About two weeks a year, it gets hot enough and humid enough here in Massachusetts to push us into turning on the air conditioning.
For the first few years of the century, after the house was built and we moved in, everything was fine, but in recent years not so much. We have different AC zones, and separate systems for each. Each year, typically, one or two of the units don’t work. Not work as in blow hot air instead of cold. I then go outside around back and discover that the fan in the outside unit isn’t spinning. Until last year, I’ve always been able to fix this problem by reaching through the grill and unsticking the fan with a screwdriver, or in the worst case, by taking the fan and motor off and whaling on it with a hammer. Evidently, enough moisture gets into the motor bearings over the winter to seize them beyond the motor’s starting torque’s ability to spin.
Brief Digression on AC
Air conditioners work by expanding a high pressure gas or fluid like freon through a nozzle into a low pressure gas. As a consequence of the ideal gas law, the expanding gas gets cold. It is then run through a heat exchanger inside the house, where the cold gas absorbs heat from the room air. (There is usually a fan to push the room air through the radiator fins of the heat exchanger. The expanded gas is then piped outside to a compressor. The compressor squeezes the working fluid, which according to the gas law, heats it up. Because heat was absorbed from the room, the compressed gas is now hotter than it was originally. It is then run through the outside heat exchanger, when a fan blows warm outside air past it to absorb the heat from the (hot) compressed gas. (I am using “gas” and “working fluid” interchangeably here. In fact, I think freon is one of those things that turns into a liquid at high pressure, so there is a phase change involved as well.) if the outside fan doesn’t work, then the there is nothing to cool off the compressed gas, and the whole outside unit eventually gets so hot that the thermal overload switch in the compressor shuts it off. This is why fixing the outside fan fixes the whole AC.
Well last year, one unit’s fan wasn’t spinning, but wasn’t stuck either. There are only three reasons why that could be: no power, bad motor, or bad capacitor. I was able to measure that the power was present, and it was cheaper to replace the capacitor, and that fixed it. Except that my measurements seemed to indicate there was nothing wrong with the old capacitor. I had fixed a loose push-on connector, so I wrote off the experience.
This year, same problem, same unit. The motor was not stuck, but wasn’t spinning either.
Brief digression about induction motors
Electric motors work by having a spinning magnet (the rotor) driven by a stationary magnet (the field). Now the magnets are going to want to line up north pole opposite south pole, and stay that way, so there also has to be something that makes “north” spin. Some motors have the rotor or the field be a permanent magnets with the other being an electromagnet, while other motors have electromagnets for both field and rotor. If the rotor is an electromagnet, there will often be brushes to supply power to the rotor. An induction motor is kind of strange, in that both the field and the rotor are electromagnets, but the power for the rotor is supplied by induction, with no physical connection.
A three phase induction motor is fairly easy to understand. The field has three windings, fed by the three phases. They are rotated with respect to on another by 120 degrees. As the current in phase “A” dies down, the current in phase “B” is picking up, and as a consequence the direction of North in the field windings rotates by 120 degrees. With three phases, you get a nice rotating field, and the rotor follows it, with just enough lag to generate an induced current in the rotor to create the rotor magnetic field. A single phase induction motor is different, the field merely reverses 120 times a second. If the rotor is spinning, then it will keep spinning, but there is nothing to get it started! To solve this problem, single phase induction motors have a capacitor. The capacitor is connected in series with another field winding that is rotated with respect to the main winding. Due to the properties of capacitors, the current in this starting winding will be advanced with respect to the current in the main winding. This gives enough of a rotating field to get the rotor started spinning. In fact, if you have an open circuit starting capacitor, you can sometimes start the motor by hand by giving it a spin yourself.
Because it seemed really unlikely that the new capacitor failed over the winter, I resolved to replace the motor. The problem was that I could not get the fan off the motor shaft!. The steel shaft was pretty well rusted together with the steel fan hub into a single glob. Repeated application of WD40 and hammers and so forth did nothing. By suitable pounding, I could move the fan axially towards the motor. By supporting the fan and pounding on the shaft, I could move it back, but hammering on the shaft was mushrooming the end of the shaft, so there would be no way to get the fan off. The usual tool for this problem is a gear puller, but a two-fingered gear puller won’t work with a three bladed fan. I have some nice pipe wrenches with which to twist the shaft against the hub, but the fan was too close to the motor for the wrench to fit, and the motor shaft didn’t come out the other end of the motor.
My solution to this is somewhat destructive! I used my angle grinder with a metal cutting wheel to take the motor apart. By grinding off six rivets I was able to get the back of the motor off, but there was nothing to grab with the wrench. I then used the cutting wheel to cut all the way around the fan end of the motor housing, at which point the field assembly came off, revealing the rotor. I could then grab the rotor with one wrench and the fan hub with the other and twist them apart.
This whole exercise was destructive and messy, and no doubt a new fan would be less trouble overall, but it sure was fun.