The Professor: Digital Capacity Control for Compressors

November 3, 2008
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FIGURE 1. Second-generation digital scroll with internal solenoid and piping. (Photos courtesy of Emerson Climate Technologies.)

[Editor’s note: This is the third in a series of three columns about scroll compressors that began in the Sept. 1 issue of The NEWS and continued in the Oct. 6 issue.]

Compressor capacity control is desirable for optimum system performance when loads vary over a wide range. Compressor capacity control through modulation can reduce power consumption, produce better dehumidification control, and reduce compressor cycling along with smaller compressor starting currents.

This is especially important if you are trying to cool an area that has different cooling needs throughout the day. Constant temperatures of ±0.5°F can be accomplished in every room, at any time of the day, whether the room is mostly empty or standing room only with a digital capacity scroll doing the cooling.


If the mating scroll members of a scroll compressor are separated axially, there will be no refrigerant gas compressed and only 10 percent power usage will be realized. If varying the amount of time they are separated can control axial separation of the mating scrolls, capacity control can be achieved between 10 and 100 percent.

The separation of the mating scrolls is achieved by bypassing a controlled amount of discharge gas to the suction side of the compressor through a solenoid valve. In a second-generation digital scroll with internal piping and solenoid (Figure 1), the pressure in the modulating chamber is lowered by energizing the solenoid valve.

Discharge gas is metered through a bleed hole and the scrolls separate axially. No flow of refrigerant gas takes place when the mating scrolls are separated. This axial separation causes the scroll compressor to be unloaded.

FIGURE 2. A controller used with the digital scroll compressor.


Modulating capacity is achieved by either energizing or de-energizing the solenoid valve. An energized solenoid will unload the compressor by axially separating the scrolls and the compressor’s capacity is 0. When the solenoid valve is de-energized, the compressor’s capacity is 100 percent.

Solenoid cycle times between 10 and 30 seconds should be used to minimize solenoid valve cycling and to make the system more responsive. One complete “cycle time” is a combination of solenoid valve energized (unloaded) time plus de-energized (loaded) time.

It is suggested to never de-energize the solenoid less than 10 percent of the cycle time, to ensure enough refrigerant gas flow for motor cooling, because the digital scroll is a refrigerant-cooled compressor.

Example: If you have a 30-second cycle time and the solenoid is de-energized for 20 seconds, then energized for 10 seconds, the resulting capacity will be:

(20 ÷ 30) = 66.6 percent.

Any normal control parameter, including surrounding air temperature, humidity, or suction pressure, can unload the compressor. A compressor discharge line thermistor is required with a cutout temperature of 280°. The solenoid valve must have 15 watts of power at the appropriate voltage.


A compressor controller is used with the digital scroll compressor, which offers many protective features like phase control, short-cycling control, amperage and voltage unbalance, and high-amperage monitoring, to name a few. The controller can also supply a variable voltage to the unloading solenoid valve for open/close time intervals (Figure 2).

In summary, the digital scroll compressor can offer:

• The ability to hold a precise temperature and humidity level.

• Efficient full-load and part-load efficiencies.

• Thirty percent more efficient than traditional methods of compressor modulation.

• Less compressor cycling for longer compressor lives.

• Maximum comfort, efficiency, and reliability in one compressor.

Publication date: 11/03/2008

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