# motor diameter ?



## dougingraham (Jul 26, 2011)

They do. An example of this would be Netgain which makes 9, 11, and 13 inch diameter motors. The down side of a larger diameter is an increase in the forces which generally will limit the RPM. You can increase the diameter at the cost of max rpm. You can also increase the torque by increasing the length of the motor. The downside of this is an increase in copper and iron losses. Motor design is a series of trade offs.


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## poprock (Apr 29, 2010)

Hi, Also to be considered are the limitations on diameter imposed by front drive transaxles, my Daihatsu will only allow 11" max. I do recall one poster saying that he had to grind part of the motor casing( a Geo?). On rear drives the limiting factor on older vehicles is the steering track rod , which usually was level with the bottom of the tranny bell housing. On later rack & pinion steering models ground clearance and engine mounting crossmember or bracing bracket would be the only inhibiting factors.


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## Efiero (Feb 7, 2009)

I was thinking 20 to 24 inch diameter and thinking RWD


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## poprock (Apr 29, 2010)

Only achievable by using Subaru Utility vehicle or similar where diff is mounted to body with swing axles ; using belt or chain drive with reduction and motor mounted above diff pinion flange. I had a trial arrangement on my Toyota Dyna 150, but with motor mounted on a mount plate on diff housing. Chain drive requires an efficient lubrication system. Belt drive would be better. Positive: Under bonnet space free for batteries.Negative:Intrusion of your monster motor into load compartment.


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## major (Apr 4, 2008)

Efiero said:


> How come no one tries to use a large diameter on the motor to make more torque with the same amount of magnetic force.


 Hi Ef,

Your assumption is flawed. If, as you say, the amount of magnetic force remains constant, increased diameter does not increase torque. I know it is counter intuitive to your logic, but there is no equation for a motor which has the torque dependent on diameter, without that dimension affecting the magnetic force. The most basic torque equation is the vector cross product of the flux and current. There is no length factor (radius or diameter).

major


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## Ivansgarage (Sep 3, 2011)

In all cases, torque is the result of force applied at a radius. In all motors, the radius increases directly with the increase in diameter of the motor, resulting in a linear increase in torque with diameter solely due to the increased radius.

The available force is directly proportional to the ((flux-carrying capability)) of the stator. So, as diameter increases and the amount of flux that can be carried in the stator increases, the torque of a motor will also increase.

Therefore, in a radial motor, torque increases with the square of diameter and also increases with additional length of the motor, resulting in an overall cubic function for torque. 

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## major (Apr 4, 2008)

Ivansgarage said:


> In all cases, torque is the result of force applied at a radius. In all motors, the radius increases directly with the increase in diameter of the motor, resulting in a linear increase in torque with diameter solely due to the increased radius.
> 
> The available force is directly proportional to the ((flux-carrying capability)) of the stator. So, as diameter increases and the amount of flux that can be carried in the stator increases, the torque of a motor will also increase.
> 
> ...


Ivan,

Nobody believes me when I tell them this, so join the club  If you're so inclined, please investigate further what I said. The text I used years ago calls it very well on pages 313-316, section 7.3.4, The Motor Torque. Book is titled Basic Electric Power Engineering by Olle I. Elgerd. ISBN 0-201-01717-2. 

The author derives the basic torque equation from the basic forces and geometry. The result is that diameter cancels out of the final expression. It is only the flux and the armature current along with a machine constant which amounts to the number of armature conductors. 

So while larger diameter motors may appear to have greater torque, which they often do, it is not solely due to the increase in any particular dimension, but rather an increase in "magnetic force" which the OP qualified to be held "the same".

Regards,

major


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## Ivansgarage (Sep 3, 2011)

Major Don't feel bad, Nobody believes me when I tell them about Kevins
truck.

In reference to: Kevins truck
http://topekaelectricmotor.com/electric-vehicles/ac-project


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