At the risk of being redundant:
- MOV = Metal Oxide Varistor. As described, it doesn't conduct significantly below a well-defined applied voltage and conducts heavily if the voltage rises above that threshold, .thus "clamping" the voltage to the threshold. They protect electronics from moderately short electrical high-voltage spikes, like those you get when lightning strikes nearby, by conducting heavily when the voltage is higher than their threshold, which you pick to be lower than the rated voltage of the protected device. Semiconductors, like your mill's rectifiers, have a "peak reverse voltage", and if the voltage rises above that they die. So you hook a varistor (or string of them) across the output, with the threshold voltage somewhere above runaway-mill-max and below the peak-reverse. If you get a surge induced in the wiring between the mill and your house/battery/whatever, they MAY protect the diodes. (They may also die, themselves, in the process, and need replacing, or disconnection until you get a replacement, to get things running again.)
A hall effect sensor is a magnetic field detector. The sensor proper produces a small signal and the module has an amplifier to produce either an output voltage proportional to the field or a voltage that jumps up or down if the field is above a below a certain value. You want the proportional type. You don't hook it into your mill's wiring. You hook a battery to the power input pin, a meter to the output pin, and the other side of both to the common/ground pin. Then you use the sensor as a probe to measure the field from your coils, reading the field strength on your meter. (Or you would if you hadn't already found out the coils were OK.) The sensor measures the component of the magnetic field penetrating it along a particular axis. So not only do you have to put it in the equivalent location on each coil, but you also have to orient it the same way, preferably with the sensor axis aligned with the line through the middle of the coil. Shorted turns don't contribute to the field, so if you feed a given DC current into each coil, a coil that has a short across 20% of the turns will only produce 80% of the field produced by an unshorted coil of the same number of turns.
My ranch house is being painted, an a couple weeks ago the painter discovered a large beehive on the outside. It was entirely on the outside beams under the roof overhang (a rare occurrence). Of course it had to be removed so the painting could be finished before the weather changes. Rather than have it killed off, I contacted a local bee breeder who wanted them for his post-colony-collapse breeding program, which is based on the feral bees in the region. The bees in the problem hive were very productive (going from nothing to a basketball-sized hive in less than three months) and non-aggressive (not bothering the painter while he took picktures from a couple feet away). The breeder is trading removal service, a small bottle of their honey, and a "nuc" from his program next spring, for the bees and their comb. (A "nuc" is the nucleus of a new hive: A few combs on frames with a queen and a starter set of workers.)