2

tl;dr - How close to the compressor LRA / RLA and fan motor FLA should the meter readings be compared to the values on the plate? Do you also need to consider if multiple readings are off or too far together?


I want to understand what are the tolerance ranges around the values on the nameplate for my AC unit. I finally got someone to come out who actually would check my compressor and fan related to my AC tripping my breaker (which is now almost certain on the 2nd cycle, we're keeping it off most of the time).

Unlike the other companies I had out, this tech actually thought the compressor was going to be the issue based on what I described to their intake person. Again, unlike the other techs he unscrewed the top and actually checked the compressor ohms, the fan, etc. I cannot recall if the other tech's actually checked the RLA and LRA, but when he did, he was like "oh yeah, yep, your compressor's going, you're still under warranty right?"

I wasn't taking photos, but I think the LRA was like 115 and the nameplate shows LRA 125. Is the LRA number on the nameplate, the "max", the "expected", something else? He made it seem like this was really bad, and I confirmed with him the nameplate actually specified 125 (I think when I saw some videos they were saying average LRA was much lower, but this is a 5-ton unit).

Do you also need to consider time? e.g., if it was longer than a second and you were still near the LRA value?

What I want to understand is if I am trying to diagnose this on my own with a clamp meter to measure the inrush, how do I use my reading versus the nameplate. If my AC was not tripping the breaker would that make a difference? e.g., If it was reading 115, but I had no problem that would be acceptable? However, if the AC is tripping the breaker, the capacitor is good, the refrigerant is good, then it is more concerning? My research around this (and hard start / soft start) seems to indicate that in all cases you are better the lower the inrush.

The fan, which also just sounds bad (audio/sound-wise), was reading 1.1 and the nampleate say fan motor FLA at 1.5. Is that way off? The RLA reading was I think around 16 and the nameplate specifies 22.1; is that too low (I thought lower was just better)?

One other point of reference about my specific situation - the tech tried to install a hard start kit, but when he checked inrush it actually went higher. The hard start looked like one of the cheaper temp ones and he said it was used, he was just going to leave it on as a band aide until the parts could be ordered, etc. But seeing the rating jump, he took it off. Not sure if this impacts the question of reading the values, but if this is relevant in my specific case to the troubleshooting, please let me know.


There is clearly something wrong, and this is the first guy to actually point to something besides a capacitor, but I want to understand the logic behind the decision. I don't want to replace the fan and the capacitor (under warranty, just paying for labor) and find out there is another problem that might make them go bad I also need them to consider (let's figure it ALL out while under warranty). I also want to know how to check for similar problems in the future.

My gut and other research tells me the compressor and fan should go anyway: fan appears to be leaking, fan sounds bad running compared to my other unit; first time I had a problem with the AC not working, it didn't trip the breaker, the fan was just stalled and seemed to be thermal lockout solved by cooling down. But do these readings tell me this? I assume so because the tech said all he needed to do was tell the manufacturer these readings and it would be a valid warranty claim.

9
  • Has anyone measured voltage at the compressor during peak LRA? If your line voltage is 240 but the motor is seeing, say, less than 200V during LRA, that could explain your troubles. If so, repeat at the mains to see if the cause is inadequate wiring to the AC or a utility issue. What about short cycling? Is your control board allowing compressor to start less than 5 min after it stops? Is the compressor's own overload cutout not working or bypassed? All basic troubleshooting steps that I haven't seen mentioned.
    – MTA
    Commented May 28 at 13:03
  • @MTA - I asked the last guy about cycles and programming the thermostat and he told me my furnace was responsible and checking that (but he didn't do any actual testing). Can you explain what would be considered inadequate wiring? It's a 40amp breaker which is within range for the 50amp max breaker specified on nameplate. The other guys didn't tell me what they were doing at each step, but I would hope at least one out of 3 would have checked the voltage; I know they all did a few tests with clamping meters on the line. When they come back to install replacement, i'll ask them to check again.
    – HelpEric
    Commented May 28 at 13:38
  • 1
    Short cycling: Get a helper to man the thermostat and you go stand by the outdoor unit. Use phones. Have helper turn A/C to low temp. Observe startup and let run 5 minutes. Have helper turn thermostat to high temp. Observe compressor shutdown. Within 1 minute or less, have helper turn thermostat back down to low temp. If compressor attempts to start, A/C has no short cycle protection and this could be your entire problem. If compressor’s internal overload is defective or bypassed, this compounds the problem.
    – MTA
    Commented May 28 at 14:32
  • 1
    (Continued) If compressor does not attempt to start, leave the thermostat at low temp until compressor first attempts to start and note the time. This is your short cycle lockout time. If it's less than 5 minutes, it's a problem.
    – MTA
    Commented May 28 at 14:37
  • 1
    Short cycle protection is not in the compressor, is usually in the outdoor unit's control board and is sometimes in the thermostat too, to accommodate old units without protection. Overcurrent protection is most often in/on the compressor but accessible and replaceable from outside the sealed unit.
    – MTA
    Commented May 28 at 15:54

1 Answer 1

3

Let's start by talking a little about electric motors. The compressor and the fan both use a type called a two-pole induction motor. In this kind of motor there are two "windings" of wire nested into the shell ("stator") of the motor. AC mains power goes directly to one of the poles - the "main" winding - and creates a magnetic field. The moving part in the core of the motor ("rotor") has windings too, but there's no electrical connection from these to the stationary parts of the motor. Electric current is induced in the rotor's windings - it is a spinning transformer, basically.

LRA (locked rotor amps) is the amount of current drawn by the main winding when the rotor is not spinning. When the rotor spins it creates a magnetic field that opposes the field from the AC mains power, with the effect that it reduces the current that flows in the main winding. The "locked rotor amps" condition is fleeting - current drops off quickly when the rotor starts to move.

Measurement of the LRA could be a little tricky. You'll have to use the "max" mode on a meter to have any hope of catching it. Even still the meter may have a certain sample rate and, if it doesn't sample at exactly the right moment, it won't catch the absolute peak of current. Another factor is that the LRA is measured at a certain line voltage; your actual line voltage - especially when that number of amps are being drawn - may be lower.

TLDR: If your measurement of LRA is kinda close, say within 10 or 15 or maybe 20 percent, and especially if you're measuring less than the nameplate, it's probably fine.

RLA (rated load amps) is the result of a calculation based on measurements of the actual current drawn under certain laboratory conditions; it's not a lot of use to us so far as I can tell.

Earlier I mentioned the motor has two windings. The second winding is called the "start" winding (even though it is powered all the time). It is powered from AC mains with a capacitor in series. The capacitor causes a phase shift in the AC voltage and the magnetic field; this influences the rotor so that it always starts spinning the right way. The amount of capacitance affects the amount of current that flows in the start winding and the strength of the resulting magnetic field. A hard start kit plays on this by switching in a different capacitor briefly at startup. This alternate capacitor allows a higher current to flow in the start winding, giving it a stronger jolt to get the motor going sooner. That jolt is a "hard" start, as compared to a "soft" start in which current is gently ramped up to accelerate the motor slowly. It's not surprising, especially if the tech measured total motor current including the start winding, that he would measure a higher current with the hard start kit in place.

3
  • > Another factor is that the LRA is measured at a certain line voltage; your actual line voltage - especially when that number of amps are being drawn - may be lower. Can you explain that a bit more? Does this mean its not just the LRA on the nameplate but "LRA x line voltage" or some other math that should be taken into account - does the multimeter do this for you?
    – HelpEric
    Commented May 28 at 13:40
  • @HelpEric No the multimeter doesn't do any kind of compensating or calculation of what the current might be at some other voltage. I took a motor control theory class a few years back and should remember the math better - but I don't. I think calculating current would involve the applied voltage, inductance of the winding, the mutual inductance between stator and rotor, and probably will be a differential or integral equation.. It's unlikely to be a simple linear relationship, though across a small range of voltages, a linear approximation might work. I don't recall.
    – Greg Hill
    Commented May 28 at 15:39
  • @HelpEric The takeaway is that reduced voltage results lower starting torque and more prolonged LRA. When a utility does an intentional brownout, a small percentage of A/C units will become unable to start and will get into a perpetual cycle of popping their overcurrent protection in the compressor, then waiting for the protection to cool down and try to start again. If protection is not working or is absent, the breaker eventually pops. Undersize wiring is the equivalent of a brownout but primarily at start-up. The voltage can slump so much that starting torque is insufficient.
    – MTA
    Commented May 28 at 17:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.