Motor Stall - 3Ton HVAC

Below is an example of an unsolved motor stall:

How was your device detected?:
Through the app I saw an alert for an all-time high energy peak of ~23000w.

What device has a potential issue?
After seeing the alert I went outside and saw the fan of my HVAC spinning and would hear a periodic loud click sound each time the compressor tried to turn on. The high wattage alert along with the frequent running of the AC made me think to check there first.

Screenshots from Sense Labs: Motor Stall

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Hi @senseinaz. Thanks for submitting all these details here.
Our Data Science lead is taking a look at your motor stall data. We’ll share an update once we have more insight.

I have seen this behavior 1 other time in the past and posted on it here: Strange Electricity Spikes

Hi @senseinaz. We had our Data Science lead take a look at the stall data here and had a few notes. Although we’re not HVAC experts, there are some additional findings we want to share with you here so other community members can provide insight and hopefully provide some more direction on next steps for you.

  1. We did see an undetected stall at shut-off.
    Note: The controller inside an A/C is supposed to protect against this. Sometimes until the pressures somewhat equalize between the two sides of the compressor, the compressor won’t have enough torque to start up again, causing a stall.

  2. We did see multiple stalls that do not look like the stalls that are typically caused by a faulty capacitor, which we can recognize rather well.
    Note: Start capacitors are what we’ve primarily been detecting faults in (they are cheap to replace). Other faults that are not linked to the capacitor could be more costly.

  3. We are seeing some abnormal stalls here. They’re abnormal compared to what we typically see here (as i mentioned above, typically we see capacitor faults).
    Note: Some “abnormal” behaviors are one-offs, so we recommend watching for it happening again moving forward.

I’ve included additional screenshots for community members below

This is the signature we typically see for a stall:

image (6)

This is a back-end view from Data Science of your stalls with the “abnormalities” circled:

image (8)

@pswired Welcome your thoughts here :slight_smile:

@senseinaz when you heard the clicking while the compressor was trying to start, could you tell if the compressor did actually start, or did it just sit there and hum?

Also, are you sure the indoor blower was turning on when this happened?

I didn’t hear any humming sounds just the click. The air handler was running at the time so air was coming out of the vents inside, just not cold air. The system seemed like it was stuck in an infinite loop, it would click but not run then click again. After hearing this 4-5 times I cut the power at the breaker, waited a few minutes, turned the power back on, then tried again in about 10 minutes. The system worked that time without any issues and has been good ever since.

I see. Well, unless your system voltage was low when that episode occurred (looking forward to when Sense tells us this!), the situations that could cause that are a failing start cap, a burnt contactor, or a mechanically failing compressor. The cap and contactor are easy for a HVAC tech to diagnose and relatively cheap fixes. The compressor mechanical failure will probably mean a new system, but your HVAC company could install a hard start kit (an additional capacitor that will assist with the motor start) to extend its life a bit.

I’d have your HVAC service company take a look to rule out the easy fixes, and if that’s not the problem throw on a start kit and begin weighing your options for a system replacement when the day eventually comes.

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Was any maintenance recently performed on the unit such are adding refrigerant? An overcharged system can cause this to occur.

One more note from Data Science - we did look at voltage and did not see a voltage drop here.

This unit was last serviced ~3 years ago, its an older unit so whenever someone comes out they try to upsell us on a new AC. So far so good, if it acts up again I’ll try buying a new capacitor for it.


You might want to pull the cover now, and get the numbers off the capacitor. Typically they are $12 off Amazon, and $20-25 at the local supply house. Might we worth having on hand for the cost, and if yours is more than 10-15 years old.


The capacitor has been replaced and so far so good. I’ve had a few stalls since then but have found a way to prevent them for now. The stalls only happen when leaving the AC on a very low setting over a long period of time (under 70). The most recent stalls from a few weeks back have not shown up on sense labs and I have since raised the thermostat.