Hermetic Compressors for Refrigeration

Formerly known as Danfoss Compressors, Secop is one of the founding fathers of modern cooling compressor technology and an expert in advanced refrigeration compressor technologies and cooling solutions in commercial refrigeration. The basic operation of reciprocating refrigeration compressors and a refrigeration circuit can be found on this page. The hermetic cooling compressor is at the very heart of the refrigeration unit, producing around 2,900 revolutions (at 50 Hz) or 3,500 revolutions (at 60 Hz) every minute.

Summary of topics:

  • Secop KAPPA Compressor
    Secop K-Series (formerly KAPPA) R600a compressor
  • Secop KAPPA Compressor
    Secop K-Series (GTK, HKK, HMK, HTK, HXK, HZK)
  • Secop KAPPA Compressor
    Secop K-Series compressor with evaporation tray

The Compressor, Its Components, and Their Function

Type label

The type label that can be found on every hermetic compressor contains critical information for the customer or user, as well as for the dispatch process. Details include the type of compressor, the refrigerant which it is designed for, the frequency, and the mains voltage.


The suction connector is used to suck the refrigerant into the compressor with the process and discharge connectors located on opposite sides. Compressed refrigerant is discharged from the compressor via the discharge connector. The process connector allows the cooling system to be charged with refrigerant after the compressor has been installed. This process would usually take place in a final stage at the manufacturer’s assembly line.

Electrical connection

The electrical connection can be seen on the outside of the compressor. Depending upon the type of device being purchased, starting equipment is sometimes preassembled together with an appropriate protective plastic cover.

Compressor housing

The compressor shell is constructed from a steel sheet with the top cover welded together with the bottom housing. That connection is hermetically sealed, ensuring that refrigerant cannot leak to the outside. In order to guarantee that the mechanical unit (motor, cylinder, and valve) of the hermetic compressor remains within the center of its housing, four springs are used to keep it in its dedicated position. The external interfaces of the shell include two baseplates that enable the compressor to be mounted within the appliance.

Motor and stator

The motor is of key importance to the compressor and consists of a stator, rotor, and power cable. It is a priority to ensure that noise levels are kept to a minimum, and the motor is mounted on springs for this reason which results in reduced levels of vibration.
The stator consists of a stator stack, together with two windings of copper wires. The stator stack is constructed using sheet metal. The windings are suitably protected to ensure that damage from loose wires can be avoided. By contrast, the rotor has an iron core, cast in aluminum.

Electrical start equipment

The provided electrical starting device must be suitable for the individual compressor. The starting device is responsible for starting the hermetic compressor and does so by supplying the necessary power to enable the start winding to be activated. Once the compressor has started to rotate, power is supplied via the motor to the main winding. A flexible connection to the motor is provided via the power cable.

Pump unit

The piston and cylinder unit contains four distinct elements: a block, discharge tube, crankshaft, and piston. The discharge tube is mounted upon the block, with an oil pump mounted at the bottom of the crankshaft. The piston has a piston rod which is connected via a piston pin. Two discharge chambers are contained within the block and are used to enable compressed refrigerant to make its way to the discharge connector.
The flexibility of the tube itself is increased by turns of the discharge tube. The crankshaft is securely connected to the rotor, transforming the rotations of the motor into strokes of the piston. As a result of these piston strokes, the cylinder moves up and down. It's this movement that allows refrigerant to be sucked in, then compressed, and finally discharged.
The valve unit makes use of a discharge valve and suction unit, which are both installed on the main valve plate. The valves assist with opening and closing the valve plates during the suction and discharge processes.
This action allows the hermetically compressed refrigerant to make its way into the discharge chambers. Finally, a muffler is used to reduce the noise that is created by the suction process. It is fitted between the suction connector and the suction side of the pump unit.

  • Hermetic Compressor
    Glass model of a Secop TL compressor
  • Hermetic Compressor
    Glass model of a Secop TL compressor w/o top cover
  • Pump and motor of a Secop TL compressor

Parts of a Hermetic Refrigeration Compressor

  • 1 – Housing with connectors and baseplates
  • 2 – Top covers
  • 3 – Block with a stator bracket
  • 4 – Stators (with screws)
  • 5 – Rotors
  • 6 – Valve units (screws, cylinder cover, gaskets, valve plate)
  • 7 – Crankshafts with grommet
  • 8 – Connecting rodes with a piston
  • 9 – Oil pickup tubes
  • 10 – Springs with suspensions
  • 11 – Internal discharge tubes (screw, washer, gasket)
  • 12 – Start equipment (PTC device, cover, cord relief)
  • Hermetic Refrigeration Compressor
    Parts of a Secop TL-G compressor

The Refrigeration Circuit and Compressor

How Does a Refrigerator Work?

Part I: The Refrigerant Circuit – Evaporative Cooling

In order to show how a cooling system functions, this film first explains the principle of evaporative cooling. How does it look like in our refrigerator? Here it is just this principle of the closed refrigerant circuit.

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English Chinese

How Does a Compressor Work?

Part II: The Compressor – Its Components and Their Function

The compressor – the heart of the refrigerator – pumps the refrigerant through the refrigerating circuit with about 3,000 revolutions per minute. With its innovative and environmentally favorable compressors, Secop belongs to one of the leading manufacturers of hermetic compressors worldwide.

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English Chinese

Hermetic Reciprocating Compressors – Exploded Views

Secop Compressors FAQ – General

Find our answers to the most frequently asked questions on general topics.

What type and what amount of oil are Secop compressors charged with?

The amount of oil we charge our compressors with can be found in the data sheet of the individual compressor. The data sheets can be found and downloaded via the Data Sheet Selector on our website.

More information can be found in our catalogs which contain an introductory section with some information on use, oil type, starting equipment, and application limits for the Secop compressors.

Compressor oils used by Secop include:

For R134a compressors:

  • PL and TL: 15 cSt POE oil
  • NL and FR: 19 cSt POE oil
  • SC: 22 cSt POE oil
Are Material Safety Data Sheets (MSDS) available for the compressor oils used by Secop?

Secop has MSDS sheets for the original oil that will be handed out to customers on request. Please contact your local Secop company.

What parameters determine the lifetime of a compressor and what can typically be expected?

Important factors determining the lifetime of hermetic compressors are:

  • System dimensioning and system lifetime
  • System temperature, esp. valve and winding temperature
  • System pressure
  • Purity of system components
  • Quality of refrigerant and oil
  • Lifetime of other system components
  • Quality of main power supply (cutouts, voltage drops, etc.)
  • Application duty cycle

In a typical application where the compressor is used within the specified limits given in the actual product literature, a lifetime of 10–15 years can be expected as a rule of thumb.

The lifetime of a compressor used in a household application may be longer than the lifetime in light commercial application.

Do you have a mathematical equation and coefficients for the calculation of the compressor data?

Yes, we publish our compressor performance data according to the ARI standard 540-91 and EN 12900.

We will gladly provide data for specific compressors if requested. Please contact us via our contact form

What determines the noise level of a cooling system?

The most relevant factors are:

  • Combination of components in the cooling system (layout and installation)
  • Pulsations and turbulences in the refrigerant gas related to the specific system
  • Actual level of system insulation
  • Function of machine compartment
  • Actual system vibration prevention
  • Suction and/or pressure mufflers 
What is the noise level of a Secop compressor?

The Secop compressors are extremely low noised compared to the average products from the competition.

How does horsepower compare to capacity in W or Btu/h?

Horsepower or hp is a historic designation for the engine performance. There is no official standard relation to refrigerating capacity. All manufacturers have developed their own formula. Therefore, the data stated in hp by different manufacturers cannot be compared.

Furthermore, a capacity given in hp is dependent on the application range of the compressor, i.e. 1 hp of an air-conditioning compressor is different from 1 hp of a LBP compressor.

If the capacity in W or BTU/h at given conditions is known, this should be preferred for comparison.

Why is the compressor housing hot?

Secop compressors are not cooled by suction gas but with the oil circulating inside the compressor housing. The housing then transports the heat to the surroundings.

What is the meaning of the serial number on the compressor?

Serial numbers were introduced in July 2004. The serial number is individual to each single compressor. There is no relation between serial number and production date, shipment day, or the like. The serial number has been introduced to help our customers use the individual compressor serial number in the documentation for their appliances. The production date can still be found in the stamped code on the compressor housing. 

How can I read the date stamp on the compressor?

What information does the date stamp on the compressor housing provide?

Secop compressors have a manufacturing date code stamp or a needle print on the housing. The content of the coding is set in two lines.

For more information, see Product Bulletin: Date Code Format, Country of Origin, and Labels

How can the type code on a Secop compressor be interpreted?

Please refer to our brochure: Key to Compressor Type Designation

Secop Compressors FAQ – DC-Powered

Find our answers to the most frequently asked questions on DC-Powered topics.

What is the difference between the BD Solar and the normal BD types?

The BD solar compressors can be connected to a solar panel without the use of any additional electronics or the use of a battery. The BD Solar compressor electronic unit includes an option to adapt to the voltage range of solar modules (9.6 V to 31.5 V).

Can I use the refrigerant R404a in BD35F or BD50F compressors?

No, using R404A with the BD compressors in their current design is not recommended or supported by Secop.

The specifications of the motor as well as of the oil prohibit the use of R404a in these compressors.

The power consumption in general and the starting current needed are very high using R404A. That will make the start of the compressor questionable. Even if the compressor was used in low-temperature applications (LBP) only, it would not be able to make the pull down.

What are the instructions to check the BD35/50F electronics?

Generally, the customer can use the LED for failure detection. Errors are indicated by the failure LED. For the details, please refer to the instructions which can be found on our website. Additionally the following can be checked:

If the compressor does not start at all the first time:

  • Check connections on compressor and back of battery.
  • Check for loose connections in plug or leads. 
  • Check poles on the battery (verdigris on poles or contacts?).
  • Check correct setting of standard or resistor changed battery voltage protection (see instructions, wiring diagram and description).
  • Check whether battery voltage is within the battery protection limits.
  • Check thermostat and connection to ports C and T.
  • Check fuse and polarity: Has the fuse blown because of wrong polarity? If so, change the fuse and try again with the correct polarity.
  • Test resistance between all three current lead-in pins? If the value between all three pins is approximately the same, the motor is most likely still okay. Otherwise, the compressor must be changed and investigated closer by Secop.
  • If the compressor is so far okay and no external faults have been detected, the electronics have to be closer investigated by Secop. 
  • If the compressor starts but operates only for a short time: the problem can be a dimensioning problem of the cooling circuit (evaporator or condenser size, charge). 
  • If the compressor stops approximately 2 seconds after starting the fan (2 flashes on LED), then the fan is overloading the electronics and the fan fault has to be solved or a smaller fan has to be used. In this case, solve the fan fault or use a smaller fan. 

For further information, refer to our page Electronic Units Instructions.

Should a 12 V or a 24 V fan be used with the BD35/50F compressor?

If a fan is required, the 12 V as well as the 24 V direct current systems have to be equipped with a 12 V fan.

A question concerning the resistor in BD35F and BD50F

A 692 Ohm non-standard resistor is recommended for the 3,000 RPM setting of the BD35F and BD50F compressors. Can a standard resistor of 680 Ohm be used instead?

If you use a 680 Ohm resistor in the control (thermostat) circuit, the motor speed will just be a little less than 3,000 rpm. However, the difference between the two resistance values is so small that no difference in performance would be noticeable in practice.

For more information on the resistance as well as the electronic unit, please view the page "Electronic Units Instructions" on our website.

Why would a BD35F or BD50F compressor that is operated on a converter not start at all, or start and stop in 2–3 second intervals every now and then?

Depending on the selected setting, the controller of the compressor's motor has a cutoff voltage of 9.6 V to 10.4 V on a 12 V power supply. On a 24 V power supply, the cutoff value is 21.3 V - 22.8 V (for details refer to table "Optional battery protection settings" in the Instructions of those units). The starting current is approximately 14 A.

If the compressor has not reached the minimum speed or the minimum voltage is not available 1–2 seconds after starting, it will be switched off again.

Because the undervoltage of a ripple voltage supply is defined by the minimum value, the compressor's electronics are not suitable for the operation with a battery charging station or not stabilized power supplies. The power supply must be able to handle a current of up to 7.5 A and a starting current of up to ca. 14 A in order not to reach the minimum voltage, even if there are ripples. Since normally the battery would buffer this, connecting a small battery, for example, of a lawn mower or a motorcycle, should be sufficient to solve this problem.

How can an existing Secop 12 V DC refrigeration system be run on 220 V or 110 V AC?

Secop does not stock any AC/DC converters, nor do we recommend any. But since those converters are widely available, it should be rather easy to buy a unit anywhere.

A simple device like a traditional battery charger is sufficient if the converter is connected to a battery. The compressor will then be powered by the battery. An output power of about 70 W (6 A) will be needed.
Running the compressor directly from the AC/DC converter is more complicated and only possible with the DC compressors BD35F or BD50F. The converter output voltage needs to be an approximate DC with a maximum ripple of 0.5 V peak to peak. Furthermore, the unit must be able to yield a current of 14 A for 1 second at a minimum voltage of about 11 V.

We strongly recommend running the compressor with a battery since this will ensure a stable refrigeration system operation.

For more information on the electronic unit, please refer to our website.

What are the specifications for the diagnostic diode on BD35F/50F/80F compressors?

The diode should be rated 10 mA. The electronic unit will send a flashing current signal of approx. 4 mA constantly into the circuit in case of a fault.

Can BD compressors be run without electronic units for test reasons?

No, that is not advisable.

BD compressors are designed to be operated on a DC power supply. To run the compressor, the electric equipment needs to generate the magnetic field that basically turns the rotor. If the compressor is used without an electronic unit, it can be destroyed. For more information on the electronic unit, please see the page Electronic Units Instructions on our website.

Can I use leak detection dyes in refrigeration systems?

No. Secop does not endorse the use of any substance except the refrigerant and the oil designed for use in the specific compressor.

Can ammonia be used as a refrigerant in a Secop compressor if combined with a compatible oil? Does Secop have any experience with the use of ammonia in hermetic compressors?

No, it cannot.

All Secop hermetic compressors use motor windings of enamel copper wire, and some other small parts of copper or copper alloys. The copper will be attacked by ammonia, so the compressors will certainly be damaged.
All materials used for the internal parts of Secop compressors are only tested for chemical compatibility with the oils and refrigerants they are made for. Changing the oil, maybe even changing the producer of the oil, might lead to problems. 

Ammonia generally is not used in small hermetic systems because ammonia develops very high temperatures when compressed.

Where can I find replacement parts for BD2, BD2.5, BD3?

The compressors are end of life since more than 20 years. All genuine first party replacement part stocks are empty. Please consider to upgrade to a newer compressor.

Which motor controller can I use to replace my defect one?

You can find all compatible controllers to your compressor in the compressors data sheet. All compressor data sheets are available in our Product Selector.

My compressor cannot start. It could start before the winter, now after not using it for a few months it fails.

Re-charge or replace the 12 V battery that supplies the compressor with energy. Batteries degrade over time and can be discharged after some time without usage.

Secop Compressors FAQ – AC-Powered

Find our answers to the most frequently asked questions on AC-Powered topics.

What determines the noise level of a cooling system?

The most relevant factors are:

  • Combination of components in the cooling system (layout and installation)
  • Pulsations and turbulences in the refrigerant gas related to the specific system
  • Actual level of system insulation
  • Function of machine compartment
  • Actual system vibration prevention
  • Suction and/or pressure mufflers 
There are no data sheets for R12 or R22 compressor in the data sheet selector.

We don't have the data sheets of our older compressor and condensing unit types on our website, but we will gladly send you the documentation by mail or e-mail on request.

What is the function of a PTC?

The PTC protects the start winding by switching it off.

When can a PTC be used?

A compressor can be started using a PTC if the appliance has a standstill period of a minimum period of seven minutes in order to ensure pressure equalization via the capillary tube.

Will the wiring in the compressor automatically fail when the PTC starting device burns out? Can the compressor be ohmed out to check for shortings?

Generally, the compressor should be still okay even if the PTC starting device burns out – we recommend using a new PTC instead.

The nominal values for the main winding and for the start winding at a temperature of 25°C are stated in the data sheet. Depending on the actual ambient temperature, this nominal value has to be corrected by a factor from the following table.































Example: For the NL7CNK compressor, the value given in the data sheet is 14.7 Ohm @ 25°C. If your ambient temperature is 40°C, you have to multiply this by 1.058 as declared in the table. So your resistance would be 1.058 x 14.7 Ohm = 15.6 Ohm. Please note that these values apply only if the resistance is measured directly at the stator – you should include a tolerance of about +/- 10%.

What is the advantage of an ePTC?

The main advantage of the ePTC solution is the easy way of saving energy in existing and future fridge and freezer designs.

This is due to the following factors:

  • Reduction of 2 W in power consumption
  • Full PTC protection of start winding
  • Can be operated with a run capacitor
  • Silent operation
  • Short recovery time
  • Compatibility to all Secop 103N PTC starters

 In special cases, an ePTC can replace a PTC/run capacitor combination.

What does the stamping on a capacitor mean, for example, AC 220V 1.7% ED SD = 3MIN AC 280V 0.55% ED SD = 1MIN

In this example, the capacitor can operate at 220 V AC for a maximum time of 1.7% within three minutes. More exactly, this means the capacitor should be operated for a maximum of 3.1 seconds and afterwards be turned off for 176.9 seconds. At 280 V AC, the capacitor can be used for 0.3 seconds in a period of one minute.

Can capacitors with a different rating than specified in your literature be used?

Yes, that is possible. The voltage rating of all capacitors must be the same as, or higher than, the specified voltage.

Can I replace the standard run capacitor delivered by Secop with another type having the same electrical data but with safety class P0?

For safety reasons, Secop only distributes run capacitors of safety class P2 (with aluminum housing) instead of safety class P0 (with plastic housing) with its compressors. A P0 run capacitor is normally not used by, for example, German manufacturers because of safety and insurance reasons.

On the other hand, we know that P0 run capacitors are used by several manufacturers elsewhere. If a P0 run capacitor is used, it is very important to follow the instructions from the supplier regarding safety conception to prevent any fire in case the flammable capacitor, which is not protected, fails – for example, the distance to flammable parts should be at least 100 mm or the capacitor should be encapsulated.

So when technical matters are ok on the P0 capacitor we cannot see why it should not be operated with a Secop compressor. Also, please consider that the Secop run capacitor holder probably will not fit the capacitor. Compact installation including earth connection cannot be ensured because it cannot be installed directly on the compressing housing.

Can existing starting equipment be used with another compressor?

Generally no, it cannot.

The Secop Engineering department has selected optimized starting equipment for each compressor which meets the requirements and specifications. We, therefore, strongly advise using only the starting equipment that is recommended by Secop.

Especially when using starting relays, the cut-in and cutout current values have to match the motor requirements precisely. A wrong relay can destroy the motor or the start capacitor and cause severe safety problems.

Does the designation "198 V to 254 V" state the start voltage range or the working voltage range?

When we state a voltage range of 198 V to 254 V in our catalogs, it means that the compressor can both run and start at voltages within this range.

Can Secop recommend substitutes for failing compressors in old appliances?

Example: A cooler from 1977 is equipped with a PW7.5K14 compressor. For several months, the compressor ran longer and longer and finally worked continuously. Since the thermostat is operational, it is probable that the compressor has reached the end of its lifecycle. Is there any appropriate substitute?

No, Secop will does not recommend substitutes for failing old devices. If a compressor that has been used in an appliance for ten or more years finally fails, replacing the compressor is not recommended, even if it is technically feasible. After ten or more years of use, the insulation and the sealing of a refrigerator and freezer are used up. This would lead to significantly increased energy consumption.

Furthermore, the refrigerant R12 is no longer allowed in most countries. Replacing it with another refrigerant will become quite expensive.

The sensible recommendation is to buy a new appliance.

What is the recommended position to transport a refrigerator horizontally over a short distance?

The positions in which each compressor is allowed to be shipped are stated in the Product Bulletin – Shipment of Refrigeration Appliances in the Horizontal Position. If the unit has been shipped in the correct position in accordance with the Technical Information, we recommend waiting 30 minutes before switching on the refrigerator.

Is the refrigerator ready to run once it is returned to the upright position or should it rest for a certain period of time before it is turned on?

The positions in which each compressor is allowed to be put are stated in the Product Bulletin – Shipment of Refrigeration Appliances in the Horizontal Position. 

 If the unit has been shipped in the correct position in accordance with the Technical Information, we recommend waiting 30 minutes before switching on the refrigerator.

What is the difference between the standard compressor and the tropical compressor?

There are two main differences between the tropical compressor and the corresponding standard compressor:

  • All tropical compressors can be operated at a maximum ambient temperature of 43°C while most standard compressors are designed to work at temperatures of up to 38°C.
  • While the tropical compressors can be operated at a voltage range from 187 V to 254 V, the corresponding standard compressors have a smaller range of 198 V to 254V.

These factors require the tropical compressors to be equipped with a stronger motor which slightly decreases the COP. 

What do crankcase heaters do and when is the use of a crankcase heater recommended?

The crankcase heater is a heating element which is placed around the compressor to heat the compressor oil during standstill periods.

Depending on temperature and pressure, the oil in the compressor absorbs bigger or smaller amounts of refrigerant. During standstill periods, the oil level in the compressor rises due to the refrigerant solution in the oil. On start-up of the compressor, the pressure on the oil is reduced and the refrigerant dissolves in the form of vapor. This causes the oil to foam. The foam can even reach the compressor suction tube and the cylinder, which can cause the hydraulic pressure in the compressor cylinder. This can result in damaged valves and gaskets.

Especially during the first start and defrost, when the compressor is colder than the evaporator, the oil absorbs a lot of refrigerant. By using a crankcase heater during standstill periods, the compressor can be maintained at a steady temperature so that only a small amount of refrigerant is absorbed by the oil. When starting it for the first time, and in cases where refrigeration systems use a large charge, a crankcase heater must be cut in two to three hours before starting the compressor.There are two cases where a crankcase heater is recommended:

  1. Properly using a crankcase heater can reduce the risk of liquid hammer in cases where the maximum refrigerant charge as given in the compressor's data sheet cannot be complied with. If a crankcase heater is used, the maximum refrigerant charge can be increased by up to 70%.
  2. If the compressor is located where it may get colder than the evaporator, it is always recommended to use a crankcase heater. In the case of extremely low ambient temperatures, oil viscosity can become relatively high. If this causes starting problems, it may be advisable to use a crankcase heater even on domestic appliances.
Is there any Secop compressor suitable for R407C?

Yes there is. Please check our Product Selector for available types.

What determines the noise level of a cooling system?

The most relevant factors are:

  • combination of components in the cooling system (layout and installation)
  • pulsations and turbulences in the refrigerant gas related to the specific system
  • actual level of system insulation
  • fx of machine compartment
  • actual system vibration prevention
  • suction and/or pressure mufflers 

Compressor Service for R600a, R290 – Step by Step

Secop teams up with VULKAN Lokring – manufacturer of solder-free tube connections for the refrigeration and air-conditioning industry to deliver a educational film "Compressor Service for R600a, R290 – Step by Step". Servicing and repair for R600a and R290 systems is possible for skilled and well trained service technicians.

Refrigerant Circuit Poster and Technical Literature

Phantom View Images of Hermetic Compressors

  • Secop BD Compressor (BD-P)
  • Secop T-Series Compressor (TL)
  • Secop N-Series Compressor (NL)
  • Secop F-Series Compressor (FR)
  • Secop S-Series Compressor (SC)
  • Secop G-Series Compressor (GS)