dc motor and torque

[toxicmouse]

I have a small model boat that is powered by a speed 400 dc motor rated at 7,2V. the motor turns a propellor through a gearbox. the power supply is 6X 1,2V NiMH batteries connected by a relay only.

the problem is that the boat needs to move a bit faster. i dont have easy access to larger motors, but i do have a few more speed 400 motors.

there are obviously a few combinations that i can put together, but to save me on building gearboxes i was hoping a kind soul would impart some wisdom on a good solution.

i am thinking of adding another motor, joining the drive shafts and sending that through one gearbox. also attach them to the same power source. i think that this will increase the torque, meaning the prop should run faster- if too fast then it is easy to put on a larger prop. is this logical, or is it better to add a separate power source to each motor? or is it better to keep the one motor and add the two power sources in parallel to reduce the internal resistances?

many thanks

 

[davyjoneslocker]

You can use another speed 400, which should increase torque considerably. You will probably have to make a gearbox or belt drive to achieve this however, which may offset any savings of having to purchase another motor!

Andy

 

[Matt_C]

I had a picojet that ran on 480 motor and was supplied by 9.6V so you may be able to up the volts. Check the specs on your motor to find the highest volt rate it can take. You can over volt a motor but the more you over volt the shorter the life of the motor. You will get extra heat with this. Unless you know what your doing I would only take it to it's max volt limit.
Some other ideas for you to try -
Try is a different gearbox with different gearing.
Try direct drive.
Try a direct replacement 400 brushless motor but this will require a new speed controller and you could try lipos with this. Just keep the lipos sealed dry and read up on lipos before you buy them.

 

[BeanieBots]

Or, just try a larger prop.
Combining the output of two motors into one shaft will be quite a challenge
Over-volting is OK as long as you don't over-rev or over power as well.
Another trick, if you are feeling brave, is to remove a few turns of wire from each segment of the armature. The stall torque will be slightly reduced but the revs per volt will increase. Adding turns will have the opposite effect.

Don't forget, the gearbox was designed for that motor. Just like those who add turbos to their cars without up-rating the transmission, it could end in tears

 

[Wrenow]

Toxicmouse,

What size batteries are you using? If AA, you are probably overdrawing the capacity with a speed 400. If a good SubC or better, you are probably fine there.

Tinkering with the gear ratio (up or down) can make a considerable difference. The trick is to find the best speed on water to efficiency band of the motor. Changing the pitch of the prop (tweaking) can also help. As noted, prop size and design can have an effect. Surprisingly, for many scale boats, you may be better off with a 2 or 3 blade prop than 4 (the 4 bladed reportedly have a greater tendency to cavitate, thoug I have never confirmed that - I run a lot of 4 bladed props and have no issues).

Cheers,

Wreno

 

[toxicmouse]

i am using AA size batteries, but they seemed fine. i ran the motor in the water yesterday for about 20 minutes and they were not even hot, and surprisingly not particularly discharged! i dont think it is too wise to use the same supply for two speed 400 motors though, but i will give it a try out of curiosity.

 

[Michael 2727]

I had a model RC boat years ago, 7.2V motor, just over 2'
foot long, you could walk beside it as chugged along.
Then one day I replaced the prop with one that was only
2/3 the original size. Well did that thing take off, the RPM
went up by 2,000 to 4,000 and it went 3x faster.
I took it out of the creek and over to the beach where it
would punch through the small waves and get airborn just
like a real boat hitting a surf break. The difference was
truly amazing.
Keep your RPM high and you will get less heat and the
battery will last much longer (heat is wasted power).

 

[toxicmouse]

feedback

i just tested out a couple combinations and it seems Michael 2727 was right.

yesterday i had one motor

today i tried two motors on the same power supply and the same prop. the boat went significantly faster than yesterday.

then i used a prop about half the size and the boat went faster still.

the speed is currently around 4 seconds per metre.

next i will get a few more props of different sizes.

suggestions welcome.

 

[jpyle1]

Toxicmouse, When you get your props tweaked for the best run, you might want to try a trick I use on my Pittman motors to increase overall speed\torque. I use the strongest pair of permanent magnets that I can find (from old hard drives), to stick to metal case of the motor. Try different locations on the on the case, 180 degrees opposed, and best over the existing field magnets. When in the correct positions, the field strenght is increased, resulting in more speed, power and current draw. I find that on my Pittmans, there is a 10% to 12% amperage increase overall, with speed and torque boosted also. More power in = more power out. Done this for four+ years on 3 motors with no ill effect. Works for that little bit of extra punch when every thing else is tweaked for me. jpyle