The Impact of Static Pressure on Fan and Blower Motors w/ Rick Streacker
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- Опубліковано 29 чер 2022
- This video is of Rick Streacker’s HVACR Training Symposium presentation: “The Impact of Static Pressure on Fan and Blower Motors” Static pressure acts on both sides of a blower, so it plays a major role in the operation of fan and blower motors.
Fans and blowers help HVAC/R systems move air over the coils, which assists with heat transfer. We use fans on the condenser side in refrigeration and HVAC applications, but we also use fans on the evaporator side in refrigeration whereas we use blowers in HVAC. Blowers allow the air to move a greater distance (i.e., through ductwork) than a fan.
Common fans include axial fans, which have propeller blades that rotate on an axis (and are affected by static pressure). Free air with minimal static pressure on the fan blade results in a low load on the motor. However, adding more static pressure increases the load on a motor, which would reduce the speed of a PSC motor and increase the amperage. Static pressure, amps, and the load on the motor are all linked. Motor speed and temperature are also affected by those three factors.
A residential condensing fan might only have a static pressure of about 0.3”w.c, and higher static pressure is often associated with a dirty coil and a higher amp draw on the motor. A motor with a higher load due to increased static pressure will likely run slower and hotter.
Centrifugal fans are also known as forward-curve blowers. Unlike the axial fan, a centrifugal fan brings air through the inlet, changes its direction by ~90 degrees, pressurizes it, and discharges it. When there is no static on the blower, it will operate at a higher amperage, more slowly, and hotter. Centrifugal fans need higher static pressure to work optimally.
We can think of the load on the motor as “boxes of air” (CFM), and that value will be influenced by horsepower, restrictions, and system design. Centrifugal fans that have to work harder to move more boxes of air will run higher loads, which may lead to overload conditions. The filter or furnace door will provide a restriction and increase the static pressure, decreasing the load on the motor, making the amps drop, and helping the motor run more quickly.
In most cases, the HVAC/R system design is tailored to a specific load and has a relatively small operating window, and the fans and blowers need to operate within that envelope. That’s why we like low amps, but amperages that are too low to move air effectively are undesirable. If the load is above or below what the motor has been designed for, the motor’s performance and longevity will drop. Even if a restriction keeps the fan or blower motor in its intended operating envelope, a dirty filter or coil could also affect the capacitor’s longevity.
Sometimes, technicians replace blower wheels but keep the housing. A mismatch can cause the performance to drop, so we want the replacement blowers to match the characteristics of the first blower wheel. To pick a good replacement, we need to be able to tell the wholesaler what our target CFM and static pressure are.
In many cases, we’re dealing with air-over motors, which means that the air itself is what keeps the motor from overheating (compared to belt-drive motors, which are self-cooling). The right load needs to be on the motor; otherwise, the motor will overheat. So, the replacement motor should be equal to the amps of the original motor but not more than 25% stronger. Upon replacement, we need to check the motor amps with all of the components in place.
ECMs (electrically commutated motors) are sometimes variable-speed motors, but they are all air-over motors and rely on the proper load for cooling. However, ECMs tend to be easier to design for the appropriate static pressure. So, we can factor loads into the design more easily, whether the ECMs are used in fans or blowers. In fans, ECMs tend to be constant-speed motors, but their designs can be refined for blowers and tend to be more variable than PSC motors, but they can’t overcome very bad static conditions. Fans and blowers react to static pressure the same way, but the motors themselves react differently.
Although we can’t correct bad static conditions with ECMs, we can modify the ductwork to correct the bad static conditions.
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Very, very important presentation. Thx Brian
Blowers are blowing my mind with how counter intuitive they are
lots of this is more engineering than a tech on the field , replace what you took out !
thank you for the class
Can anyone explain why the fan blades on walk in cooler/freezer have the shaft hub and lock-screws facing the motor sides? It’s so hard to remove those fan blades. Why those fan blades are not made the same as the condenser fan blades?
Very informative presentation! Usually not a technician strong point in troubleshooting. Thank you very much for publishing. I should add that this now emphasizes the importance of proper duct sizing when replacing/retrofitting ducts. Drop in replacements and retrofits modify the ducting and airflow. This presentation emphasizes the importance of proper engineering of the ducting.
extremely important presentation. keep it up
I finally get the inverse relationship between fans and blowers. For blowers the best way to think about it is with a straw. If say you want to move a piece of paper that is 3 feet away from you, you can blow try really hard and exert a lot of power (amps) to move the paper. You even purse your lips to increase static pressure. Now if you blow through the straw, you have static pressure and you can easily puff into the straw with less power and direct that to the paper with ease.
Best video I've ever seen on this subject and explains a lot. As far as amp draw when static pressure increases, I have the documentation for my Ruud AHU, which has an ECM X13 constant torque motor, and when static increases, RPMs go up and so does the wattage, so there is clearly a difference between a PSC and ECM motor.
thnks u somuch papa hvac
Think of a vacuum cleaner - block off inlet, motor speeds up - not moving any air so current goes down - try it with vacuum cleaner plugged into a kill-a-watt device.
Same thing with a vacuum pump - draws more current when initially pumping air - current on motor goes down as air volume goes down as vacuum is reached.
nice analogy!
Excellent 👌👌
very good video loveit
It has been my experience, when restricting supply air to a blower or fan it cavitates, has has less load, and amps decrease. The motor may actually increase in temperature, and system pressure will increase, causing higher than normal amps on the compressor and decreased efficiency overall.
This is true and can be shown on the blower curve, but this is only when the cfms are cut way back. In fact, the lowest load is against a fully closed system.
So with blowers (vs. fans) higher static pressure leads to lower amps (but higher speed). Counter intuitive. But the lower amps are actually bad for motor longevity because the lower CFM isn't cooling the motor enough. That blew my brain windings.
For real life isn't what it seems anymore.
He ruined my feeling of having common sense.
This is true for PSC motors, but with something like a ECM X13 Constant Torque motor, both RPM and Amps go up with increased static.
QUESTION: what are the mostly likely issues that would cause the delta T at one register to be only 8 F, but 16 F at another??
Background info: the ac unit is in the attic, there are two intake registers in the ceiling and 14 outlet registers.
the 16 F register is closer than the 8 F register. the ducting in the attic are R8, so i dont see how the ducts would be so hot as to heat up the air more than 2 F.
I though maybe there is a leak, but the air flow from the 8 F seems ok and because it is on the upstream side of the blower any cooled air would go into the attic, not the other way around.
Check your CFMs at both registers. The 8F my be significantly lower than the 16F.
Also check your “wet bulb” at both return register locations also.
Hi, related question I hope...........
I have a fuji aru24 RGLX medium duct system.
As you may or may not know the two ton unit comes with the default setting of 31 for Function 26 -> Static pressure which = .18 inWG.
Settings go from 03 to 20, ranging from 03 = .12 inWG by increments of 1, up to 20 = .8 inWG
There is also a setting of 32 = Automatic Air Flow Adjustment.
Does this vary the static pressure or something else. Is it reasonable to use this if the installers clearly have not made any calculations other than the system is designed for that unit and thus the default setting is fine ( no measurements of lengths of ductwork type of duct, number of turns, height of and length of copper etc….). I am assuming that this setting basically looks at the supply fan load ( amperage ) to decide the optimal setting..........??
A bit confused: Question,
If you remove the blower door shouldn’t the amount of available air( boxes of air) increase thus less static pressure( less air restriction ) ,thus higher amps (blower works harder) and therefore more air moved across the blower?
I would imagine it would drop the static pressure. If it is a PSC motor it would lower the amp.
@@mackenziegray2090 No, with PSC, the amps go up because there is more air to move.
Hoping to pick your brain.
I have a 100,000 (input) btu 80% Gas furnace in my 2 story home that is 2500 sq. ft. but there’s a 3 ton AC unit here as well but both units do great heating and cooling all season(s). Since this furnace specs are 5ton, 2000 cfm ECM motor, should my fan be set at 1200 cfm for the cooling speed? I realize the AC is undersized for whatever reason but it works great. Anything here that you would see as being an issue?
Thank you for your time.
Yes, you need less CFM for cooling. You are picking up very little superheat with that oversized blower, and my guess too much subcooling.
Why would a blower motor's external static pressure regularily be 0.4 to 0.8 W.C ? dont most furnaces call for 0.5 W.C max?
Depends on the manufacturer,. I have a Ruud AHU, that lists specs for statics as high as .70. I also have seen specs from other manufacturers that list specs up to 1.0 static. Your design goals for duct should try to keep static as low as possible.
0.8" w.c. I call it "Oh Shit Mode"
bye bye ecm blower bye bye
Most manufacturers air handler tables for total CFM at various static pressures are listed in blower motor watts.
To calculate watts from measured motor current you must convert measured V*A to watts by multiplying VA by motor's power factor. Best if you measure PF directly as it changes with motor load, but if unable assume a PF of 0.85 for PSC motor and 0.65 for ECM motor. For ECM motors you should use a true rms clamp-on amp meter. If you have a meter to measure PF it will usually give you watts readout directly.