How Inverters Work Basics

The new way for ultra-efficient air conditioners and heat pumps have modulating compressors and motors. A few years ago, it was rare to hear the question about how inverters work, but now the question is coming up more and more, so I wanted to write an article to explain how inverters work.

Inverters are used in air conditioning to modulate compressors and fan motors from 120% down to a minimum of 20%, although the minimum will vary from manufacturer to manufacturer according to part selection and engineering limitations. To truly understand how inverters work, one must have a basic understanding of electronics.

In electronics, a rectifier is used to change AC power (alternating current) to DC power (direct current). The difference between AC power and DC power is the sine wave produced. A rectifier is used to change the available power (AC) to DC.

The DC power can then use pulse-width-modulation of the DC current to change the frequency after the DC current is run through an inverter changing the DC power into AC power once again.  By changing the frequency of the AC power being used by the electric motor the speed can be changed thereby allowing the compressor and any other motor utilizing an inverter to be modulated from a minimum speed to a maximum speed.

The motors used in this process are brushless induction motors that should be specially rated for inverter duty use.


Modulation versus 100 Percent All the Time

If you find it difficult to understand the above just grasp the concept that the motor can be modulated from a minimum speed to a maximum speed. The minimum is usually 10 to 20 percent and the maximum is 100 percent of the motors rated RPMs. It is possible to run the motor over 100% of the rated RPM’s however, the service factor of the motor should never be exceeded. Having the ability to modulate the system one can see the advantages of having a demand-based system that only uses the amount of energy needed to get the job done.

Also added to this efficiency is the ability to soft start compressors and fan motors eliminating Locked Rotor Amps (LRA) of a motor and overcoming the initial resistance of going from a dead stop to 100% in a split second. This occurs with standard PSC motors. By eliminating the LRA or Locked Rotor Amps and by modulating the system based on load or demand the air conditioner or heat pump will run longer. Some people may say this hurts efficiency.


Cycling Issues and Wear and Tear

The answer to these doubters we must look at the usage of the air conditioner or heat pump system. In the heat of summer when maximum demand is necessary a properly sized air conditioner or heat pump will cycle on and off 4 to 6 times per hour. Of course, this depends on many factors and cycle times depend a lot on the system. Some inefficient systems may not turn off at all while an oversized system in a tight house will cycle more. Regardless, to get an accurate calculation of savings from one system to another the actual watts used must be measured with the conventional air conditioner or heat pump system.

A comparison to inverter air conditioners or heat pumps can then be used to calculate savings from one system to the next. Inverter systems with soft start control eliminate LRA and the system running at full blast 100% during the typical cooling cycles.  Temperature swings are also narrowed with inverter air conditioners and heat pumps. Less temperature swing results in better comfort with more efficiency.


Inverter Control – Modulating Compressors and Motors

Now that we understand the basics of how inverters work how is an inverter controlled.  I can only surmise that the typical HVAC thermostat is only a part of the control as we need to know when to modulate the compressor and the fan motor down to a lower RPM or speed based on demand. The bottom line on this is it has to be based on superheating on the evaporator coil to protect the compressor from liquid slugging. As the compressor ramps down the condenser fan motor respond to the same signal as well as the indoor blow so they match the speed of the compressor.

If the compressor ramps up the condenser fan motor and the indoor blower motor ramp up and vice versa. Of course, manufacturers will have their own individual methods for controlling the speed of the compressor, the condenser fan motor, and the indoor blower motor but this is a minimum for control as it protects the compressor from liquid slugging. These new methods that use modulation are referred to as variable capacity refrigeration or variable capacity flow refrigeration.  Modulation gives you added comfort and efficiency.