In previous articles I have mentioned that the only two moving parts in an air conditioner are the compressor and the refrigerant. Why didn't I mention fan motors? Simply because they are external to the refrigerant circuit, and they aren't absolutely necessary. Many refrigerators, for example, have no fan motors at all. And air conditioners could be designed without them too. However, the condenser and evaporator coils would have to be very large. So in a nutshell, the purpose of fan motors in cooling systems is to reduce the size of the coils and to properly deliver the cooling to the area where it is needed.
Two different types of fans are commonly used in air-conditioning systems: the propeller type, and what is commonly referred to as the “squirrel cage.” Each has a different application and uses a different motor type.
Propeller fans are usually applied as condenser (outside) fan motors. The reason for this is that propeller fans work best in a free flow of air. That's because, as the pressure across them rises, they draw more electrical current. So they are generally applied where there is no static pressure to overcome. They are also fairly noisy, so outdoors is a good place for them. And although they can be designed for less noise (with larger blades and slower fan motors), most of the motors operate in the 3400-rpm range.
Squirrel-cage fans: The most commonly used squirrel-cage fans have the opposite operating characteristics when compared to propeller fan blades. As the static pressure across them rises, they draw less electrical current. So they are best applied to move air in ductwork. And actually, using a squirrel-cage fan without an operating pressure difference can cause its motor to overload electrically. This type of fan also tends to (and should) operate more quietly. And since heating cycles usually require less air than cooling cycles, most fan motors for this application have multiple speeds and operate in the quieter 900 to 1600-rpm range.
There are many types of fans and fan motors used in the HVACR industry. It would be foolish to try to cover every possible application. What I'm covering here has to do with what you as a supply-house person will most commonly see and sell.
Types Of Fan Motors
The most common fan-motor types used in our industry can be broken into four groups:-- Shaded pole
-- Capacitor start
-- Permanent split capacitor (PSC)
-- Three phase.
Shaded-pole motors: Because of energy regulations, you don't see many shaded-pole motors anymore. They are very inefficient and were primarily applied as condenser fan motors in smaller air-conditioning systems. The design is the same as a clock motor in that they truly operate “in sync” with the power, at 3600-rpm (60 cycles X 60 seconds). Most have now been replaced with the more efficient PSC motor type today.
Capacitor start motors are used whenever a drive belt is employed to operate an evaporator (or furnace) fan. You don't find this type of motor used as much anymore either, because variable fan speeds and higher efficiencies are the law of the land. So most replacements are for older heating systems. The main advantage of this type of motor is that it has a high starting torque, which is what a belt drive requires.
Permanent split capacitor (PSC) motors are the most common types used for residential and light-commercial applications today, both as condenser and evaporator fan motors. This is because they are the most efficient motor types, although they don't have much starting torque. The most common sizes range between 1/4- to 1/2-horsepower. Most are designed for 230V operation, and the ones with variable speed taps are for use indoors. Motors with speed taps usually operate at half the speed of condenser fan motors, so they shouldn't be used in that application.
Three-phase motors are the most efficient types, and they have excellent starting torque. However, three-phase motors may only be used where three-phase power is available, in commercial locations. But that also means these motors are available in higher horsepowers and at commercial voltages such as 208V and 480V. Three-phase motors are synchronous with the A/C power and are available mostly in single speeds of 1800 and 3600 rpm. Varying the speed is usually handled through solid-state motor-drive electronics.
One problem with three-phase fan motors is that they will run backwards whenever the systems are misconnected to the power line. And although this doesn't hurt three-phase piston-type compressor operation (three-phase piston compressors pump in both directions), a fan running backwards can cause problems. For that reason, many manufacturers of commercial systems prefer to use single-phase PSC motors to operate fans on three-phase systems.
Selecting Fan Motors
A very important consideration when selecting fan motors - especially condenser fan motors - is water protection. Condenser fan motors usually face up and can be damaged by rain, so motors for this application are enclosed on the topside for protection. However, some condenser fan motors are mounted upside down, so motors for this purpose are usually enclosed on the bottom side. For this reason, the best motor type to carry in stock is one that is totally enclosed, so it can be used in either configuration.Another fan-motor consideration is the type of bearings that are used. Less-expensive motors use sleeve bearings, which don't last too long unless they are regularly lubricated. Ball-bearing motors are more expensive, but they last longer and require less lubrication.
Of course, proper motor rotation is also important, because you'll find that there is no standard direction used in the industry. That's why I've always preferred to stock motors that can be wired to run in either direction.
There are also differences in motor efficiencies. Most of this has to do with the way the iron in the motor core is designed and assembled. Motor losses are created by eddy currents in the core material. So when motors are designed for lower losses they usually employ thinner individual core-laminate segments, which are secured or fastened together in a way to prevent completed circuits. Welding the core material creates short circuits. So, not all motors are created equal.
Troubleshooting
When motors fail early, what are some things to look for? Well, applying the wrong voltage, speed, or horsepower are obvious. Other, less obvious things depend on the application. Replacement condenser fan motors usually fail because of water getting into them, or because they are working against too much pressure drop. I remember, for example, one job I looked at several years ago where we lost several condenser fan motors in rapid succession. The problem? Although the condenser coil looked clean as a whistle on the outside, it was actually stopped up with debris that couldn't be seen. However, an out-of-balance fan blade or short cycling can also cause problems.
Evaporator fan motors suffer when there isn't enough ductwork to give it the required operating static pressure. Dirty filters will never cause a failure, although dirty fan blades will. And you always have to check to make sure that evaporator fan motors are configured to run in the proper direction. You see, when they run backwards, they also blow, but not as well. I can't tell you how many times I've looked at problem jobs that nobody else could figure out, just to find the fan motor running backward.
