MAGNETIC CIRCUITS

An air gap is usually inserted in magnetic circuits to

• A)Increase m.m.f

• B)Increase the flux

• C)Prevent saturation

• D)None of the above

The relative permeability of a ferromagnetic material is

• A)Less than one

• B)More than one

• C)More than 10

• D)More than 100 or 1000

The unit of magnetic flux is

• A)Henry

• B)Weber

• C)Ampere-turn/Weber

• D)Ampere/meter

Permeability of a magnetic circuit corresponds to………. In an electrical circuit

• A)Resistance

• B)Resistivity

• C) Conductivity

• D)Conductance

Point out the wrong statement.

Magnetic leakage is undesirable inn electric machine because it

• A)Lower their power efficiency

• B)Increases their cost of manufacture

• C)Leads to their increased weight

• D)Produces fringing

The magnetising force (H) and magnetic flux density (B) are connected by the relation

• A)B = µrH/µ0

• B)B = µH

• C) B = H/µrµ0

• D)B = µr/Hµ0

Permanent magnets are normally made of

• A)Alnico alloys

• B)Aluminium

• C)Cast iron

• D)Wrought iron

Energy stored by a coil is doubled when its current is increased by ………. percent.

• A)25

• B)50

• C)41.4

• D)100

Those magnetic materials are best suited for making armature and transformer cores which have …. permeability and …… hysteresis loss.

• A)High, high

• B)Low, high

• C)High, Low

• D)Low, Low

The rate of rise of current through an inductive coil is maximum

• A) At 63.2% of its maximum steady value

• B)

  At the start of the current flow

• C)

  After one time constant

• D)

  Near the final maximum value of current