Motor Rotation

Many single phase motors can only turn in one direction. For example, pump motors and fan motors. Since the pumps and fans they operate only work in one direction, the motors that drive them re usually built for one direction.  This can pose a problem for service techs when replacing these motors. Often, service motors solve this problem by being reversible. However, OEM replacement motors are generally not reversible, so you must specify the correct motor rotation. To do this you need to understand the terminology that is used to describe motor direction.

There are only two possible rotations: clockwise and counter-clockwise. However, there are also two perspectives: looking at the shaft end of the motor or looking at the lead end (opposite the shaft end). A motor which turns clockwise looking at the shaft end is turning counter-clockwise when viewed from the lead end! The point is that just stating a direction is not good enough. You must also identify a perspective.

There are several names for the two possible perspectives. The most common are shaft end and lead end. The shaft end can also be called the output end, drives end, or pulley end. The lead end is sometimes referred to by placing “opposite” in front of whatever phrase is used to describe the shaft end; such as, “opposite drive end.”

Normally these descriptions are abbreviated, which tends to add to the confusion. Below is  list of some of the abbreviation used. The graphic above each group uses an arrow to show the rotation looking at the motor shaft.

CCWSE Counterclockwise shaft end
CCWOE Counterclockwise output end
CCWDE Counterclockwise drive end
CCWPE Counterclockwise pulley end
CWLE   Clockwise lead end


CWSE  Clockwise  shaft end
CWOE Clockwise output end
CWDE Clockwise drive end
CWPE Clockwise pulley end
CCWLE Counterclockwise lead end

DOE Small Motor Efficiency Standards for 2015

You may have missed the news about required small motor efficiency minimum standards while you were focused on refrigerant changes; regional efficiency standards for air conditioners, furnaces, and heat pumps; and trying to stay ahead of the latest news on climate change. I know I did. I was looking at the Grainger web site the other day and up popped a bulletin regarding the Department of Energy requirement that certain small electric motors 3 horsepower and under meet a new minimum efficiency requirement beginning in March, 2015. I immediately thought “oh bother,” that would include most of the motors we deal with in residential HVAC. Then I looked up some details and breathed a bit easier.  Motors which are exempt include multispeed, enclosed, specific purpose, or special mounting bracket. That just exempted most of the motors in residential air conditioning equipment. If you deal with regular mount, general purpose, 42, 48 or 56 frame capacitor start or capacitor start-run motors, you WILL be affected. An example might be ventilation fan motors. Grainger was not saying replacement motors would not be available, just that they would now have run capacitors and would be longer due to the extra windings required to meet the new efficiencies. Basically, this DOE ruling makes what was once the premium, high efficiency motors the minimum motor design for certain motor types defined by NEMA. I found several links with discussion and details on this requirement. They are listed below. You might want to take a look at some of these links to determine if you work with any of the affected motors. That way you can develop a replacement strategy before you get to a job and find that you will not be able to replace the old motor with another just like it.

Baldor http://www.baldor.com/news/article.asp?id=1014

US Department of Energy  http://www1.eere.energy.gov/buildings/appliance_standards/product.aspx/productid/40

Weg Industries http://ecatalog.weg.net/files/wegnet/WEG-motor-global-energy-efficiency-legislations-2015-and-beyond-energyeffleg-training-english.pdf

Design News   http://www.designnews.com/author.asp?section_id=1365&doc_id=271471

Replacing Condenser Fan Motors

Hot weather always brings on a lot of condenser fan motor replacements. Although it is not too difficult to change a condenser fan motor, it is still possible to do it incorrectly. Of course you need a motor that operates at the same voltage, phase, and frequency of the original motor and it should also be the same horsepower. However, I would like to mention a few items to check that I often find techs have overlooked. First, make sure the motor you plan to use is actually designed for a condensing fan motor. Specifically, is it sealed and is it designed for vertical shaft installation (assuming a typical vertical mount, induced draft condenser fan). Motors with air vents on the sides or top should not be used outside. Also, all motors are not rated for vertical shaft installation. Some motors are designed for horizontal shaft mount only because they do not have the necessary thrust bearing to take the downward force against the bearing when the fan pushes air up. Next, is the motor the correct RPM? Some condenser fan motors turn 1075 RPM while others turn around 850 RPM. They are not interchangeable. If you can’t read the label on the old motor, take it apart and look at the number of poles. A 6 pole motor will be a 1075 RPM motor and an 8 pole motor will be an 850 RPM motor. Does the new motor turn in the correct direction? If you cannot see the markings on the old motor, you can tell by looking at the fan blade. First, the cups of the blades should be facing up on an upflow blower. Next, the blade should turn in a direction that allows the lower part of the blade to hit the air first. If the blade turns clockwise looking down on it, this is called clockwise shaft end, or counter clockwise lead end because the lead end rotation is always opposite the shaft end. Some motors specify the rotation looking at the shaft while others specify looking at the opposite end where the leads come out. If you are using a general purpose replacement motor, such as a Rescue Motor, you can change the motor rotation if you need to. Some of these motors use different size capacitors for different horsepower ranges. Make sure the capacitor you are using fits the application. Most likely, the capacitor for the old motor will not be the correct size. Most sealed condenser fan motors have weep holes on the bottom that come plugged with plastic caps. You should remove these when installing the motor. The idea is to let condensation out. The motor will last longer if you open the bottom weep holes.   After installing the motor and fan, turn the unit on and then off again to see which way the blade is turning. If it is turning in the wrong direction, change the rotation leads on the motor. Changing the power leads will have no effect. Once you establish that the fan is turning in the correct direction, operate the unit and check the motor amp draw. If it operates close to  the manufacturer’s FLA specification for your particular application, you should be good to go.

Checking ECM Motors

Electronically Commutated Motors (ECM) are pretty common these days. You can’t troubleshoot them the same way you do a standard PSC motor. The ECM motor is really two components – a motor and a control module. One big difference in checking these motors is that is that they receive voltage all the time, not just when the motor is operating. Really, it is the module that is powered all the time – the module then controls when the motor receives power. The first thing you want to establish is if the problem is in the controls to the motor/module, or the motor/module itself. A device called a TECINSPECT helps you do this. Turn off power to the unit. This step is important, not just for your safety, but for the motor’s safety. Unplugging connectors from the motor while the unit is powered up can cause the module to arc out. Unplug the control plug from the module and connect the TECINSPECT. Connect the two tecmate alligator clips to 24 volts, such as the R and C terminals. Turn power back on to the unit. The LED on the TECINSPECT should light to show it is receiving 24 volts. Flip the TECINSPECT switch on and the motor should operate. If it does, the problem is in the control board or wiring harness from the unit. If the motor does not operate, the problem is in the motor or module. If the problem is in the motor or module, the odds are that it is the module. Turn the power to the unit back off and wait 5 minutes for the high voltage capacitors inside the module to discharge. Remove the module and unplug it from the motor. Ohm out the motor. The resistance between any two of the three motor leads should be approximately 5 ohms. If the readings check out, the problem is in the module. You must replace the module with one designed for that specific unit because the mmodules are programmed for the blowers they control.  

Understanding Centrifugal Fan Motor Performance

Since air is what we work with it makes sense to insure that our students understand airflow and fan performance. Fan motor performance is one of the most often misunderstood aspects air conditioning systems. The amp draw on a centrifugal fan goes down as resistance to airflow is increased. For most people this seems counterintuitive. It is easy to picture the fan motor pushing harder to overcome the resistance and increasing in amp draw. However, this is exactly backwards. Centrifugal fans move air by throwing the air outwards through centrifugal force. The amount of air the fan is moving decreases as the resistance to airflow increases. If the fan blades are moving less air, they can actually spin easier because there is less air to sling. This causes the motor RPM to increase and the motor amp draw to decrease.

The most convincing way to teach this concept is to have students figure it out for themselves using a centrifugal blower. Have them operate a centrifugal blower in free air with no restriction and measure both the amp draw and the fan RPM. Note that most centrifugal blowers cannot operate in free air for an extended time without overheating, so try and keep the free air operating time to a minimum. Next have them block one side of the air intake with a piece of cardboard and recheck the amp draw and RPM. Typically the increase in RPM is immediately obvious, but measurements prove the point. Have them slide the cardboard to block the intake only half way while watching the amp draw. A few minutes of experimentation will convince the students that blocking the intake actually causes an increase in RPM and a decrease in the motor amp draw. Next have them partially block the fan outlet while checking the amp draw. Once again, the amp draw will decrease. Allow them a few minutes of play time to convince themselves. This experiment does more to explain centrifugal blower motor performance than a week’s worth of lectures.

Now that you have them hooked, refer them to Unit 56 Fans and Airflow inFundamentals of HVAC/R where they can read about the characteristics of different type of fans used in the industry and the basic principles of airflow. There the students can see examples of the different types of fans and read about their performance characteristics. Unit 56 Fans and Airflow wraps up with a discussion of the fan laws and using fan perfromance tables and curves. As always, examples show in detail how to apply each of these concepts.

Note that what I have been discussing assumes a "regular" PSC blower motor. ECM blower motors behave differently because they are programmed to adjust their output according to the resistance they are working against, but that is an entirely new discussion which I will save for another article.

Safety note: If you are not sure all of your students understand that it will hurt to put their hands into a moving fan blade, you should put a wire gaurd over the intake and exhaust to keep hands and fingers out. For a more polished trainer build sliding sheet metal baffles for both the intake and exhaust and mount the blower to a stand.

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