This is the tale of how I found myself stuck on a service call for over 12 hours on a weekend, due to my failure to re-diagnose an issue. I was working for a service company that had many accounts with local gas stations. These were large customers, and we did everything we could to keep them happy.

One Friday as I was gearing up for my on-call weekend, I was informed I must travel an hour and a half away early the next morning to a gas station where another technician had diagnosed a faulty X-13 blower motor. The technician didn’t have the right blower motor for the repair, so the system was still down. The catch was: no one knew what size blower motor was supposed to go in. No model numbers, no detailed notes, nothing. So I grab every size aftermarket X-13 motor I could find in the shop. I had up to ¾ HP. 

I arrived to find this location had two 5-ton package units mounted atop stands lifted 8 feet off the ground. After I setup the ladder and double-check the motor size, I realized it was 1 HP. I began calling all the parts houses in the area, hoping someone answered on a Saturday morning at 8am. No luck. I called parts houses in my local area and my co-workers to try and find a 1 HP X-13 motor that would work. Finally, I got in touch with one of my local suppliers. He had a motor that would fit the system I was working on, but he was an hour away from the supply house, and I was an additional hour and half away. Luckily, my employer at the time picked it up for me, and I had the part within a couple hours. I still had not re-diagnosed the system at that time.

Once I had the motor in hand, I quickly replaced it and had everything back together in a snap. I re-energize the system and….I curse loudly. The motor wouldn’t run. NOW I start re-diagnosing, a step I should have taken when I first arrived. Turns out, the original motor was just fine. The motor was not receiving 24v to the motor module, due to a faulty fan relay. I swapped out the 90-340 relay in the electrical compartment, restarted the system, and the blower ran beautifully. I hated myself.

I entertained the idea of packing up, walking away, and calling it complete, but I knew too well how that plays out. I began running complete system diagnostics, and found the system charge to be very low. I started leak searching the system with Big Blue from Refrigeration Technologies, and discovered a micro leak on the mechanical connection between the distributor tube and the TXV. No rubs outs were apparent, and it wasn’t a super loose connection, but it was clearly leaking. This was a package unit, remember, so I had to recover the entire system charge before I could make any repairs. 


Once recovered, I found the connection was just coming loose. A healthy dab of Nylogon the fitting connection and a torque wrench was all I needed to pass a nitrogen pressure test. Of course, the repair process was time-consuming, but eventually, I had the system evacuated, cleaned, recharged, and operating in peak condition under the current load.

I still would have needed to make the leak repair no matter what, but I could have easily saved 3.5 hours of time if i had re-diagnosed the system first when I arrived to the job. One could argue I was simply distracted by the chaos of the call, which would be true. However, a good technician should be able to follow the proper processes in spite of disorganization and frustration. I learned the importance of always checking behind yourself and others when you arrive to make a repair. Since, I have found the real causes for issues that were previously (either by me, or another technician) diagnosed as bad TXVs, reversing valves, motors, etc. 


ALWAYS double-check your work and other people’s work. You never know how valuable it is until you fail to do it, and it costs you time and money.

— Kaleb



Ever since Nikola Tesla invented the modern induction motor we have been struggling with varying the speed of motors in an efficient and reliable way. The trouble in the HVAC industry is that there are several different types of technologies in play and they can easily get confused.

ECM (electronically commutated motor)

In residential and light commercial HVAC we have seen ECM (Variable Speed / X13) motors for years, primarily in blower motors but sometimes even in condenser fan motors. The first thing to know is that an ECM motor is “Brushless” DC motor. Most traditional DC motors require brushes to provide power to the motor rotor (spinning part). Brushes are notorious for wearing out over time making DC motors unreliable in constant duty applications. An ECM motor uses a permanent magnet rotor which eliminates the need for power to be fed to the rotor through brushes.

An ECM motor is a DC 3 phase motor with a permanent magnet rotor where the cycle rate is controlled by the motor module. Here is a great video on how they work.


VFD (Variable Frequency Drive) 

For existing A.C. (Alternating Current)  3 phase motors the only way to change the speed reliably and efficiently is the alter the “frequency” of the power applied to the motor to something other than 60 hz (60 cycles per second). A VFD intercepts the power applied to a motor, changes it to DC power with a bank of diodes (rectifier) also called a CONVERTER. It then smooths out the power using capacitors before feeding that power to a bank of transistors called an INVERTER which is constantly switching the power from DC back to a form of power called PWM (Pulse Width Modulation) which replicates frequency change to the motor. The drive needs to be able to provide this PWM power at the correct voltage and current in order to control a 3 phase motor properly.

Inverter / Inverter Drive

Many A/C systems are coming with Converters, Capacitor Smoothing (Intermediate Circuit) and then the Inverter all built in to the equipment itself to drive a compressor or compressors. This inverter technology is essentially an intelligent and specifically designed VFD built into the equipment itself. The Carrier Infinity system is one of many systems that utilize inverters.

These technologies are constantly evolving and changing and while they may be similar, the different names describe different types and applications of technology all designed with e end goal of making motors go more than one speed with the best efficiency and reliability.

— Bryan

First, let’s cover the basics. X13 is a brand name for the Regal Beloit / Genteq brand of constant torque motors, there are other manufacturers who make them, but the term “X13” has become pretty much synonymous for the fractional horsepower HVAC constant torque motor.

Also, this article is specifically discussing the common residential/light commercial motors. There are other types of variable and constant torque motors and equipment not being addressed here.

Both variable speed and X13 motors are ECM or “Electronically Commutated Motors,” This means the DC power that drives them is electronically switched from positive to negative to spin the motor. Both are more efficient than the typical PSC motor with ECM motors commonly being about 80% efficient and PSC being about 60%.

Both X13 and variable speed motors are DC (or alternating DC if you prefer), 3 phase, permanent magnet rotor motors that use back EMF to determine motor torque and adjust to load conditions.

The primary difference is the type of inputs to the motor control. A variable speed motor is programmed for a specific piece of equipment to produce a set amount of airflow based on the particular static pressure profile of that system as well as based on the inputs from the air handler circuit board or system controller. In other words, a variable speed motor can ramp up down based on the static pressure as well as the staging of the equipment, pin/dip switch or controller settings for desired airflow output and “comfort profiles” that can be set up to allow the blower to ramp up or down for enhanced dehumidification and comfort.

An X13 motor is programmed to produce a set motor torque based on which input it is receiving 24v. This means that while an X13 motor is more efficient than a PSC motor and does a better job of ramping up to overcome static pressure increase it does not have the level of control that a variable speed has and it also does not produce an exact airflow output across the full range of static pressure.

This is why when you check the blower charts on a unit with a variable speed motor the CFM will remain the same over a wide range of static points, but when you look at an X13 system, the CFM will drop as the static pressure increases.

— Bryan

First, let’s give proper credit. Most of the best practices and tools for the diagnosis of ECM and X13 motors come from Regal / Genteq and their site and their app the dealer toolbelt. Your best bet is to follow the practices shown there and use their TECInspect diagnostic tool shown below.

Here is the general process to follow when checking an ECM and X13 motor that isn’t running. Most of it is very common practices you would follow with any motor.

  1. Check for proper line voltage and 24v calls to the proper terminals in the equipment
  2. Check for proper control signal entering the motor from the 24v field wiring or boards. The fan speed selections and programming will vary by manufacturer but the intent is to see if there is a proper control input signal. This can be a bit a challenging and is the primary purpose of the TECInspect tool.
  3. Disconnect power and remove the blower housing
  4. Check for abnormal sounds and side to side bearing play. Because these motors have permanent magnets on the rotor they won’t spin freely like a normal motor and you will get an “indexing” feel on the shaft as you turn it.
  5. Look for signs of overheating, burned spots etc…
  6. Remove the module from the motor and disconnect the plug that connects the motor to the module. 
  7. Measure winding to winding on the plug feeding the motor (called phase to phase below). resistances should be less than 20 ohms and nearly the same between all phases/windings.
  8. Measure from each winding to ground on the casing and you should see no less than 100k ohms to ground.
  9. If the motor checks out OK and the module is receiving inputs but the motor still isn’t running then it is the module that needs to be replaced

The diagnosis of ECM and X13 motors is actually pretty easy if you do a bit of reading and take a practical “process of elimination” approach.

— Bryan

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