Variable Speed ECM Condenser Fan Motors are use in condensers for the high efficient models so that the fan speed for the condenser fan motor can be modulate according to the load of the system. The variable speed ECM condenser fan motors are used in condensers that either have a modulating compressor or condensers that have two compressors where one compressor is small and one compressor is large. The variable speed ECM condenser fan motor adds to the efficiency of the system by modulating the condenser fan motor to match the compressor use needed to satisfy the load.
On very hot days you need more air conditioning capacity to satisfy the heating load demand put on your home in the summer. In milder weather you do not need to run the air conditioner at 100% to satisfy the higher demand. HVAC equipment manufacturers have introduced systems that have the ability to run in two-stages so that on the hotter days you can satisfy the demand and on the cooler days you can run the system at a lower setting thereby using less energy.
Using less energy is the objective so how do we do this? The solution is two-fold for having an efficient running system. One is to reduce the heat gain by insulating the home and adding things to the home such as attic fans. These things help reduce heat gain to the living areas that you want to condition with cool less humid air. The second thing you can do is to purchase a higher efficient system that can modulate or offers staging from a high level for higher demand to lower level for lower demand.
HVAC Manufacturers introduced two-stage compressors and in the future (offered currently in commercial systems) modulating compressors that will run according to the demand based on exactly what you need to condition your space. Since the compressor would stage based on demand they needed a condenser fan motor that would also stage based on demand. Some manufacturers use a standard multi-speed condenser fan motor while others use a variable speed ECM condenser fan motor to facilitate a higher speed for the increased demand when the system is calling for higher demand and a lower speed when the system is calling for a lower demand. So when the compressor is running at a higher speed the condenser fan motor will also run at a high speed and when the compressor is running at a lower speed the condenser fan motor will run at a lower speed.