Seaview

[greenman407]

Also, the aftermarket modified motors, for the most part are not useable in this situation because the shaft is too short and its got a flat running all the way up it so that a shaft seal wont work on it.

[greenman407]

Way over my head. OK so I took some new 12 volt motors that I got from Caswells and decided to try to break them in a little better. I took two D cells , glued them together and soldered them up in series. Next I took a can of water and dropped the motor in it. Within 2 minutes the water had turned black. I let it run for at least 45 minutes and then blew it out with compressed air. I let it run too long because when I checked the brushes.......they were all gone.
Ok, so lets not let them run so long. The next two I did the same way except I only let them run for three minutes. That did the trick. Now the brushes appear to fully conform to the commutator. Should see an improvement. If you look in at the brushes you can see light shining thru between the brushes and the commutator. Not Good. After the break in procedure, that went away.

[Subculture]

It has to do with the current, power is voltage times current, for example, your motor has 60 watt of power, the current at 12 volt will be 5A, at 6 volt this will be 10A, this high current will eat away your brushes.

If you halve the voltage you halve the current (ohms law), unless you alter the windings and stator of the motor, as the resistance remains a constant. A 12 volt motor running on 6 volts will draw roughly half the current. I say roughly because many factors affect voltage scaling on a brushed motor, brushless motors scale more accurately- no brush drag.

So the motors will only produce about a quarter of the power if you halve the voltage, which will affect both the torque and RPM of the motor.

12*5= 60 watts
6*2.5= 15 watts

Flipping that around if you take a motor wound for 6 volts and run it on 12, you are now putting four times the power through those windings. Not surprisingly, it's going to get a little warm unless you do something about it, either by increasing the resistance to the windings or chopping the current.

So in a nutshell I agree that running a motor at or near it's nominal voltage is required if you want the best performance from it, but bear in mind that current will not double in a motor when you halve the voltage.

[greenman407]

Im going to call Mr. C and see if he has a source for more motors.

[greenman407]

The search goes on. Thanks Manfred.

[MFR1964]

That's a good point Mark, good ventilation is a must, normally you can use the rule, the diameter of your propellor may not be bigger than the diameter of your motor, don't know if you can apply this when using a gearbox, but using direct drive it's a marker.
I would suspect that you can go a little bigger when using a gearbox, this way i run my type VII, it has gearboxes ratio 3:1, no problems sofar.

Manfred.

[greenman407]

Another thing that we are dealing with is lack of ventilation. Our motors are all sealed up inside the Subdriver. It has been said that for good health an RC motor should not go above 130 degrees F. Motors in your RC car get good ventalation going down the track. So it would seem that you need a motor that can handle your prop with power to spare so that you dont get too much heat buildup.

[MFR1964]

Yes, that can be done, but not for long periods on full throttle, most of the time we don't drive on full throttle, so in general your 6v motor will last for a longer period.

Manfred.

[trout]

I got the math!
Power = voltage X current
60w = 12v X 5amp.
60w = 6v X 10amp.
60w = 7.2v X 8.3amp
does running a 6v motor at 7.2v cause a problem?

[MFR1964]

That is looking familiar, happened to me with my balance device inside the type VII, was trying to run a12v motor on 6 volts, it did work for some time, only eating away the brushes.
It has to do with the current, power is voltage times current, for example, your motor has 60 watt of power, the current at 12 volt will be 5A, at 6 volt this will be 10A, this high current will eat away your brushes.
Always try to get the voltage of your battery as near as possible to the pecs of your motor, 7.2 volts will kill your motor on the short term, unless you use a 6v motor.
Replaced my 12v for a 6v motor and it is still running great, done this 5 years ago.

Manfred.

[greenman407]

I just have a hard time understanding why those 12 volt motors have such a hardtime turning those little empellors, especially at 3 to 1. I took one of the motors apart and looked at the brushes. You can see where the brushes were only touching the comm. in two small places. That has to effect motor operation. Maybe I just need to break in the new Caswell 12 volt motors and install them and note the improvement.

[Subculture]

Very easy to work out a ballpark figure for what you're looking for too.

Simple bit of mathmatics- multiply the pitch of the prop in inches by the RPM, then divide by 1408. This gives you a mph figure equal to 25% prop slip.

So for instance lets say your props are about 1:1 pitch to diameter ratio (many model props are but pitch can vary from about 0.8-1.6 times diameter)

1.56*5000/1408= 5.53mph

This is very simplistic, as lots of other factors like the drag of the hull and prop blade area alter the balance, but it gives you a good basis to select motors. Generally for direct drive, the motor can should be 75% or greater the diameter of the prop for it to spin it easily. Again there are other factors that can alter that, especially if using brushless motors, but for the type of motors being used here it works fairly well.

[greenman407]

Well, I was, but it wont hurt anything, if need be to take out a 3s batt and put in a 2s and retrim. If need be. As it is , it looks like I wont have any choice. I like those EBAY/UK motors that you posted a link to but I cant buy them.

[Subculture]

I thought you were running a 12 volt system- that's what I was basing my figures on.

[greenman407]

At another site, their 35 turn turns 1450 RPM per volt. So thats 10,150 divided by three gives us 3383 RPM, perhaps a little slow. I dont know.