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LESSON

4 LECTURE

VOLT DROP IN-INTERNAL RESISTANCE OF A GENERATOR SERIES & PARALLEL GENERATORS SUB - OBJECTIVE

At the end of this lesson, the trainee will be able to: 1. 2. 1.0

Measure the internal resistance of a generator. Measure potential difference in series & parallel generators.

VOLTAGE DROP & INTERNAL RESISTANCE OF A GENERATOR We shall consider the circuit in Fig. 4-1.

Fig.4-1. The generated emf (E), which is connected to the resistance (R), gets a current (I) to circulate in the circuit. From Ohms Law, we know that if a current (I) flows in a resistance (R) a potential difference or electrical voltage ( V = R x I ) appears at its terminals. The same potential difference or electrical voltage will of course appear at the generator terminals as well.

ADVANCED COURSE COURSE LESSON 4 PAGE 1

ELECTRICAL MAINTENANCE BASICS OF DIRECT CURRENT

In order to differentiate them, the potential difference or electric voltage appearing at the terminals of the resistance will be called the volt drop, whereas the potential difference appearing at the terminals of the generator will keep the original denomination. The measuring unit used for the volt drop will be the Volt the same as for the potential difference and the emf. We have already noted that when the circuit is dead, the potential difference appearing at the terminals of the generator is equal to the emf produced by the generator. V

=

E

On the other hand, when the circuit is alive, the potential difference appearing at the terminals of the generator is less than the emf. V

<

E

This comes from the fact that the generator shows a resistance r i inside when the electrons flow from the positive to the negative terminals. (Conventionally, the current flows from the negative to the positive terminal inside the generator). According to Ohms Law, if a current (I) flows in the internal resistance r i of the generator, we may obtain the volt drop by the equation: VI

=

ri x I

To be deduced from the emf (E) inside the generator. Indeed, the potential difference at the terminals of the generator with an internal resistance r i and producing a current (I) is given by the equation: V

=

E - ri x I

Such a potential difference is equal to the voltage drop appearing at the terminals of the resistance R : V

=

RxI

NOTE When the internal resistance of the generator and the current produced are small, the internal volt drop can be neglected. V

=

E

To calculate the internal resistance of the generator, we should apply the following formula : rI

ELECTRICAL MAINTENANCE COURSE COURSE BASICS OF DIRECT CURRENT

=

E-V I

ADVANCED LESSON 4 PAGE 2

2.0

SERIES AND PARALLEL GENERATORS Two or more generators can be series or parallel connected. 2.1

A series connection can be made in two different ways: 1.

Using the circuit diagram in Fig4-2. connect the negative terminal of the first generator to the positive terminal of the second generator: in that case , both emf do not tally and the total potential difference is equal to the sum of the potential differences of each generator when off/circuit or live.

Fig.4-2.Series Connected Generators 2.

Using the circuit diagram in Fig4-3. connect the negative terminal of the first generator to the negative terminal of the second generator: Then, both emf do not tally and the total potential difference is equal to the difference between the potential differences of the generators, when off circuit or live.

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ELECTRICAL MAINTENANCE BASICS OF DIRECT CURRENT

Fig.4-3. Opposed Series Connected Generators This connection is said to be in opposition. When V1 > V2 then the total potential difference is positive. (positive terminal upwards). When V1 = V2 the total potential difference is null. When V1 < V2 then the total potential difference is negative. (positive terminal downward). 2.2

PARALLEL CONNECTIONS Using the circuit diagram in Fig.4-4. If we consider the most common case, which is when the generators have the same emf and the same internal resistance’s, the parallel connection is made by connecting together the positive terminals and connecting together the negative terminals separately.

ELECTRICAL MAINTENANCE COURSE COURSE BASICS OF DIRECT CURRENT

ADVANCED LESSON 4 PAGE 4

Fig.4-4.Parallel Connected Generators In this case, the potential difference at the common terminals is equal to the potential difference of each generator, when off circuit or lives. PRACTICAL TEST # 1 Using the circuit diagram in Fig.4-5. 1.

Connect the supply to the loop with the plug.

2.

Set the output voltages to zero (0) on the variable transformer.

3.

Build the circuit shown in Fig.4-5.

Fig.4-5.

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ELECTRICAL MAINTENANCE BASICS OF DIRECT CURRENT

EQUIPMENT REQUIRED A

=

Tester used as an ammeter

V

=

Tester used as a voltmeter

J

=

Switch

R

=

Resistance’s 120 5 ‫ظ‬W Supply

NOTES Both testers should be used for measuring direct currents or voltages. Attention should be paid to the polarity signs when connecting the instruments into the circuit. Make sure the switch is turned off. 4

Press the magneto thermal switch and after having selected the output indicated on the circuit diagram, turn the variable transformer clockwise to its maximum. "Note" the potential difference V = E shown on the voltmeter.

5.

Turn the switch on and note the new potential difference: V

=

E - ri x I < E shown on the voltmeter and the current (I) shown on the ammeter.

6.

Calculate the internal resistance of the supply.

7.

Set the variable transformer to zero (O). PRACTICAL TEST # 2 Using the circuit diagram in Fig.4-6.

1.

Build the circuit shown in Fig.4-6.

ELECTRICAL MAINTENANCE COURSE COURSE BASICS OF DIRECT CURRENT

ADVANCED LESSON 4 PAGE 6

Fig.4-6. EQUIPMENT REQUIRED A

=

Tester used as an ammeter.

V

=

Tester used as a voltmeter.

J

=

Switch.

R

=

Resistance 22 18 ‫ظ‬W. Supply

NOTES See practical test # 1. 2.

Repeat the same tests and calculations as for the practical test # 1.

3.

Set the Variable transformer to zero (0), turn off the magneto thermal switch and after having dismantled the circuit, remove the supply from the loop.

PRACTICAL TEST # 3

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ELECTRICAL MAINTENANCE BASICS OF DIRECT CURRENT

Using circuit diagram in Fig.4-7. 1.

Build up the circuit as shown in Fig.4-7.

Fig.4-7. EQUIPMENT REQUIRED A

=

Tester used as an ammeter

V

=

Tester used as a voltmeter

R

=

Resistance 12 5 ‫ظ‬W

J

=

Switch

P

=

2 Element / Battery (1.5V each)

NOTES See practical test # 1. 2.

Repeat the same tests and calculations as that of practical test # 1.

3.

Turn off the switch J and dismantle the circuit.

PRACTICAL TEST # 4 Using the circuit diagrams shown in Figs. 4-8, 4-9 & 4-10.

ELECTRICAL MAINTENANCE COURSE COURSE BASICS OF DIRECT CURRENT

ADVANCED LESSON 4 PAGE 8

1.

Build up the circuit shown in Fig.4-8.

Fig.4-8. EQUIPMENT REQUIRED V

=

Tester used as a voltmeter

P

=

2 element/battery (1.5V each)

NOTES The tester should be used for measuring direct voltage, paying attention to the polarity signs when connecting the instrument. 2

By reading the tester indication, note that the total potential difference is twice that of each battery (series connection of two equal generators).

3.

Build up the circuit as shown in Fig.4-9.

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ELECTRICAL MAINTENANCE BASICS OF DIRECT CURRENT

Fig.4-9. 4.

By reading the tester indication, note that the total potential difference is equal to the difference in potential difference between the two batteries (series connection of two generators said to be in opposition).

5.

Build up the circuit shown in Fig.4-10.

Fig.4-10. 6.

By reading the tester indication, note that the total potential difference is equal to that of each battery. (Parallel connection of two equal generators.

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ADVANCED LESSON 4 PAGE