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brown.jacob.p
08-26-2010, 01:09 AM
hi. can anybody tell me what max efficiency current is? the question came up when i was looking at these motors. thanks

http://www.hobbyking.com/hobbyking/store/tp_motor.html

http://www.hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?catname=Towerpro+ Combos&idCategory=179&ParentCat=192

dumo01
08-26-2010, 01:50 AM
I am hoping someone else will see it and jump in to confirm or correct, but I would take that as the current draw on any given motor that results in the best usable power out of the motor vs. power into the motor. There will always be some amount of the power input to the motor that is lost as heat generation. The exact amount will vary with the physical construction characteristics of each motor, and I think what they are giving you is what they think the "sweet spot" should be for that motor. The closer you can come to that current based on the prop you use etc, the more efficiently you are able to use your fuel supply, battery in this case.

MustangMan
08-26-2010, 01:52 AM
Motor efficiency is a measure of how much of the electrical energy going into the motor come out as mechanical work.

There are many different ways in which energy can be wasted in an electric motor. There are several mechanical losses such as bearing friction, vibration, and air drag. Non-mechanical losses include wire resistance, eddy currents, loop currents (in delta wind motors), magnet saturation, and magnetic hysteresis. Many of these losses are non-linear, meaning they don't start at 0 and increase in direct proportion to the motor current. The result is that there is an operating point (current) at which the maximum amount of electrical energy is converted to mechanical energy, thus giving the highest efficiency. Pushing the motor above this point causes the losses to increase more rapidly than the current is increasing. The result is that the output continues to increase but the losses are increasing faster. In our models we typically operate our motors above the maximum efficiency current when at full throttle, but hopefully we're pretty close to maximum efficiency most of the time (assuming you're willing to fly at less than full throttle).

kyleservicetech
08-26-2010, 03:27 AM
hi. can anybody tell me what max efficiency current is? the question came up when i was looking at these motors. thanks


If you were to draw up a graph of the current input to a motor, and the corresponding horsepower (or watts output) on the motor shaft, you would wind up with a "Bell" shaped graph.

Starting with zero load on your motor, since the motor is not putting out any horsepower to a load, your efficiency is zero. And, if you load down that motor until it stalls, again your efficiency is zero, since the motor is not turning. (And, in the process will release lots of smoke, since the entire battery input to your motor winds up as heat in the motor windings.)

Now, if you slowly increase the load to the motor, you will notice the efficiency increases with the load. That is, until you reach the point where the motor is running at its best performance. That would be the "Top of the Bell" graph, and its maximum efficiency. If you keep loading the motor with even more load, the motors horsepower output will continue to increase. But, and this is a big BUT, the motor efficiency starts to drop from its best value. And as you increase the load to the motor, more and more of the current (and with it wattage input) turns into heating up the motor windings.

And, eventually, you get to the point where the windings (and perhaps the motor magnets) go past the point of no return, and burn off the insulation on the motor windings. At this point, you are looking at buying a new motor.

The peak motor efficiency will usually be at a current somthing like 20 or 40 times the the motors no load current. Different motors will have different values.

That's the nasty thing about these electric motors. They will happily turn over far oversized propellers, with little warning, until you get smoke, and damage.

This is why so many electric model flyers have a wattmeter to see exactly what is going on with their motor, ESC and battery setup. First time you do NOT burn up a motor, you've paid for the wattmeter.

mesh
08-26-2010, 06:00 PM
I'll note that we often run motors a bit harder than the maximum efficiency would dictate, essentially trading flight time for power.

Here is an efficiency vs. watts graph from the free program Drive Calculator, it is fairly typical.
The red line is watts at the propeller, the green line is efficiency %.
The red dots are the motors watt-limit, and the green dots are performance with the selected propeller.
As you can see, maximum efficiency with that motor would result in uninspiring performance.

Dr Kiwi
08-27-2010, 01:45 PM
Just for interest here is the "power curve" for a high quality Plettenberg motor... almost flat, showing that it maintains high efficiency across a wide range of power input.

flydiver
08-27-2010, 06:45 PM
The TP motors have less of max eff. than min. eff. They are cheap but a fair amount of their power consumption is wasted.

That chart from TP should be considered optimistic, maybe even fanciful as are a lot of Chinese specs. For example the 2408-21 is spec'ed for 13.8A max eff. and 16.2 max current. The reality is it starts to fry at 13A and fries quickly at 16A (unless rewound).

brown.jacob.p
08-28-2010, 12:03 AM
The TP motors have less of max eff. than min. eff. They are cheap but a fair amount of their power consumption is wasted.

That chart from TP should be considered optimistic, maybe even fanciful as are a lot of Chinese specs. For example the 2408-21 is spec'ed for 13.8A max eff. and 16.2 max current. The reality is it starts to fry at 13A and fries quickly at 16A (unless rewound).

Would you happen to know some brands that have more efficient motors? (and also somewhat cheap)

thanks

flydiver
08-28-2010, 12:21 AM
What's your application? It's most useful if you are specific > size, weight, speed, etc, not just 'a foamie'.

brown.jacob.p
08-28-2010, 12:57 AM
What's your application? It's most useful if you are specific > size, weight, speed, etc, not just 'a foamie'.

Umm not quite sure about the weight(I'm guessing around 16 oz). I plan on using the same power setup for a number of different projects so I'm not going to name a certain type of plane(warbird, jet, trainer, etc.) but the wingspan for most planes will probably be around 36 inches. I want some decent power and speed. Around 60-75mph. I plan on making an imitation of the stormlauncher, and making an extra 300 type plane, just something that will be really aerobatic, also maybe a flying wing eventually.

Hope this is specific enough.
Thanks

kyleservicetech
08-28-2010, 01:51 AM
Would you happen to know some brands that have more efficient motors? (and also somewhat cheap)

thanks

Quite often, cheap and high efficiency don't go together. But, check out the Hacker A20 series. These run about $40 or so, and will do what the mfg says they will.

http://www.aero-model.com/Hacker-Brushless-Motors-A20-Series.aspx

One of these, the Hacker A20-26M will turn a 9-4.7 prop at 8800 RPM on a 3S Lipo.

I've got an A30 series, two A40 series, and three A50 series motors. Every one will perform very close to what their specifications are. And, you can run them at maximum power without worrying about overheating.

flydiver
08-28-2010, 02:20 AM
Cheap + efficient + wing + 300 extra + decent power AND speed = unrealistic expectations

No single motor will fill that bill.
You need more information than can be reasonably typed by this guy in a reasonable period of time.

Beginners Guide to Motor and Prop Selection (http://www.rcgroups.com/forums/showthread.php?t=1136470)

Start studying.

DKNguyen
01-14-2011, 10:26 PM
Rule of thumb is 1/7th (1/6th to 18th) of the stall torque and therefore stall current.