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Old Apr 03, 2008, 07:45 AM
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High KV motor or Low KV motor?

Hi guys,

To achieve a given headspeed, is it generally better to run a 3000 KV + 15 tooth pinion or a 3500 KV motor + 13 T pinion. (for a 150 size main gear)

What are the differences in performance?

Thanks!
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Old Apr 03, 2008, 08:14 AM
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khaoz's Avatar
Randburg
Joined Jul 2005
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I've want to understand this for years now however I don't think that there is a straight forward answer to this perfectly sensible question.

Given the same motor in two configuration one with more windings and one with less, the fewer the number of windings the lower the torque and the higher the KV rating. One might therefore presume that using a lower KV motor would be better because of the increased torque except that the gearing roughly negates this.

In addition the other factors than the KV rating affect a motors performance, the wiring configuration delta or wye. Delta configurations have more torque at higher rpms and Wye configurations are exactly opposite. So if you're comparing two different motors this effect may come into play. Internal resistance of the motor also affects efficiency.

Perhaps somebody who understands this more fully can help.
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Old Apr 03, 2008, 08:32 AM
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Joined Feb 2007
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I will not pretend to understand this any better than either of you but only chime in to offer this.

If you go with a lower KV motor you may possibly be able to reduce your pinion size and go to 4S if you choose to do so later. I am backed into a 3S corner on both my 450 helis and am finding myself entertaining the thought of 4S where when my skills were less I thought I would never want to do so.
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Old Apr 03, 2008, 01:38 PM
LaurenceGough's Avatar
Reading, UK
Joined Sep 2007
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Doesn't the motor work as a flywheel at all? Higher speed = more force to stop?

Just the idea I always thought!

No idea if this even makes a difference, but it's another point out there.
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Old Apr 03, 2008, 01:54 PM
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helifrek's Avatar
United States, TN, Clarksville
Joined Feb 2004
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simple formula for calculating headspeed is

(KVxV)/(main/pinion) x throttle = headspeed

for example

(3000KV x 11.1) / (150/15) x 100%throttle = 3330

of course this doesn't factor in the drag, efficiency of the motor, battery voltage under load etc etc. just a rough idea. hope this is of some help.

Brandon
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Old Apr 03, 2008, 02:36 PM
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San Carlos, California, United States
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Quote:
Originally Posted by learninginverted
Hi guys,

To achieve a given headspeed, is it generally better to run a 3000 KV + 15 tooth pinion or a 3500 KV motor + 13 T pinion. (for a 150 size main gear)

What are the differences in performance?

Thanks!
From my experience:

Given two motors which are identical (same model) except one has a higher Kv than the other, you can basically infer a few things:

1. The higher Kv motor will have either thicker wire or fewer turns on the windings, or both. This means it will have lower resistance than the lower Kv motor.

2. The lower resistance of the higher Kv motor means it is capable of handling higher power levels than the lower Kv motor, so it will have more "punch" as long as your battery and ESC can handle it.

3. The lower resistance of the higher Kv motor means it will have a higher Io (no-load running current) than the lower Kv motor.

4. Since the higher Kv motor has a higher Io, it will be less efficient at lower power levels because more of the power is consumed by the overhead of just simply spinning the motor.

5. Also, since the Io is higher, the sweet spot for efficiency (range of maximum efficiency) will occur at a higher power level than the low Kv motor.

So:

I generally try to pick a motor so the maximum efficiency of the motor occurs at hovering power, or maybe a little above hovering power, but the motor still has enough performance so it has good climbout. Since the motor has pretty good efficiency around hover, it tends to stay cooler becauase wasted power in a motor is dissipated as heat.

If you pick a motor strictly for performance, then the "sweet spot" for efficiency will occur way above the hovering current. This means the motor will be inefficient at around hover, and your flight times will be pretty short, and the motor may become very hot.

The better quality motors (Lehner, Actro, Neu, etc) tend to be more efficient, and also be more efficient across a wider power range. So it's easier to pick a motor which has good performance, but also has a decent amount of flight time.

Couple more notes:

If you use a pinion with a very small tooth count (the smallest you can get for the shaft size) then the transfer of power from the pinion to the main gear will be inefficient because this increases the angle between the teeth. I generally try to avoid the smallest tooth count pinion for this reason, and try to get the smallest + 1.

Also, small pinions tend to be noisy because of the tooth angle between the pinion and main gear. If noise is a concern, you probably want to pick a pinion size maybe +3 or +4 from the smallest tooth count to reduce noise.

Toshi
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Old Apr 03, 2008, 07:29 PM
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Thanks. I think u guys addressed most of the points. Innovative designs did provide some feedback, but he stopped short and only said that the higher kv motor will be more efficient. (Thanks for your input lucien, it was helpful nontheless)

There is also a balance where using a lower kv motor and a big pinion might result in not enough torque to counter fast changing loads over quick pitch changes.

The motors in question are the Scorpion 2221-8 (3560kv) and the 2221-10 (3000kv).

I did decide to go with the higher kv motor, not also least because its almost sold out everywhere, i guess that might suggest something about its suitability for a 450 size heli.

Next question: What is the lowest headspeed i can run to maximise flighttime while still maintaining enough cyclic control do perform reasonably quick flips? i am not really at the level of hard 3D yet. I know its a personal preference and i would probably have to experiment on this. But what are you guys running out there? Thanks for all your input!!!
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Old Apr 03, 2008, 07:33 PM
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Quote:
Originally Posted by HeliSmith
I will not pretend to understand this any better than either of you but only chime in to offer this.

If you go with a lower KV motor you may possibly be able to reduce your pinion size and go to 4S if you choose to do so later. I am backed into a 3S corner on both my 450 helis and am finding myself entertaining the thought of 4S where when my skills were less I thought I would never want to do so.
Is a 4S any heavier? I also have another issue at hand when upgrading from a King II to a 450. I used to run thunderpower prolite 2100 MAH 15C/20C batteries which were great as they gave me 12 minute flight times per pack in the King.

I am now worried that these packs might not have enough discharge capability to feed a scorpion motor running 2700-3000 rpm.

Should this be a concern? the LHS is advising that it will result in very hot packs if the current draw is always above 20C

If this is so, it looks like i'd have to sell the batteries and move on, in which i could consider the 4S packs. (then again, i just ordered the motor )
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Old Apr 03, 2008, 08:16 PM
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Maryland, USA
Joined Jan 2007
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The power draw required by the power systems is fairly constant for a given heli and head speed. The power is in Watts though, current * voltage. The heli may draw, for example, 120W in hover; on 3S, nominally 10.5 to 11V under load, this is about 11A current. With 4S at about 15.5V, it would be 8A or a little less. So going from a system driven by a 2100mAh 3S battery, you can switch to 4S 1500mAh and have the same total power capacity but at lower current, and the weight comes out pretty close.

The ProLites on my Dragonus do just fine driving it at 2800 RPM, but it's about at the top of their comfortable range. It's nowhere near 20C- I don't know where that came from -but discharging the pack repeatedly anywhere near it's rated capacity is going to wear it out quickly.
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Old Apr 03, 2008, 10:59 PM
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Sorry if i was not clear initially. The prolites are rated 15C cont and max of 25c. (i thought it was 20)

Do you have any problems with the prolites on your dragonus (i presume its a 450 size)

If it is still managable, i will just keep them rather than selling them 2nd hand at a substantial loss.
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Old Apr 03, 2008, 11:02 PM
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I checked again and the max continous current is 31A.

Considering we do about 11A current as you mentioned under hover, and perhaps double of that under hard loads (22A) it sounds like its adequate, for a newbie like me that is..
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Old Apr 03, 2008, 11:45 PM
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Maryland, USA
Joined Jan 2007
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No problem at all; they're very good batteries in my experience. Yes, the Dragonus is a 450-sized heli. I think the highest I've measured in current was 29A in full throttle, full pitch punchouts, but that is never more than a second or two.

Do please note that the numbers I quoted above were more for comparison than anything else; they're realistic, but a lot of things can change your current draw. The real test is to try flying them and measure the flight times and temps (or use an inflight logger like the EagleTree). That said, I've flown the PL2100s as high as 15-16A continuous without them heating excessively, but typical for my 450s is somewhere between 10 and 14A or so.
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Old Apr 03, 2008, 11:51 PM
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Joined Mar 2002
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Quote:
The motors in question are the Scorpion 2221-8 (3560kv) and the 2221-10 (3000kv).

I just got that same motor today. Thinking of using 12t pinion.



I don't no what all the kv is all about, more rpm less torque? But i do know that the Neu, for me 1107 & 1105 didnt have enough power on 3s. Maybe it has something to do w/ my altitude of 5000ft. Using Neu 2100 packs, i only get about 8-9min on fff and some inverted 3d. Motor and batts are hardly warm. I guess its efficient at the cost of power?

I also got the Medusa motor today also. We'll see how the two new motors stack up to Neu in power.

Im willing to bet that the Scorpion has more torque/power because its an outrunner.
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Old Apr 04, 2008, 12:01 AM
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kansas city
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Hey Jason you package went out yesterday. Please let me know what you think against the Neu and others.
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Old Apr 04, 2008, 01:07 AM
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thazy let me know how the performance is with the 12 t, im still deciding on the pinion so your feedback is appreciated. My goal : longest flight time with the lowest head speed possible to do mild 3d. of course a higher headspeed is always better but i hope to achieve the long(est) flight time possible.
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