whilst trying to drive motors in parallel? (at least relative to in series) hmmm...

Wow, this caused quite a discussion. From listening to everone's comments, I think what I'm going to do is toss out my 6-cell battery idea and put in 2 battery holders each containing 2 "C" or "D" batteries bringing me up to 4.8 VDC so I can run my electronics as well.

Weight is not a problem so far as the entire ship with elecronics probably weighs in at 1/2 a pound. So I'll probably put in "D" batteries.

The problem I ran into with the motors from Radioshack is that they basically 2 types. My 3VDC motors run at 8700RPM and this other super high speed ones run at 11000RPM on 9VDC. If you think I should switch over to the larger ones, I can do that no problem. Stuff in my LST is meant to be easily replaced.

If you get 3 C- or D-cell Ni-Cads (or NiMH or whatever), you should have enough juice for running around the pond in an LST. Just join them together (the batteries) in series, and use a MAG throttle (the kind with a servo moving microswitches)(also v.cheap to make), and you'll be scooting around in no time. Just run a separate receiver battery, and that'll power the throttle and rudder servos. Easy-peasy.

If you run the motors in parallel, then you'll need to figure out something to slow the motors down. If you run them in series, they'll be running at half the battery voltage, and will seriously slow down without wasting power in a big resistor. Assuming that they'll run at that voltage :) test them and see, can't hurt anything to test.

klems, have you checked whether your club actually requires you to run at 11 knots, or do they have a higher minimum speed? If they have a higher minimum speed, then you may not need to change anything at all.

Right now, I'm independent but building according to the WWCC rules as that is my closest club and most probable that I would battle with. I just saw the updated 2008 ruleset:

"Any vessel whose prototype had a top speed of less than 25 knots may run at 25 knots in scale speed (see speed/time & radar chart below)."

Kotori are you guys serious? If so this thing is going to haul butt.

I'm sure Klems will no longer need this discussion.. but Tuggy & I have been on this course for since the beginning of the board. :) I think we know each other well enough not to offend each other, so lets have it out buddy! :)

SOooo... Tuggy... Nope. Sorry. I completely disagree with you. :) hehe :)

Firstly, it is power (Watts)that moves a motor not current (I). Really high current and really low voltage or really high voltage and really low current will do the exact same thing.. nothing at all, because the power will remain the same ~0.

Lets just do it this way.


Here is the best case scenario, both motors are matched perfectly in manufacture, speed, resistance, and (CEMF) load.

As expected, each motor is dissipating the exact same power, 7.5W.



Here are the same motors only the top motor now has a higher (CEMF) load. For example, it got a few blades of grass stuck in it which is causing more resistance against the water.

The resistance of the motor goes down, lets say by 1/3, trying to compensate for the increased load. This will reduce the overall resistance of the system, which increases current. Which is good.

HOWEVER, the top motor is now at .1 ohms. Which means it drops (V=IR) .75V and therefore 5.625 watts. Therefore it will slow down (lower power, and higher load).

ADDITIONALLY, the lower motor is still at .3 ohms. And with the increased current, will drop 2.25V & 16.875W. Therefore it will speed up (to probably over twice as fast).

So, as load is added to a motor in series it dissipates less power therefore providing less torque to the props. Looking at these results, it is no wonder Carl's Scharny drove in circles.

Additionally,

Both of these configurations are valid. However the first one is probably the best of the two. Less dependencies = less problems. Addtionally, if the motors eat all their power the cannons can still fire.

The grounds are connected for a reason. Remember to do so. Floating grounds are never a good thing.

Note: you can run whatever AH you want.. I just put something down.

Ooops.. scratch that the bottom one is bad b/c of the noise it would incur on the electronics.