Watts is watts?

[Serum]

Finnster is mixing up certain things;

efficiency is efficiency. With the hottest setups i used (120A plus nibbling from the batts) , i never got hot wires/plugs. Heat means less efficient, it's not true that a high current setup runs less efficient. and it's not all about internal resistance either. a HV setup runs more efficient on the same resistance than a HC setup.

Like i said earlier, with a higher voltage, the resistance becomes less relevant. Look at the HV cords hanging over land. they are made of aluminum, while copper has got a lower resistance. (but not a better weight/resistance factor) but due to the high voltages this rather high resistant aluminum can be used. If they put 110V on it it would be gone in a few km.

[BrianG]

In your "over land" example, I don't quite agree. Resistance in wire produces a voltage drop with a certain amount of current. Increases in current will increase the voltage drop across the wire and decrease the available voltage at the end of the wire. So, to help minimize this, they use very high voltage at relatively low currents and then use transformers to step down the voltage and step up the current nearer the load. Transformers can sorta be considered "power" transfer devices - the output power will equal the input power (minus some losses). So, if the load (at the xformer output) is pulling 10A @ 120v, that's 1200W. If the primary of the transformer is at 1200v, that's only 1A (plus a little extra for losses).

That said, we don't use such high voltage or A/C transformers in R/C, nor do we transfer power over such long distances. The only thing we really gain with using HV setups is the reduced voltage drop on the wiring/connectors due to the heavy current, don't need such beefy connectors and wiring, and maybe some benefit for the ESC as fewer FETs are needed (becauses it's the current that determines how many parallel FETs are needed). But, this assumes you have a suitably high turn motor for that HV...

[Serum]

That's what i said in the first place;

on a higher voltage the resistance becomes less of an issue.

Something else to keep in mind is motor efficiency. DC brushless motors run at optimal efficiency (ie. highest mechanical power output for least electrical power input) when copper and iron losses are balanced. Copper losses increase as current increases while iron losses increase as motor RPM increases. Spinning a motor too slowly and over gearing can cause copper losses to be too high resulting in an efficiency loss. Spinning a motor too fast and under gearing can cause iron losses to be too high resulting in an efficiency loss.

For modeling purposes where the motor's energy is being transferred into kinetic energy, weight is a factor as well. Running a motor too big for the power required would be inefficient because the model will not benefit from the full power potential of the motor and so a larger percentage of the power produced is consumed in accelerating the mass of the motor. Running a motor too small will result in copper and/or iron losses to increase too far because of the model's high demand causing temperatures to run away.

Getting the most efficient system is really a matter of matching the motor, speedo, and ESC all together for the model in question given weight, gearing constraints, power requirements, and battery space.

BTW, I've read many references here to 30000 RPM being an optimal motor RPM. From where is this number born? Is this specific to Lehner motors? Or is it a number based on common car gearing?

[BrianG]

Originally Posted by Serum

That's what i said in the first place;

on a higher voltage the resistance becomes less of an issue.

Oh, ok then. :) I guess I just misinterpreted your previous post.

[Serum]

From where is this number born?

from the hundreds of setups that are gathered here. based on their temperature.

btw, you are talking about copper losses and iron losses, but i think the statisfactionpoint of the magnet becomes more of an issue with our motors. this in combination with the max. power of the electric magnetic field. (which is the result of copper/iron losses or lack of power in the batteries)

A brushless motor is anything but a DC motor bytheway.

What do you mean by the max power of the magnetic field?

My understanding is that to maximise torque delivered to the wheels one needs to maximise the torque obtained from the motor. By running it at a high RPM and gearing further down, you're effectively increasing torque through the gearing. So a motor spinning at 60k RPM geared down to 3k RPM will result in more torque at the wheels than the same motor geared from 30k to 3k.

Originally Posted by Serum

A brushless motor is anything but a DC motor bytheway.

Yup, I know. The motors we use are similar in operation to AC motors, but they are still referred to as Brushless DC motors.

[Serum]

a magnetic field is not unlimited strong.

A lower KV motor with the same size rotor has got more torque than the very same motor with a high KV.

but a 60K spinning motor is most likely out of its efficient range and not necessarily more powerful than the 30K spinning motor. I do get your point though, but this is something to consider;

In an ideal world motors would spin all the time on 60K, but in RC vehicles motors spin up/down up/down.

it takes more time/energy to spin up/down a motor on 60K than on 30K usually a higher turning motor is a smaller one (so more nimble in spinning up/down)

But a vehicle which is geared from 60K to 3K on the wheels has got a higher gearing ratio (20:1) than a 30K to 3K (10:1), this gearing ratio makes it harder to accelerate and to brake. But this time it's not the motor which is holding things up, it's the speed of the mass. But...... what is the use for more torque? if a 30K rpm motor has got enough torque to do an instant wheel-spin on 4 wheels, with more than enough power to launch the truck to warp speed in the blink of an eye, and pops a wheelie like nothing else?

Ask Novak, they use 4400kv motors on their hvmaxx, which are not known as the best and most powerful setups available. they have got a 2-2.5 more kv than most other motors we advice.

the balance lays somewhere in the middle. Which is around 30-35000 rpm.

[Serum]

wikipedia is not the most reliable source. A bl motor might be named DC motor but it is all but a dc motor.

It runs from a DC source, but that's it. Theoretical Yes, DC.. since it doesn't hit bellow 0V

it's pure pwm, steered by its emf.

My point was you refered to iron and copper losses, i assumed you took these from a standard DC motor, since you referred to that.

Yeh, true about the useful torque point.

It would take more time to spin a motor up to 60k RPM. The question is how much more time I guess? In the heli world where most of my experience lies that is not an issue as the motor is running at a fairly constant RPM. My main point was that by running a motor at a higher RPM, closer to its limit, you gain torque through the gearing. If the resulting gain of torque is more than you need, then its possible you can use a smaller (and lighter) motor to do the job and gain some effiency by saving weight.

The iron/copper losses I refer to does pertain to the brushless motors we use (and DC brushed motors afaik). I found this article on iron losses which gets pretty technical, but probably explains it well. Found this too.

[Serum]

Yeah, nice that you mention it;

The eddy currents are responsible for the motor not being too efficient on a partial load. (segmented magnets don't suffer as much as solid magnets (or the three thing 'segments' of a feigao/wanderer/nemesis motor, which is considered a non-segmented magnet).

Like you say;

a heli runs on a rather constant RPM. (CP's at least, and i assume you are talking about CP instead of FP choppers) the pitch of the blades determine the load the motor gets. due to that the RPM vary a bit. Not as much as with cars.

Quite simple; my lehner XL2400 powered maxx felt more agile on 3S than my Lehner 2250 powered savage. Both had about the same rpm. The small motors spin up fast. (noticeable for acceleration)

Cool. :)

BTW, what do you think of the Neu 1900 series that I plan to test in my buggy?

[zeropointbug]

I don't think rotor mass has anything to do with acceleration with a vehicle. Seems to me that if you let an unloaded motor spin up to full speed, it takes what, a mere 20-30ms to do so?

So i don't think this should matter in a truck that weights 10-12 lbs loaded. As that mass of the rotor becomes far INSIGNIFICANT in the acceleration of the vehicle.

The physical weight of the motor, say what you said Serum, about an XL2400 vs. your 2250 motor, i think that's all total mass there.

Just my thoughts.

[Serum]

Try it, i wasn't a believer either, but it simple is the truth.

Mike kept on raving about his 1930, and as simple as that, it is the very truth. I takes way more before it's propelled to max RPM, it might look quick unloaded, but with a load it simple works as described.