Electrotechnics N6

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Electrotechnics N6

This document includes important informa on about the revised curriculum for Electrotechnics N6, scheduled for implementa on in 2021.

Main changes to the curriculum

• There have not been many fundamental changes to this curriculum for 2021. • It has, however, been broken down into weighted modules with learning content and detailed, numbered learning outcomes. • Lecturers of this subject will know that M6–M10 from the old syllabus and some learning outcomes from the other modules have not been examined in years. These outcomes have now been completely removed from the new 2021 syllabus. • Learning outcomes not explicitly stated in the old syllabus, but regularly examined, have now been clearly defi ned in the 2021 syllabus document.

Modules and weigh ng

The new syllabus has been broken down into modules with weigh ngs as follows:

New curriculum (2021)

Calcula on ques ons Theory ques ons (incl. drawings)

Total percentage

Module 1 DC machines ±10% ±5% ±15%

Module 2 AC theory ±13% ±2% ±15%

Module 3 Transformers ±10% ±5% ±15%

Module 4 AC machines ±25% ±5% ±30%

Module 5 Genera on and supply of AC power ±20% ±5% ±25%

TOTAL ±78% ±22% 100%

Summary of main changes

Overview/highlights

• The syllabus is largely unchanged. • Modules have now been weighted. • Learning outcomes have been numbered and listed in more detail. • Many learning outcomes not explicitly listed in the old syllabus, but regularly tested in examina ons, have now been included in the syllabus document.

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Electrotechnics N6

• A new learning outcome on the Prony brake effi ciency test has been added to the DC machine module. • New learning outcomes on star ng of synchronous motors, and characteris cs and applica ons of synchronous motors have been added. • Short transmission lines are covered in N5. • Old modules 6–10, which had not been examined in years, are now all removed: – Measuring instruments (old M6) – Switchgear and protec ve devices (old M7) – Sta c control (old M8) – Arc-furnace transformers, switchgear and control systems, types of electrodes and types of furnaces (old M9) – Installa on, care, opera on, maintenance, supervision and inspec on of AC machines, generators, synchronous motors and induc on motors (old M10)

Detailed

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes:

Module 1: DC machines

Weight ±15% LEARNING CONTENT REMAINING: • Series-parallel speed control • Calcula on of torque and power • Load sharing of generators working in parallel • Equalising bars and crossconnec on fi elds • Tests – direct and indirect;

Hopkinson; back-to-back;

Swinburne test

CONTENT REMOVED: • Trac on – choice of motors and generators • Bridge and shunt transi on • Dynamic breaking • Plugging control • Regenera ve braking SPECIFIC LEARNING OUTCOMES IN NEW SYLLABUS:

Speed control

• Explain normal speed, below-normal speed and above normal speed of DC motors. • Explain how speed control of a DC motor is achieved by: – varying the voltage across the armature – series-parallel control method – the Ward-Leonard control system – varying the value of the main magne c fl ux/ fi eld current. • Perform calcula ons with regards to speed control of shunt and series motors. Speed control being achieved by inser ng a resistor in: – series with the shunt fi eld – series with the armature – parallel with the series fi eld. • Interpret the magne sa on curve of a DC motor.

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • Determine the speed, current, EMF and torque of a DC motor (a er inser ng the resistor) by using the formulae. • Interpret the following condi ons: – Flux suddenly changes. – Current drawn from the mains is to remain unchanged. – The ohmic voltage drops are negligible.

Load sharing of generators working in parallel

• Explain with the aid of a diagram the procedure to parallel two generators. • Explain how the total load is shared by the two generators. • Perform calcula ons to determine the por on of total load supplied by each generator. • Show by means of circuit diagrams how the following generators are used to supply a common load: – Two shunt generators – Two series generators – Two compound generators

Effi ciency tes ng of DC machines

• List, classify and calculate the diff erent losses that occur in DC machines. • Determine the effi ciency of any small DC machine by using the following direct method: – Rope brake – Prony brake • Determine the effi ciency of large shunt and compound wound machines by using the following indirect methods: – Summa on of losses or Swinburne method – Regenera ve or Hopkinson back-to-back method

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • State the condi on for maximum effi ciency and also calculate maximum effi ciency.

Module 2: AC theory

Weight ±15% LEARNING CONTENT REMAINING: • Complex waveforms • Breaking down of the fundamental and harmonics • Calcula ons using complex nota ons and phasors for mixed circuits

CONTENT REMOVED: • (None) SPECIFIC LEARNING OUTCOMES IN NEW SYLLABUS:

Unbalanced three-phase loads

• Calculate the phase currents, line currents, power per phase, impedance and total power drawn by three-phase unbalanced star and deltaconnected loads. • Use vector diagrams to determine the line currents drawn by three-phase unbalanced star and delta-connected loads.

Unbalanced star-connected load with a missing neutral

• Explain the impact of a missing neutral on a three-phase circuit. • List the common causes of a missing neutral. • Calculate the poten al diff erence between the star point of the load and the neutral point of the supply. • Determine the poten al diff erence across each phase of the load. • Determine the current fl owing through each line or phase of the load.

Complex waves

• Explain the concept ‘complex wave’. • Draw a complex wave comprising the fundamental component together with any harmonic component. • List the causes and eff ects of harmonic components on circuits. • Write the mathema cal expression of complex waves.

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • Calculate the following a er a complex wave is applied across a series or parallel or seriesparallel circuit: – Expression for current – Expression for voltage – RMS value of current – RMS value of voltage – Average value of current – Average value of voltage – Power drawn by circuits – Power factor of circuits – Energy consumed by circuits • Calculate the power drawn by circuits that resonate at the frequency of any of the harmonic components.

Module 3: Transformers

Weight ±15% LEARNING CONTENT REMAINING: • Calcula ons on load using equivalent circuit • Regula on • Tests – back-to-back; delta-delta;

Sco connec on; harmonics

CONTENT REMOVED: • Tests – economics; zig-zag connec on; ter ary windings SPECIFIC LEARNING OUTCOMES IN NEW SYLLABUS:

Equivalent circuit diagrams of a single-phase transformer

• Draw the equivalent circuit diagram and vector diagram of a transformer on no-load. • Draw the equivalent circuit diagram and vector diagram of a transformer on full-load. • Determine the equivalent resistance, reactance and impedance of a transformer referred to primary side and secondary side, using any of the following: – Method of referring with primary values, secondary values and transformer ra on – From given percentage or per unit values – Results obtained from the short- circuit test

Voltage regula on of a transformer

• Defi ne voltage regula on of a transformer.

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • Calculate the voltage regula on of transformers supplying loads working at unity, lagging and leading power factor using any of the following: – Method of referring with primary values, secondary values and transformer ra on – From given percentage or per unit values – Results obtained from the short-circuit test

Effi ciency of transformers

• List, classify and calculate the various losses that occur in a transformer. • Explain with the aid of circuit diagrams how the open-circuit and short-circuit tests are conducted on a single-phase transformer. • Determine the effi ciency of a transformer at any load. • Determine the maximum effi ciency of a transformer.

Transformer connec ons

• Draw circuit diagrams to illustrate the following types of transformer connec ons: – Star-star – Delta-delta – Star-delta – Delta-star – Open delta – Sco connec on

Heat run tests on transformers

• State the func on of heat run tests. • Explain using a suitable circuit diagram how the open-delta heat run test is conducted. • Explain using a circuit diagram how the Sumpner back-to-back test is conducted.

Harmonics in transformers

• State the eff ects of harmonic voltages and harmonic currents on transformers.

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes:

Autotransformers

• Draw circuit diagrams to illustrate a step-down and a step-up transformer. • State the advantages and disadvantages of an autotransformer.

Module 4: AC machines

Weight ±30% LEARNING CONTENT REMAINING: • Alternators and synchronous motor: back EMF; armature reac on; load angle; calcula ons of regula on and phasor diagrams; varying excita on;

V-curves • Induc on motor: calcula ons using slip and torque; star ng resistances (slip-ring motors); double cage rotors; dc injec on; plugging frequency in stator and rotor

CONTENT REMOVED: • (None) SPECIFIC LEARNING OUTCOMES IN NEW SYLLABUS:

The EMF equa on of a synchronous alternator

• Defi ne and calculate distribu on factor, pitch factor and form factor. • Calculate the open-circuit voltage generated by single-phase and three-phase synchronous alternators.

Parallel opera on of alternators

• State the factors to be sa sfi ed before two alternators are made to operate in parallel. • Calculate the synchronising power of two singlephase alternators working in parallel or two three-phase alternators working in parallel.

Voltage regula on of alternators

• Defi ne voltage regula on of an alternator. • Explain by means of circuit diagrams how the open-circuit and short-circuit tests are conducted on an alternator. • Determine the voltage regula on of an alternator using the ampere turn and synchronous impedance methods. • Determine the voltage regula on of alternators opera ng at the following power factors: – Unity – Lagging – Leading • Explain the concept of infi nite busbars.

Synchronous motors

• Explain how synchronous motors are started.

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • State the characteris cs and applica ons of synchronous motors. • Explain by means of vector diagrams the opera on of a synchronous motor under the condi on of constant excita on and varying load. • Explain by means of vector diagrams the opera on of a synchronous motor under the condi on of constant load and varying excita on. • Calculate using vector diagrams the generated EMF, load angle, power developed, and the torque developed by a synchronous motor. • Explain the concept of hun ng of a synchronous motor and also its causes and eff ects. • Explain what V-curves are with regards to synchronous motors.

Three-phase induc on motors

• Explain using the energy fl ow diagram the fl ow of energy from the supply to the motor’s output. • Calculate the rotor current and rotor copper loss at any condi on of slip. • Calculate the torque developed by the rotor. • State the condi on for maximum torque and also calculate maximum torque. • Draw the circuit diagram, simplifi ed circuit diagram and the vector diagram of an induc on motor. • Explain how using a double cage rotor improves the star ng torque of an induc on motor. • Explain braking of induc on motors by the methods of plugging and DC injec on. • Use the results obtained from the no-load and locked rotor tests conducted on an induc on motor to construct the circle diagram for the following condi ons: – Maximum output – Full-load

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes:

Module 5: Genera on and supply of AC power

Weight ±25% LEARNING CONTENT REMAINING: • Power factor correc on – causes of low power factor; reasons for improvement; capacitors and synchronous motors; kVA and kVA reac ve; calcula ons and phasor diagrams

CONTENT REMOVED • (None) SPECIFIC LEARNING OUTCOMES IN NEW SYLLABUS:

Power factor correc on

• State the eff ects of a poor power factor and the methods used to improve power factor. • Draw the power triangle (vector diagram) of a plant before and a er power factor correc on. • Determine the ra ng of capacitor banks required to improve power factor to any desired value. • Determine the size of individual capacitors required to build single-phase and three-phase (star and delta-connected) capacitor banks. • Draw the power triangle (vector diagram) of a plant before and a er the installa on of a synchronous condenser.

– Maximum torque • Determine from the circle diagram the following: – Line current of the motor – Constant losses – Stator copper loss at stands ll – Rotor copper loss at stands ll – Stator copper loss – Rotor copper loss – Total losses – Rotor output – Rotor input – Effi ciency – Slip – Power factor – Torque in synchronous wa s – Input power – Torque developed

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Electrotechnics N6

Module Previous curriculum New content

Learning outcomes updated to match examina on tradi ons with the following specifi c outcomes: • Determine the overall power factor of a plant a er the installa on of a synchronous condenser working at either unity power factor or a leading power factor. • Draw the vector diagram to determine the fi nal connected load of a plant by combining the power triangles of all known loads. • Calculate the fi nal connected load of a plant.

Medium transmission lines

• Defi ne voltage regula on of a transmission line. • Draw a circuit diagram and determine the effi ciency of transmission lines using the nominal ‘T’ method. • Draw a circuit diagram and determine the effi ciency of transmission lines using the nominal ‘∏’ method.

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Sunil Chuturgoon