18091 Demonstrate knowledge of three-phase theory for ...



|Title |Demonstrate knowledge of three-phase theory for electrical service technicians |

|Level |4 |Credits |3 |

|Purpose |This unit standard covers three-phase theory. It is intended for use in the training of electrical |

| |service technicians. |

| | |

| |People credited with this unit standard are able to demonstrate knowledge of: |

| |– three-phase generation; |

| |– three-phase load connections; and |

| |– neutral current in three-phase loads. |

|Classification |Electrical Engineering > Electrical Service Technicians |

|Available grade |Achieved |

Guidance Information

1 This unit standard has been developed for learning and assessment off-job.

2 Range

a Candidates are expected to use the Système International (SI) units, including the multiples and sub-multiples, for voltage, current, resistance, inductance, capacitance, reactance, impedance, power, and energy, and the qualitative relationships among these quantities in alternating current (a.c.) circuits.

b Use of a calculator during assessment is permitted.

Outcomes and performance criteria

Outcome 1

Demonstrate knowledge of three-phase generation.

Performance criteria

1.1 A simple three-phase a.c. generator (alternator) is sketched and its operation described.

Range three coils in stator distributed at 120° in space, rotating magnetic pole pair, flux linkage with stationary coil, induced voltages proportional to rate of change of flux.

1.2 The alternating voltages induced in the three stationary coils by at least one full rotation of the pole pair are sketched.

Range voltage waveforms sinusoidal in shape with equal maximum values, separated by 120° phase displacement, voltage zero when pole pair normal to each coil, voltage maximum when pole pair is in line with each coil.

1.3 Advantages of three-phase over single-phase supply systems are stated.

Range efficient generation, transmission, transformation and distribution; conductor number and size; induction motor construction, starting requirements, power output to size ratio.

1.4 The relationship between generator or motor poles, speed, and frequency is stated for a 50 Hz supply system.

Range f = p(N/60;

where f = frequency in Hz

p = number of pole pairs

N = speed of rotation in revolutions per minute.

Outcome 2

Demonstrate knowledge of three-phase load connections.

Performance criteria

2.1 Three-phase terms are defined and explained with the aid of sketches.

Range line (nominal three-phase), phase, balanced, unbalanced, star, delta, phase sequence.

2.2 Relationship between line and phase voltages, and between line and phase currents are explained for star and delta connected balanced loads.

Range star connection – VL = 1.732(VPH; IL = IPH;

delta connection – VL = VPH; IL = 1.732(IPH.

2.3 The expressions for the power drawn are given for both star and delta connected balanced loads.

Range P = 1.732(VL(IL = 3(VPH(IPH (for resistive loads).

Outcome 3

Demonstrate knowledge of neutral current in three-phase loads.

Performance criteria

3.1 The effects of balanced and unbalanced loads on the neutral current are explained in terms of the sum of the three-phase currents.

3.2 The need for a neutral conductor on an unbalanced star connected load is explained.

This unit standard is expiring. Assessment against the standard must take place by the last date for assessment set out below.

Status information and last date for assessment for superseded versions

|Process |Version |Date |Last Date for Assessment |

|Registration |1 |28 January 2001 |31 December 2013 |

|Review |2 |20 June 2006 |31 December 2022 |

|Rollover and Revision |3 |20 September 2012 |31 December 2022 |

|Revision |4 |15 January 2014 |31 December 2022 |

|Review |5 |28 January 2021 |31 December 2022 |

|Consent and Moderation Requirements (CMR) reference |0003 |

This CMR can be accessed at .

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