# Homework Help: What are the reluctance values for this DC Motor?

1. Feb 20, 2016

### TheRedDevil18

1. The problem statement, all variables and given/known data

2. Relevant equations

3. 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 ?

2. Feb 20, 2016

### Staff: Mentor

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.

3. Feb 20, 2016

### Staff: Mentor

And BTW, your title calls this a motor. Is the cylindrical piece in the middle magnetized?

4. Feb 20, 2016

### TheRedDevil18

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

5. Feb 20, 2016

### Staff: Mentor

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?

6. Feb 20, 2016

### TheRedDevil18

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 ?

7. Feb 20, 2016

### Staff: Mentor

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?

8. Feb 20, 2016

### TheRedDevil18

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

9. Feb 27, 2016

### rude man

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 ...