Btu Buddy 148: A Badly Burned Compressor Motor
Bob is a service technician who is well trained and nationally certified. However, he has sometimes suffered from the same confusion that all technicians occasionally do — the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has had something that no one else has. He recalled his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminded him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim were at a retail store where the manager had called and said there was no cooling. The system was a 5-ton heat pump.
After talking to the manager, Bob and Tim turned the thermostat to the “fan on” position to see if there was power to the unit and the indoor fan started. The heat pump was a split system, so they went up to the roof.
When they got to the roof, they discovered the breaker was tripped. Tim started to reset it when Bob stopped him and said, “Let’s do some checking around before we try to start the unit. There hasn’t been any lightning lately so there should not be any random breaker trips. Get the volt-ohmmeter and let’s see if there is a ground somewhere in the system.”
Tim had the volt-ohmmeter and Bob said, “Make sure that the breaker has all of the power off of the system with the volt meter and then check the ohm reading to ground using the RX1 scale at the load side of the contactor.”
Tim said, “The reading shows a dead short to ground somewhere. I think I should disconnect the fan motor and hope it is the problem.”
Tim checked the fan motor and said, “It shows no circuit to ground. I will try the compressor motor.”
He disconnected the compressor motor leads and checked it to ground and said, “It shows a dead short to ground. Here is our problem. What do you want to do next?”
Bob said, “The next thing is to tell the store manager and get the approval to move forward. Then let’s recover the refrigerant and see if it is badly contaminated (Figure 1).”
They got permission from the manager to start the job and started the recovery of the refrigerant. While the recovery process was being done, they wrote down all of the numbers off of the outdoor unit and called the supply house with a list of supplies including the compressor.
They did an oil acid test sample of the oil and it showed high acidity. A small amount of refrigerant vented while disconnecting the recovery unit and Bob said, “Wow, that is a very smelly burn.”
When the recovery was complete, they went to the supply house and then to lunch where Tim asked, “Why did you order all of those elbows for the suction line?”
Bob explained, “I think this is a really bad burn and we will need to install an acid removing filter in the suction line. We won’t know until we disconnect the line, but I suspect that the compressor burned on startup and, if so, soot from the motor burn will blow backwards down the suction line. If the new refrigerant isn’t filtered before it reaches the compressor, it will pollute the oil in the new compressor on startup. We don’t want that to happen. We may have to install the suction line filter outside of the unit’s cabinet; it is very tight inside the unit. I want to be prepared. Remember, on a heat pump, the suction line filter must be installed in the short suction line between the four-way valve and the compressor (Figure 2).”
They removed the old compressor and set the new one in place. They filed the silver solder off of the discharge line so that it would fit up correctly, then they cut the suction line off and piped in the suction line filter by locating it down low inside the cabinet.
When they cut the suction line, Tim commented, “Boy, that is dirty inside. There is loose soot residue in the line. Can we blow it out?”
Bob said, “When we start the unit up, the filter will catch any soot. Flowing refrigerant has a great scrubbing action. If we tried to blow it out with nitrogen alone, we could not get enough velocity to flush it out. Some companies make solvents that can be pushed through the lines and will evaporate during evacuation, but I don’t think that this burn is bad enough for that action. As you pointed out, the soot is like a light powder.”
They installed a liquid line filter at the last minute to keep it from becoming polluted. The system was all back together and they pressurized it with dry nitrogen and a trace of R-22 so the leak detector would detect. They then did a leak check on all of the connections that they had completed. They left the nitrogen pressure on at 150 psig while they cleaned up and the pressure didn’t drop so they declared the system leak-free and let the pressure down to atmospheric pressure and started the vacuum pump. They changed the contactor and the capacitor because of the strain of the motor burn. They had an oversized suction line on the vacuum pump and had removed the valve stems out of the Schrader valve connectors so the evacuation went quickly, down to 500 microns.
Bob said, “Let the pressure build up to atmospheric pressure with nitrogen and replace the Schrader valve cores and start the vacuum pump for another evacuation.”
The vacuum pulled down to 500 microns again. They let the system stand under 500 microns of vacuum for 15 minutes and the vacuum held. They were ready to add refrigerant and start the system up.
Tim had connected the refrigerant cylinder to the manifold and the cylinder was on the scales and the scales were set to “0.“ Everything was ready to go.
They valved the vacuum pump off and charged liquid refrigerant into the liquid line until the liquid stopped flowing.
They started the system up and charged the rest of the refrigerant into the suction line in the vapor state.
The system was running well and Tim asked, “How long are we going to leave the filter in the line?”
Bob said, “When it runs and filters, the filter may filter out enough debris to start having pressure drop. If that pressure drop is more than 2 psig, we will change it and see if the new filter starts restricting. Chances are that the first filter will not become restricted. They have a large capacity to hold a lot of particles; look at the large diameter of the filter (Figure 3).”
Bob then said, “We will come back in a few days and check the pressure drop. If it is 2 psig or less, we will leave the filter in the line permanently. The filter must have some running time on it to do a good job.”
They returned in three days and checked the filter and it was not clogged.
Tim said, “It seems like I learn something new every day or a new twist on the same thing. Thanks for your teaching help.”
Bob said, “We technicians must always help other technicians learn what we know. Knowledge is power; share the power you have with others and it will come back to you someday when you least expect it. I never will forget helping a new technician from another company solve a problem. Six months later I was caught out of town on a job when I needed to cut a hardened steel bolt off and didn’t have anything to cut it with. Sure enough, along came Stan with a Dremel tool with a small cutting wheel. He offered to cut it off for me. I told him that it would not do the job. He told me to watch and he cut that bolt off in record time. That is why we have a Dremel tool in the truck compartment. There is a good reason for every tool in this truck. Many of them have similar stories. We don’t have enough time in our lives to learn all the shortcuts on our own; let’s share and others will share with us.”
Tim said, “Great idea.”
Publication date: 7/20/2015