One of my A/Cs has just failed (Not).

In the Greater Montreal Area (Canada), we have been subject to a prolonged heat-wave, with daily high-temperatures of 35⁰C (+), for approximately a week in a row now. This is expected to continue at least until tomorrow (Thursday, July 5). Luckily, my own home has been protected by two working, 8500BTU air-conditioners until now.

The way an A/C works is such, that it has a compressor-motor, the windings of which are cooled by the return-flow of refrigerant in its gaseous form, after that refrigerant has evaporated in the evaporator, and done its job cooling the home. Yet, these motors are not designed to run with 100% duty, 24/7. They need to cycle off periodically, and one reason for which they normally cycle, is that their function has achieved some sort of (low) target temperature. But, another reason fw the compressor-motor can switch off, is the possibility that its windings themselves, which are linked to yet another temperature-sensor, have overheated.

Even worse than to have the temperature-protection trip once, because of overheated windings, is the very common problem that eventually, the enamel-insulation of the windings may itself fail, causing a permanently defective motor! This tends to happen eventually, because of the cheap way the motors are made.

If that happens, certain turns of the enameled wire, within the motor-windings, will act as if they were the secondary winding of a transformer, to which the still-healthy turns would form the primary winding. A heavy current flows through the short-circuited turns in this way, that can be hard to detect, unless one also measures the exact amount of current drawn by a running motor, and compares it to a known, correct amount of current, which I do not know for the motor in question. But if a winding has in fact started to short in this way, the amount of heat that builds up inside it becomes more acute, of course.

What I am used to from my A/Cs, is that they will run for about 15 minutes, if they fail to reach a target temperature, before their compressors cycle ‘off’. But the A/C in my bedroom, where the temperature is 24⁰C right now, has started to run for only 5 minutes, before turning off. And I have it set to achieve an evaporator-temperature of 20⁰C.

I’ve decided to switch off both my A/Cs temporarily, even though the temperature outside is 35⁰ at the moment, in hopes that they will recuperate. As they are switched off, of course it will start to get warmer in all parts of my home, including in the computer room.

If I should not be able to keep my indoor temperatures under control, I will need to shut down my actual computers next, which are more important to me than the A/Cs, or than my own, personal comfort. In such an event, my blog will also go offline. For the moment, my site and blog are still accessible. But depending on what happens next, there could be some downtime.

(Edit 07/04/2018, 23h05 : )

Apparently, my A/C is still fine. But in order for me to understand this strange behavior, I need to take into account the peculiar way in which my present A/Cs are designed. They are both indoor, portable A/Cs, which have air-ducts that send warm air, with the waste heat, out a window.

What’s peculiar about both A/Cs, that are of the same design, is that they have reached their target temperature, just when their evaporators have been cooled down to the set temperature, not when the intake-air has gone down to the set temperature.

The temperature of the intake air, is also the air-temperature in the room, and the traditional way A/Cs were ‘programmed’, was that the compressor would keep running, and the evaporator could get freezing cold, until the intake temperature had gone down to the set temperature, and therefore the room-temperature had done so as well. With the additional fact that more-traditional A/Cs were window-mounted meant that at the same time, their condensers would become considerably hotter, than the outdoor temperature.

This design used to burn out frequently, if just set to operate at one setting continuously, for a whole Summer.

The fact that my present A/Cs shut off their compressors when the evaporator had merely ‘become as cold as requested’, actually makes it much less likely that these would burn out. They keep cycling.

Further, since the indoor temperature in question was 24⁰C, it could happen easily, that the evaporator would cool down to 20⁰C, after a 5 minute cycle, especially if since the previous cycle, the evaporator was still filled with liquid refrigerant.

The design just saved both A/Cs.

So, as soon as I reactivated the A/Cs, the one in my bedroom ran for 50 minutes straight, which it could not have done, had any of the turns of the wiring, of its compressor-motor, been shorted. During a 35⁰C heatwave, that A/C had just been spectacularly successful, before. :-)

Dirk

 

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