Inverter Compressors: Improving the Efficiency of Refrigerators & Freezers

By John Balazs, Mike Marcario, Jennifer Archibald

As the “heart” of the refrigeration system, the compressor is the workhorse behind the stable operation 

of the refrigerator or freezer. Most home-use refrigerators and freezers today utilize single speed 

compressors which are either “on” or “off” and can only operate at one speed This type of performance 

does not allow flexible operation to adapt to the different usage conditions such appliances experience 

during the day, nor does it efficiently utilize electric power once the unit is at a steady state.

In 1996, Panasonic began to utilize inverter compressors in its refrigerators as a solution for providing 

more flexible operation of the compressor, thereby significantly increasing the overall appliance 

efficiency. Compared to the traditional single speed compressor, the inverter compressor can run at 

a number of desired speeds, depending on how the refrigerator is being used by the consumer. This 

provides options for quick pull down or freezing, or to run at a very low speed once the cabinet is at a 

steady state, thereby significantly reducing energy consumption.

Panasonic has recently introduced its ENI and EFI inverter 

compressors to the North American market. These 

compressors make use of the same inverter basic inverter 

technology that Panasonic introduced over 10 years ago, 

however it offers this performance in a smaller package and 

with better efficiency than the products from last decade.

When applying an inverter compressor in a refrigeration system, one must take a number of factors 

into account. The cooling load requirement of the cabinet must be considered. In addition, in order to 

maintain the durability and reliability of the compressor, one needs to make sure of the pressures and 

temperatures of the compressor, particularly at low speeds.

Generally refrigerators are designed to secure their specific cooling performance when the load is 

highest (i.e. Summer). Therefore it is necessary to select a compressor with the freezing capacity 

equivalent to the maximum cooling load of the static cooling load plus the “door open-shut” load (see 

Figure 1). Historically then refrigerators were being designed with compressors to accommodate the 

maximum freezing capacity that the refrigeration system would require. In reality, however, the actual 

load on the refrigerator is not this large at all times. In fact, the load seen by the unit for most of the year is 

actually much smaller. But the freezing capacity of the compressor is constant irrespective of the actual 

load, so it will be excessive 

under the usual or more normal 

cooling operation (see Figure 

1). This results in a great loss 

of efficiency due to the high 

compression ratio. It’s wasted 

energy. 

In order to maximize on the 

efficiency of the refrigerator, the 

ideal solution would be to reduce 

the standard static cooling load 

of the refrigerator. Careful consideration of the design can accomplish this, including the adoption of 

better insulation as well as vacuum insulation panels. By reducing the static load requirement, the 

cylinder volume of the compressor can be reduced as less capacity will be required, even at peak 

load conditions. Aside from this downsizing, under usual load conditions, less freezing capacity is 

Door open -

shut load Static cooling load

Compressor freezing capacity

Compressor freezing capacity

Static cooling load

Fig. 1 Conventional Design

A. Peak Load

B. Usual Load

Door open -

shut load Static cooling load

Compressor freezing capacity

Compressor freezing capacity

Static cooling load

Fig. 1 Conventional Design

A. Peak Load

B. Usual Load
required. A normal single speed 

compressor can only operate at 

one speed with one capacity. By 

varying the speed of the inverter 

compressor, to a lower speed 

so as to match the cooling load 

that is required, you maximize on 

efficiency (see Figure 2). 

Even in cases where the cylinder 

volume or displacement size of 

the compressor can not be reduced due to design limitations in reducing the static load, the benefit of 

employing an inverter compressor can be realized (see Figure 3). Figure 3 illustrates a comparison 

of a conventional single speed compressor versus an inverter compressor where you can vary the 

speed. A single speed compressor must cycle on and off based on the cooling capacity needs of the 

refrigeration system. Regardless of the time of day or the cooling capacity required, the compressor 

can only rotate at one speed. So during the daytime, when there is more activity and more cooling 

capacity is required, the compressor can only run at one speed and deliver a set cooling capacity. 

By varying the speed of the inverter compressor, the cooling capacity can be tailored to the cabinet’s 

need at that time. For example, if the door is left open for a period of time due to loading of the cabinet, 

the compressor can run at a high speed to provide maximum cooling, in some cases possibly more 

than what the conventional single speed compressor could deliver. Likewise the compressor can 

run at gradually lower speeds as the need for cooling capacity diminishes. Since the conventional 

compressor can only operate at one speed, it is during these periods that there is a significant benefit in 

terms of energy efficiency. At night when there is little activity, a conventional single speed compressor 

would still have to cycle on and off, again at a constant cooling capacity, when the freezer/refrigerator 

compartment temperature goes below a certain level. The inverter compressor can operate at a 

very low speed during this time, providing just enough cooling capacity to maintain the cabinet’s 

temperature. It is during these periods where the maximum energy efficiency benefit can be realized 

(see figure 3). 

Door open -

shut load Static cooling load

Compressor freezing capacity

Compressor

capacity

Static

cooling load

Fig. 2 Approach to energy -saving

A. Peak Load

B. Usual Load

Load reduction

Capacity control

(Inverter)

Adoption of small capacity

cylinder in compressor

Door open -

shut load Static cooling load

Compressor freezing capacity

Compressor

capacity

Static

cooling load

Fig. 2 Approach to energy -saving

A. Peak Load

B. Usual Load

Load reduction

Capacity control

(Inverter)

Adoption of small capacity

cylinder in compressor

Daytime Night time 

When internal 

temperature 

rises 

dramatically, 

high-power 

operation kicks 

in 

(e.g., Leaving 

the door open 

for a long 

period; 

defrosting) 

(rpm)

Energy saving 

After stabilizing 

internal temperature, 

low-rotation operation 

automatically kicks in. 

Fig. 3 
All compressors will cycle on and off based on the temperature level in the cabinet and the cooling 

capacity required. An inverter compressor, by running at a low speed, can actually be run for a longer 

period of time, but the result would still be reduced energy consumption as the input watts required for 

the lower speed operation are much lower. With the on and off cycling of the compressor, and with 

only one large cooling capacity available, the temperature fluctuates quite a bit. The temperature can 

be kept much more stable by employing an inverter compressor which can be run at a much lower 

speed and for longer cycle times. 

This combination of low speed operation and longer cycle times also improves the overall noise 

levels of the refrigerator. Single speed compressors typically produce noticeable start-up noise as 

they cycle from off to full speed operation and again from full speed to off at the end of the cycle. The 

inverter compressor starts up at 

low speed so the transition noise 

is minimal. The long cycle times 

at low speed produce obvious 

noise advantages compared to 

the high speed operation of the 

single speed compressor. (see 

Figures 4A & 4B) 

In designing the inverter 

compressor, it was clear that 

operation at low rotational speeds 

could provide many benefits. 

However it is at these low rotation 

speeds that stresses can act 

upon the compressor, resulting 

in wear due to a shortage of 

oil supply. Therefore it was 

critical to design the inverter 

compressor in such a way to 

allow for proper lubrication, even 

at lower speeds. Great care was 

taken in designing the oil pick up 

system as well as the pathway 

for distribution of the oil over 

all critical parts. For each type 

and size of inverter compressor, 

there are clear specifications for 

the condensing, and evaporating 

pressures, as well as the Motor 

winding and discharge 2” 

temperatures, for each speed 

range of the compressor. As 

the oil circulation is greater at 

higher speeds, the pressures 

and temperatures the compressor can withstand are greater. As the compressor speed slows, 

correspondingly the maximum allowable pressures and temperatures decrease. The refrigeration 

design must account for these specifications to insure reliability of the compressor.
If one takes these design specifications into account, it is possible to design a very effective and efficient 

refrigeration system using an inverter compressor. Being able to vary the speed, and thereby the 

capacity, to accommodate only the cooling load that is necessary provides for much better temperature 

performance in the cabinet, and much lower energy usage. Further design enhancements which 

could allow for lowering the static load requirements of the system would also provide for employment 

of smaller displacement compressors, further reducing energy consumption. This combined with 

utilization of superior insulation technology such as vacuum insulation panels and installation of 

variable speed fan motors can lead to ultra high efficiency refrigeration systems.

References

(1) “High Efficiency Refrigerator Cooling System Using an Inverter Compressor”, S. Kanaoka, T. 

Noda, H. Hamano, Y. Kubota.

(2) Panasonic Corporation Refrigeration Devices Business Unit, Technical Bulletin No. 

D-B16Ef12-E, Dec. 25, 2009.