Reducing voltage

[Steppenrig]

Hey Guys,

I'm investigating options to drop the voltage from 12 to 6 for my main motors to reduce RPM and allow the motors to run cooler

My 1:144 Cruiser has 2 motors that I are 540 size, 13000rpm no load , no load current 0.47A and max efficiency 2.883A.

1 Prop per shaft that are 45mm in diameter, and a 12v 7Ah battery (motorbike type). Speed controller (one per motor) is a mechanical variable resistor actuated by servo. they are getting most likely being ditched soon in favour of ESC's.

Every time I've run her, the motors get very hot (too hot to touch) and after a bit of research I think the solution is either reduce voltage or gear motors down. I'm leaning towards both gearing down and reducing voltage.

This is my first R/C ship, I've only taken her out a couple of times as I'm still building her and have disassembed the drivetrain to make her easier to work on.

See: viewtopic.php?f=13&t=37275

Regards

Dave

[Sean Hert]

Yeah, I'd knock that voltage down- uncooled 540's on 12V are gonna get real hot, real quick. I'd consider cooling the motors anyway, but definitely drop that voltage down.

Personally, I'm a KISS principle kinda guy, and avoid gearing my props; they can be very noisy, expensive and difficult to align properly. Drop you voltage, get some ESC's and make some simple water cooling jackets for those motors.

[Steppenrig]

Hey guys,

Sean, cheers, I'd made my mind up 75% to drop voltage, you've just confirmed my decision.

Next, can someone give me an idea how to achieve this? I've got a big (fairly expensive) 12v battery that is brand spanking new, maybe 2 hours use on it max so far. I don't want to ditch it in favour of a 6v one

Cheers

Dave

[Cadman]

I'm not all that familiar with RC running gear, but you could wire the motors in series which will effectively half the voltage going to each one. Note that this is only practical if both motors are designed to run together all the time.

[FrancisMcN]

Dave,

My suggestion would be to consider using a slower turning but higher torque motor like a 545 instead of a 540 which might well handle the 12v without overheating.

Francis Macnaughton

[Sean Hert]

You can try running them in series like Tim suggested; that's probably the best option. The problem is, trying to reduce the voltage, particularly that much, means creating lots of heat.

An ESC can help with that, but you're still talking about a lot of heat generated; you're just turning your unused energy into heat. Resistors get hot (an electric space heater is basically a large resistor), and ESC's pulse the available voltage so it appears to be less- any excess goes to the heat sinks on the ESC. It's going to depend a lot on the current needed to run your boat at the speeds you want.

Get an ESC rated for your voltages, you can get one of the large Traxxas ones meant for duel motors, and it's also meant for 14.4v, so 12V won't be a problem for it. But, even the Traxxas EVX-2 is setup for water cooling- again with the heat!

The other option is to get a different battery, which you don't want to do.

FYI in my experience, 12V R/C boats are usually much larger than 1/44; like 1/96 or 1/72. 1/144 boats typically run 6V gel cell, or 7.2V-9V systems.

[Tachikaze]

I do combat warships in 1/72nd scale. I looked over your original post on this project and have several recommendations.
First, with regards to your motors, the 540 size refers to the motor size, not the voltage rating of the motor. We often use 540 size motors that are rated at 12v. If your motors are 6v motors, with a 13000RPM rating at 6v, your have way way way too much motor here. In our club, our ships are expected to go " scale " speed, and I have noted, that on all my ships, be it the German Schnelleboote, or the 1.9m IJN Shimakaze, the no load RPMs are some where between 4500 - 4800 RPMs. Even if your motor is rated at 12V with that many RPMs, it would probably be too fast. And if you drop the voltage too low, you actually start to run the motor more inefficiently. If you wish to use the motor, my recommendation is to gear it and water cool it, but I agree with Mr. Hart, and prefer to only gear motors in my tugs, not the warships.
We use 12v - 14v Pittman motors and Maxon motors. You can often find them on ebay or places like AllEletronics.com.
We run all of our ships on twin motors, set in parallel, NOT SERIES. If you place them in series, they are going to draw as one big 12v motor, and you amp draw will be huge. With the two pittman motors, set in parallel, we often run off of Vantac 412 ESCs, or any R/C car ESC with a 12v/ 20Amp rating. The Mtronik ESCs are a great deal also. You really need to get rid of the mechanical speed control and get an ESC.
Look at getting that battery lower in the boat, its location with regards to where it sets in the boat looks good, you want your object of greatest weight to be just forward of the axis of momentum.