The majority of compressors supplied today incorporate an enclosed motor and this avoids any possible slight leakage of refrigerant through an open-drive shaft gland. The wide use of small refrigeration systems led to the evolution of methods of avoiding shaft seals, provided that the working fluid is compatible with the materials of electric motors and has a high dielectric strength.

The semi-hermetic or accessible-hermetic compressor (Figure 4.12) has the rotor of its drive motor integral with an extended crankshaft, and the sta­tor is fitted within an extension of the crankcase. Suction gas passes over the motor windings to remove waste heat in all but some of the smallest machines where forced cooling by air or water jackets is used. All starting switches must be outside the crankcase, since any sparking would lead to decomposition of the refrigerant. Electrical leads pass through ceramic or glass seals. Semi­hermetic compressors are built in a very wide range of sizes for the commercial and industrial markets. The motor is specified to suit the compressor and as such can be designed for best efficiency over the application range. Effective refrigerant cooling has the advantage that the motor can be more compact than the corresponding ‘stand alone’ electric motor.


Figure 4.12 Exterior view of semi-hermetic piston compressor (Bitzer)

Small compressors can be fully hermetic, i. e. having the motor and all working parts sealed within a steel shell (Figure 4.13), and so not accessible for repair or maintenance. ‘White goods’ in the form of domestic refrigerators and freezers accounts for many millions of hermetic compressors, and the concept can be applied in sizes of up to tens of kW. They are generally lighter and more compact than semi-hermetic types and usually operate at two-pole synchro­nous motor speed (2900 rpm for 50 Hz supply), whereas semi-hermetics most often run at four-pole speed (1450 rpm). Below approximately 5kW power input single-phase motors are used in locations where a three-phase supply is not available. The upper size limit of the hermetic compressor is determined by




Interna pressui relief v;





Terminal box

Suction inlet

Motor winding

Figure 4.13 Hermetic piston compressor (Danfoss)

The practicality of manufacture — the welded shell design lends itself to volume production methods.

The failure of an in-built motor is likely to lead to products of decomposi­tion and serious contamination of the system, which must then be thoroughly cleaned. Internal and external motor protection devices are fitted with the object of switching off the supply before such damage occurs.

Electronic motor power and speed controls provide the means to vary the speed of synchronous motors by generating electrical waveforms of controllable frequency. These devices are commonly called inverters and they can be very useful, particularly in close temperature control applications where stop/start or stepwise capacity changes give rise to unwanted fluctuations. Some over speed­ing may be possible if the motor is suitable. Care must be taken that both the compressor motor and the compressor itself are suitable for inverter application. There is likely to be a minimum speed limitation due to lubrication requirements. Inverters have internal electrical losses, and because the signal to the motor devi­ates from the pure waveform, additional motor losses and overheating can occur.

DC motors are now used in some small compressors and a converter is required to convert the supply from the AC source. An advantage of this approach is that only one motor is required for each model regardless of the local power supply voltage. The traditional AC synchronous motor together with its protection system is developed in different versions for the various voltages and frequencies existing in different countries. This results in a single compres­sor model having a number of motor options, which have to be manufactured

And stocked. The DC motor is universal and additionally provides variable speed capability.

There is a need for very small compressors to be driven from low-voltage DC supplies. Typical cases are batteries on small boats and mobile homes, where these do not have a mains voltage alternator. It is also possible to obtain such a supply from a bank of solar cells. This requirement has been met in the past by diaphragm compressors driven by a crank and piston rod from a DC motor, or by vibrating solenoids. Electronic devices now make it possible to obtain the mains voltage AC supply for hermetic compressors from low-voltage DC.

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