Tag: motor

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

Service factor is an interesting motor rating that you will see on many motor data tags. It simply means how much additional “work” a motor can do or “load” it may be placed under for short periods of time without failure or overload.

For example. The FLA or Full Load Amps of the motor above is 10.8 amps at 115 volts

The Service Factor or S.F. is 1.5, which makes the Service Factor Amps 16.2 (rounded down to 16 on the motor tag)at 115v because 10.8 x 1.5 = 16.2

Don’t confuse SFA with LRA (Locked Rotor Amps). LRA is the current the motor will draw when the rotor is stationary, such as during startup. Service Factor is simply a short term “fudge factor” that the motor has for short periods of higher than normal load.

When a motor is running above its Full Load Amps and in the Service Factor range it may function but its operational life will be shorter and it will generally run at lower efficiency and power factor.

In other words, only go into the “service factor” range when necessary, not as a matter of normal operation.

— Bryan

The point of this article is to give you a full understanding of the role fuses, overloads and circuit breakers play in the protection of HVAC/R equipment. If you skim read or jump to conclusions you will be tempted to argue. Be patient, if you want to understand you will need to read all the way through and possibly even watch the videos at the end. This topic is WIDELY misunderstood so the odds are when you first read it you may think I’m crazy. Do your own detailed research once you get to the end if you still dispute what is contained here.


There are a few topics in HVAC/R that get widely confused and result in a lot of misinformation because of the similarity of the concepts. If you have two terms that have SIMILAR meaning but get used interchangeably you can come to completely logical sounding (though totally incorrect) conclusions.

For example, a tech could say a particular circuit is reading “no Ohms to ground” and by that he could mean zero Ohms or he could mean the meter is reading OL which means infinite Ohms.

In the same way, I often hear people say something is “shorted” and what they really mean is it’s not working, or something inexplicable is happening. So let’s define some terms starting with one of the most often confused. Here is the dictionary definition.

Short Circuit

In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.

When a professional uses the term short or short circuit, they can mean an electrical path with lower than designed resistance or they can also mean any unintended path.
For example, if two conductors in a cable are compromised and touching one another a tech will often say they are shorted even if there is not a low resistance overall in the circuit.
Because of this the term “Short” has become a broad term and must be used carefully.

Overload

Place too much a load on

Pretty simple, when you put too much load in the bed of your truck it bottoms out, when you place too much load on an electrical circuit or device, it fails. In the case of a conductor, this load is in the form of amperage, more amperage than designed and the conductor will fail due to overheating.

In the case of a motor, this same thing holds true but actual load (opposing force) on the motor results in increased amperage load which causes increased amperage and overheating.

This is why a compressor with failing bearings will draw higher amperage, the motor slips due to the additional mechanical load, this drops the impedance (resistance) in the motor windings, resulting in higher amperage.

Ground Fault

The momentary, usually accidental, connection of a current carrying conductor to ground or other point of differing potential

A ground fault occurs when an electrical conductor or device that is electrically charged comes in direct contact to ground, a grounded assembly or substance, usually resulting in large current spikes until either a protection device opens the circuit or the circuit itself fails open (breaks) due to heat.

I say USUALLY  because there are cases when a ground fault may exist with no spike in amperage, such as when you are using an ungrounded, two-prong appliance like a hair dryer or an old drill (or a drill that you cut off the ground plug in order to use on a two prong cord). If the internal windings on the device short to the casing there will be no path from the casing to the ground unless something else makes a path, like say , YOUR BODY. Then when your hand touches the drill casing and connects to ground, some current will leak to ground through the very high resistance load that is your flesh and organs. The circuit will not “overload” because it will not be drawing abnormally high amps but you may still die from the incident. This is why ground fault circuit interrupters (GFCI) are used in some high-risk applications, to break the circuit when a ground fault exists, even if that ground fault does not result in an overcurrent condition.

Overcurrent Protection

A form of protection in an electrical circuit that prevents excessive current usually at a predetermined value – Usually refers to a type of protection designed to deal with instantaneous spikes in current

Over-current protection can be used as a broad term that can include circuit breakers, fuses, etc… basically anything that prevents a current from rising above a predetermined value. It CAN be a pretty broad term in some circles, HOWEVER, in the electrical community when overcurrent protection is used it is generally referring to short circuit or ground fault conditions.

Any condition that results in quick, massive spikes in current is addressed by overcurrent protection. If you want to argue read THIS from Siemens.

Overload protection

Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment

Overload protection deals with higher current resultant from too much current being pulled by a load. When the compressor goes out on overload after one second because it is locked, that is an example of overload.

When a condenser fan goes off after running with a blade that it has too steep of a pitch, that is an example of overload. Overload in a motor is dealt with by the overload in the motor, not by the overcurrent protection/circuit breaker/fuses.  In the case of motor loads specifically, if the overload were to fail, the overcurrent protection would usually break the circuit eventually, but that is not it’s primary design function in most cases. Again, read THIS from Siemens if you are getting riled up at this point.

When a manufacturer writes their system specs and prints their equipment labels they use guidelines provided by the National Electrical Code (NEC) and they refer to articles 430 and 440 of the code to calculate the required minimum conductor size and maximum overcurrent size.  This is how they come up with the MOCP, or Max Breaker / Fuse size and the MCA or minimum circuit ampacity/conductor size required.

Here is some manufacturer electrical data from a Carrier 25HCC condensing unit –


Notice the maximum breaker or fuse is 40 amps on the 4 ton and the MCA is 26.1 with an allowable wire size of  #10 on an assembly rated at 75°C and a 60°C circuit

Yep…

On this system it is perfectly acceptable by the National Electrical Code and the manufacturer to run #10 wire and a 40 amp breaker so long as the wire run is a properly rated copper conductor under 70 feet (on this specific unit because of spec shown above). Wire length has an impact on voltage drop which is only addressed as a suggestion in the code but is clearly laid out by the manufacturer either directly or based on the minimum voltage if you do a voltage drop calculation. In this case, the voltage must by 197v or above

This is because the circuit breaker or fuse is providing the overcurrent protection (as well as some backup overload protection) and the motor overloads are providing the overload protection.

Now.. If you would like, you are allowed to put in a lower rated breaker than the max so long as you don’t go below the MCA rating because the breaker itself also needs to be able to handle the rated capacity. Just be aware that the lower you go the more likely you will be to have nuisance tripping.

You can also install larger wire if you like, just be aware that in some cases the equipment lugs may not be rated to hold/connect the larger wire. Also, keep in mind that you must also upsize the grounding conductor when arbitrarily upsizing the current carrying conductors.

If you would like some more discussion on the topic you can see the three videos I did on this HERE, HERE , HERE and HERE

Finally, before you start leaving comments about what is written, please see the two videos below. If you watch these videos, read the reference material and STILL think that what I’m saying is false in some way, feel free to join in the conversation.

— Bryan

P.S.- Mike Holt (shown below) is a friend of mine and considered the authority on NEC and electrical training in the US.

Can a single phase motor run backward when start and run are swapped? The answer is (generally) yes. Is the motor designed to run backward by simply swapping run and start? The answer is (generally) no with a few notable exceptions.

Before we jump in, this article has two purposes. #1 – It helps you understand a compressor design you may find in the field and #2 – It will help new techs with reading and understanding wiring schematics and diagrams.

If it gets too technical for you, jump down to the bottom and just watch the videos before you get fed up and move on.


In modern residential air conditioning, we see this design where the motor can run forward and backward depending on the wiring of start and run in the two-stage compressors made by Bristol shown in the USPTO drawing above which activates the full stroke of both pistons in one direction and only one piston in the other direction. This design allows two distinct capacities from a single compressor with no special unloaders, speed changes or bypass.

This is an extension of an earlier design by Westinghouse shown in the image above. The diagram on this one is pretty vague, but the general idea is a swapping of the phases to the compressor motor R & S to reverse the rotation. Now you may be thinking-

On single phase 240v power the two phases are the same and swapping them makes no difference

You would be totally correct in this assertion other than the purpose of the start (aux) winding is to have a force at play on the motor that is out of phase to an extent to provide the necessary starting torque as well as the improved efficiency and power quality that comes along with the constant phase shift provided by the run capacitor.

In layman’s terms –

We are trying to make single phase motors as close as we can to 3-phase motors and capacitors are our best tool to try and get close

Single phase motors are like a two-handed juggler trying to compete with a three-handed juggler by optimizing our toss and catching angles. I’m running out of metaphors here so I hope you’re getting it…

In order to make a motor that works in either direction the run and start windings need to both be designed to carry the continuous amperage that is usually reserved for run. You may think that the start winding draws higher amperage than start because START sounds like it would take the bulk of the amps during start. Actually, the start winding is generally a smaller, higher resistance winding and its amperage is limited by the connected capacitor. In order for a compressor like the one shown below to work, it needs to have a start winding engineered to function as a run winding and vice versa.

This diagram from Bristol really simplifies how they initially envisioned it, I also like how they give directional arrows so you can follow the circuits in both high and low modes. Obviously, it is alternating current so it doesn’t travel in only one direction but it helps you see how the capacitor is connected to Start in High on top and the Run winding on the bottom.

Here is a diagram from a Carrier 38YDB that used this compressor in the early 2000’s and this diagram shows it in the usual schematic form with the addition of a start capacitor and a potential relay.

Look at the left side, CH is the “compressor high” contact and CL is “compressor low”. When CH is Closed, CL needs to be open and the unit will be in high-speed. When CL is closed CH needs to be open and it will be in low-speed. If you trace it out you will see that in low-speed L1 is connected directly to start and in high-speed, L1 is connected directly to run. From there the opposite side is then only capacitively coupled to L1 through the run and start capacitors. This swap in phase is what causes the motor to run in one direction in high which grabs both pistons and the other in low which only pumps one.

Here are two videos that I did recently. One of a teardown of this compressor and another going through the schematic shown above.

— Bryan

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