There are 3 main types of motors used in heating and air conditioning applications. These three types of motors are Permanent Split Capacitor (PSC), Variable Speed and Variable Frequency. Variable Frequent drives are not in residential applications so we will skip them.
Permanent Split Capacitor Motors are the most common alternating current (AC) motor used in heating and air conditioning applications. They can be single speed or as many as 5 speeds are common. They can be reversible by wiring or they can be single direction. These are low end motors, not the lowest, but they are considered standard and “standard” is always low end. In fan mode or air conditioning mode a PSC motor that is ½ horsepower will consume about $65 a month in electricity here in Southern California at 11 to 14 cents per kilowatt when it is running 24 hours a day 7 days a week. Every speed on the motor is fixed at a certain rate. The lower the speed, the lower the horsepower of the motor. Speed is controlled by applying electricity to a number of the nodes on the motor. As more parts of the motor (nodes) receive electricity the motor increases speed and horsepower. These are usually direct drive in modern furnace applications. If the motor is under loaded or over loaded by more than about 10% the motor will overheat and burn out.
Variable Speed Motors are Direct Current (DC) motors (batteries are DC). These are high end motors and often cost more than a standard or low end furnace all by themselves. Since these motors run off of DC voltage and your home only has AC they come equipped with an end bell cap full of capacitors. These capacitors simulate DC voltage using the available AC current.
These motors can be equipped with a great many speeds and often ramp up well beyond the standard speeds of a PSC motor. This motor runs at full horsepower without regard to its speed. Speed is controlled by varying the voltage to the motor. This is done by the circuit board measuring the amperage draw (the amount of electricity) used by the motor for how it is set up.
These motors can be set up to deliver a more accurate airflow for the air conditioning. We can set them for 350-450 cubic feet of air per minute per Ton of cooling (12,000 btus). This is a tremendous help as these blowers can help to correct undersized ducting situations or long duct runs.
If we set this motor up to move a certain amount of airflow and the ducting is much too small the motor will attempt to ramp up to satisfy the required air demand. This can result in significantly higher electrical bills and more noise, but if the ducting is oversized this motor will slow down and you could see significant savings. If your ducting is the correct size this motor will not make any different in electrical usage when set at 400 cubic feet per ton (the normal setting)
Here is the Real Savings
The real savings of a variable speed motor is in its constant on recirculating mode. This is used to help even out the temperature from home to room in a home, particularly useful for upstairs/downstairs applications. In circulating mode this motor will cost about $5 a month to operate versus the PSC motor costing $65 a month.
The variable speed motor has a slow ramp up meaning there will be no swoosh or banging ductwork when it turns on. The evaporator coil in cooling mode will be allowed to cool down before the motor starts and therefore deliver cooler air quicker than a PSC motor that turns on just as soon as the thermostat calls for cooling. This is more comfortable as well as quieter.