Basic Electricity - Senior Secondary 1 - Source of AC supply

TERM: 2^ND TERM

WEEK SEVEN

Class: Senior Secondary School 1

Age: 15 years

Duration: 40 minutes of 5 periods each

Date:

Subject: BASIC ELECTRICITY

Topic: SOURCE OF AC SUPPLY

SPECIFIC OBJECTIVES: At the end of the lesson, pupils should be able to

I.) Define alternators

II.) Describe the principles of operation of alternators

III.) Discuss the relationship between speed, number of poles and frequency

INSTRUCTIONAL TECHNIQUES: Identification, explanation, questions and answers, demonstration, videos from source

INSTRUCTIONAL MATERIALS: Videos, loud speaker, textbook, pictures,

INSTRUCTIONAL PROCEDURES

PERIOD 1-2

NOTE

SOURCE OF AC SUPPLY

Alternators are devices used in vehicles and power generation systems to convert mechanical energy into electrical energy. They are essentially generators that produce alternating current (AC).

Principles of operation of alternators

The principles of operation of alternators involve electromagnetic induction, similar to generators. Here's how they work:

1. Rotor: The rotor is the rotating component of the alternator. It consists of a coil of wire (usually wrapped around an iron core) that rotates within a magnetic field.
2. Stator: The stator is the stationary component surrounding the rotor. It contains another set of coils of wire. These coils are connected to the output terminals of the alternator.
3. Magnetic Field: A magnetic field is created within the alternator, typically by electromagnets or permanent magnets. This magnetic field remains stationary in the stator.
4. Rotation: When the rotor rotates within the magnetic field, it induces a changing magnetic flux. According to Faraday's law of electromagnetic induction, this changing magnetic flux induces an electromotive force (EMF) or voltage in the coils of the stator.
5. Output: As the rotor continues to rotate, it continuously generates alternating current (AC) in the coils of the stator. This AC output can then be rectified to produce direct current (DC) if needed or used directly in AC-powered devices.

Relationship between speed, number of poles and frequency

The relationship between speed, number of poles, and frequency is fundamental in understanding the operation of alternators and synchronous motors.

1. Speed (N): Speed refers to the rotational speed of the rotor or the shaft of the alternator or motor, usually measured in revolutions per minute (RPM) or radians per second (rad/s).
2. Number of Poles (P): The number of poles in an alternator or motor refers to the number of pairs of north and south poles on the rotor or stator. For example, if an alternator has four poles, it means there are two pairs of poles, with alternating polarity.

3.Frequency (f): Frequency refers to the rate at which the alternating current changes direction in an alternator's output. It's measured in Hertz (Hz), which represents the number of cycles per second.

The relationship between these factors can be described by the formula:

Frequency, f = PN

                     120                

Where:

- f = Frequency of the output voltage (Hz)

- P = Number of poles

- N = Rotational speed of the alternator (RPM)

This formula shows that the frequency of the output voltage is directly proportional to both the number of poles and the rotational speed of the alternator.

- Increasing the number of poles (P) increases the frequency of the output voltage.

- Increasing the rotational speed (N) also increases the frequency of the output voltage.

Conversely, decreasing the rotational speed or the number of poles will decrease the frequency of the output voltage.

EVALUATION: 1. Define alternators

2. Discuss the principles of operation of alternators
3. Discuss briefly the relationship between speed, number of poles and frequency

CLASSWORK: As in evaluation

CONCLUSION: The teacher commends the students positively