Guys,
Looking for ways to reduce amp draw with a particular prop. Would a motor with a larger diameter can same Kv draw less amps then a small diameter can motor for the same prop/rpm?

Best,  DennisT

Not sure how much change you are looking for. And I can’t say how much my suggestions would reduce the amp draw. Experiment and find out.

1) Adjust the timing.

2) Use a more efficient motor.

3) Add a cell and use the same motor in a lower kv.

4) Use the same prop but twist another inch of pitch into it.

Mike

From my experience in my plane I run a 4250 1150W rated motor. I  use a 2B XOAR wood prop and use 1300mAh per flight. Flight time is 5min 30sec. Average A is in the 14.2A. I run my motor on 6S.

I'm curious -- what brand is the motor? What size is the prop?

Are you downsizing the battery?  The only 4250 motors I can find are 8s turbofan motors.  If this works then a 6s 2200 would more that make up for the extra motor weight.  What is the "kv" of the motor?

Ken

Traian is using a Joker 4250 510kv motor.  It's a 3520 by another measurement method. 
Joker motors are likely made or relabled to their specs by DualSky. 
https://www.lindinger.at/en/AIRPLANES/ELECTRIC-POWERPLANTS/Motors/PLANET-HOBBY-JOKER-4250-8-V3-510-KV-BRUSHLESS-MOTOR/9741926

Something doesn't add up. If the moment arm of the magnets is longer shouldn't it require less amps for the same conditions as a shorter moment arm from a smaller can diameter, all else being the same (prop, Kv, battery, rpm setting)? If the smaller diameter could turn the same prop why would you put in a larger heavier motor?

Best,   DennisT

The motor is a black box. It converts electrical power to mechanical. A given prop at a given RPM requires a given amount of power. The only possibility is to improve efficiency. If the motor is already ~80% efficient, how much room is there for improvement when max in this size is around 90%?

It's possible a higher quality motor can do better with better copper fill and reduced iron loss. It's possible a more expensive motor is more efficient (but not absolute). Small size variations have little impact. If you had good specs for a lot of motors, you may be able to find something a little better.

Motor designers juggle the physical variables to take advantage of very small factors to get the results they want. About the only thing we can do is rewind a motor trying to improve copper fill.

Are you downsizing the battery?  The only 4250 motors I can find are 8s turbofan motors.  If this works then a 6s 2200 would more that make up for the extra motor weight.  What is the "kv" of the motor?

Ken

Ken that is definitely a viable option if you need to make the nose lighter. In my case I need the nose weight and as an additional bonus after the flight the battery is straight in to the storage charge values.

Yesterday I finally put two full patterns on the Prowler. I run a https://www.lindinger.at/en/AIRPLANES/ELECTRIC-POWERPLANTS/Motors/PLANET-HOBBY-JOKER-3548-4-5-V3-850-KV-BRUSHLESS-MOTOR/9741922 with XOAR 10X6 on a 4S 2200mAh. After the flight, which was set to 5min 15sec, I checked the battery and it read 3.82V per cell which corresponds to 45 percent left in the pack. I used 1210mAh for the flight.

My obvious conclusion is that a larger can motor will use less energy to pull the plane through a pattern compared to a smaller one. In addition I noticed no heat build up in the motor ESC or the battery, that means no wasted energy. Perhaps here is the big difference, because calculated theoretical data doesn't include this heat generated and wasted by the system.

Two motors turning the same prop at the same RPM will have to spend the same amount of energy to accomplish that. The difference comes from the small motor heating up due to power A demand to accomplish that so it consumes the energy to spin the prop plus the heat wasted energy. This ca be observed when you have motors so hot after you land, that you cant touch them with your hand. I didn't even touch the heat in the ESC or the battery but that is also wasted energy that adds to this.

Here's some follow up tests. The first post was kind of opportunity based and I just went and grabbed data on the 9 x 4.7 prop I decided was the correct prop based on previous testing. So I went back and installed the 10 x 5 on both motors and repeated the test. Unfortunately the 2221 can't pull the 10 x 5 for very long at the first test point of 10k RPM so I backed off to 9400. There's a bunch of things going on in propellers pitch and pitch speed being one. Anyway, the results are fairly evident. When the propeller becomes larger the bigger motor is necessary to run it. When it is small enough either motor performs relatively the same. It's really no different than running an IC engine big bore v small bore. The big bore handles the loads better. I think the results speak for themselves. The 2820 pulled the 10x5 at 9400 with 236 Watts while the 2221 pulled the 10x5 at 9400 with 260 Watts. That's not insignificant.

Big motors loafing do a better job of handling the additional loads such as occur during maneuvering. So if the first smaller motor is just managing the pattern, switching to a larger motor would be a good move. If the smaller motor is more than adequate for the pattern the larger motor doesn't have any benefit. It truly depends on the off point operation. We can see that in my first post where both motors were handling the smaller load just fine, neither has a big advantage operationally. In the second case which is equivalent to off point operation the advantage goes to the larger motor.

BTW, my go to motor would generally be the 2820 for this size application because of the available power overhead. But this installation case requirements necessitated the smaller power motor.

Mark do us a favor and run both motors on the same way on the same plane. After that see how much do you put back in the pack. Just make sure you run the same RPM. Doing stunts is irrelevant, flying level should be sufficient. Also if you can check temperatures on the motor battery and ESC.

Thanks Traian

You mean, in essence, exactly what I did. I ran both motors using the same ESC, timer and battery at the same RPM with a load meter in line. In case one the propeller was a 9 x 4.7 and both configurations ran basically the same 200 watts at 10,000 RPM. In case two the 10x5 the smaller motor drew 30 more watts at the same 9400 RPM. The load (RPM) had to be reduced because the smaller motor was beyond it's efficiency rage and it consume more power for the same output power. Therefor I reduced the RPM into a zone the motor wouldn't melt and took a power comparison 236 Watts for the larger motor and 263 Watts for the smaller motor. I could have run at 10k RPM but the smaller motor was pushed up above 300 Watts which is above it 280 Watts continuous rating and I didn't want to hurt it. This fact along begins to tell a story.

I could have gone back and reduced the first test to 9400 RPM and taken readings again but the result would very similar again. Both motors would be pulling roughly the same wattage from the battery at a lower wattage than the original test point which would result in a greater power difference between case 1 and case 2. I don't see value added in taking the time to do that. I don't think that has much benefit. If it does to you, then I would suggest going out and doing the test. I personally have a solid understanding of the physics involved and I just did this testing because it was convenient timing.

Putting both on the same airplane wouldn't change the results much either. Agreed that the unloading when airborne might cause, in this case, the 10x5 prop power consumption to drop more than the 9x4.7 but what you would find is that the total power consumed over the flight for the smaller motor would still be greater which I think is the point of the first question, would switching to a larger motor help. The answer is maybe, if the smaller motor is being taxed and is running beyond it's efficiency zone then yes, otherwise no. The performance of the airplane is a different question. Yes, the airplane would work better in this case with the 10x5 but it works the way this category airplane should with either.

Here's the take away. If the motor being flown is operating within it's performance range, switching to a larger motor isn't going to make much of an impact. If on the other hand the first motor is operating in a range where it is demanded, as in being loaded above it's efficiency range, moving to a larger motor would be beneficial. Again, if on the other hand the motor is already "oversized" for its task, then switching up isn't going to make a big difference. The corollary to that is that going with the next size up in diameter isn't going to hurt and will benefit from bragging rights which, all told are very important.

End of the day the propeller requires X amount of power which comes from the motor. The battery has to provide that much power plus whatever is lost along the way. As Igor pointed out in many constants and much physics interpretation required, as the torque required increases the resulting current demand increases differently for each motor. The smaller motor increases at a higher rate than the larger motor. At the propeller that power is torque times RPM = current times voltage. Between the battery and the motor output a crap ton of power is lost to heat where the power lost is current squared times the resistance along the current path. So the little motor looses because torque is a function of current which drives the power loss by a square function of that current. The battery has to provide the power for the propeller I*V plus the power lost I^2*R. Think of the power loss as that heat gun you use to shrink moneycote. It uses 1500 ish watts. That's 6 times the power of the 10x5 propeller running at 950 RPM. 

 

Nice Igor

I am not an electric motor guru, only a powerplants guy. It doesn't jell in my mind that for a constant Kv a change in diameter results in constant current for a constant power output. Intuitively that doesn't work. In fact, it boggles my brain cells. But it is. The propeller requires specific amount of power input. The work done by the propeller over the flight is that power times the duration of the flight. That doesn't change for a given configuration. For a lossless system Power out = Power in which means the power that the propeller needs comes from the motor and translates to (toque x RPM)/k = Current x voltage. Given two motors of the same Kv running at the same RPM means they are also running at the same voltage thus the current must be the same for both. That was demonstrated by my test case 1. The battery must supply the power required by the propeller plus losses to the system which is what the power meter reads. If I did this on a dynamometer we would find the power output to the propeller would be something different due losses to heat. Given the current between the two motors for running the propeller is the same the power losses would be the same as well. There is a small difference between my two motors 940 Kv vs 970 Kv which would account for the small power difference between them. This was test and demonstration of your previous input.

My test case 2 is an off point condition. In the practical world two otherwise identical motors of different diameters are hard to find so we , whom do things empirically, do the next best thing, find two motors with the same Kv and different diameters. Unfortunately the size comes along with it. I happened to have on hand two 4S cell sized  motors which is why I did what I did. If I had a 280 Watt Cobra 28xx/y to use that's what I would have done but they don't make one and I don't have one. The only option is get a bigger motor. This test test shows the impact of efficiency and losses. What happens is the smaller motor is running near it's peak efficiency while motor two is down it's curve. As more load is applied the smaller motor falls off and requires more input power to overcome it's losses. This is the practical side of empirical development. What it shows is that if the currently installed motor is performing within it's efficiency range range changing to a larger diameter won't make an impact on the work provided by the battery, ie "how many milliamp hours put back in". Actually Watt hours...

The point is, as long as the currently installed motor is running within it's useful range, 60% - 80% rated power, changing to a larger diameter motor won't make a big impact on the power. Test case 1. If, the currently installed motor is running near it's peak rated power, changing to a larger motor may make and impact. Test case 2. The dependency is based on whether the maneuvering  loads (power requirement) are pushing the motor into a higher loss or not.

50g isn't much in the grand scheme of things, but these are real tradeoffs made in designs.

If we take the mass position and rotation to the equation, it makes the difference. Motor is in tip of the nose, while battery can be at or behind 50% of the distance from CG. Therefore it has much smaller impact to moment of inertia and in case of nose heavy model also to mass at all.

The other thing is rotating 50g especially on larger diameter also makes problems - 1/ it limits quick RPM changes 2/ causes precession 3/ has stabilizing effect (like flywheel) so I personally prefer those 50g in the battery, not in the motor :- )

The dependency is based on whether the maneuvering  loads (power requirement) are pushing the motor into a higher loss or not.

Unfortunately it does. I found that really hard corner pushes power train very close to its static run. But it is hard to to simulate, because it depends on prop geometry. Especially low pitch props making high static thrust load motor during maneuvering more.

But there is another problem, and it is PWM. While iron loses depends more on mechanical properties, they are simply to estimate. But copper loses depending on current are not so easy. If we have 50% PWM in governor mode, then loses in copper are 4x higher compared to 100% PWM of the same motor and load - that is because of that square at current - if it takes 10A 100% PWM on 1 ohm (1*10^2 =100W), chopped feeding by 50% on and then 50% off, will make 20A in duty cycle what is 400W, what makes motor overloaded during duty cycle. That makes motor run as it is overloaded (compared to iron loses). It heats only by 200W, but it has no impact on its working point choice.

Fortunately duty cycle gets longer on loaded motor, so that low efficiency regime is not so harmful, it happens only on unloaded motor.   

But there is another problem, and it is PWM. While iron loses depends more on mechanical properties, they are simply to estimate. But copper loses depending on current are not so easy. If we have 50% PWM in governor mode, then loses in copper are 4x higher compared to 100% PWM of the same motor and load - that is because of that square at current - if it takes 10A 100% PWM on 1 ohm (1*10^2 =100W), chopped feeding by 50% on and then 50% off, will make 20A in duty cycle what is 400W, what makes motor overloaded during duty cycle. That makes motor run as it is overloaded (compared to iron loses). It heats only by 200W, but it has no impact on its working point choice.

Fortunately duty cycle gets longer on loaded motor, so that low efficiency regime is not so harmful, it happens only on unloaded motor.

But this doesn't represent a motor. 50hz frame rate ESC of a lifetime ago were as you describe.



Current continues to flow during FET off time during the PWM cycle. There is some additional copper loss due to motor current fluctuation. Part throttle current will never exceed static current (ignoring stalled props at low speed), so part throttle losses in the motor will never exceed full throttle. Doubling source voltage with ~50% duty cycle to get the same RPM, results in more loss due to the variation in the current waveform, but far less than 4x. I did some of these tests probably 20 years ago with RC equipment. In one test doubling voltage and reducing duty cycle(approx. 50%, I didn't measure duty cycle) at fixed RPM resulted in 19% more power from the source. This was only 100W input to a large motor and ESC so as to not burn anything up. ESC design and characteristics have an influence also. 50% duty cycle is the worst case for PWM induced losses in a motor.