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Jan 26, 2012, 06:36 PM
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th OP's questions are
Quote:
Originally Posted by Go4it
So why a disc motor???
What do they have to offer over other brushless motors??
and i replied with the correct answer
Quote:
Originally Posted by Adverse Effects
the wider the motor the more toque it has but the slower it spins

this is exactly what you want/need in a multicopter

as a general rule a disk motor is a lot wider then a normal barrel type of motor this is where it gets its power from and still stays light not to mention its more air contact on the windings to cool it
i should have said torque not power
he also made a comment
Quote:
Originally Posted by Go4it
I did notice the price of thes disc motors are more costly then others used on multi rotor copters
and the reply to this is

because they can charge more for the reason of mass production
not as many made cost more to make it

the more torque the bigger the prop it can spin and the higher the pitch the blades can be
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Jan 26, 2012, 07:25 PM
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Quote:
Kt = T * L * D^2
Where T is a torque-constant for the motor reliant mostly on the magnet strength and airgap. L is the length of the stator stack and D is the diameter of the stack.

There is another Kt formula based on the actual values instead of just a constant.
Kt = 1/Kv
but depending on the Kv units it may become:
Kt = 60/(2*pi*Kv)
This turns a rpm/volts Kv into a radians/second/volts so that Kt becomes Nm/A.

An interesting thing about the first formula is it shows that, as mentioned, The relationship between Kt and Kv is inverse proportional and that length of stack has a linear relationship with Kt and Kv. What it also shows is that the diameter has a squared relationship! This is important if you need more torque. For a in-wheel motor, maximizing diameter will increase torque by D^2 while increasing width only adds L.
https://www.rcgroups.com/forums/show...6&postcount=26
Last edited by Erknie; Jan 26, 2012 at 09:31 PM.
Jan 26, 2012, 07:46 PM
Registered User
Quote:
Originally Posted by Adverse Effects
th OP's questions are

and i replied with the correct answer

i should have said torque not power
he also made a comment

and the reply to this is

because they can charge more for the reason of mass production
not as many made cost more to make it

the more torque the bigger the prop it can spin and the higher the pitch the blades can be

You're still wrong. How do you know it has more torque? Just because it has a larger diameter? You're still not listening, nor did you read the OP fully. He was wondering, if it has the same kv, what is the difference? The answer is not more torque.
Jan 26, 2012, 08:00 PM
Registered User
ok rogue277 i wasnt sure b4 but now i am sure of it

your nothing more than a troll and your just talking bullsh!t to start a fight

go away and do some research

thes are just examples

11.1VDC 10AMP in the long and narrow will spin DAM fast but have little torque (this is why they use reduction gearbox on most of them)

11.1VDC 10AMP in the short and wide will spin slow but have a dam lot of torque
Jan 26, 2012, 08:23 PM
Registered User
Quote:
Originally Posted by Adverse Effects
ok rogue277 i wasnt sure b4 but now i am sure of it

your nothing more than a troll and your just talking bullsh!t to start a fight

go away and do some research

thes are just examples

11.1VDC 10AMP in the long and narrow will spin DAM fast but have little torque (this is why they use reduction gearbox on most of them)

11.1VDC 10AMP in the short and wide will spin slow but have a dam lot of torque
Saying the same thing twice after it's been disproven doesn't magically make it right. Perhaps I do need to do more research. I've been thinking actually that there's no way my knowledge of the governing equations could be based on research. In fact I must be making them all up as I go along.
Jan 26, 2012, 09:19 PM
Registered User
Hey Kids,

why can't you all just get along and learn from each other?

rogue277 - you are coming at this from a practicalish point of view - two motors with the same specs but different designs should perform very similarly.

adverse effects - you are coming at this from a more theoretical point of view - the disc design has more potential for torque from light weight, but how the manufacturers design the motor and how it performs in the flesh will be borne out in the specs.

So you are both right, but you're not arguing the same point...
Jan 26, 2012, 10:05 PM
Registered User
I think everyone needs to take a step back and look at the basics again:

power = torque x rotational speed x (scaling factor)
The scaling factor changes depending on the units being used (watts vs. HP, in-lbs. vs. ft-lbs, etc.).

Any two motors designed to provide the same kv and draw the same level of power will produce the same torque. Neither diameter or number of poles are part of the eqution.

Diameter only enters the mix when you start to worry about the mechanical and physical design of the motor.

The bottom line is that it is possible to design a 1" diameter motor with exactly the same performace (power, torque and kV) as a 2" diameter motor. One of these designs will simply be easier and less costly to manufacture.
Jan 26, 2012, 10:17 PM
Registered User
rimshotcopter's Avatar
Quote:
Originally Posted by Bill Vassallo
I think everyone needs to take a step back and look at the basics again:

power = torque x rotational speed x (scaling factor)
The scaling factor changes depending on the units being used (watts vs. HP, in-lbs. vs. ft-lbs, etc.).

Any two motors designed to provide the same kv and draw the same level of power will produce the same torque. Neither diameter or number of poles are part of the eqution.

Diameter only enters the mix when you start to worry about the mechanical and physical design of the motor.

The bottom line is that it is possible to design a 1" diameter motor with exactly the same performace (power, torque and kV) as a 2" diameter motor. One of these designs will simply be easier and less costly to manufacture.
How so? I really don't understand how this would be possible.
Jan 26, 2012, 10:27 PM
Registered User
Welcome s2156945 and Bill Vassallo.
Two first time posters agreeing in succession, how curious.

I suggest you both have a read of "Brushless Permanent Magnet Motor Design" by Dr Duane Hanselman

If you can't be bothered then at least read this thread on motor formulas

https://www.rcgroups.com/forums/showthread.php?t=587549
Jan 26, 2012, 10:33 PM
Registered User
1" diameter motor with same torque/power/kv as 2" motor simple needs stronger magnetic fields. This is a manufacturing and material selection issue, not a physics issue. You can get stronger magnetic fields by using better magnets.

I do admit that you will probably not find my theoretical 1" motor at your local hobby dealer. I simply point out that the power/torque/rpm equation does not care about diameter. The diameter is dictated by physical design, not physics.

Maybe I should not have jumped in the middle of this discussion, but I wanted to point out that some of the posts seemed to be mixing design issues with the actual physics.
Jan 26, 2012, 10:38 PM
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rimshotcopter's Avatar
I get the point however, a motor in my opinion is totally useless unless it is able to transfer the energy to the physical world in the form of motion. So it would be easier for the 2" motor to do this over the theatrical 1" motor.
Jan 26, 2012, 10:45 PM
Registered User
I agree with you, rimshotcopter. I merely point out that "easier" apllies to the manufacturing, not the physics.

Cheers
Jan 26, 2012, 11:00 PM
Registered User
Erknie -

There is some good information in the thread you refernced - thanks. It does not change or contradict the basic equation governing rotating systems:
power = torque x rotational speed x (scaling factor)

And yes - I am a fist time poster, but with over 35 years of electrical engineering experience.
Jan 26, 2012, 11:14 PM
Registered User
welcome guys ;-)
Mar 21, 2014, 01:01 AM
Registered User

settle science


I can see this i as settled science as global warming.


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