2205B0JE-DA-J-N_2014.05.
2 Structure and Specifications of the Compressor
Screw Compressor J-series
2.8 Variable Vi Control
2-29
2.8.2 Reasons for Adjusting Vi According to Operating Conditions
Operating conditions for the compressor differs with applications. The same compressor may be
operated under a variety of pressure conditions such as air conditioning, cold storage and freezing.
Also, the discharge pressure may vary depending on the type of condenser or climate conditions. Even
in the same application, some conditions may vary. For example, a compressor for air conditioning has
different operating conditions for the cooling mode and the heating mode. And a compressor for
refrigeration may have different operating conditions depending on the storage room temperature.
Compressors are always required to operate with maximum efficiency in these varying operating
conditions.
For a compressor without a variable Vi mechanism, power is wasted when the fixed Vi does not match
the operating conditions (refer to Figure 2-25). For example, if a compressor with a high Vi discharge
port (suitable for a high compression ratio) is operated at low compression ratio conditions, the gas
between the rotor lobes increases internal pressure to a value higher than the discharge pressure
before it is released to the discharge port, this results in extra compression power. On the other hand, if
a compressor with a low Vi discharge port is operated at high compression ratio conditions, the gas
between the rotor lobes is released to the discharge port before the internal pressure reaches
discharge pressure. This results in backflow of the high pressure gas to the discharge side which
wastes extra power to push back the backflow gas. These characteristics are inevitable for a fixed Vi
screw compressor and become obvious when the discharge port Vi does not match the actual
operation conditions.
The J-series has a structure that can vary Vi of the discharge port according to operating conditions,
therefore, it provides the advantage of operating at a wide range of conditions with high efficiency.
2.8.3 Details of Variable Vi Mechanism
The J-series varies the Vi by moving the variable Vi slide valve with a hydraulic cylinder thus changing
the radial discharge port located in the unloader slide valve.
The Vi is increased when oil is supplied to the Vi increase side of the hydraulic cylinder. The variable Vi
slide valve moves towards the discharge side, and the unloader slide valve moves so that the radial
discharge port becomes smaller and Vi is increased.
On the other hand, the Vi is decreased when oil is supplied to the Vi decrease side of the hydraulic
cylinder and the variable Vi slide valve moves towards the suction side. The radial discharge port
becomes large and Vi is decreased.
The position of the variable Vi slide valve can be detected by the Vi position sensor. The J-series can
always control Vi to an optimum value with a specified controller according to measured operating
conditions (such as suction and discharge pressure).
2.8.3.1 Automatic Variable Vi Mode
In the automatic variable Vi mode, the J-series calculates the optimum Vi from the measured operating
pressures using a specified controller and adjusts the hydraulic pressure on the Vi increasing side or
the Vi decreasing side of the hydraulic cylinder to move the variable Vi slide valve to a suitable position
for the calculated Vi.
Note 1: Basically, the automatic variable Vi control is achieved in three steps, L, M, and H.
（
Not continuously
）
Note 2: Do not frequently vary the Vi automatically. This may lead to deterioration of reliability such as early
wear to the related components.
Figure 2-26
