Welcome to the Solutions page
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavor to enlighten
Andre asks: Hi, I feel that electrical work forms part of the trade but not much attention is given to this
area. So Grant how about it? What is a Watt? Single phase versus three phase, why? Capacitors, what
for? Can you let us have the basics applicable air conditioning? This will be of great help, Thanks
Hi Andre, In the last issue we went back to basics (electrical) and we now look at circuits.
SERIES CIRCUITS
In a series circuit the electricity goes through the one circuit first and then through the other circuit(s).
The circuit shown below is a series circuit; the electricity must go through the switch(es) first and then
through the lamp.
If more switches, relays or interlocks (such as door switches) would be introduced into the same wire as
the present switch, all these switches etc, would be in series.
A fault in one item in a series circuit will affect the entire circuit.
In air conditioning and refrigeration applications we commonly use series circuits in the control circuit.
Used in this manner should any fault develop, the protection device associated with that fault will activate turning off that piece of equipment.
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Series Circuit
PARALLEL CIRCUITS
In a parallel circuit the power can go through any circuit. A typical parallel circuit is shown in figure below.
In a parallel circuit a fault in one circuit does not affect the other circuits.
If switches are installed before each lamp the individual lamps can be switched off without affecting the
others. This method is used for ordinary house wiring.
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A parallel circuit.
SERIES/PARALLEL CIRCUITS
It therefore stands to reason that given the complex nature of air conditioning and refrigeration equipment that many would utilise a combination of both series and parallel circuits.
A series / parallel circuit is thus a combination of both the series and the parallel circuits.
A typical example is shown in figure below.
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A Series / Parallel Circuit
The examples given above are all taken from household wiring.
The same rules do however apply for all refrigeration and air conditioning circuits.
TYPICAL ELECTRICAL CIRCUITS
A REFRIGERATOR
To start off with “electrical air conditioning and refrigeration circuits” let’s look at a circuit for a simple
household fridge.
• A circuit is a path for the electrical current to flow from the source, through the load and back.
• A load is the equipment or the appliance in the circuit that will convert the electrical power into a
useful commodity such as work, light or heat.
• A switch is a device with which we can interrupt the path of the electricity, or break the circuit.
A household fridge contains the following loads:
• A compressor and a light.
In addition there are the following switching devices:
• A thermostat to switch the compressor ON and OFF.
• A starting relay to energise the start winding of the compressor and an overload to stop the
compressor when it overheats or draws too much current,
• A switch for the light.
With the above information we can make up the wiring diagram.
The electrical wiring diagram shows the actual internal wiring of the unit. It shows all the electrical components (such as loads and switches) together with the actual path of the wires from one component to
another. It often even shows the colours of the wires.
This diagram may be used to locate specific wires or components during service work.
It does not give an indication of the purpose of the components, but it does enable the serviceman to
determine and locate the position(s) where he needs to connect his meter probes for checking any wire
or component that he wants to test.
Actual drawings or look-alike pictures of the device are frequently used on the wiring diagram.
This diagram is a very simple one and can easily be followed. Wiring diagrams of bigger, or more complex plants can, however, become very involved.
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Simple wiring diagram
Although the wiring diagrams as shown are of value, the schematic wiring diagram is even
more important because it tells him “how and why” the unit works as it does. It is also much
easier to follow when it comes to complex diagrams.
A schematic diagram is also called a “ladder diagram”.
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Symbols should comply with IEC (International Electromechanical Commission) standards.
Other symbols are in use in South Africa but as these are not according to the above standards
they are not dealt with.
Switches, contacts and loads must be marked to indicate what they are.
A legend explaining all the markings must also be shown.
From the above ladder diagram we can determine the following:
• If the circuit is “live” the light can be switched “ON” by the door switch.
• If the thermostat “makes” and the overload is made, the compressor will be energised.
• Starting relay “R” will be energised and the NO contact “R” will “make”.
Please note that the starting relay is also a load as it does convert electricity into magnetism for
the relay. As this load is “in series” with the compressor and the compressor winding requires
230 volts the voltage drop across the starting relay may only be minimal; for this reason it has
very thick windings.
The starting relay described is of the “current” type. At start-up the compressor will draw a high
current which, when flowing through the relay, produces sufficient magnetism to pull-in the relay. As soon as the compressor runs the current drawn will reduce and the relay will drop out.
(This will be dealt with in detail later).
It is important to note that all electrical diagrams show the circuit with the power OFF.
For this reason all relays and contactor contacts are in the position they would be in if the relay
or contactor was not connected to the circuit at all.
If a contact is “open” when the relay or contactor is de-energised it is called a “normally open
contact”, this is abbreviated as “NO”.
When it is “closed” when the relay or contactor is de-energised it is called a “normally
closed contact”, abbreviated as “NC”.
Andre in the next issue we look at an electrical circuit applicable to air conditioning
Thank you for all your questions. Send your problems (and sometimes your creative solutions) to
[email protected] with “Solutions Page” in the subject line. You may include pictures.
References:
SAIRAC
ASHRAE
Modern Refrigeration
Merseta Training
ACRA