New Turnigy Electric outboard project 6KW

[Bazaki]

Thanks again, i need the side covers of this housing part exactly the same to keep it waterproof with the other ends. So what about making gaps and lay around some of this epoxy stuff around it to make it waterproof. Or should it really be air to avoid eddy currents ?

Besides......at the moment my motor is really sticking to the housing, so this is no eddy current but really magnetic, so I think I need to make out of aluminum or plastic anyway.

[nieles]

any electrical insulating material will work.
the slots only need to cover the part of the housing where the rotor will be.

is the housing a structural component in the assembly right now?
if so, the strength will be compromised after you cut the slots in it.

but do the tests first, maybe it isn't even necessary to cut the slots.

[Honk]

How is your project going?

Have you calculated what the phase resistance will come at the 55KV rewound Turnigy motor?
At 6000W you must have pretty low resistance to not overheat the windings by the current needed.
2100 RPM at 55KV motor comes to approx 38.4V battery. (rpm estimated from an of the shelf prop)
6000W at 38.4V = 156amps, losses not included. Perhaps you can reach 75% motor efficiency if lucky.
So in real life your looking at 8000W, meaning 2000W in losses has to be cooled somehow, which can
be troublesome unless you have really good thermal connection to the outside water passing by.

[Jeremy Harris]

I can confirm that an 80-100 wound for 75 Kv is only OK to about 2 kW or so continuous, and even then gets pretty hot. It'll peak at around 4 or 5 kW, but that's not much use for a boat that runs at fairly high power most of the time.

An 80-100 rewound for something like 55 kV would have a lower continuous power limit, because it'd have a higher winding resistance and so higher losses. My best guess is that you'd struggle to get more than about 1 to 1.5 kW continuous from such a motor, even with a water cooled outer case.

[Bazaki]

I think both of you are right and that takes me to the next step.

A 365cm inflatable boat with one person getting in plane will cost at least 5KW input power, with 2kW of heat inside a metal housing is going to last very long I am afraid.

The next step is then make an electric surfboard, I think ( just a wild guess ) that about 2.5KW input power will get a big surfboard incl. a person in plane.
That sounds realistic right ?

About the outboardmotor, the Torqeedo 4.0 can deliver 4.0KW continuous, has that something to do with the gearing reduction inside so the motor has a lower resistance winding ?

[Jeremy Harris]

About the outboardmotor, the Torqeedo 4.0 can deliver 4.0KW continuous, has that something to do with the gearing reduction inside so the motor has a lower resistance winding ?

Yes. it's current that heats motors up, and current is directly proportional to torque. Power is proportional to torque x rpm, so if you keep the torque (and hence current) constant and increase rpm you increase power. If you take a motor that is OK for 30 A continuous without overheating and double the voltage, you will roughly get double the power and a similar amount of heat. This is a bit of a crude approximation, because if you increase rpm enough then other losses start to make the motor get hot, but it's good enough to try and explain the principle.

The torqeedo uses a small outrunner running at high rpm and a planetary gearbox to get the prop speed down. This allows the motor to spin fast and produce a lot of power without cooking. There's more on this topic in the later posts in this thread: viewtopic.php?f=39&t=38897

[Honk]

I have some info from an email conversation with the man (Jochen) behind the Torcman 685 motor that is used in Torqeedo 4.0.
He stated that the rewinded Torcman 685 is no more efficent than 75% at 4KW output. At good midrange (1.5KW) it peaked at 85%.

That means the Torqeedo 4.0 has to dissipate approx 1333W at 4KW full blast, consuming 5333W from the battery bank.
I wonder if Torqeedo 4.0 has some kind of time limit recomendations for the use of full power?
The Torcman 685 is an outrunner and they are even more difficult to cool as the windings sit far away from the water cooled casing.

A few years back ago I found some statements at various forums from a couple of guys that had burnt their Torqeedo's.
Some guy burnt the electronic controller, another guy the motor.....maybe they are safer today?

At 2:12 in the Torqeedo 4.0 video at youtube it can be seen consuming 5459W when going at planning speeds with approx 4KW output.
http://www.youtube.com/watch?v=2gIACjevcRw
This confirmes the inefficiency of the electric motor used by Torqeedo.
They should develop a motor of their own that was at least 95% efficient at top speeds......that is the minimum
efficiency I'd expect from their highly priced outboards if I was to be interested in a deal.