Saturday, August 29, 2009

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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  1. I use the following analogy:

    1. I find the biggest student in the classroom and have the students visualize this student holding a wheelbarrow with all the student inside- "fully loaded.. I ask,"Is he able to move very fast and what kind of effort would have to be put forth?" Answer: Not very fast and lots of energy to move the wheelbarrow.

    When all the panels are off the unit and filter removed, the centrifugal blower is "fully loaded" - lower rpm and high current. Demonstrate this at a furnace - take BEMF reading at Run Capacitor and current reading at Common wire.

    2. Now, visualize some of the students jumping out of the wheelbarrow - "partially unloaded."
    "Is he able to move the wheelbarrow faster and
    at what effort?" Answer: Yes, moving faster and less effort.

    Put the panels on the unit and filter in place -
    operates now at "partially unloaded" condition.
    BEMF will go up (higher rpm) and current will go down.

    3. Now, visualize all but the smallest student jumping out of the wheelbarrow - "unloaded condition". "What happens to speed and effort?"
    Answer: Big guy is now running and with little effort.

    Gradually block the Return Air opening or Supply Air opening on the furnace. BEMF to capacitor may actually go over the capacitor's voltage rating (i.e. >370VAC) when totally blocked and the current will drop.

    I explain that this condition represents a very dirty filter, blocked evap coil, crushed ductwork, and such. At the condensing unit, it might mean a "loss of charge" - unloaded compressor. Unloaded conditions could lead to start winding failure, blown capacitor (over voltage), burt wires/terminals,and such.

    See my ppt entitled, Back EMF and PSC Motors, at

    1. howdy, your websites are really good. I appreciate your work. furnace replacement

  2. I suspect the perception that a fan motor is working "harder" when air is restricted may come from our common experience with blocked nozzles on vacuum cleaners...the motor spins up and certainly "sounds" like it's working harder, but as been pointed out so eloquently above, it's loafing since the only load now becomes friction loss. Been in touch with the Filtrete folks who still have not corrected a video on their site in which a tech notes that the more restrictive filter must be using more energy since the fan motor "sounds" like it's working harder...NOT

    Great job on the explanations and analogies!

    Steve Waclo
    Carson City

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  11. I am desperate to get a question answered. I got chewed out for hauling a piece of diesel equipment 600+ miles without closing off the exhaust. The boss's thought is "the exhaust is exposed to the moving air (70 MPH wind) and as I drive, which turns the exhaust turbo (essentially a box fan) with no oil to lubricate, I am essentially frying the turbo." I try to explain as a mechanical engineer that there is no way that air could flow through an internal combustion engine in reverse that would allow a turbo to burn up. I then proceeded to say that a turbo (box fan) does not work in reverse. Am I wrong in that a box fan will not work properly if the flow direction is reversed? I feel confident in my first point but less so in the second point.

    1. It is possible but highly unlikely for air to flow through an engine backwards and spin the turbo. I would close the exhaust mostly to keep stuff out. The turbo can spin in either direction but would not be efficient in reverse.

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  19. Your blog is a one stop solution to all the questions which were coming into my mind about the performance of centrifugal blower fans. Studying this with a practical demonstration of the blower or fan is the most understandable and easiest way to get into this concept. Thanks for sharing!

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