# help with revision for exams

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1.1. Two batteries having an emf of 50V and 30V, an internal resistance of 0,4ohms and 1,2 ohms respectively are to be charged by a generator having an EMF of 60V and an internal resistance of 0,2ohms.

Use kirchoff’s law to determine the following:

1.1.1. the value of current supplied by the generator

1.1.2. the value of the current through battery A

1.1.3. the value of the current through batter B.

1.2. a 50 ohm resistor is connected across a rheostat which is then connected in series with a heater element of 500W. What must the value of  the rheostat be so that the heater draws 10A when the whole circuit is connected across a 150V supply?

1.3. On what does the magnitude of the hysteresis loss depends?

1.4. what is the reason that certain alloys are used in the manufacturing of standard resistors for the use in measuring instruments?

2.1. two capacitors of 10 microfarads and 15 microfarads respectively, are connected in series across a 30V supply.

Calculate the following:

2.1.1. the total capacitance

2.1.2. the pd across each capacitor

2.2. State Lenz Law.

2.3. an aluminium conductor 22m long is connected in parallel with a copper conductor, having the same length. When a current of 25A passed through the combination it is found that the current through the copper conductor is 15A. the diameter of the aluminium is 4mm.

Calculate the following:

2.3.1.  Diameter odf copper conductor if the resistivity of copper is 0,017 micro-ohm  meters and that of the aluminium is 0,028 micr-ohm meters.

2.3.2. voltage drop across the conductors

2.4. the resistance of a coil of wire increases from 125 ohms at 50 deg C to 250 ohms at 300 deg C. Find the temperature co-efficient of the conductor.

3.1. the open circuit characteristics of a separately excited DC-Generator are given as follows:

 Terminal voltage (V) 4 8 13 15,5 16 16,5 Field Current (A) 0,2 0,4 0,9 1,7 2,0 2,4

Plot a graph to determine the following:

3.1.1. the voltage to which the machine will excite on no-load when shunt-connected and the total field resistance is 8 ohms

3.1.2. the value of the critical resistance

3.2. a short shunt compound generator supplies a load current of 50A. it has a shunt filed resistance of 11 ohms, an armature resistance of 0,1 ohms and a field resistance of 0,2ohms.

Calculate the armature EMF, if the terminal voltage is 100V.

3.3. how can the field coils of DC machines be connected with self-excitation?

3.4. What is the chief purpose of a DC motor starter?

4.1. a single phase 13 320/1 332V, 60Hz transformer has a net area of 50cm² and a maximum flux density of 1 Tesla. Estimate the number of turns in each winding.

4.2. A sinusoidal AC supply has a maximum value of 400V and a frequency of 50Hz.

Calculate the following:

4.2.1. the RMS value of the voltage

4.2.2. the periodic time in milliseconds.

4.3. in a certain circuit of THREE parallel branches, the instantaneous branch currents are represented by:

4.3.1. calculate the magnitude of the supply current and write it in the form:

4.3.2. represent these currents by drawing a phasor diagram

4.4. an impedance of -3 –j3 ohms and an impedance of 6+j6 ohms are connected in series across a 240V, 50Hz supply.

Calculate the following:

4.4.1. the current flowing in the circuit

4.4.2. the power factor of the circuit

5.1. a 230V DC supply is connected across a circuit of 300 ohms resistor in series with a resistor of unknown value. A Voltmeter having a resistance of 300ohms is connected across the 300 ohms resistor and shows a reading of 30 Volt. Calculate the value of the unknown resistor.

5.2. what is the function of a Buchholz device on a transformer?

5.3. what is the purpose of a capacitor in a single phase capacitor motor?

5.4. what crystals are commonly used in the breather of a transformer and what purpose do they serve?

5.5.1. Name two types of feeders used in transmission systems

5.5.2. in which system does failure of ONE interconnecting feeder interrupt the supply to any of the other substations?

5.6. an ideal 30kVA transformer has 1200 primary turns and 40 secondary turns. The primary windings is connected to a 3kV, 60Hz supply.

Calculate the following:

5.6.1. secondary voltage

5.6.2. value of the secondary current and the primary currents on full load.

5.6.3. Maximum core flux

1.1. two capacitors of 15 microfarads and 4 microfarads respectively are connected in series across a 10 volt supply.

Calculate the following:

1.1.1. the total capacitance

1.1.2. the pd across each capacitor

1.2. why are certain currents that circulate in transformer cores undesirable and what are they called?

1.3. an aluminium conductor, 1km long, is connected in parallel with a copper conductor, having the same length. When a current of 350A passed through the combination, it is found that the current through the copper conductor is 150A. the diameter of the aluminium is 3m.

Calculate the following:

1.3.1. the diameter of the copper conductor if the resistivity of copper is 0,018 micro-ohm meters and that of the aluminium is 0,028 micro-ohm meters.

1.3.2. the voltage drop across the conductors

1.4. the field coil of a motor has a resistance of 300 ohms at 30 deg C. by how much will the resistance increase if the motor reached a temperature of 40 deg C when running? Take the temperature coefficient of the resistance to be 0,004 per deg C at 30 deg C.

2.1. a DC generator of EMF 75V and an internal resistance of 0,5 ohms are connected in parallel with a battery of EMF of 41V and internal resistance of 0,3 ohms. The combination is used to supply a load having a resistance of 2,5 ohms.

Use kirchhoffs law to determine the following:

2.1.1. the value and direction of the current through the generator

2.1.2. the  value and direction of the current through the battery

2.1.3. the terminal voltage across the load

2.3. define the ampere.

2.4. what is the reason that certain alloys are used in the manufacturing of standard resistors for the use in measuring instruments?

3.1. the open circuit characteristics of a separately excited DC-Generator are given as follows:

 Field Current (A) 10 20 30 40 85 100 120 EMF (V) 200 400 500 600 775 800 825

Plot a graph to determine the following:

3.1.1. the voltage to which the machine will excite on no-load when a shunt connected if the total field resistance is 8 ohms

3.1.2. the value of the critical resistance

3.2. a short shunt compound generator supplies a load current of 80A. it has a shunt resistance of 40 ohms, an armature resistance of 0,2 ohms and a field resistance of 0,5 ohms.

Calculate the armature EMF if the terminal voltage is 360 V.

3.3. name threes methods that can be used to overcome the effects of armature reaction.

3.4. what is the chief purpose of a DC motor starter?

4.1. a single phase 222/2 220V, 50 Hz transformer has an area of 100sm² and 200 turns on the primary windings. Calculate the maximum flux density in Tesla and the number of turns in the secondary windings.

4.2. a sinusoidal AC supply has a RMS value of 197,96 V and a periodic time of 30 milliseconds.

Calculate the following:

4.2.1. the maximum value of the voltage

4.2.2. the frequency of the supply.

4.3. in a certain circuit of THREE parallel branches, the instantaneous branch currents are represented by:

4.3.1. calculate the magnitude of the supply current and write it in the form:

4.4. an impedance of 15+j10 ohms and an impedance of 15-j10 ohms are connected in parallel across a 270, 825V, 50 Hz supply.

Calculate the following:

4.4.1. the total impedance of the circuit

4.4.2. the total current

4.4.3. the current in each branch

4.4.4. the total power

4.4.5. the power factor of the circuit

5.1. a single phase transformer has 20 secondary turns and 40 primary turns. The secondary winding is connected to a 200 Volt supply.

Calculate the following:

5.1.1. primary voltage

5.1.2 value of the primary current when the secondary current is 20A

5.1.3. primary power if the load has a power factor of 0,6.

5.2. name three types of capacitors

5.3. what is the purpose of the capacitor in a single phase capacitor motor

5.4. name two of the most common methods of cooling transformers

5.5. name two types of feeders used in transmission systems

5.6. a 400V DC supply is connected across a circuit of a 200 ohm resistor in series with a resistor of unknown value. A voltmeter having a resistance of 800 ohms is connected across the 200 ohm resistor and shows a reading of 200V.

Calculate the value of the unknown.