Talking technically
INCORPORATING COLD CHAIN
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy
(ACRA) in Edenvale. He holds a Bachelor of Business administration and an associate
degree in educational administration. He has a National Technical Diploma and completed
an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical.
He has been involved with SAIRAC for 17 years and has been a Johannesburg committee
member for the past eight (chairman in 2011 and 2012. Currently he is the vice-chairman
(Johannesburg council) as well as vice-president (national council).
Electricity and
magnetism (Part 2)
By Grant Laidlaw
U
nderstanding the electrical
measured to the meter. In the case of
aspects of refrigeration is
a single-phase power supply, these are
essential. Read the first part of
the phase and the neutral or the phase
this article in the September/October
and the earth. When the power supply
edition of Cold Link Africa.
is three-phase, the voltage can be
measured from phase to phase or from
Voltmeter
phase to neutral. The normal voltages
Although many of the clamp-
measured were discussed previously.
on ammeters have connections
By using a switch on the meter,
to measure the voltage and the
different voltage ranges can be
resistance, the voltage and the
selected. Always select a range that
resistance are usually measured with
will put the reading towards the middle
a multimeter. This traditional type of
and upper third of the scale, if possible.
instrument is illustrated in Figure 1, and
When in doubt of the expected
has a needle that points at the voltage
reading, start with the highest range
measured. It’s called an analogue type
and progress downward to obtain the
meter.
most accurate reading.
Modern multimeters are often of the
digital type, indicating the measured
Ohmmeter
values in digits. Two-metre leads are
The ohmmeter is usually incorporated
required to connect the voltage to be
in the ammeter or the multimeter. The
Figure 1: The clamp-on ammeter and the method of measuring. Modern clamp-on
ammeters are also available with digital read-out.
Figure 2: An ohmmeter showing the component or circuit resistance.
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