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e-Electric General Discussion > Interesting question about motor efficiency...

Prophet
Senior Heliman
Location: N/A

Alright, please tell me which motor/pinion choice will be more efficient... if possible.

I have 2 motors made by the same company, one motor has 2580kv and the other motor has 3000kv.
Both will be run at 100% throttle on 4S (14.8v)

To get the headspeed I want I could either run the 2580kv motor with 14 tooth pinion or run the 3000kv motor with 12 tooth pinion.

Which motor will be more efficient? The lower rpm motor that is higher geared? Or the higher rpm motor that is lower geared?

.

04-14-2008 08:47 PM

nappyroots2182
Key Veteran
Location: Moline, il

higher kv is always better as your dropping the size of the pinion making the heli have more torque which will work that system less.

DX7, Trex 600N, YS50, JR770 gyro, trex 500

04-14-2008 11:24 PM

Prophet
Senior Heliman
Location: N/A

Ok thanks, thats actually what I was thinking.

04-15-2008 06:40 AM

snobdrs
Senior Heliman
Location: coatesville,pa-usa

what if the lower KV motor has more torque?

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04-15-2008 04:21 PM

trunkmunki
Senior Heliman
Location: Bangor

Because it is turning slower to produce that torque, it will draw more current.

04-15-2008 04:25 PM

HugeOne
Key Veteran
Location: Quebec, Canada

Lower KV motor tend to have lower Io as well, therefore being more efficient at a given voltage. Also the slower spinning motor will have less iron loss.

-Hugo

Raptor e620 w/V-bar, Tango 45-06, Phoenix 85HV, TrueRC 12S1P 4000mAh

04-15-2008 07:21 PM

snobdrs
Senior Heliman
Location: coatesville,pa-usa

thats not necessairly true trunkmunki

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04-15-2008 10:32 PM

trunkmunki
Senior Heliman
Location: Bangor

That's my mistake, I made one too many assumptions before I made that statement. What needs to be considered (and I could be wrong) is if you are aiming for the same headspeed, it is generally more efficient to use the mechanical advantage of the gearing to use a lower power, but higher RPM, motor to produce the same amount of work, which is headspeed at a given load (could be read as blade pitch in this case). As for iron loss, my understanding is it is generally load dependant based on both voltage and current. A faster turning motor with a high gear ratio and lower torque, compared to another motor both running at 100%, should achieve the same headspeed more efficiently.

04-15-2008 11:27 PM

concept1
Key Veteran
Location: Youngstown, OH

that makes sence, my assumption is you really want to pick a motor that you can run at 100% throttle with the gearing to get the right headspeed. this is tricky! if the motor has too low Kv, and you need to run a larger pinion it can really suck some amps. also if the motor has too High KV it can spin way over it's max rpm and become very inefficient! and if you drop the throttle curve you can also start to loose some efficency. one test I did was to run my motor @ 100% curve and a small pinion which put the motor way over its recomended max rpm, then I ran it again with a larger pinion and droped the curve down to 85% to get the same head speed, I was supprized this acctually was more effiecient. So my thoughts are I should get a motor with slightly less Kv and run at 100% with similiar gearing

04-16-2008 02:10 PM

snobdrs
Senior Heliman
Location: coatesville,pa-usa

are we talking 100% or 100% in gov mode?

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04-16-2008 04:49 PM

concept1
Key Veteran
Location: Youngstown, OH

100% non gov

04-16-2008 04:50 PM

snobdrs
Senior Heliman
Location: coatesville,pa-usa

with brushless motors does a given % of throttle equate to a set motor speed or a set current or a set voltage? It would seem to me if your running 100% non gov mode say at hover and you load the heli up where does the extra juice come from?

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04-17-2008 05:36 PM

concept1
Key Veteran
Location: Youngstown, OH

AMPS!
at hover lets say I am pulling 10 amps and at full pitch 60 amps. Have you ever blown a cicruit breaker in the house running a table saw,or any power tool, cutting a 1/4 inch piece of wood is no problem, but 3 inch thick piece of oak blows my circuit breaker, both cuts get same 110v, but the thicker piece will suck more amps!
that is what is so nice about electric heli's, you really don't need Gov's if the motor and gearing is good, nitro is a different story, you have fuel and air mixture etc...

in generic terms the electric heli motor at 100% throttle will spin so fast, and by adding pitch it will try to keep the same rpm by sucking more amps, with the right gearing and right motor it will stay constant, or change very little. if you are running a throttle curve, now you are also bleading off voltage thru your esc loosing efficiency, and creating heat in your esc. any heat is efficency loss. but for each application you would need to see just how much efficency you loose by using a throttle curve. for example if you are over reving your motor it will probably be more efficient to lower the curve. my motor has a max rpm of 25000, and I found that over 27000 (by calculations)it is very very inefficient! it runs hot and I loose flight time. so I have lowerd my curve, but my goal is to get a motor with slighlty less Kv so I can go back to 100% throttle, I have found the best gearing for my motor, a larger pinion I loose 30 seconds of time and a smaller pinion I over speed my motor even worse and loose flight time.

04-17-2008 06:33 PM

trunkmunki
Senior Heliman
Location: Bangor

I was referring to 100% rpm, not necessarily gov./non-gov. And if you are running 100% rpm and need more power to handle a higher load, that is where amperage comes in.

04-17-2008 06:37 PM

HugeOne
Key Veteran
Location: Quebec, Canada

Quote
you really don't need Gov's if the motor and gearing is good

Quote
in generic terms the electric heli motor at 100% throttle will spin so fast, and by adding pitch it will try to keep the same rpm by sucking more amps, with the right gearing and right motor it will stay constant, or change very little.

*Electromotice force: As the motor spin, it produce voltage. At no load this voltage is almost equal to battery's voltage, pushing against it, so little current flow in the motor.

The motor will draw more amps not to keep rpm, but because its rpm is going down. As RPM goes down the back EMF* goes down as well, letting current flow in the motor. The more the load the more the RPM drop the more current and indeed the more the power.

Gov, will keep your rpm steady, by giving headroom. You just have to gear right so at 75% (non-gov) you get your desired HS. Then program that HS in gov mode. This will give 25% throttle headroom for the gov to play with.

If you gear for example 2100rpm at 100% throttle, the hs may have to drop tp 1900rpm so enough power flow to sustain high pitch. If you want ot keep 2100rpm at high pitch you'll then gear up to 2300rpm at no pitch. 2300rpm may be a bit to much, this where the gov take action. CC for example suggest to gear to obtain your desired HS at 75-85% throttle in fixed end-point, leaving the gov 15-25% throttle headroom to keep your HS steady.

-Hugo

Raptor e620 w/V-bar, Tango 45-06, Phoenix 85HV, TrueRC 12S1P 4000mAh

04-18-2008 02:37 PM

snobdrs
Senior Heliman
Location: coatesville,pa-usa

thanks hugo. thats what my thinking was

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04-18-2008 05:00 PM

JKos
Elite Veteran
Location: Town of California in the state of Maryland

There is no easy answer to Prophet's question. No matter how similar two motors are, all it takes is a small change in the winding procedure to change the efficiency under a given operating scenario. Given two motors with obviously different windings since they have different kVs, there is simply no easy answer.

Motor construction and winding is a lot math and a little black magic. Simply put, sometimes designs that look incredible on paper turn out to be complete duds in real life.

The first step is to look for existing data from other users on the setup you are contemplating. Sometimes it becomes immediately obvious that certain setups just don't work. The next step is to run the numbers on both motors through something like the famous K.C. Power System Comparison spreadsheet. Finally, only real life testing gives the final answer.

- John

Protos -- Logo 14 -- Logo 10

04-18-2008 07:33 PM

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e-Electric General Discussion > Interesting question about motor efficiency...

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