What are the reluctance values for this DC Motor?

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    Dc Dc motor Motor
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Discussion Overview

The discussion centers around calculating the reluctance values for a DC motor, specifically addressing the components of the magnetic circuit, including the rotor and air gaps. Participants explore the complexities involved in determining these values, considering both theoretical and practical aspects.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory

Main Points Raised

  • Some participants note the presence of reluctance values for the core and air gap but question the reluctance of the rotor.
  • There is a suggestion that Finite Element Analysis might be necessary due to the complexity of the geometry involved.
  • Participants discuss the assumption of approximating the air gaps as flat, which could simplify calculations.
  • Questions arise about the material properties of the rotor, specifically whether it shares the same permeability as the armature.
  • One participant mentions that the reluctance of metal is typically much smaller than that of air gaps, suggesting that air gap reluctances might dominate in calculations.
  • There is uncertainty about how to approach the rotor's reluctance without specific information about its material properties.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the rotor's reluctance and whether it can be approximated as a cube. There is no consensus on how to proceed with the calculations, and multiple viewpoints on the assumptions needed remain unresolved.

Contextual Notes

Participants highlight limitations in the information provided, such as the lack of details about the rotor material and its permeability, which affects the calculation of reluctance.

TheRedDevil18
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Homework Statement


motor.PNG


Homework Equations

The Attempt at a Solution



I know from a basic magnetic circuit that their will be an Rcore and an Rairgap but what about the rotor in the middle ?
 
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TheRedDevil18 said:

Homework Statement


View attachment 96183

Homework Equations

The Attempt at a Solution



I know from a basic magnetic circuit that their will be an Rcore and an Rairgap but what about the rotor in the middle ?
Wow, that looks pretty non-trivial. Are you supposed to use Finite Element Analysis to come up with the answer? If it were just flat air gaps, that you should be able to calculate. But having that cylindrical piece and round air gaps makes this very complicated, IMO.
 
And BTW, your title calls this a motor. Is the cylindrical piece in the middle magnetized?
 
berkeman said:
And BTW, your title calls this a motor. Is the cylindrical piece in the middle magnetized?

The cylindrical piece in the middle is the rotor and the core is the stator. They said assume that the area of each air gap is 18 cm^2
 
TheRedDevil18 said:
The cylindrical piece in the middle is the rotor and the core is the stator. They said assume that the area of each air gap is 18 cm^2
Oh, so approximate it as two flat air gaps? That simplifies things a lot. What equations do you use to calculate the reluctance of the different magnetic path sections?
 
berkeman said:
Oh, so approximate it as two flat air gaps? That simplifies things a lot. What equations do you use to calculate the reluctance of the different magnetic path sections?

R = l/u*A

I know how to calculate the reluctance for the core and the air gap but I'm not too sure about the rotor part. Is their supposed to be a reluctance for the rotor ?
 
TheRedDevil18 said:
Is their supposed to be a reluctance for the rotor ?
Yes, but it would be good to see how they ask the question. Are you given a different μ for the rotor material? Or are you supposed to assume the same μ as the armature? If they say you can approximate the air gaps as flat, do they say you can approximate the cylindrical rotor as a cube?
 
berkeman said:
Yes, but it would be good to see how they ask the question. Are you given a different μ for the rotor material? Or are you supposed to assume the same μ as the armature? If they say you can approximate the air gaps as flat, do they say you can approximate the cylindrical rotor as a cube?

They don't say anything about the rotors material so I think it's the same as the core. They do give the diameter of the rotor core to be 4 cm. I think it has to be approximated as a cube
 
Typically, the reluctance of metal is so much smaller than that of the air gaps that one would just use the air gap reluctances to determine the total reluctance. However, since you're given all those dimensions I guess you're not supposed to do that. I hope they gave you the permeability of the metal ... then as berkeman says you have a bigger job determining the effective path areas.
BTW you never posed the question in the first place ...
 

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