Alternator Disassembly and Maintenance

Alternator disassembly

Wind turbines use moving parts, and all moving parts need regular inspection, maintenance, and lubrication. Once a year or so it makes good sense to disassemble your wind turbine alternator, so you can clean and inspect the bearings, electrical connections, and other parts. You might also have to do this procedure if you change your system / battery bank voltage and need to install a new stator in your alternator to make it compatibile with your new system.

The alternator consists of a stator, with a magnet rotor on each side. The magnet rotors are attached to a wheel hub that rotates on a spindle, which is fixed to the wind turbine frame assembly. The bearings are common tapered roller bearings. The 10-foot diameter wind turbine utilizes a Dexter #8-91A wheel hub and common L44643 tapered roller bearings. The spindle on which the inner races for the bearings fit is a Dexter BT8 spindle. These are all common parts.

If you upgrade your power system and change system (and battery bank) voltage, you'll need a new stator. And on occasion, it also becomes necessary to replace a damaged stator. This is usually due to improper loading of the wind turbine, or some issue with the furling system. If the tail is too heavy or the geometry of the furling system is incorrect, the machine may produce too much power for sustained periods of time and overheat the stator.

Before you start...

Before starting, inspect the alternator. Are the magnet rotors clean, or are there bits of “stuff” stuck to the magnets? You'll want to carefully clean the magnet rotors after disassembly. It's important to check for that now, as once the magnet rotors are separated oftentimes small bits of ferrous material will lay down flat against the magnets, but when the alternator is together these will stand up and often rub the stator, eventually wearing through the resin in the stator and possibly causing damage to the coils.

Check the wheel bearing. Does the alternator spin freely? Often a worn bearing will spin freely but make a bit of a growling sound – be sure to check for that. Check for play in the bearing. A properly adjusted wheel bearing on this alternator should be slightly loose, so that there is slight play in the bearing but not more than 1/32 inch of play at the end of the magnet rotor. If the play is more than that, the bearing may need adjustment or you may choose to replace it. If in doubt, replace it – bearings are cheap (less than US$10)

Following are detailed instructions to disassemble the alternator.

Jacking screws

The stator sits between two magnet rotors, both of which are very much attracted to one another! The force needed to separate them is extreme, and there is no good way to do this without jacking screws.

The first step is to manufacture the 4 jacking screws.

Materials required:
1/2 inch - 13tpi threaded rod 10 inches long, qty 4
1/2 inch- 13tpi nuts, qty 4

Instructions

Cut 4 pieces of the threaded rod 10 inches long. Weld a nut to one end of each threaded rod (basically you're making 4 very long bolts here...). If you don't have a welder, you could double nut it (drive two nuts tightly together at one end of the threaded rod).

Chamfer the other end of each jacking screw with a grinder (see the results in the picture). This is an important step. If you try to use jacking screws that are not chamfered at the end, you will damage the threaded holes in the front magnet rotor as you remove them. Once the ends of the jacking screws are chamfered, clean up the threads with a file or a die and check that the threads work well (run a nut over it or test them in the front magnet rotor).

Separating the magnet rotors

The magnet rotors must fit together a certain way, so that wherever there is a magnetic pole on one rotor, the other one has an opposite magnetic pole. The first step is to put an index mark on each rotor so that when you disassemble the alternator, you can be sure to put it back together the same way. Use a permanent marker to make a mark on the edge of each rotor, each directly across from one other.

There should be 4 nuts holding the front rotor down to the alternator, they are between the blades and the alternator. Remove them before you attempt to remove the front magnet rotor.

Put a drop of oil on the tapered end of each jacking screw, and screw them into the 4 threaded holes in the front magnet rotor. Run them all the way in until they stop against the hub.

Once there is an even gap between the front rotor and the stator of about 3 inches, the magnetic attraction between the rotors is very little and you should be able to firmly hold the 4 jacking screws and pull the front rotor away from the alternator.

Once you pull the front rotor off, bring it to a safe place, away from anything it might be attracted to, where nobody else will mess with it. Hanging on a nail on the wall, away from anything made of steel, is a good choice.

Remove the stator

The stator is attached to the steel stator bracket via 3 pieces of stainless steel threaded rod. It is clamped between two nuts (usually an acorn nut on the front and a standard nut behind it). The first step is to loosen the three nuts directly behind the stator. It's important to loosen those nuts so the studs don't bind up in the stator bracket. Next remove the nuts, washers, and lock washers behind the steel stator bracket. At this point you can remove the stator along with the three studs attached to it.

Inspect the stator

Look carefully at the stator to see if it shows signs of overheating. If the resin over the coils is blackened, or cracked – this is a sign of overheating. If the machine was still running well with good output but you see these signs, it's an indication that perhaps the tail should be lightened (so that it furls earlier) - or – the geometry of the furling system was not correct to begin with. Improper loading of the wind turbine could also cause stator overheating, we've seen this caused by rectifier failure, line failure (both cases could cause the machine to run single phase), and battery problems.

Look for signs that magnet rotors have been rubbing on the stator. If that appears to be the case, there are a couple possibilities. It's possible that the stator was never properly centered between the magnet rotors to begin with. It's also possible that the stator overheated and warped – check to see if the stator is flat. If the stator is warped, it may still be possible to use it as is with careful adjustment.

A slightly damaged stator may still work fine for years to come... it's a judgment call, if the machine still worked fine but the stator appears damaged if you should replace it, or use it as is. In some cases surface damage to the stator can be repaired with epoxy or other resins.

Inspect the magnet rotors

Now the the stator is off you can look carefully at both magnet rotors. Look to see that resin cast around the magnets is not cracked and still adhered to the magnets, and the steel rotors. If it appears to be damaged, best bet is to recast the magnet rotors. We've seen problems along these lines especially when magnet rotors were cast with Polyester – we've had no such problems with magnet rotors cast in vinyl ester or epoxy. Sometimes minor damage can be repaired with epoxy... we don't know how long such repairs might last, it's a judgment call.

Look at the magnets to see if they've been rubbing the stator. In some cases when magnets do rub on the stator the coating will wear off leading to corrosion of the magnets. If that's the case, we suggest you clean the magnet and re coat it with epoxy to slow the corrosion.

See that the magnets are clean! Again, if there are ferrous materials stuck to them now, they may 'stand up' and rub the stator when you reassemble the alternator. A good way to clean the magnet rotors is to push all the material to an edge on the magnet, and pinch it/remove it with your thumb and forefinger. That removes most of it, the rest can be removed with sticky tape.

Inspect the bearings

If the alternator spins freely and silently the bearings are probably fine. You may choose to remove the grease cap, cotter pin, and the castle nut – and remove the hub with the back magnet rotor to inspect the bearings more closely. It also never hurts to clean them with a good solvent (gasoline works fine) and repack them with grease. If you go to that trouble (which we suggest you do every two years), carefully look at the bearings, and the races. If you can see any pitting or other signs of wear consider the bearing shot, and replace it. If all looks good, simply repack them and reassemble the machine, keeping in mind as you tighten the nut on the spindle, that there needs to be a bit of play in a tapered roller bearing. Good practice for tightening the castle nut is to tighten it all the way down with a wrench, or channel locks, and the back it off to the next position into which you can fit the cotter pin. If that's still to tight, take the next position down. There should be some play, but not more than 1/32 inch if you measure it at the end of the magnet rotor (6 inch radius from the spindle).

Reassembly

Reassembly of the alternator is pretty straight forward, the opposite of the disassembly. Once the back magnet rotor/wheel hub assembly is on the spindle and properly adjusted you need to refit the stator. When installing the stator, the studs should be still fixed to the stator but slightly loose, and the nuts that come against the stator bracket should still be in place. Fit the stator onto the wind turbine and install the washers, lock washers and nuts behind the stator bracket. Tighten them with your fingers but not more. Next tighten the hardware on both sides of the stator (the acorn nut in the very front and the nut behind the stator). You don't want to try adjusting the stator yet – not until the front magnet rotor is installed.

Presumably the jacking screws are still in the front magnet rotor, if not screw them in about 4 inches. Measure the distance they stick out so they're all perfectly even (the magnet rotor needs to go on flat or else it will bind up – same as when you removed it). Hold the front magnet rotor by the jacking screws (same as you did when you removed it) and rotate it so that its index mark lines up with the corresponding mark on the back rotor (you need to be sure it goes on in the same position that it came off). Carefully lower the front magnet rotor onto the wind turbine. Do not every put fingers or hands between the magnet rotors! Use the jacking screws (1 turn at a time with each screw) to slowly lower the front rotor down to its resting position on the alternator. Once the rotor comes to rest in its proper position you can remove the jacking screws. Replace the 4 nuts that hold the front magnet rotor in place and tighten them. (these are the 4 nuts that exist between the alternator and the blades)

Next step is to adjust the position of the stator. It should be fairly perfectly centered between the magnet rotors. Make adjustments to the stators position using the two nuts on the stator bracket, do not make adjustments with the nuts that hold the stator itself (you've already tightened these and they shouldn't be messed with anymore).

If the stator seems well adjusted you can tighten all the hardware. Give the alternator a spin to see that there isn't too much wobble and that there is plenty of clearance between the magnet rotors and the stator.

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