placement online test 4

1. What is the effect of increasing the cross-sectional area of a magnetic core, assuming other parameters are constant?

• A) Increases reluctance

• B) Decreases reluctance

• C) Increases magnetic flux leakage

• D) Has no effect

Answer explanation

B) Decreases reluctance

Reluctance is inversely proportional to the cross-sectional area of the magnetic core.

Reluctance=l/𝜇 A

So, increasing area → decreases reluctance.

2. The magnetic field intensity (H) is defined as:

• A) Magneto-motive force per unit length

• B) Magnetic flux per unit area

• C) Permeability times flux density

• D) Magnetic flux times reluctance

Answer explanation

A) Magneto-motive force per unit length

Magnetic field intensity is defined as:

H=F/l=MMf/length

where MMF = magneto-motive force

3. Which of the following is NOT a unit of magnetic field strength (H)?

• A) A/m

• B) Ampere per meter

• C) N/Wb

• D) Tesla

Answer explanation

D) Tesla

Tesla (T) is the unit of magnetic flux density (B), not magnetic field intensity (H).

H is measured in A/m.

4. What is the purpose of using laminated cores in electrical machines?

• A) To reduce magnetic flux

• B) To increase reluctance

• C) To reduce eddy current losses

• D) To increase inductance

Answer explanation

C) To reduce eddy current losses

Laminated cores reduce the loop area for eddy currents, thereby minimizing these losses.

5. In which material is magnetic hysteresis loss lowest?

• A) Hard steel

• B) Cast iron

• C) Soft iron

• D) Copper

Answer explanation

C) Soft iron

Soft iron has low hysteresis loss due to its narrow hysteresis loop.

It's commonly used in transformer cores to minimize energy loss.