Page History: Unisaw Bullet Motor Repair
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Page Revision: 2012/01/18 00:52
This page shows some of the main internal components of a single-phase Unisaw repulsion-start/induction run (RI) "bullet" motor. The page covers three major elements in troubleshooting and repairing internal components:
- Turning the commutator and undercutting the mica
- Fabricating and replacing the shorting necklace
- Replacing the brushes
The approaches and tools shown in the original entry on this page were done in a hobbiest garage-shop, using moderate level tools such as a lathe, and do not require significant machinist skills or equipment.
Turning the Commutator and Undercutting the Mica¶
The first place to start when any repulsion-start/induction-run motor is having starting difficulties is to turn the commutator to eliminate any carbon build-up on the copper bars and to even out any imbalances or uneven surfaces. This is a fairly common procedure that only takes a few minutes with the rotor assembly mounted on a metal lathe.
The space between the copper bars on the commutator is a softer, non-conductive material called "mica". In order to ensure clean contact between carbon brush surfaces and the copper bars, the top surface of the mica should be slightly recessed below the surface of the commutator bars. After turning the bars down, a user often needs to then remove a small top layer of the mica to create that recess. This process is called undercutting the mica.
Here is the commutator end of the rotor from a 1hp RI bullet motor. The carbon build-up on the copper surface is obvious, and it's also apparent that the commutator has been turned down several times over the many years.
After spending a few minutes on the lathe turning the commutator, it is much cleaner and smoother. However, in this instance, the surface of the mica has also become essentially flush with the top of the bars. Depending on the size of the gap, you can use a small blade and hand cut the grooves lengthwise. In this case, the photo below shows using the tip of a metal blade from a reciprocating saw.
Once done, the commutator looks far better than it did half an hour earlier.
Fabricating and Replacing the Shorting Necklace¶
The "shorting necklace" inside the commutator is often one of the most common failure points on a Unisaw RI bullet motor. It is also one of the most troublesome parts to either locate for replacement. In addition to showing one method for fabricating a new shorting necklace from small extension spring stock, the page also shows demonstrates briefly the turning the commutator and undercut the mica, as well as replacing the brushes