Bearing Failure Leads to Cooked Winding

I ran across a failed capacitor start motor on an air compressor recently. It is obvious that the start winding got barbecued (see picture).

A student asked me how I knew right off that it was the start winding.  Notice that the winding which is burned has smaller gauge wire than the winding that appears OK. The start winding in single phase motors is constructed of smaller gauge wire than the run winding and has fewer turns. It is tempting to call this an electrical failure after seeing the cooked winding. However, most motor failures can be traced back to bearing failures.

Disassembling the motor we saw that the rotor had been dragging – a sure sign of bearing failure. (see picture)

The lead end bearing was to blame in this case. You can see that the dragging all took place on the lead end of the motor. Taking a close look at the stator you can see where the rotor has been rubbing the stator. (see picture)

This can cause two types of winding failures: one where the rotor knocks some of the metal layers into the slots where the windings are, and another where the rotor stays magnetically locked down at startup, which is what I believe happened here. Even though the rotor turns easily by hand and there is no play that can be casually observed by hand, it is obvious the bearing was allowing the rotor to touch the stator. If this happens on startup, the reaction will be like two magnets with opposite polarity pulling together. The motor will lock down, draw high current, and heat up.

Toggle Switch Failure in a Crawl Space

A very common problem encountered in crawl spaces is for the toggle switch used as a furnace disconnect to die. Often you turn it off, and it will never turn on again. They can also fail on their own over time. Toggle switches are not really made for the environment found in most crawl spaces. The contacts and the switch mechanism get corroded. The contact corrosion causes voltage drop and heat, further degrading the switch. 

The picture shows what is left of the toggle switch that was serving as a furnace disconnect. The switch had failed, keeping power from reaching the furnace. It was relatively easy to diagnose: nothing happening with any thermostat setting, no transformer hum, and no LEDs glowing. My non-contact voltage detector showed power entering the switch but not leaving it. When I started to remove it so I could check it, it just fell apart.

To avoid similar problems, toggle switches in crawl spaces should be installed inside a weather proof switch box. I also recommend spending a few extra dollars to get a commercial, heavy duty toggle switch instead of the normal light duty residential toggle switch. In fact, the NEC requires the weatherproof box, but it is often overlooked. The relevant NEC sections are listed below. These are from the 2017 edition, but I do not think these particular sections have changed from previous code versions.

“404.4 Damp or Wet Locations

(A) Surface-Mounted Switch or Circuit Breaker. A surface mounted switch or circuit breaker shall be enclosed in a weatherproof enclosure or cabinet that complies with 312.2.

312.2 Damp and Wet Locations.

In damp or wet locations, surface-type enclosures within the scope of this article shall be placed or equipped so as to prevent moisture or water from entering and accumulating within the cabinet or cutout box, and shall be mounted so there is at least 6-mm (1∕4-in.) airspace between the enclosure and the wall or other supporting surface. Enclosures installed in wet locations shall be weatherproof. For enclosures in wet locations, raceways or cables entering above the level of uninsulated live parts shall use fittings listed for wet locations.

Exception: Nonmetallic enclosures shall be permitted to be installed without the airspace on a concrete, masonry, tile, or similar surface.”

Entropy, oh Entropy, What Art Thou?

Ok, so Shakespeare did not wax lyrical about entropy. I was talking with Bryan Orr the other day and the subject of entropy came up. Bryan does HVACR podcasts and organizes educational resources for HVACR. His website is hvacrschool.com. The quickest way to discover that you don’t really know everything you should about a subject is to try and explain it to someone else. In talking with Bryan, I realized my grasp on entropy was a bit tenuous.

Anyone looking carefully at a refrigerant pressure-enthalpy diagram might have noticed the steep, diagonal lines on the right side of the saturation curve labeled entropy. Entropy is a measure of the level of disorganization in something. Entropy is a natural process: everything tends to become less and less organized over time. The inside of most service technician’s trucks towards the end of the week is an example of increasing entropy. Parts and tools scattered about, service tickets and coffee cups in the dash, and a copy of Fundamentals of HVACR, 3rd edition open in the front seat. So how does this concept of increased randomness have anything to do with air conditioning?

In researching for an answer to that very question I came upon many explanations that were honestly a bit beyond me. It seems I am not the only person who struggles with exactly what entropy is. The definition that I found which came the closest to something which made sense from an HVACR point of view was “Entropy, the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. Because work is obtained from ordered molecular motion, the amount of entropy is also a measure of the molecular disorder, or randomness, of a system.” Another idea that helped was “Thermodynamic entropy is part of the science of heat energy. It is a measure of how organized or disorganized energy is in a system of atoms or molecules.” From these descriptions I can visualize that entropy is a measure of the amount of energy required to keep something at its present condition and state. That energy is unavailable for useful work because that amount of energy is required just for the substance to remain as it is.

If you think about the different physical states and the way molecules are arranged, you can see that solids have a low entropy – the molecules have very few possible arrangements, liquids have a higher entropy – the molecules move freely around, and gasses have the highest entropy – the molecules are whizzing around nearly independently. Extending the mental concept a bit further, increasing the temperature of a gas increases the entropy because now the molecules are moving more, so there are more possible arrangements. On the other hand, increasing the pressure of a gas decreases its entropy because the molecules are packed in more tightly, decreasing their ability to move around. Increasing the volume of a gas increases the entropy because the molecules have more room to play, and thus, more arrangements.

OK, so we still have not really nailed down how this has anything practical to do with a refrigeration system. Entropy is measured in BTU per pound per degree. Basically, that is the definition of specific heat. Notice that if you follow any line of constant entropy from left to right, the gas increases in temperature and pressure. That is exactly what happens in a compressor. These two changes counteract each other in terms of the effect they have on entropy, leaving entropy unchanged. When gas is compressed its entropy remains the same. Mechanical energy is converted into heat energy, but the amount of heat per degree for each pound of refrigerant stays the same.

Next time you are on a job and want to plot the system operation on an enthalpy diagram, remember that you can use the lines of constant entropy to make it a bit easier. The compressor line starts where the evaporator pressure line intersects the suction temperature line. Compression will follow the lines of constant entropy up until you intersect with the condenser pressure line.