NZQA registered unit standard
18091 version 4
Page 1 of 3
Title
Demonstrate knowledge of three-phase theory for electrical service
technicians
Level
4
Credits
3
This unit standard covers three-phase theory. It is intended for
use in the training of electrical service technicians.
Purpose
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
Explanatory notes
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 evidence requirements
Outcome 1
Demonstrate knowledge of three-phase generation.
Evidence requirements
1.1
A simple three-phase a.c. generator (alternator) is sketched and its operation
described.
Range
1.2
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.
The alternating voltages induced in the three stationary coils by at least one full
rotation of the pole pair are sketched.
The Skills Organisation
SSB Code 100401
© New Zealand Qualifications Authority 2014
NZQA registered unit standard
Range
1.3
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.
Advantages of three-phase over single-phase supply systems are stated.
Range
1.4
18091 version 4
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efficient generation, transmission, transformation and distribution;
conductor number and size; induction motor construction, starting
requirements, power output to size ratio.
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.
Evidence requirements
2.1
Three-phase terms are defined and explained with the aid of sketches.
Range
2.2
Relationship between line and phase voltages, and between line and phase
currents are explained for star and delta connected balanced loads.
Range
2.3
line (nominal three-phase), phase, balanced, unbalanced, star,
delta, phase sequence.
star connection – VL = 1.732×VPH; IL = IPH;
delta connection – VL = VPH; IL = 1.732×IPH.
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.
Evidence requirements
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.
The Skills Organisation
SSB Code 100401
© New Zealand Qualifications Authority 2014
NZQA registered unit standard
3.2
18091 version 4
Page 3 of 3
The need for a neutral conductor on an unbalanced star connected load is
explained.
Planned review date
31 December 2014
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
N/A
Rollover and
Revision
3
20 September 2012
N/A
Revision
4
15 January 2014
N/A
0003
Consent and Moderation Requirements (CMR) reference
This CMR can be accessed at http://www.nzqa.govt.nz/framework/search/index.do.
Please note
Providers must be granted consent to assess against standards (accredited) by NZQA,
before they can report credits from assessment against unit standards or deliver courses
of study leading to that assessment.
Industry Training Organisations must be granted consent to assess against standards by
NZQA before they can register credits from assessment against unit standards.
Providers and Industry Training Organisations, which have been granted consent and
which are assessing against unit standards must engage with the moderation system that
applies to those standards.
Requirements for consent to assess and an outline of the moderation system that applies
to this standard are outlined in the Consent and Moderation Requirements (CMR). The
CMR also includes useful information about special requirements for organisations wishing
to develop education and training programmes, such as minimum qualifications for tutors
and assessors, and special resource requirements.
Comments on this unit standard
Please contact The Skills Organisation reviewcomments@skills.org.nz if you wish to
suggest changes to the content of this unit standard.
The Skills Organisation
SSB Code 100401
© New Zealand Qualifications Authority 2014