Originally Posted by glennjanie
To test the compressor for short-out, disconnect the three wires to it, use an ohmmeter on each tab to the case. It works best with a digital ohmmeter. Record the reading on each tab and two of them should add up to the third one, in no special order. If two of the readings do not equal the third one, there is a dead short in it.
I think what you meant to say was:
To test the compressor for short-out, disconnect the three wires to it, use an ohmmeter to read the voltage between each pair of tabs. (With three tabs (1, 2 and 3), there will be three resistances; (1 to 2), (2 to 3) and (1 to 3)). It works best with a digital ohmmeter. Record the reading across each pair and two readings should add up to the third reading, in no special order. If two of the readings do not equal the third one, there is a dead short in it.
Because, if you have continuity between one of those compressor tabs and the casing of the compressor, and that compressor is bolted to the rest of the fridge, then the whole fridge would be energized to 120 volts AC (or thereabouts) all the time the fridge compressor is running. And that would be a dangerous situation cuz touching the fridge handle would give you a shock. And, if you had continuity between all three of those tabs and the compressor casing, then the power would flow through the compressor casing instead of the motor, or at least it would until the circuit breaker tripped cuz you'd have a short circuit through the compressor casing.
That is, you SHOULDN'T have continuity between any of the motor tabs and ground. If you do, you have an electrical problem in the compressor motor.
I think the easiest way to remember how to check is to simply understand that the motor inside a fridge compressor (or dryer or washing machine) has a start winding and a run winding wired in parallel:
1. One tab on the motor goes to one end of the START winding and the other end of that start winding goes to the tab connected to the white wire that takes the electricity back to the generating station.
2. Another tab goes to one end of the RUN winding and the other end of that run winding goes to the tab connected to the white wire that takes the electricity back to the generating station.
3. The third tab is where you connect the white wire that takes the electricity back to the generating station.
So, by measuring between 1 and 3 and 2 and 3, you're measuring the resistance of each the start winding and the run winding individually. By measuring between 1 and 2 you're measuring the resistance of the two windings in series. That's why the resistances of each winding individually add up to the third resistance, which is measuring the two windings in series.
In fact, you can tell which is the neutral tab that takes the electricity back to the generating station. Just note which pair of tabs have the highest resistance between them, and it'll be the tab left out. The white wire will be connected to that left out tab.
PS: You don't need to know the rest:
The one characteristic of all electric motors is that they all have a stator that tries to produce a rotating magnetic field. If the rotor in the motor has magnets in it, or is magnetized because it has windings of it's own, it will try to follow that rotating magnetic field produced by the stator.
In electric motors that operate on three phase power, it's simple as mud to produce a rotating magnetic field. You just arrange three pair of windings equidistant around the stator and connect each phase of your power to each pair of windings. Since the windings are offset by 120 degree intervals around the circumference of the stator, and the power going to the windings is out of phase by 120 degrees, the magnetic field produced by that stator will rotate.
You can do the same thing with two phase power.
When you get to single phase power (that is 120 volts AC) then you simply have a single pair of windings which cannot create a rotating magnetic field, only an oscillating magnetic field that will just push and pull the rotor back and forth, but not make it rotate.
ALL OF THE ELECTRIC MOTORS you've probably ever heard of (such as shaded pole motors, split phase motors, capacitor start motors, capacitor run motors, resistance motors, etc.) ALL use some form of trick in the stator to create the illusion of a rotating magnetic field for the rotor to follow.
That is, the reason there is only one kind of three phase motor, but so many different kinds of 120 VAC single phase electric motors is that each different kind of single phase 120 VAC motor uses a different trick to produce the illusion of a rotating magnetic by the stator out of what would otherwise be a purely oscillating magnetic field produced by the stator.
For example, in a split phase motor, different kinds of wire are used for the start and run winding. One winding has many turns of a thin wire whereas the other has fewer turns of a thick wire, and that means that each winding has a different inductance, and therefore changes the amount by which the current sine wave lags behind the applied voltage sine wave. That time difference in the current sine waves between the two windings means that the two windings develop their magnetic fields at different times. Since the two windings are spaced equidistantly around the circumference of the stator, they create the illusion of a rotating magnetic field. And that's one way to produce something that looks like a rotating magnetic field to the rotor to follow.
Capacitor start motors, on the other hand use a capacitor wired in series with the start winding.
...flying off on a tangent...
In a resistor, current out of the resistor is highest when the voltage across it is highest, so the current and voltage is in synch in a resistor. However, the current out of a capacitor is highest when the voltage across it is changing the fastest, and that occurs when the voltage is 0 (zilch) VAC on the voltage sine wave, and so the current sine wave out of a capacitor will be 90 degrees out of synch with the applied voltage sine wave.
...coming back to the subject at hand...
That means the start winding in series with that capacitor will receive it's current sine wave at a different time than the run winding (which doesn't have a capacitor connected to it), and so the two windings will develop their magnetic fields at different times. Since the start and run windings are arranged around the circumference of the stator, the stator produces the illusion of a rotating magnetic field for the rotor to follow.
Shaded pole motors use a different trick. They actually use a coil of wire on each pole of the stator to cause different sides of the same pole to develop their magnetic fields at different times, again producing the illusion of a rotating magnetic field for the rotor.
So, all the different kinds of 120 volt single phase motors on the market have the same thing in common. They each use a different trick to produce the illusion of a rotating magnetic field for the rotor to follow. Without that trick, all would produce an oscillating magnetic field which would simply cause the rotor to shift back and forth inside it's bearings until the motor overheated.
Thus, to understand electric motors completely, you merely have to understand all the different tricks used in 120 VAC motors to create the illusion of a rotating magnetic field in the stator.
Your US military is one of the best educational institutions in the world. It takes kids straight off the streets and teaches them technical skills you'd need to pay to go to college to learn. Since the US military is owned by the American people, the learning material it uses to educate it's recruits with is also owned by the American people. Integrated Publishing is a company that provides those same learning materials to the general public. Integrated Publishing can't charge US citizens for this information (because you guys own it already), so it makes it's living selling CD's of the US military's training manuals to US citizens. That is, Integrated Publishing doesn't charge for the information, it charges for the service of burning a CD with all of the information on it for you. The information is free, but you pay for that service.
However, you don't need to buy the CD to access the information.
Take a look at this web page:
Click on Electronics,
then click on Electrical Engineering (NEETS), which is short for Navy Electrical Engineering Training Series
then, click on NEETS Module 5 - Introduction to Generators and Motors.
and learn all about the ONE three phase, the ONE two phase and all the different kinds of single phase 120 VAC electric motors and how each different kind creates the illusion of a rotating magnetic field.
You can, quite literally get a college education at home in electrical technology from this web page.
And, there is more knowledge than any one person can absorb about so many different technologies at that same Integrated Publishing web site. It's really a US Military university education all rolled up in a single web page.