Tag: fan motor

Fan laws and fan curves are a deep subject with a lot of nuance and variation. Just to get our heads around the subject let’s focus on two different types of fans that we see all the time in HVAC, the prop/axial type and the radial/centrifugal type. Centrifugal / Blower With a typical PSC blower motor (non-variable/ECM) as we increase the pressure differential across it due to any variety of factors (small ducts, dirty filter, dirty coils) the blower moves less air and it uses less power to do it. The easiest way to test this is to measure the amperage of a PSC blower with a blower door off and then measure again with it on. The current will be higher with the panel off because the static pressure is LOWER and the blower is moving more air. Take a look at this chart which shows the huge impact static pressure (and input voltage) have on airflow If you do the same test (door on then off) on an ECM constant torque or constant airflow motor the amperage will go DOWN with the door off but this is because of the motor characteristics ramping the RPM down not because of the blower wheel properties. Take a look at this chart for an air handler that uses an ECM motor. The lines of airflow to static pressure are pretty constant until the static gets above the 0.5″wc If you were to check amperage on an ECM blower you will notice it draws higher amperage the higher the static pressure across it gets due to the motor ramping up to maintain the designed flow or torque. The PSC motor is the opposite, if we increase static the airflow, amperage and wattage all drop due to the characteristics of the centrifugal blower. To Summarize –

A blower wheel decreases in power used as static pressure increases UNLESS there is a ECM motor changing the RPM to compensate

Axial / Prop A prop fan performs in an opposite way in relation to pressure. As pressure differential across it increases the power used INCREASES even as the airflow it produces decreases. This means that if you block a condenser coil the fan will move less air and draw higher amps… illustrating again why keeping condenser coils clean has a big impact on performance. To Summarize –

A prop fan “loads” more based on pressure while a centrifugal fan “loads” based on mass flow

Again… this is a simplification but for a technician understanding these relationships can help you diagnose and understand system issues. Read specific system fan charts and curves for a better look at how a particular fan performs. — Bryan

One aspect of motor selection that can get overlooked is motor bearings and it can have big consequences.

Sleeve bearings are most common in residential and light commercial applications because they are less expensive and quieter. They don’t have rolling “balls” but rather rely on a thin film of oil on metal sleeves.

Sleeve bearings work well when noise is a consideration and when the shaft load isn’t high which is especially true in residential direct driven blower and condenser fan applications.

Ball bearings are the best choice when the motor is under greater shaft tension or when fan speed controllers are in place. This is common in larger commercial and industrial applications especially when the motor is driving a belt that pulls on the motor shaft.

If you accidentally use a sleeve bearing in a situation where there should be a ball bearing the motor will fail early,

If you use a ball bearing motor where there should be a sleeve bearing it may be bothersome from a noise standpoint.

— Bryan

This article is written by technician and HVAC School community star Kenneth Casebier… Thanks Kenneth!

When looking at replacing a single phase A/C motor with an aftermarket motor from your van, there’s a few things you should know and pay attention to.

First, the factory OEM motor is always going to be the best option especially when talking about blower motors in a furnace or fan coil unit as that motor was specifically designed for the static pressure and application of the unit. Sometimes this isn’t always an option and for those times there’s a few guidelines that will aid in ensuring the motor of choice will be a good decision for both the tech and the consumer.

The first hard fast rule when selecting a motor is going to be frame and size. The frame of the motor needs to match the application, the last thing you want to do is modify a piece of equipment just to install a motor that may fail because of improper installation and now the equipment may not accommodate the right motor because of the modifications made. The actual depth of a condenser fan motor is very important as an aftermarket motor where the body of the motor is taller than the original, it can create a situation where the blade wont be positioned properly in the cabinet/shroud leading to incorrect amounts of airflow and potentially causing issues with obtaining the correct amp draws for reliable performance. Blade position can be EXTREMELY important to condenser airflow and should be carefully considered especially when up-sizing the HP of a motor.

The next most important consideration is amps. The amp draws need to be similar to the factory motor. Always check the data plate as the motor you’re removing may have been changed with an improperly matched motor thus why you are there now. A good rule to follow is keep amp draws within + or – 5% of the original but as close as possible or exact is a best practice. You have to keep in mind the blade, blower wheel, and duct work or shroud are going to affect the ability to properly load up a motor. If you choose a motor too far out of specs for the application you may find yourself in a potential situation for a prematurely failing motor.

(Note from Bryan: If you are replacing an OEM motor with a more efficient motor such as replacing a PSC with an ECM the amperage may go down in those cases and still be acceptable)

Horsepower is the biggest value that there seems to be some confusion on. An easy way to make a wise choice when selecting an aftermarket motor is NEVER DECREASE HORSEPOWER! Keeping the HP the same or increasing by no more than 1 value is a safe practice that will keep you from going back and replacing the motor again.

An example of this would be if you have a failed OEM ¼ hp motor, a “like” 1/3 hp would be an acceptable option, however a ½ or ¾ hp motor may work but will likely cause size issues and will be more costly to operate for the end user and therefore a bad choice.

The last major consideration when selecting a replacement motor is RPM, in PSC motors you want the match to be exact. A 1075 RPM PSC motor is 6 pole motor with a synchronous speed of 1200 RPM and a 825 RPM motor is an 8 pole pole motor with a synchronous speed of 900 RPM. Some motor manufacturers will use slightly different RPM ratings such as 1100 vs. 1075 but this is still a 6-pole motor and the 1075 can replace the 1100.

Additionally it is good to look at the bearing type used when replacing with ball bearings having a longer life but often noisier than sleeve bearings. Also consider the ambient temperature rating of the motor and chooser higher temp rated motors in more extreme ambient conditions where appropriate.

Always use the proper sized capacitor when replacing a motor and it is a good idea to replace capacitors with motors as a precaution.

Remember, no after market motor is going to be an “EXACT” replacement and for that reason I always recommend the factory OEM when possible. In extreme temperatures I know getting the equipment operational can be a driving factor in the decision making process and I’ve even “loaned” a motor to someone until I could order and return with the OEM. This can incur extra costs to the owner but it’s still better and sometimes cheaper in the long run than leaving an improperly applied motor in a system.

— Kenneth Casebier

Diagram courtesy of Emerson

How to wire a condensing fan motor for 3 vs. 4 wires is a common question by new techs. Jesse Grandbois submitted this tech tip to help make it simple. Thanks, Jesse!.

This is a quick one on the difference between wiring universal condenser fan motors and why brown+white is the same wire as white. This one seems to confuse even experienced technicians and really is very simple once you see it. Now keep in mind that wire colors ACTUALLY MEAN NOTHING, but on service replacement motors the colors do tend to be consistent. As always, refer to the wiring diagram on the particular motor you are using.

I’ll provide a diagram and explain the wires below.

Here’s the 3 wire method:
– White wire from the condenser fan motor to one side of power on the contactor (T1) and jumped to one side of the fan capacitor. This is AC power and not a dual capacitor, so the terminal side does not matter
– Black wire from the condenser fan motor to other side of power on the contactor (T2)
– Brown wire from the condenser fan motor to the other side of the capacitor opposite the jumper wire.
– Cap off brown+white (unused)

Now for your 4 wire method:
– White wire from the condenser fan motor to one side of power on the contactor (T1)
– Black wire from the condenser fan motor to other side of power on the contactor (T2)
– Brown wire from the condenser fan motor to the capacitor. Again, this is AC power and not a dual capacitor, so the terminal side does not matter
– Brown+white wire to the other side of the capacitor

Now as you can see the only difference is there is no jumper from the contactor to the capacitor. This is because the brown+white and the white is the same wire. They’re joined inside the motor. The brown wire with the white stripe is only there for convenience.

If you want to prove that the white and brown w/ white stripe are the same, take an ohm meter and test between the two. You will find that it either reads Zero or very low ohms proving that they are directly connected within the motor.

In this 60-second tech tip video by Brad Hicks with HVAC in SC. he shows us how and why to remove the weep port plugs on a condensing fan motor. I know from experience that motors can fail prematurely when this practice isn’t followed. Remember that motor orientation dictates which are removed. It (generally) the ports facing down that need to be removed and the ones face up stay in place.


What’s going on guys here is a quick 60-second tech tip is on changing condenser fan motors. Whenever you’re changing them, most all condenser fan motors have plugs that are supposed to be removed depending on the orientation of the motor. Since this shaft is facing down into the unit these need to be removed and basically what they do is, they open the weep holes so any condensation or moisture that can get into the motor doesn’t stay in there to corrode the windings and in turn prematurely make the motor fail. So make sure you take those plugs out, if you don’t, like that motor over there you’ll be back within a couple years to replace it again. Just a quick tip make sure you take those plugs out like I said this motor is oriented this way so you want to take the plugs out of the bottom like I just did and your motor will last much longer. There you go thanks for watching.
— Brad Hicks


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