4The operating principle of electromagnetic devices
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# 4The operating principle of electromagnetic devices

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docx
07.05.2020
4The operating principle of electromagnetic devices.docx

Theoretical material for the lesson, definitions for concepts

Our lesson today is…….

How is wind power converted into electricity?

Most of the electricity we use is generated by electromagnetic induction. This process goes on in the generators at work in power stations, in wind turbines (Figure) and, on a much smaller scale, in bicycle dynamos. It is the process whereby a conductor and a magnetic field are moved relative to each other to induce, or generate, a current or electromotive force (e.m.f.).

## AC Generator

An AC generator is an electric generator that converts mechanical energy into electrical energy in form of alternative emf or alternating current. AC generator works on the principle of ”Electromagnetic Induction”.

### Parts of an AC Generator

An Ac generator consists of two poles i.e is the north pole and south pole of a magnet so that we can have a uniform magnetic field. There is also a coil which is rectangular in shape that is the armature. These coils are connected to the slip rings and attached to them are carbon brushes.

The slip rings are made of metal and are insulated from each other. The brushes are carbon brushes and one end of each brush connects to the ring and other connects to the circuit. The rectangular coils rotate about an axis which is perpendicular to the magnetic field.  There is also a shaft which rotates rapidly.

### Working of an AC Generator

When the armature rotates between the poles of the magnet upon an axis perpendicular to the magnetic field, the flux which links with the armature changes continuously. Due to this, an emf is induced in the armature. This produces an electric current through the galvanometer and the slip rings and brushes.

The galvanometer swings between the positive and negative values. This indicates that there is an alternating current flowing through the galvanometer.

Emf induced in an AC generator

If the coil of turn and area A  is rotated at v  revolutions per second in a uniform magnetic field B, then the motional emf produced is e = NBA(2πv)sin(2πv)t, where we assume that at time t = 0 s, the coil is perpendicular to the field. The direction of the induced emf is given by Fleming’s right-hand rule or the Lenz’s law.

Fleming’s right-hand rule states that, stretch the forefinger, the middle finger and the thumb of the right hand such that they are manually perpendicular to each other. If the forefinger indicates the direction of the magnetic field, rhumb indicates the direction of the motion of the conductor. The middle finger indicates the direction of the induced current in the conductor.

3-Phase AC generator

In a symmetric three-phase power supply system, three conductors each carry an alternating current of the same frequency and voltage amplitude relative to a common reference but with a phase difference of one third the period. The common reference usually connects to ground and often to a current-carrying conductor that is neutral.

Due to the phase difference, the voltage on any conductor reaches its peak at one-third of a cycle after one of the other conductors and one-third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load. It also makes it possible to produce a rotating magnetic field in an electric motor and generate other phase arrangements using transformers

Electromagnetic Relay Working Principle

How does an electromagnetic relay work?
As the below figure showed, electromagnetic relay consists of electromagnet, armature, spring, movable contact and stationary contact.
Usually an electromagnetic relay has two circuits, low-voltage control circuit and high-voltage working circuit.The low-voltage control circuit includes an electromagnetic relay coil, a low-voltage power supply and a switch. The high-voltage working circuit includes a high-voltage power supply, a motor and the contacts of the electromagnetic relay.
The working principle of electromagnetic relays is not complicated, and it operates mainly according to the principle of electromagnetic induction. Switching on the power in the low-voltage control circuit, the current goes through the coil of the electromagnet to generate a magnetic field. Then the armature generates a suction force to making the movable contact and stationary contact touching. Thus the working circuit is powered on and the motor begins to work. When switching off the power in the low-voltage control circuit, the current in the coil will disappear and the armature under the action of the spring will separate the movable contact and stationary contact. The working circuit is disconnected and the motor stops working.

Generally speaking, an electromagnetic relay uses electromagnet to control “on” or “off” status of the operating circuit. When placing voltage to both ends of a coil, the coil will be flowed with current and generate electromagnetic effect. The electromagnet will attract armature to the iron core against tension of spring, so as to pull the movable contact of the armature to the stationary contact (normally open contact, or NO). When cutting off power, attraction of the electromagnet will disappear and the armature will restore its position under tension of the spring to release the movable contract from the stationary contact (normally closed contract or NC). The pulling and releasing are used to control opening and closing of the circuit. Normally open and closed contacts respectively refer to the stationary contract is in the state of “on” when the coil is cut off from power and the stationery contract is in the state of “off” when the coil is connected to power.

One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

The reason for transforming the voltage to a much higher level is that higher distribution voltages implies lower currents for the same power and therefore lower I2*R losses along the networked grid of cables. These higher AC transmission voltages and currents can then be reduced to a much lower, safer and usable voltage level where it can be used to supply electrical equipment in our homes and workplaces, and all this is possible thanks to the basic Voltage Transformer.

A Typical Voltage Transformer

The Voltage Transformer can be thought of as an electrical component rather than an electronic component. A transformer basically is very simple static (or stationary) electro-magnetic passive electrical device that works on the principle of Faraday’s law of induction by converting electrical energy from one value to another.

The transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer itself. A transformer operates on the principals of “electromagnetic induction”, in the form of  Mutual Induction.

Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Then we can say that transformers work in the “magnetic domain”, and transformers get their name from the fact that they “transform” one voltage or current level into another.

Transformers are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of electrical power being transferred from one winding to another via the magnetic circuit.

A single phase voltage transformer basically consists of two electrical coils of wire, one called the “Primary Winding” and another called the “Secondary Winding”. For this tutorial we will define the “primary” side of the transformer as the side that usually takes power, and the “secondary” as the side that usually delivers power. In a single-phase voltage transformer the primary is usually the side with the higher voltage.

These two coils are not in electrical contact with each other but are instead wrapped together around a common closed magnetic iron circuit called the “core”. This soft iron core is not solid but made up of individual laminations connected together to help reduce the core’s losses.

The two coil windings are electrically isolated from each other but are magnetically linked through the common core allowing electrical power to be transferred from one coil to the other. When an electric current passed through the primary winding, a magnetic field is developed which induces a voltage into the secondary winding as shown.

### Single Phase Voltage Transformer

In other words, for a transformer there is no direct electrical connection between the two coil windings, thereby giving it the name also of an Isolation Transformer. Generally, the primary winding of a transformer is connected to the input voltage supply and converts or transforms the electrical power into a magnetic field. While the job of the secondary winding is to convert this alternating magnetic field into electrical power producing the required output voltage as shown.

### Transformer Construction (single-phase)

·         Where:

·           VP  -  is the Primary Voltage

·           VS  -  is the Secondary Voltage

·           NP  -  is the Number of Primary Windings

·           NS  -  is the Number of Secondary Windings

·           Φ (phi)  -  is the Flux Linkage

Notice that the two coil windings are not electrically connected but are only linked magnetically. A single-phase transformer can operate to either increase or decrease the voltage applied to the primary winding. When a transformer is used to “increase” the voltage on its secondary winding with respect to the primary, it is called a Step-up transformer. When it is used to “decrease” the voltage on the secondary winding with respect to the primary it is called a Step-down transformer.

However, a third condition exists in which a transformer produces the same voltage on its secondary as is applied to its primary winding. In other words, its output is identical with respect to voltage, current and power transferred. This type of transformer is called an “Impedance Transformer” and is mainly used for impedance matching or the isolation of adjoining electrical circuits.

The difference in voltage between the primary and the secondary windings is achieved by changing the number of coil turns in the primary winding ( NP ) compared to the number of coil turns on the secondary winding ( NS ).

As the transformer is basically a linear device, a ratio now exists between the number of turns of the primary coil divided by the number of turns of the secondary coil. This ratio, called the ratio of transformation, more commonly known as a transformers “turns ratio”, ( TR ). This turns ratio value dictates the operation of the transformer and the corresponding voltage available on the secondary winding.

It is necessary to know the ratio of the number of turns of wire on the primary winding compared to the secondary winding. The turns ratio, which has no units, compares the two windings in order and is written with a colon, such as 3:1 (3-to-1). This means in this example, that if there are 3 volts on the primary winding there will be 1 volt on the secondary winding, 3 volts-to-1 volt. Then we can see that if the ratio between the number of turns changes the resulting voltages must also change by the same ratio, and this is true.

Transformers are all about “ratios”. The ratio of the primary to the secondary, the ratio of the input to the output, and the turns ratio of any given transformer will be the same as its voltage ratio. In other words for a transformer: “turns ratio = voltage ratio”. The actual number of turns of wire on any winding is generally not important, just the turns ratio and this relationship is given as:

### A Transformers Turns Ratio

Assuming an ideal transformer and the phase angles:  ΦP ≡ ΦS

Note that the order of the numbers when expressing a transformers turns ratio value is very important as the turns ratio 3:1 expresses a very different transformer relationship and output voltage than one in which the turns ratio is given as: 1:3.

Instructions for demonstrations and safety

Warning: experiments should be performed under the supervision of teachers or students followthe instructions of safety procedures.

Additional guidelines for organizing a lesson

1.      Organization moment. Establishing emotional state. Checking for absent students.

2.      Teacher provides a class discussion for: How is wind power converted into electricity?

Individual students are called on to respond to questions and share their own opinions/thoughts. Then she explains that electricity is generated by electromagnetic induction. This process goes on in the generators at work in power stations, in wind turbines.

Teacher introduces the topic and objectives of the lesson, assess criteria.

3.      Teacher asks learners to divide into 3 groups and study the operational principle of electromagnetic relay, generator and transformer based on a given model.

4.      Groups assess each other’s presentations and provide fair and helpful feedback by using a assessment criteria.

5.      Teacher asks Learners individually answer the MisConceptual Questions. They can use A, B, C, D and E cards to show their answers. The questions are shown at the interactive board.

Laminate a set of cards so every member of the class has five, with A,B,C, D and E written

Encourage them not to look at other people’s responses so as to copy.

6.      The Teacher checks and assesses each student’s answer and provides fair and helpful feedback.

7.      At the end of the lesson students are encouraged to reflect on what they have learned and what they need to improve.

Recommendations for formative assessment

Activity 1.Students discuss learning objectives and assess criteria.

Activity 2.A class discussion about: How is wind power converted into electricity?

Individual students are called on to respond to questions and share their own

opinions/thoughts.

Activity 3. Learners are  divided into 3  groups and study the operational principle of

electromagnetic relay, generator and transformer based on a given model. The

results of group works should be given in the form of presentations to be defended

by learners. Assessment criteria should be agreed in advance.

Activity 4. Groups assess each other’s presentations and provide fair and helpful feedback by

using a assessment criteria.

Activity 5. Learners individually answer the MisConceptual Questions. They can use A, B, C,

D and E cards to show their answers. The questions are shown at the interactive

board. Laminate a set of cards so every member of the class has five, with A,B,C, D

and E written on them. Ask questions with five answers, and then learners show you

their answer. Encourage them not to look at other people’s responses so as to copy.

Activity 6. The Teacher checks and assesses each student’s answer and provide fair and helpful

feedback.

Activity 7. At the end of the lesson students are encouraged to reflect on what they have learned

and what they need to improve.

Assessment criteria for group work

Application

Use Faraday’s law of electromagnetic induction to explain the operating principle of electromagnetic relay, generator and transformer;

Analysis

Explain the working principle of electromagnetic relay;

Explain the working principle generator;

Explain the working principle of transformer;

List of useful links and literature

Douglas  C. Giancoli, Physics Principles with Applications, Seventh edition  2014.

Keith Johnson Physics for You IGCSE Updated Edition 2011

https://tap.iop.org/fields/electromagnetism/414/page_46948.html

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