Development of large capacity rotary compressor with three cylinders


The heat pump, which is the principle of air conditioners, is expected to be widely used in larger capacity heat utilization systems toward the realization of carbon neutrality as an energy-saving technology that can effectively utilize the heat of the earth. The compressor, which is an essential part for heat pump, is also strongly required to have a large capacity. To meet thesedemands, we have developed the world’s first three-cylinder rotary compressor with an increased number of cylinders from the conventional two to three. The maximum capacity of this compressor is one of the largest in the world for positive displacement variable speed models. In addition, by distributing the flow rate and load per cylinder, high efficiency was obtained in a wide operating range and a remarkable vibration reduction effect was obtained compared to the conventional two-cylinder model.

Fig.1 Three-cylinder rotary compressor


2.1 Three-cylinder rotary structure

As shown in Fig. 2, three cylinders are arranged in the axial direction, an angle formed by the eccentric directions of cranks of the crankshaft is set to 120° each, and compression is performed three times per rotation   . As a result, the vibration can be reduced despite the large capacity by significantly reducing the rotational torque fluctuation compared to the conventional two-cylinder compression. Fig. 3 shows an example of theoretical calculation of the torque fluctuation rate of a conventional two-cylinder rotary and a three-cylinder rotary assuming the same suction volume. The torque fluctuation range of the three-cylinder is reduced to about 30% of that of the two-cylinder.

The problems of the three-cylinder rotary structure are a large deflection of the crankshaft that occurs especially near the second cylinder and the great difficulty of aligning the inner diameter of the second cylinder with high accuracy on the center of the rotation axis. As a countermeasure, we have developed a structure in which the middle bearing shown in Fig. 2 is installed between the first cylinder and the second cylinder, and the second cylinder is aligned and fixed to the middle bearing based on the inner diameter of the middle bearing in advance. Thanks to this structure, the maximum amount of shaft deflection has been halved and the reliability of the bearings has been greatly improved.

Fig.2 Three-cylinder rotary structure

Fig.3 Torque fluctuation patterns

2.2 Multi-discharge structure

As the flow rate of the refrigerant increases in the rotary compressor, the flow path loss increases significantly due to the presence of the discharge valve at the discharge port, which is the main cause of efficiency deterioration. Therefore, as shown in Fig. 4, we have developed a multi-discharge structure, in which discharge ports are provided on both the upper and lower sides of the cylinder, the cross-sectional areas of the two ports are intentionally made different, and the responsiveness of the two discharge valves is also different.

Fig. 5 shows the results of actual measurement of the behavior of the discharge valves at large flow rate and small flow rate. Thanks to the different responsiveness of the two discharge valves, overcompression loss and flow path loss can be effectively suppressed, and efficiency can be improved over a wide operating range. In addition, the noise reduction effect is obtained by reducing the discharge pulsation.

Fig.4 Multi-discharge structure

Fig.5 Discharge valves behavior


2.3 High efficiency wide range motor (Open winding motor system)

We have developed the industry’s first open winding motor system that drives one large output motor with two inverters to cope with the increase in the capacity of the compression section. This system can increase the applied voltage by about 1.7 times and can operate up to the high load range without deterioration of efficiency. In contrast, in the low load range, the loss was suppressed by driving with one inverter, enabling high-efficiency operation in the entire range.


The developed model is installed in a multi-air conditioning system for buildings, and the number of compressors in the outdoor unit is integrated into one, contributing to the miniaturization and large capacity of the system, as well as energy saving and resource saving. We would like to contribute to the development of heat pump equipment by using the developed compressor to further increase the capacity and expand the operating temperature range.

Hirayama Takuya,
Member, Toshiba Carrier Corporation

Hirano Koji,
Member, Toshiba Carrier Corporation

Shida Shogo,
Toshiba Carrier Corporation

Taema Yoshihiro,
Toshiba Carrier Corporation

Shishimoto Tomohide,
Toshiba Carrier Corporation