This document provides a detailed overview of reciprocating hermetic compressors, including their parts, operation, applications, and controls. It discusses the key components of the compressor such as the crankshaft, pistons, valves, and lubrication systems. Applications like household refrigerators, air conditioners, and commercial refrigeration are reviewed. Control components like overload protection, start devices, and motor windings are examined in depth. Operating characteristics like compression ratio and discharge temperature are also covered.

1. Research Project: Reciprocating Hermetic Compressor Aaron Doyle 100929860
Many different hermetic compressors are used to move refrigerant in an air
conditioning or refrigeration system. The one I will be discussing has been most common since
the beginning of refrigerant based systems and hermetic designs; the reciprocating hermetic
compressor. I will be discussing the parts that make up the compressor, what a hermetic
compressor is and why it is used, the application of the reciprocating hermetic compressor,
possible power supplies for different applications, with schematics: control protection
components, start and run windings with control components and motor control circuits used. I
will also include how refrigerant vapour is compressed in the compressor itself, the different
refrigerants that are used and their possible applications and some operating characteristics
that include compression ratio, discharge temperature, operating temperature, liquid entering
the compressor and the change of suction/discharge direction.
The reciprocating hermetic compressor uses an electric motor with its drive shaft
directly connected to the crankshaft. The crankshaft is responsible for changing the circular
motion of the motor drive shaft into a back and forth or reciprocating motion of the pistons.
The crankshaft is a moving part that is subject to a high amount of rpm, thus it requires
lubrication. On smaller reciprocating hermetic motors a splash systemis used where an amount
of lubricating oil is picked up by a dipper on the piston down-stroke. It is then slung to the
outside of the crankshaft surface when the compressor runs. Other larger systems use a
pressure lubricated systemwhere the shafts are drilled and lubricated with a pressure
lubrication system. These compressors have an oil pump mounted on the end of the crankshaft
that turns with the crankshaft. When these compressors first start they are not yet lubricated
until, they are running up to speed.
Reciprocating hermetic; crankshaft and piston orientation, pressure lubricated
Example of splash lubrication (not hermetic in example)
Next I will discuss the connecting rods, piston, cylinder valves, valve plate and
compressor head. The connecting rods quite simply connect the crankshaft to the piston. They
are usually made of iron, brass or aluminum. The piston is a cylindrical assembly that is exposed
to the high pressure refrigerant during the compression. The piston slides up and down to
pump the refrigerant. Because pistons are exposed to very high pressure they need to be
sealed to prevent refrigerant from entering the crankcase. The smaller compressors use oil to
seal the piston where larger systems have compression rings to seal the piston. The valves at

2. the top of the cylinder determine the flow of refrigerant in a system. On the down-stroke one
side will open and the other will open on the up-stroke as the other closes. The opposite
opening and closing of each valve creates the direction of flow. Two types of cylinder valves are
flapper type or ring type. Ring type is circular and uses a spring underneath to open and close
on discharge. The flapper type is held down on one end and this provides enough spring action
to close the valve when there is a reversal of flow. The valve plate holds the suction and
discharge flapper valves. It is placed in between the head of compressor and the top of the
cylinder wall. The head of the compressor is on top of the cylinder wall and valve plate. The
high pressure gas moves through the compressor head. The parts I have discussed thus far
complement the basic function of almost any reciprocating compressor. This leads me to how
refrigerant vapour is compressed.
With the piston at the top of the cylinder, it begins its backward or withdrawing force.
As it moves in this direction (away from the top of the cylinder), a lower pressure is created in
the cylinder than the suction line. This will cause the suction side flapper valve to open. The
remaining portion of the stroke pulls gas from the suction line into the cylinder until the piston
has reached its dead bottom position. At this point the flapper to the suction line will have
closed. The piston proceeds on the up stroke, compressing the now full cylinder. On the up
stroke the flapper valve on the discharge side will open when the pressure inside the cylinder is
greater than the pressure in the suction line. It may actually need several more psi to open the
discharge line flapper as it must overcome the force of the flapper itself. With the flapper open
the cylinder releases the now high pressure gas into the discharge line.
As I am reporting specifically on a hermetic compressor, next I will discuss some specific
qualities pertaining to hermetic reciprocating compressors. A hermetic compressor is one
where the motor and compressor are sealed in a single welded shell. On site repairs of a
hermetic compressor is not typically viable. Larger more valuable hermetic compressors can be
sent away to specialized companies for repair. Smaller hermetic compressors are often referred
to as “throw away” compressors as they are usually discarded and replaced in the case of a
severe condition. A very unique aspect of the hermetic compressor in a refrigerant system is
the cooling method. The cool suction gas received from the evaporator is used to cool the
motor windings. The entire compressor shell is then considered a low side device as it contains
the low temperature gas that has left the evaporator. The materials within the compressor shell
have to be compatible with the refrigerant being used. The windings on the motor must be
coated with the proper material. Any current conducted through the windings into the
refrigerant would cause acid and sludge to form in the system. Typically a reciprocating
hermetic will have its pistons working at at a right angle from the crankshaft. The motor will
usually be found at the top of the shell. This compressor type is typically found in fractional
horse power in residential refrigerators and freezers. The most common applications are small
and medium sized commercial refrigeration systems. Another quality unique to the hermetic
compressor type is its electrical terminals. They need to be sealed from the refrigerant within
the compressor shell and have proper insulation to prevent current upon the shell. In the case
of current leakage arcing may occur. Any electrical current encountering refrigerant will cause
the refrigerant gas to deteriorate and become acidic. This will lead to sludge and a whole host
of problems can occur throughout the system at this point.

3. Some specific applications of the hermetic reciprocating compressor include: household
refrigerators and freezers, room air conditioning units, and air conditioning systems. The
household refrigerator was probably the first most common use of this compressor type. In
refrigerators, freezers and small room A/C systems refrigerant R-134A is typically found. A
nominal low side pressure for R-134A in a fridge may be around 10psi whereas a room A/C will
run approximately 40psi on the low side. Power supplied to these types of units is typically
115v, 60hz AC. An air conditioning systemfor a home or small to medium commercial building
may also use the hermetic reciprocating compressor. Common refrigerants for these larger
systems may be R-410A, R-410C or R-22. It should be noted that R-22 has been banned from
any newly constructed equipment as of 2010. These refrigerants operate at a much higher
pressure than R-134A so it is expected that the compressor not only has compatible
components for the required refrigerant but also is designed to withstand a pressure above
operating pressure at the low side. It is expected that the rise in low side temperature can be
endured by the compressor shell. It may be expected to withstand pressures above 200psi.
Power for an A/C unit is typically designed for the capacity of the system. On a normal
household system 230v single phase is adequate. Light commercial use may be of the same
voltage but includes the possibility of a 3-phase power supply. The voltage on a 3-phase system
can even be 208V, 460v, or 600V if the demand is high enough but does not to require multi-
stage compressors. The power being used by a compressor should be within 10% of the rated
power. If it is rated at 120v AC, the compressor should draw no more than 132v or less than
108v. On any small fractional horse power, residential use equipment the power supply will be
120V single phase at 60Hz.
Now that I have discussed the inner workings and application of the hermetic
compressor, I will move on to more specific controls that allow the proper function and
operation therein. Protection components guard the compressor motor from overload
conditions. They can be categorized into two types: internal protection and external protection.
The internal protection device is typically a temperature sensing disc or thermal overloads in
the motor windings. The line current of the motor passes through the device, when current
exceeds normal operating conditions the line to motor winding connection will be opened.
External protection is applied to the device that relays the power to the motor or motor starter.
A typical motor relay will have an overload circuit built in.
Internal circuit opening on overload condition:
External circuit with overload protection:
Overload condition will draw heat in the circuit at the
heater, the overload coil will open the line connection
de-energising the coil and opening the contacts to the motor.

4. Another type of external overload protection that is not part of the motor starter circuit
is a magnetic overload device. The magnetic device reacts to amperes only, so ambient
conditions where it is located have no effect on how the device functions.
Now, taking a deeper look into how the motor functions we will examine the start and
run windings with the appropriate components. Electric motors run on the principal of
electromagnetism. A stator, which has wire wound around it, generates electromagnetic force.
The rotor is made of a magnetic material and is place in between stators. When two stators are
placed opposite to one another with reverse poles they will cause the continuous circular
motion of the rotor when momentum has been induced. To induce motion a start winding
around offset stators is added to the motor. The start winding is also used to increase starting
torque in higher load applications. In such scenarios a capacitor is added to the start winding
and on some applications the run winding as well. The start winding, being designed only to
begin the movement of the rotor has to drop out of the circuit some time before FLA (full load
amperage) is achieved. Three methods of achieving this will be discussed: the potential relay,
the current relay and a positive temperature coefficient device.
The potential relay is used in situations where a high starting torque is required. They
are designed to be used with capacitor start, capacitor run motors. They are actuated via back
electromagnetic force of a running motor. When the motor reaches enough rpm that it no
longer requires starting torque the potential relay will have received enough current to open
its’ switch, removing the start capacitor from the circuit.
Potential relay wired in series with start winding:
The current relay is used in situations requiring low starting torque. Most often found
on systems with capillary tubes that will equalize pressure during an off-cycle, thus requiring
less load on the motor for start-up. It consists of a low resistance coil that when under lock
rotor amperage (LRA) it actuates a switch much like a relay and closes the contacts to energise
the start winding. When LRA lowers to FLA, the switch contacts open de-energising
the start winding.
Current relay wired in series with line and start winding
with switch:

5. The positive temperature coefficient device start device uses the LRA to create heat in
the device which creates resistance and in turn creates the necessary phase change in the start
windings to induce movement and increase torque. This device can be added to a permanent
split capacity motor to increase its torque.
PTC wired in series with start winding (low resistance):
PTC wired in series with start winding (high resistance):
As I have previously mentioned, capacitors can be used to increase motor torque. I will
now discuss several configurations of a motors start/run windings and capacitors.
The split phase motor is the base design for most motors. It contains only two separate
motor windings; the start and run winding.
Split phase Motor:
Note: The start switch can be any of the previously
described start devices.
The start winding and run winding get full line voltage
until the motor reaches adequate rpm to create enough back
electromagnetic force to open the start switch.
In a capacitive start motor, a capacitor is wired in series with the start winding. A
capacitor is chosen that will make an ideal phase angle for adding starting torque to the motor.
When a capacitor is in a circuit it creates a phase change to the amperage. Normally voltage

6. and amperage levels are synchronised. When a capacitor is introduced into a circuit, capacitive
reactance takes place and as a result the amperage will lead the voltage. Much the same as the
split phase motor the start capacitor and the start winding must be opened from the circuit
before the motor reaches FLA. This is also achieved using one of the previously mentioned
control relays.
Capacitor start motor wiring diagram:
The capacitor run, capacitor start motor has the start and run capacitor wired in parallel.
This parallel configuration is in series with the start winding. Like the capacitor start motor, a
start switch will drop the start capacitor out of the series when near FLA is achieved. The run
capacitor stays in series with the start winding. This causes the start winding to continue
running but the capacitor limits the current through the start winding during FLA.
Capacitor start, capacitor run wiring diagram:
Note: without a capacitor in series with the
run winding, the run winding would experience
FLA and overheat, eventually burning out.
The permanent split capacity motor uses a run capacitor only. The starting torque is very
low so the motor can be used only in low starting torque applications. The run capacitor is
wired in parallel to the run winding and in series with the start winding. The PSC is typically
found in fixed bore or capillary tube A/C systems that will equalize pressure on pump down.
Neutral
Line
Run Winding
Start winding

7. Lastly there is the shaded pole motor. This type of motor will not be discussed in detail
as it is not used in any type of compressor. However, a low torque fan motor typically uses the
shaded pole motor. It is likely to see one used for a smaller evaporator or condenser fan. They
are usually discarded and replaced when found to be faulty.
Lastly, I will discuss some precautions to using a reciprocating hermetic compressor
including compression ratio, discharge temperature, importance of cooling the motor, liquid in
the pistons and changing rotation direction of the compressor. The compression ratio can be
obtained by dividing the discharge pressure by the suction pressure in absolute values. A good
compression ratio would be about 3:1. A lower ratio means better efficiency as your
compressor does not have to do as much work and requires less power. The maximum ratio we
should accept is around 12:1 for a reciprocating hermetic compressor. At this point the
discharge temperature will be very high. This heat will cause the oil in the refrigerant to
breakdown turning into carbon and will create acid in the system. On an A/C system we would
not expect the discharge to exceed 220F without exception. As previously mentioned hermetic
compressors are cooled by the suction gas received from the evaporator. Without any cooling
the same effect would occur; overheating of the refrigerant and breakdown of its oil. As gas is
meant to enter the suction line, what would happen if the refrigerant was not sufficiently
evaporated? The pistons are meant to compress gas; liquid however cannot be compressed by
the piston. Encountering the force of liquid; the piston, the valves and the piston rods can
break. When a compressor is trying to compress liquid it is called slugging. Most reciprocating
compressors can have their direction changed. One important factor to take into consideration
is how the compressor is lubricated. If it is a smaller compressor that requires splash
lubrication, it may only get properly lubricated as it turns in one direction. The pressure
lubricated type will work in both directions. Taking into account what type of lubrication your
compressor uses, you can change its direction if you can gain access to the wiring of the start
and run windings. It is not possible to change the direction of a hermetic motor as the start and
run windings are not accessible.
The reciprocating hermetic compressor is widely used from small appliances to medium
sized commercial units. It is not an overly complicated device, yet demands a level of
mechanical understanding to comprehend its function. What I have discussed here hopefully
sheds some light on the workings and application of this popular piece of equipment.
Reference(s):
Whitman,W. (n.d.). Refrigeration &air conditioning technology (Seventhed.).