Size / power rating of motor, having bigger motor than needed

[trillian]

OK, for those of you familiar with the dark art of electricity and the mysteries of all that stuff to do with efficiency and amp draws under loads etc...

On any particular heli you will have an average amount of power required to hover and fly around and what it takes for a given headspeed and you'll have a certain max power draw under full pitch etc.

So what I'm curious about is this; You could have a motor that is just capable of handling all the requirements of a given heli. As if it was a .50 engine in a 50 size heli for example.

Or you could easily have a much bigger motor that is going to be spending most of its time working much less than it is capable of, say the equivalent of a .60 or .90 engine, although it is capable of much greater current draw it will never be loaded enough to produce max power or draw it's max current, it will never bog.

So between the two scenarios, is it possible that the smaller motor might actually draw bigger peaks if it is loaded heavily in relation to its max power rating? In other words is it possible to have a bigger motor actually draw less power because it is never having to work very hard, never bogged?

Will the bigger motor draw more current just in a hover simply because it's a bigger motor?

(for the purposes of this question lets ignore the weight of the motors and assume the ESC and Lipo have enough headroom for either one)

[bolders]

OK, for those of you familiar with the dark art of electricity and the mysteries of all that stuff to do with efficiency and amp draws under loads etc...

On any particular heli you will have an average amount of power required to hover and fly around and what it takes for a given headspeed and you'll have a certain max power draw under full pitch etc.

So what I'm curious about is this; You could have a motor that is just capable of handling all the requirements of a given heli. As if it was a .50 engine in a 50 size heli for example.

Or you could easily have a much bigger motor that is going to be spending most of its time working much less than it is capable of, say the equivalent of a .60 or .90 engine, although it is capable of much greater current draw it will never be loaded enough to produce max power or draw it's max current, it will never bog.

So between the two scenarios, is it possible that the smaller motor might actually draw bigger peaks if it is loaded heavily in relation to its max power rating? In other words is it possible to have a bigger motor actually draw less power because it is never having to work very hard, never bogged?

Will the bigger motor draw more current just in a hover simply because it's a bigger motor?

(for the purposes of this question lets ignore the weight of the motors and assume the ESC and Lipo have enough headroom for either one)

No I dont think it will work like this. If you take a motor and apply a load it will slow down slightly and as it slows it will draw more current. A more powerful motor will be able to draw more current to bring it back upto its rated speed than a smaller motor. As it can draw more current it will be able to produce more torque (assuming all other elements are equal such as motor efficiency etc) and so the head speed will recover faster than with a small motor.

So a small motor will draw less current but for longer and a big motor will draw more current for less time.

I guess you could liken it to 2 cars where one car has twice as much horsepower as the other. If they both do a 0-60 sprint the car with most horsepower will get there more quickly simply because it was able to deliver and use more horsepower.

[scallybert]

How do the characteristics of your bigger & smaller motors compare (eg Kv) ?

For a given motor, the output power will be roughly a scaled product of the current and revs. Probably true across different motors with the same Kv.

PWM throttling hurts efficiency. eg if you have two motors producing the same power, one at 100% throttle, the other at 50%; the 100% one should be more efficient. (If the average currents are the same, the 50% case will have double the current on the 'marks'. Hence worse I2R loss.) However, if one has lower winding resistance, that will effect this.

I reckon that a properly matched set-up, that gives 'just enough' max power, and cruises at an efficient throttle setting will be most efficient.

I2R loss is probably where you want to look, but bear in mind R will tend to decrease with Kv & also with thicker windings.


I think it would be easier to look at specific motors & configurations. (I'm tangling myself up...)

[trillian]

No I dont think it will work like this. If you take a motor and apply a load it will slow down slightly and as it slows it will draw more current. A more powerful motor will be able to draw more current to bring it back upto its rated speed than a smaller motor. As it can draw more current it will be able to produce more torque (assuming all other elements are equal such as motor efficiency etc) and so the head speed will recover faster than with a small motor.

So a small motor will draw less current but for longer and a big motor will draw more current for less time.

I guess you could liken it to 2 cars where one car has twice as much horsepower as the other. If they both do a 0-60 sprint the car with most horsepower will get there more quickly simply because it was able to deliver and use more horsepower.

Well, that's assuming that amp draw is a linear function of the load on the motor and that it is equal regardless of the percentage that draw is to the total capacity of the motor. So it's more a question of, will a motor that is pushed to near 100% of its max output draw the same current as a bigger motor pushed to only, say 70% of its max? Then the other question, looked at from a fuel economy standpoint, is if the bigger motor will draw significantly more power to do the same average work, like just outputting the wattage it takes to stay airborne.

Using the car analogy, the question is not about a sprint but about two cars driving along normally for many miles and then they encounter a hill, one has plenty of power to spare while the other is floored and grunting heavily.

I hear what you're saying, essentially putting forth the theory that it's swings and roundabouts, one draws more power for shorter duration, but this is based on it being a very linear function regardless of the load as a percentage of each motors ratings.

[trillian]

How do the characteristics of your bigger & smaller motors compare (eg Kv) ?

We're assuming the same KV. The two hypothetical motors will be flying the same heli so for the purposes of this question we'll keep the gearing the same etc.

[scallybert]

We're assuming the same KV. The two hypothetical motors will be flying the same heli so for the purposes of this question we'll keep the gearing the same etc.

Ok, what about winding resistance ?

[trillian]

Ok, what about winding resistance ?

I don't know about that stuff As I say, that's knowledge held by the dark side of the force

But I just realised, what I should really do is ask this question to the guys at Scorpion, take two of their motors as an example and the question really is about linearity of amp draw as a percentage of the motor's max and then the other issue I would think would be most pertinent is overal efficiency of the power setup due to the average throttle at the ESC.

[Paulo uk]

In a slightly less technical vein - this may answer why logo owner (myself included) always seem to run 700 size motors - it cant just be incase you want to stretch it... smoother maybe?

P

[scallybert]

I don't know about that stuff As I say, that's knowledge held by the dark side of the force

But I just realised, what I should really do is ask this question to the guys at Scorpion, take two of their motors as an example and the question really is about linearity of amp draw as a percentage of the motor's max and then the other issue I would think would be most pertinent is overal efficiency of the power setup due to the average throttle at the ESC.

AIUI (which may be wrong...) if the load, revs, LiPo, Kv & winding R are the same, the current will ~ be the same, and ~ so will efficiency.

If the winding R is different (which is likely if the motor has the same Kv, but can handle a bigger I - as it has thicker windings) , the throttling will need to change to keep the same revs. Now, I'm not sure how that pans out. Lower R -> lower throttle %age (bad); but lower R is good. It may pretty much cancel out.

Also AIUI, the fundamental calulation is quite simple. at 100% throttle, the current will be (Battery V - Back EMF)/(Winding R + Cell Internal Resistance). Your configuration needs to ensure this isn't big enough to kill ESC/LiPo/motor. The motor power rating doesn't (in itself) affect the current - but may affect winding R, or such.

[scallybert]

This may help - or not...

By definition:

Kv is 'revs per volt'

Power = V * I

power also equals torque * revs

Doing some algebra, and taking some (hopefully) slight liberties..:

The useful power is I * the Back EMF

The Back EMF is revs/Kv

Substituting, power = I * revs/Kv

Since power also = torque * revs ; then torque * revs = I * revs/Kv

Divide both sides by revs, and torque = I / Kv

[Which is what you might expect, as Kv decreases with stronger magnets, or more turns on the windings.]


So, Kv is a pretty important parameter.

It tells you the max revs you can get for a given battery voltage; and also how much torque you'll produce for a given current.

Obviously, other things come into play, like the various internal resistances; how much current the components can take; and the clever stuff, like phasing & commutation angles, etc.

But, this suggests that two motors with the same Kv value, will behave broadly the same. Even if one is rated for more power than the other.