3. Soft Magnetic Materials

Question 1

How are eddy current losses related to frequency and resistivity?

Answer 1

Losses are proportional to frequency^2 / resisitivity

Question 2

why do we want large hysteresis curves in hard magnets

Answer 2

so that the large coercivity prevents demanetising

Question 3

why do we want small hysteresis curves in soft magnets

Answer 3

we want them to be reversible (coercive force of 0) and a small hysteresis curve enclosed volume which gives low losses per cycle from moving domain walls around.

Question 4

How do hysteresis losses scale with frequency

Answer 4

Linearly

Question 5

What is the balancing act when designing magnets to avoid losses in magnets

Answer 5

Balancing between good magnetic traits that get better with purity (hysteresis losses scale linearly with frequency, and are caused by grain walls moving, so is improved with purity), and eddy current losses tat get worse with purity (larger grains/fewer obstacles increases the length scales over which eddy currents can flow)

Question 6

Can materials have preferential magnetisation directions?

Answer 6

YES

Question 7

What application could a square B-H curve be advantageous for?

Answer 7

Data storage, good for storing as 1 or 0, higher coercive field helps prevent demagnetisation

Question 8

What application could a flat B-H curve be good for?

Answer 8

Flat curve is q linear and thus has low losses and goof for transformer applications

Question 9

Why is a linear B-H curve beneficial for transmitting signals

Answer 9

If the B-H curve isn’t straight, your output B is going to b distorted from your input H, BH is like the transfer function from output and input voltages)

Question 10

How can grain size be increased?

Answer 10

Heat up and cool slowly to cool the grains, grain boundaries are an important obstacle that pin domain walls.

Question 11

How does adding Si to steel improve properties?

Answer 11

• Electrical resistivity increases, reducing eddy current losses
• Magnetorstriction decreases (the property of ferromagnetic materials which causes them to expand or contract in response to a magnetic field), reducing stress sensitivity and transformer hum
• Magnetocrystalline anisotrophy decreases (the property where it takes more energy to magnetise it in certain directions than in others. These directions are usually related to the principal axes of its crystal lattice) which increases the permeability.
• Increases grain size, reducing the coercive field
• Removes the FCC austenite iron phase which is not magnetic, so stays magnetic at higher temperatures

Question 12

What phase of iron is magnetic

Answer 12

alpha BCC, gamma FCC austenite is not magnetic