In the previous section we saw the basics about the stages in power transmission.
In the last transformer in figure 12.1. that we saw in the previous section,
• 3 lines go into that transformer.
• But there are 4 lines coming out.
In this section, we will see the reason. For convenience, fig.12.1 is shown again below:
We will write the analysis in steps:
1. In the previous chapter we saw the details about a 3 phase generator.
• We saw that, we get 3 separate currents (or phases). Details here.
2. These separate phases should marked clearly.
• For that, we use three different colours.
• Different colour codes are used in different countries. In India, the following colour code is used:
Phase 1: Red
Phase 2: Yellow
Phase 3: Blue
3. So in fig.12.1, we see these three wires coming out of the generator at the power station.
• They also go into the various transformers situated at distant places.
4. Consider the coil in fig.12.2(a) below:
• It is a coil in the secondary of the last transformer (the distribution transformer) in fig.12.1
• Since it is the secondary (see fig.11.28), the energy from it is ready to be supplied to the consumer
5. It is a source of 230 volts.
• But we will get this 230 V, only if we take out power from 'a particular end' of the coil.
• This is shown in fig. 12.2 (b).
♦ If we take power from the middle, we will get only 115 V (half of 230)
♦ If we take power from the 'other end', we will get only 0 V
♦ We can call this 'other end' as 'zero end'
6. There are separate coils for the three phases.
• So there are three coils in the secondary of the transformer.
• The 'zero ends' of all the three coils are connected at a point.
• This point is called the 'neutral point. It is represented by the letter 'N'.
• This is shown in fig.12.2 (c)
7. We can take out power from the free ends of the coils. This is shown in fig.(d)
• Household appliances need only 230 V.
• So for home use, we need only one phase. We can take current from red or yellow or blue.
8. The current entering the appliance must return to the transformer. Then only the circuit will be complete.
• For that, we take out a wire from the neutral point N.
■ This wire is called Neutral line. It's colour is black. It is shown in fig.12.3(a) below:
• If we connect a bulb between any one phase line and the neutral line, the bulb will glow.
■ So the fourth wire that we saw in fig.12.1 earlier is the 'neutral line'
9. There are two methods to connect an appliance to this system:
(i) Connect the appliance between any one phase line and the neutral line
• Then the voltage available for that appliance will be 230 V
• This is shown in fig.12.3(b)
(ii) Connect the appliance between any two phase lines
• Then the voltage available for that appliance will be 400 V
• This is shown in fig.12.4 below:
■ What is the reason for the value '400' ?
We will learn it in higher classes
■ This type of connection between three phase lines is called star connection
10. For home use, 230 V is sufficient
♦ So, from a transformer, one phase line and the neutral line goes into a home
• For industrial use, 400 V is required. This is for the working of heavy machines
♦ So from a transformer, all the three phase lines and the neutral line goes into an industrial building
11. Consider a bird sitting on a wire.
• If it's body is not touching any thing other than that wire, current will not flow through the body of the bird. So it can safely sit on the wire
• But when it begins to fly, it's wings may touch one of the phase wires or the neutral wire.
• If that happens, current will flow through it's body and it will be electrocuted
♦ Resistors in series and
♦ Resistors in parallel
• When the resistors are connected in series, there are some disadvantages. Some of them are:
♦ The net resistance of the circuit will increase
♦ Voltage available for all the resistors are not the same
We learned those details here
• Now, house hold appliances are also resistors.
• For example, the main part of a bulb is it's filament. Filament is a resistor.
♦ That is., the current encounters a resistance when it flows through the filament.
♦ As a result, heat and light are produced.
Let us write a comparison between series and parallel connections when they are used in household circuits. We will write it in steps:
1. Consider the circuit 1 in fig.12.5 below:
• Three bulbs B1, B2 and B3 and their switches S1, S2 and S3 are arranged in series.
2. If the three bulbs are of the same type, they will have the same resistance.
• Then, the available 6 volts will be split equally into 3 parts.
• So each bulb will get only 2 volts.
• Because of this 'reduction in voltage' the bulbs will become dim
3. If we decide to turn of any one bulb, say B2, we will turn the switch S2 to 'off position'
• Then the whole circuit will become open.
• None of the bulbs will glow
4. Now consider circuit 2 in fig.12.5
• Three bulbs B1, B2 and B3 and their switches S1, S2 and S3 are arranged in parallel.
5. The available 6 volts is not split up.
• Each bulb will get the same 6 volts
• So the bulbs will glow brightly
6. If we decide to turn of any one bulb, say B2, we will turn the switch S2 to 'off position'
• Even then, the current will be available for B1 and B2
• So in parallel connection, we need not worry about other appliances, while turning off any one appliance
• In other words, in parallel connection, 'appliances can be controlled individually'
7. Based on the above comparison, we can write:
■ For household electrification, parallel mode is advisable.
In the last transformer in figure 12.1. that we saw in the previous section,
• 3 lines go into that transformer.
• But there are 4 lines coming out.
In this section, we will see the reason. For convenience, fig.12.1 is shown again below:
 |
| Fig.12.1 |
1. In the previous chapter we saw the details about a 3 phase generator.
• We saw that, we get 3 separate currents (or phases). Details here.
2. These separate phases should marked clearly.
• For that, we use three different colours.
• Different colour codes are used in different countries. In India, the following colour code is used:
Phase 1: Red
Phase 2: Yellow
Phase 3: Blue
3. So in fig.12.1, we see these three wires coming out of the generator at the power station.
• They also go into the various transformers situated at distant places.
4. Consider the coil in fig.12.2(a) below:
 |
| Fig.12.2 |
• Since it is the secondary (see fig.11.28), the energy from it is ready to be supplied to the consumer
5. It is a source of 230 volts.
• But we will get this 230 V, only if we take out power from 'a particular end' of the coil.
• This is shown in fig. 12.2 (b).
♦ If we take power from the middle, we will get only 115 V (half of 230)
♦ If we take power from the 'other end', we will get only 0 V
♦ We can call this 'other end' as 'zero end'
6. There are separate coils for the three phases.
• So there are three coils in the secondary of the transformer.
• The 'zero ends' of all the three coils are connected at a point.
• This point is called the 'neutral point. It is represented by the letter 'N'.
• This is shown in fig.12.2 (c)
7. We can take out power from the free ends of the coils. This is shown in fig.(d)
• Household appliances need only 230 V.
• So for home use, we need only one phase. We can take current from red or yellow or blue.
8. The current entering the appliance must return to the transformer. Then only the circuit will be complete.
• For that, we take out a wire from the neutral point N.
■ This wire is called Neutral line. It's colour is black. It is shown in fig.12.3(a) below:
 |
| Fig.12.3 |
■ So the fourth wire that we saw in fig.12.1 earlier is the 'neutral line'
9. There are two methods to connect an appliance to this system:
(i) Connect the appliance between any one phase line and the neutral line
• Then the voltage available for that appliance will be 230 V
• This is shown in fig.12.3(b)
(ii) Connect the appliance between any two phase lines
• Then the voltage available for that appliance will be 400 V
• This is shown in fig.12.4 below:
 |
| Fig.12.4 |
We will learn it in higher classes
■ This type of connection between three phase lines is called star connection
10. For home use, 230 V is sufficient
♦ So, from a transformer, one phase line and the neutral line goes into a home
• For industrial use, 400 V is required. This is for the working of heavy machines
♦ So from a transformer, all the three phase lines and the neutral line goes into an industrial building
11. Consider a bird sitting on a wire.
• If it's body is not touching any thing other than that wire, current will not flow through the body of the bird. So it can safely sit on the wire
• But when it begins to fly, it's wings may touch one of the phase wires or the neutral wire.
• If that happens, current will flow through it's body and it will be electrocuted
Household electrification
• In an earlier chapter we learned about:♦ Resistors in series and
♦ Resistors in parallel
• When the resistors are connected in series, there are some disadvantages. Some of them are:
♦ The net resistance of the circuit will increase
♦ Voltage available for all the resistors are not the same
We learned those details here
• Now, house hold appliances are also resistors.
• For example, the main part of a bulb is it's filament. Filament is a resistor.
♦ That is., the current encounters a resistance when it flows through the filament.
♦ As a result, heat and light are produced.
Let us write a comparison between series and parallel connections when they are used in household circuits. We will write it in steps:
1. Consider the circuit 1 in fig.12.5 below:
 |
| Fig.12.5 |
2. If the three bulbs are of the same type, they will have the same resistance.
• Then, the available 6 volts will be split equally into 3 parts.
• So each bulb will get only 2 volts.
• Because of this 'reduction in voltage' the bulbs will become dim
3. If we decide to turn of any one bulb, say B2, we will turn the switch S2 to 'off position'
• Then the whole circuit will become open.
• None of the bulbs will glow
4. Now consider circuit 2 in fig.12.5
• Three bulbs B1, B2 and B3 and their switches S1, S2 and S3 are arranged in parallel.
5. The available 6 volts is not split up.
• Each bulb will get the same 6 volts
• So the bulbs will glow brightly
6. If we decide to turn of any one bulb, say B2, we will turn the switch S2 to 'off position'
• Even then, the current will be available for B1 and B2
• So in parallel connection, we need not worry about other appliances, while turning off any one appliance
• In other words, in parallel connection, 'appliances can be controlled individually'
7. Based on the above comparison, we can write:
■ For household electrification, parallel mode is advisable.
In the next section, we will see the details of an actual household circuit
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