Why Rotors Fail The rotor is one part we all hope to salvage whenever we rebuild an alter- nator (besides, of course, the housings). If we are to do this successfully, we need to know how to check them, how to repair those that can be saved, and how to pick out those that can’t. We also need to understand what causes them to fail. Excessive heat, vibrations, bad bearings and high rpm all contribute to a rotor’s death With some effort on our part, some of these can be reclaimed to make us money. The rotor consists of five basic parts: the shaft, two matching pole pieces, the bobbin (coil around an iron slug), and the slip rings. All of these must fit together tightly and be balanced. The average rotor will see in excess of 12,000 rpm, or higher. At these speeds, centrifugal forces, several thousand times the force of gravity (if my math is right), are trying to pull the coil apart and even a slight imbalance can cause severe vibration. Large coils (higher amp), result in even higher centrifugal force being put on the outside windings. The shaft needs to be straight, bearing surfaces smooth and unworn, threads clean and the spacing of the shaft in the poles must be perfect. A trained eye and a good pair of hands can check the shaft relatively quickly. If the shaft is bad, the rest of the rotor checks good, and you have a good press, many shafts can be replaced. If the rotor is inexpensive (few are these days), reshafting may not be worth your time. Only you can decide. The shaft is pressed through the pole pieces and the iron slug. The pole pieces should tightly hold the bobbin inside, and the spacing of the individual poles or fingers must be even. This is very important. Few small shops own balancing equipment. We can only assume that a used rotor with no other problems is in balance. This is not always going to be true. You can check balance to some degree with two level straightedges. Simply place the shaft ends on the straightedges and roll the rotor slowly, stopping at each pole and letting go. If the rotor stays put each time, balance is pretty close. If it always rolls in the same direction, check the level of your straightedges again. Any rotor that fails this test should definitely be rebalanced or rejected. The coil is the heart of the rotor and the part requiring the closest scrutiny. The wire should be wound tightly with no signs of overheating. First check the rotor for shorts to ground. This should be done using high voltage, which will let you know the condition of the coil’s insulation. Ifa rotor fails this test, do not throw it away. Find out why it failed. Is there copper dust between the slip rings and shaft? Has it been wet? Is one of the lead wires rubbing a pole? Can it be fixed? I find that over half the rotors that fail the short to ground test are fixable. Once you get it past the first test, the coil should be checked for amperage draw. This reading should be within spec. What is the spec? Good question. What rotor are you checking? Each different rotor will give different read- ings and the only way I know to keep up with them is by keeping records. A note- book by the rotor tester is the best way. If units are being done in batches, then comparisons are easy. Today, most of us spend half our time doing specials on an individual basis. I would suggest writing down unit number, rotor current draw and date in a book. The date is in case a problem arises later. You can look back to what the rotor originally tested at. This is the best method to collect compar- ison readings for each different rotor. Finally, a gauss meter can be used to determine the magnetic strength of the rotor. This test is a real time saver. How many times have you put together an alternator and the amperage did not meet your expectations? Gauss meter readings can practically eliminate rotor output guessing games, which can consume hours before you know it. If you build one-wire alternators, you’ll discover the gauss meter will tell you which rotors contain the greatest amount of residual magnetism, and will give you quickest turn on. This test is conducted without current using the most sensitive meter. To test the properties of the coil, slug and poles together, you must use a less sensitive gauss meter with current running through the coil. Do not use a battery as a power supply for this test, since voltage will vary and comparisons become useless. Power from a rotor tester is one way. Bob Agostin of Genco Auto Electric in Islandia, NY recently shared a trick they use. He built a jig to hold the rotor against a Denso brush holder, and powers the coil up using a solenoid coil tester through the brushes. This holds the rotor, provides a uniform amperage draw reading, and leaves the hands free to use the gauss meter to determine magnetic strength. If the rotor appears to have a high amp coil, yet readings seem low, you can use your press to bring the pole pieces closer together. Take care not to crush the bobbin,