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Electric Motors and kV

skeplin

Senior Member
#1
Now that I'm comfortable with my FT Flyer, I'm upgrading to an FT Delta and I'm building a Spitfire. Naturally, the recommended motor on the Spitfire is larger and carries more thrust but I'm still a bit confused on how kV plays into the equation. HobbyKing provides nice statistics on battery to prop to thrust ratios but I want to know more.

For example, I have a Turnigy Park300 1600kV motor on my FT Flyer. Not a ton of thrust but it flies. The Spitfire recommends a Park450 with 1300kV. Quads typically come in around the 750kV range. Does lower kV = more thrust? They have lower revolutions per volt but does that mean more torque?

Confused.
 

earthsciteach

Moderator
Moderator
#2
In general, lower KV = more torque. More torque = ability to swing bigger prop. Bigger prop = more thrust.

BUT, KV alone does not define a motor's capabilities. The diameter and length of the motor are crucial in motor selection.
 

Ak Flyer

Fly the wings off
Mentor
#3
Kv is related to voltage. Kv is rpm x volts. A lower kv motor usually requires higher voltage to achieve the optimum rpm. Without the extra voltage you won't see a huge jump in rpm. Generally the larger a motor diameter the more torque it produces. If you look a similar sized motors with different kv the lower kv motor will usually be larger in diameter. Teach is correct, there are lots of things that play into selecting the proper motor and it takes time to learn.
 

xuzme720

Dedicated foam bender
Mentor
#5
It should be KV times Volts to get top speed(RPM) of a given motor. You'd only divide it if you had the RPM value and the voltage and needed the KV rating. But the KV rating is how many RPM's each volt produces so it IS RPM per Volt but not usually expressed as "rpm/volt" as that is perceived as "RPM divided by volts"...
Does that make sense?
 
#6
Park 300, Park 370, Park 400, Park 450, ect. are an approximate guideline to the motor's wattage limits. Remember that wattage is a product of volts (the cell count from the battery, 3S = 11.1 volts and 4S = 14.8 volts) times amperage (the load put on the motor by how big and aggressive pitch the prop is).

Park 300 motors are rated for about 80 watts of power; which on a 3S battery would be about 7 amps of load. That 7 amps of load can be from a slower KV motor swinging a larger prop (1080kv motor with a 7x5E prop for 550 grams of thrust), or it can be from a faster KV motor swinging a smaller prop (1600kv motor with a 6x4E prop for 495 grams of thrust).

Park 450 motors are rated for about 160 watts of power; which on a 3S battery would be about 14.5 amps of load. That 14.5 amps of load can be from a slower KV motor swinging a larger prop (890kv motor with a 11x7.5 prop for 1080 grams of thrust), or it can be from a faster KV motor swinging a smaller prop (1200kv motor with a 8x6 prop for about 1010 grams of thrust).

More thrust can move bigger planes or smaller planes with a higher performance capacity, but more thrust comes at the expense of wattage. Guidelines are given for different planes so that you dont risk under powering or over powering them. Once you know your range of power, your motor KV really becomes a factor of if can you put a larger prop on a given plane or is a smaller prop a better idea.
 

Johan

Senior Member
#7
It should be KV times Volts to get top speed(RPM) of a given motor. You'd only divide it if you had the RPM value and the voltage and needed the KV rating. But the KV rating is how many RPM's each volt produces so it IS RPM per Volt but not usually expressed as "rpm/volt" as that is perceived as "RPM divided by volts"...
Does that make sense?
Well, not to a scientist:

Key is, you have to read rpm/volt as rpm PER volt:

so 200 Kv is 200 rpm per 1 volt
200 rpm/ 1 volt

or

400rpm per 2 volt ect

Note that 200 rpm / 1 volt is exactly the same as 400 rpm / 2 volt namely:
200 Kv = 200 rpm / volt

----------

Think about it, it matches...

If rpm equals Kv * Volts (which is correct and to which you approve), then obviously Kv should be expressed as rpm / Volt ....
That's how it works (IMHO) ..

In theory:

I get 750 rpm If I give 1 Volt to a DT750
I get 1500 rpm If I give 2 Volt to a DT750
....
I get 8325 rpm If i give 11.1 Volt to a DT 750

In other words I get 750 Kv (= 750 / 1.0 Volt) for each Volt I give to a DT750...
So you have to multiply the Kv with each Volt you give to your motor, to get the rpm, making the unit of measure rpm/Volt.

Do the calc with the fuel consumption of your car, mile to the gallon, which scientifically is also expressed mile/gallon (mile per gallon) like rpm/volt (rpm per volt)

You'd also multiply the mile/gallon with the amount of gallons consumed to get the number of miles you've travelled, just as you'd multiply the number of Kv's with the voltage to get the rpm's you've reached

Well, that is my take on it ..
 
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pgerts

Old age member
Mentor
#10
Did any of you actually measure the KV?
KV is normally defined without any load (without propeller).
A really good motor might have an actual rpm slightly lower than the calculated but another might just have half the rev with a propeller.

You will only get accurate motor data from tests done with propellers measuring the amps drawn and the real (static or dynamic in flight) rpms measured. Almost all tests are done static.
 

Johan

Senior Member
#11
Did any of you actually measure the KV?
Well, I did not. If you refer to the part where I mentioned:

"I get 750 rpm If I give 1 Volt to a DT750
I get 1500 rpm If I give 2 Volt to a DT750"

That was purely for illustration (I don't even own DT750's)
 

xuzme720

Dedicated foam bender
Mentor
#12
Well, I did not. If you refer to the part where I mentioned:

"I get 750 rpm If I give 1 Volt to a DT750
I get 1500 rpm If I give 2 Volt to a DT750"

That was purely for illustration (I don't even own DT750's)
Me either. I assumed the question was as a subjective reference to pick out a corresponding motor based on it's Kv value.