Demagnetization and Coercive Forces

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Questions and Answers

What is the formula for calculating the air gap reluctance Rg in a magnetic circuit?

Rg = $rac{ u_0 A_g}{l_g}$

Rg = $rac{l_g}{ u_0 A_g}$ (correct)

Rg = $rac{A_g}{l_g u_0}$

Rg = $rac{A_g l_g}{ u_0}$

Which parameter is NOT needed to determine the magnet operating flux density Bm?

Applied external demagnetizing magnetic field intensity (Ha)

Temperature of the magnet (correct)

Magnet area (Am)

Magnet length (lm)

Which statement about steel reluctance in magnetic circuits is true?

Steel reluctance can be negligible under certain conditions. (correct)

Steel reluctance is always significant in calculations.

Steel reluctance increases the flux density.

Steel reluctance does not affect magnet behavior.

What does the slope of the air gap line represent in the magnetic circuit analysis?

Permeance coefficient (PC) Signup and view all the answers

What does a higher value of the relative magnetic permeability (μr) indicate about a permanent magnet?

It reduces the overall reluctance. Signup and view all the answers

What happens to the magnet operating point when an external demagnetizing field is applied?

It shifts horizontally due to Ha Signup and view all the answers

In the context of magnetic circuits, what is typically considered negligible for simplifying the circuit analysis?

Leakage flux Signup and view all the answers

During no-load conditions, where does the magnet operating point occur?

At the intersection of the demagnetization curve and the air gap line Signup and view all the answers

What is the effect of the air gap line on the magnet's flux density when an external field is applied?

It reduces the magnet operating flux density Signup and view all the answers

Which aspect must be analyzed to determine the effects of reluctance in a magnetic circuit?

Air gap dimensions and material properties Signup and view all the answers

What does a non-straight demagnetization curve in the second quadrant indicate about a magnet?

The magnet is of low-grade quality. Signup and view all the answers

In the equivalent circuit of permanent magnets, what does the Norton equivalent circuit represent?

A current source of the remnant flux in parallel with internal reluctance. Signup and view all the answers

Which components make up the external reluctance of the magnet flux path?

Steel reluctance in series with air gap reluctance. Signup and view all the answers

What is represented by the voltage source of the external mmf in the magnetic circuit?

The external magnetic field acting on the magnet. Signup and view all the answers

What does the leakage reluctance branch represent in the magnet's equivalent circuit?

The leakage flux that does not cross the air gap. Signup and view all the answers

How is the intrinsic coercive force (Hci) characterized in relation to the magnet?

It signifies the force needed to completely demagnetize the magnet. Signup and view all the answers

In the context of magnetic circuits, what is the primary use of electric circuit analysis?

To calculate the magnet operating point in the magnetic circuit. Signup and view all the answers

Which of the following is NOT a source of reluctance in magnetic circuits?

Temperature resistance. Signup and view all the answers

Study Notes

Demagnetization in Permanent Magnets

A non-linear demagnetization curve indicates easier magnet demagnetization, classifying it as low-grade permanent magnet (PM).
Coercive force necessary to reduce a magnet’s intrinsic flux density to zero is termed intrinsic coercive force (Hci).
Coercivity (Hc) pertains to the normal flux density.

Equivalent Circuit of Permanent Magnets

Electric circuit analysis is utilized to determine the operating point in magnetic circuits, treating the magnet as a flux source.
Norton equivalent circuit represents the magnet as a current source for remnant flux (Φr) parallel to internal reluctance (Rm).
External reluctance includes air gap reluctance (Rg) in series with steel reluctance (Rs).
Leakage reluctance indicates leakage flux (Φl) not crossing the air gap.
Magnets can also be influenced by an external magnetic field represented as a voltage source of external magnetomotive force (Fa).

Calculating Reluctances and Flux Density

Air gap reluctance (Rg) and magnet reluctance (Rm) are calculated using:
- Rg = lg / (μ0 * Ag)
- Rm = lm / (μ0 * μr * Am)
For analytical simplification, leakage flux and steel reluctance are often considered negligible.
Operating flux density (Bm) can be represented as:
- Bm = (1 / (lg * Am * μr)) * (B − μ0 * μr * Ha) + (Φl / Φg)

Key Variables in the Equation

Am: Area of the magnet
lm: Length of the magnet
Br: Remnant flux density of the magnet
Ag: Area of the air gap
lg: Length of the air gap
μr: Relative magnetic permeability of the magnet
Ha: Applied external demagnetizing magnetic field intensity

Graphical Representation of Magnet Operating Point

The magnetic circuit can be analyzed graphically using the demagnetization curve and air gap line.
At no-load conditions, the operating point is found at the intersection of the demagnetization curve and air gap line, defining it as no-load operating point.
The slope of the air gap line is known as the permeance coefficient (PC).
Application of an external demagnetizing field shifts the air gap line horizontally, reducing the operating flux density (Bm).

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Description

This quiz explores the demagnetization curve of magnets, focusing on the characteristics of coercive force and intrinsic coercive force. Understand how these concepts relate to low-grade permanent magnets and their magnetic properties. Test your knowledge on the behavior of magnets in different conditions.