HVAC M Bilal Akbar
HVAC Instruments
HVAC instruments are required to measure the pressure and temperature of various sections of
an air conditioning system. Typical pressure gauges are Vacuum Gauges, High Pressure Gauges
and Compound Gauges. Clamp-on ammeter voltmeter is another useful device to troubleshoot
the electrical circuit of an air conditioner.
Clamp-On Ammeter Voltmeter
This is a useful instrument that can be used to measure the alternating current of a conductor
without having to disconnect the circuit and connect an ammeter in series with it. By simply
identifying a conductor with its insulator and clamping it with this instrument, you can get a
reliable and quick reading of the current that is flowing in the circuit.
This device follows the principle of a transformer where the magnetic field that is induced in the
conductor is used to determine the current that is flowing in the conductor.
This device can also be used to measure the terminal voltages of the circuit. You will need to set
the scales of the current and voltages to be measured. Typical range of current is from 10A AC
to 1,000A AC and voltage range from 175V AC to 700V AC.
Air Conditioner Motors
Air conditioner motors are some of the crucial components that are required in the operation of
the air conditioning in your house. We will discuss the various types of single-phase motors that
are used in your equipment at home. Here are four types of electric motors that are commonly
being used.
Split-Phase Air Conditioner Motor
This is the simplest design where the RUN winding and START winding are connected in
parallel and 90° electrically apart. It is usually used in small pumps, fans and blowers where the
capacity is below 1 horsepower. It has a low starting torque but high starting current. Since the
torque is low, the ability to start the motor is only practical for low load condition.
The RUN winding is make from bigger diameter wire and shorter turn for lower resistance and
high inductance properties. The START winding is make from smaller diameter wire for higher
resistance and low inductance properties.
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When power is connected to the motor, both the windings will be energized with the current in
the RUN winding lags the current in the START winding by about 30° electrically. This out-of-
phase effect on the stator produces a starting torque and causes the rotor to start rotating.
Typically the speed of the motor is 1800 rpm or 3600 rpm when running without any load. When
the load is connected, the speed can go down to 1725 rpm and 3450 rpm respectively.
The no-load speed of the motor is given by:
Speed(rpm) = (Frequency of AC power X 120)/number of poles
For example, if your supply is 60 Hz and the motor is using two-pole, the synchronous speed =
(60X120)/2
= 3600 rpm
There is a switch known as centrifugal switch which is connected in series with the START
winding. This mechanical switch will open when the motor speed reaches 75% of the rated speed
typically within 2 seconds. Once the switch opened, the START winding in circuit is
disconnected.
This is to protect the START winding from overheating. When the motor is powered off, the
switch will close the circuit to get ready for the next starting of the motor.
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These days, electronic relay is also being widely used to disconnect the START winding.
Capacitor-Start Air Conditioner Motor
This motor is similar to the split-phase motor except that there is an external capacitor that is
connected in series with the START winding. This capacitor will cause the current in the START
winding to lead the voltage.
The current in the RUN winding lags the voltage. When this happens, the phase difference
between the two windings is 90° electrically hence a true two-phase starting is achieved.
The starting torque of this motor is very high making it suitable to drive small compressor which
needs to start under full load. The capacity of this motor can go up to 1 horsepower.
Once the motor has reached 75% of the rated speed, the capacitor and the START winding will
be automatically disconnected from the circuit by using the centrifugal swith, potential or current
relay.
After the capacitor and START winding have been removed from the circuit, the magnetic field
being generated continuously will cause the motor to continue running. This type of motor is also
known as CSIR motor or Capacitor-Start-Induction-Run motor.
Capacitor-Start, Capacitor-Run Air Conditioner Motor
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This motor design is similar to the capacitor-start design except that there is a second capacitor
known as the RUN capacitor which is connected in parallel with the START capacitor and the
switch.
These capacitors are effectively connected in series with the START winding. During the
starting of the motor, both the capacitors are connected in the circuit. The START winding and
the RUN winding will remain connected to the circuit at all times.
Usually the capacitance of the RUN capacitor is lower than the START capacitor. During
starting, the effective capacitance is the combination of both capacitors causing a greater phase
angle shift between the windings.
This provides a higher starting torque and can be used to drive the compressor as well as in belt-
driven motors.
As the rotor speed reaches 75% of the rated speed, the switch will be automatically open to
disconnect the START capacitor from the circuit. The START winding remains in the circuit.
The RUN capacitor helps to correct the power factor of the circuit making it more efficient. The
capacity of this type of motor can go up to 10 horsepower and is one of the most efficient motors
used in the HVAC industry.
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Permanent Split Capacitor(PSC) Air Conditioner Motor
This motor has similar design to the phase-split motor except that there is a RUN capacitor
connected to the START and RUN windings. This motor does not have any switch and the
START winding, RUN winding and the RUN capacitor are active whenever the motor is ON.
This type of motor has low starting torque and is suitable to be used in small fan motors such as
the fan coil unit of a split air conditioning system.
Multi-speed PSC is achieved by changing the winding resistance. If high speed is required, the
terminal is connected to the least winding resistance. If low speed is required, the terminal is
connected to the highest winding resistance.
By utilizing relays to choose the terminal to be connected to the line voltage, different speed of
fan motor can be achieved.
In the diagram above, there are 4 motor speed that can be selected. Super High(SH), High(H),
Medium(M) and Low(L). The selection can be done by using electronic relays to connect L2 to
one of the four terminals depending on the speed required.
Air Conditioner Capacitors
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Air conditioner capacitors are commonly used in the electric circuit of an air conditioning
system. They are used in various circuits for different purposes. We will discuss in greater details
where these components are used. However, it is good to go to the fundamentals of a capacitor.
A capacitor is used to store energy in the form of electrical charge. In simple terms, it basically
consists of two conductor plates with an insulating material in between them. The insulating
material is also called a dielectric and can be made of paper, special non-conductive material or
simply the air. They are used in both AC and DC circuits.
The capacitance value is dependence on:
Dielectric material (The higher the dielectric K, the higher the capacitance.)
Area of the plates (The bigger the area, the higher the capacitance.)
Separation distance of the plates (The closer the plates are, the higher the capacitance.)
Number of plates (The more the number of plates used to make the capacitor, the higher
the capacitance.)
The symbols used to represent capacitors are as shown below.
The unit of measurement is Farad(F) and the common values are uF(microfarad) which is
0.000001 Farad, mF(milifarad), nF(nanofarad) and pF(picofarad).
Voltage Rating
The voltage rating of a capacitor determines the maximum voltage that should be applied during
operation. Take note also of the usable temperature range that can be used. If you are replacing a
capacitor, make sure that the capacitance is the same and the voltage rating does not go below
the old capacitor that is to be replaced. Also ensure that the type of capacitor used is similar.
Types of Air Conditioner Capacitors
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Electrolytic Capacitor
This capacitor has polarity and have leads that are marked with + or - signs. It is usually used
to smooth-en the DC voltage after the voltage has being rectified by the diodes. They helped to
reduce the ripples of the DC supply. The bigger the capacitance of the capacitor used, the less the
ripple will be. You will find this application in simple linear power supply together with a step
down transformer.
Ceramic Capacitors
Ceramic caps. are used as bypass capacitors to bypass the high frequency content of a circuit.
They could be placed parallel to the DC power intake of integrated circuits to bypass certain high
frequencies noise in the circuit that may have adverse effect on the sensitive circuitry.
As more tranformerless power supply is becoming common today due to its better efficiency and
smaller space, you will see many surface mount capacitors being used in multilayer printed
circuit boards.
Safety Capacitors
Safety caps. are commonly used in inverter control circuity of an air conditioner. They can be
classified into X or Y types and are used in the live parts of the circuit. Hence, always be careful
when dealing with this capacitors as some charge on them may cause shock even when power
supply has been cut off.
Good design should have bleeding resistors to remove the charge from the device after the air
conditioner has been switched off. Ensure proper certification marks such as VDE, UL and CE
markings are presence on the bodies of the capacitors before using them.
These capacitors together with inductor coils are used to reduce the harmonics that are generated
from the rapid switching of the AC power supply by the insulated gate bipolar transistors
(IGBT).
Start & Run Air Conditioner Capacitors
These capacitors are used in single phase electric motors. START cap. is used to shift the phase
difference between START windings and RUN windings of a capacitor-start motor. This
difference caused a starting torque to be generated that is able to start the motor with full load
connected to it.
The RUN cap. is used to further shift the phase difference between the two windings causing
even higher starting torque to be generated. There are markings on the single phase compressor
to identify the connections in the motor. The labels are R(connected to RUN winding),
S(connected to START winding) and C(common of the two windings).
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