Motors Watts vs consumption

stokke

Well-Known Member
Say we want a heli to do a headspeed of 2200 rpm.

What would consume battery capacity the most; a 4500 watt motor, or a 5000 watt motor?

Since they both are doing the same headspeed, would the stronger motor drain more juice since it's stronger? Or would the weaker motor drain more juice since it's weaker, and therefore has to try harder?

Just for sake of argument, let's say they both weigh the same and the gearing is the same - only difference is that one is stronger than the other.

What would be the best choice when it comes to battery consumption?
 

Lee

Well-Known Member
Hmmm now theres a dilemma :D
I would think there is a cross over point at some RPM, where they will be the same.
Where that point is, would only be attainable by testing or asking the manufacturer.
If you think of it like a car.
At slow speed (40mph) the less powerful car will be more efficient, and high speed (100MPH) the powerful car will.
But maybe at 60MPH they are equal. Just a rough guestimate :D
 

murankar

Staff member
At some point you would not see any benefit. if everything is staying the same then the consumption is going to be roughly the same. Whats going to save you gas so to speak is going to be in the gearing.
 

pvolcko

Well-Known Member
Let's say at 2200RPM on the 4500W motor you are having to run a throttle avg of 85%. With the 5000W motor you will maybe need to run 75% throttle to maintain that same headspeed.

All else being equal, in a stable hover, both will chew up the same power. All else being equal. Of course, in the real world all else is not equal.

One, efficiency is not same through out an electric motor's throttle range. Generally speaking they are at peak efficiency at full throttle and will get lower efficiency as you feed it less throttle. So in the example the 5000W motor being at a lower throttle than the 4500W motor to get at that same headspeed will result in the 5000W motor running in a lower efficiency region than the 4500W and thus waste more energy as heat and thus use more pack capacity than the 4500W motor at that stable headspeed.

Other factors coming into play:
- Higher power motors may have higher or lower peak efficiency compared to a lower power motor depending on various factors (winding resistance may lower due to using thicker gauge wire or it may increase due to adding windings, for instance).
- Higher power motors tend to be heavier meaning you have to add just a little more throttle for a given headspeed than ideal (and as dictated by inefficiency losses to heat).
- Rarely do people install a higher power motor to go the same headspeed. Usually they're looking for more torque, more headspeed, or some combination of the two.

It's not a simple yes/no answer, unfortunately. It depends on the motor selected, the actual specs, etc. AS a general matter, though, in an ideal situation, the two motors will draw the same power for a given headspeed. In reality, the higher power motor will draw some factor more than the 4500W motor due to higher inefficiency at the lower throttle input for the given headspeed.
 

stokke

Well-Known Member
Thanks Paul. Your input does make sense, as I also read/heard that motors are most efficient at their peak power output.

So the question comes down to this; will a 4.5 kw motor running at 90~100% draw more juice than a 5.0 kw motor running at 70~85%?

- - - Updated - - -

Btw, these are the two motors in question:

kv, rpm, power, weight, shaft, pole-count, resistance

500 25.000 4,5 kW 15.87 oz 6mm 14 15,5mΩ
500 25.000 5 kW 15.87 oz 6mm 14 9,5mΩ
 

pvolcko

Well-Known Member
My guess is that in a hover only flight the 5kw motor will end up with longer flight time. The better torque and lower overall resistance and lower throttle for the given headspeed, despite operating in a lower, less efficient throttle region, will net you some small gain in flight time. However, once you start loading the motor the higher power consumption will eat up that marginal gain quickly.
 
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