Test: Magnetic Circuits of Electrical Machines - Electrical Engineering (EE) MCQ

• SI Unit of the magnetic flux is Weber.
• It is a vector quantity.
• It is measure by the equipment called as Fluxmeter.
• The magnetic flux per unit area is called as the flux density.

Ohm’s law for magnetic circuits states that the MMF is directly proportional to the magnetic flux where reluctance is the constant of proportionality.

Ohm’s law for the magnetic circuit’s states that the MMF is directly proportional to the magnetic flux hence as the magnetic flux decreases, the MMF also decreases.

We know that:
F = ϕS
Substituting the given values from the question, we get MMF = 15At.

Power flow through the coil is: p = ei

or,

Current through the field winding,



So, energy stored in magnetic field

We know that eddy current loss,

or

• Resistance in electrical circuit corresponds to reluctance in magnetic circuit.
• Reluctance opposes the production of magnetic flux in a magnetic circuit and resistance opposes the flow of current in electrical circuit.

Magnetic materials are broadly classified into:
1. Paramagnetic
2. Diamagnetic
3. Ferromagnetic based on the intensity of magnetization.

Properties of paramagnetic materials:

• Paramagnetic substances are those which develop feeble magnetization in the direction of the magnetizing field.
• When removing the magnetizing field, the paramagnetic material lose their magnetization.
• Paramagnetic materials have weak magnetization along the direction of the magnetic field.
• The magnetic lines of force prefer to pass through these materials.
• The magnetic permeability of paramagnetic materials is slightly greater than one.

Example: Manganese, aluminum, chromium, platinum, air, etc.

From Faraday’s law of electromagnetic induction,

(minus sign is due to Lenz’s law).

Hence, emf induced is maximum when di/dt is maximum.

• It is nothing but the EMF in a conductor in a rotating magnetic field is sinusoidal and alternating.
• It may be observed from the process of EMF generation in a conductor.
• In the initial position, the conductor is parallel with magnetic flux lines. So, there is no EMF.
• When it reaches perpendicular to flux lines, EMF generation is maximum. After that again it reaches the parallel path to flux lines. So, EMF is zero now.
• This same action happens again, but it is in the reverse direction. So, again there is an EMF but is in the negative direction.
• Hence, we get the sinusoidal waveform that has a bipolar field and that is nothing but the AC.