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Answer:

(a) Principle of working of a generator:

The principle of working of a generator is based on electromagnetic induction. When a closed loop of wire, known as a coil or armature, is rotated within a magnetic field, a varying magnetic flux passes through the coil. This changing magnetic flux induces an electromotive force (EMF) or voltage across the ends of the coil. The magnitude and direction of this induced EMF depend on the rate of change of the magnetic flux and the orientation of the coil with respect to the magnetic field.

The basic components of a generator include:

1. Armature: A coil of wire or a set of coils that rotate within the magnetic field.

2. Magnetic field: Provided by permanent magnets or electromagnets that create a constant magnetic field.

3. Slip rings and brushes: Used to collect the generated electrical energy from the rotating coil.

The generated voltage can be direct current (DC) if the coil connections remain constant, or it can be alternating current (AC) if the connections change as the coil rotates.

Here's a labelled diagram of a simple DC generator:

+--------- Slip Rings ---------+

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+----|----+ +---- Armature ----+

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+---------+ +------------------+

Field Magnets Commutator

(b) To increase the strength of the current generated by a DC generator, the following changes can be made:

1. Increase the number of turns in the armature coil: More turns in the coil will lead to a higher induced voltage for a given magnetic field strength.

2. Increase the strength of the magnetic field: This can be achieved by using stronger permanent magnets or increasing the current through the field electromagnets. A stronger magnetic field will result in a higher induced EMF in the armature coil.

3. Increase the rotation speed: Higher rotation speed will lead to a higher rate of change of magnetic flux and, consequently, a higher induced EMF.

4. Improve the design and efficiency: Reducing losses, such as resistive and magnetic losses, in the generator can help maximize the output current.

(c) We mention the frequency of alternating current (AC) but not that of direct current (DC) because:

1. AC frequency: AC is characterized by its frequency, which is the number of cycles (complete oscillations) of the current per second. The standard unit of frequency is Hertz (Hz). The frequency of AC is important for power generation, transmission, and distribution, as it determines the rotational speed of generators and the behavior of electrical devices connected to the grid.

2. DC frequency: DC, being a constant and unidirectional flow of current, does not have a frequency in the same sense as AC. DC current does not oscillate or change direction, so it does not have a frequency value associated with it. Instead, the voltage level of DC can be specified, but its "frequency" is always considered to be zero.

In summary, AC changes direction periodically and has a measurable frequency, while DC maintains a constant direction, and thus its frequency is always zero.