“I never knew how much work was involved!”
That was a comment made by a visitor to our workshops earlier this week when we stopped by this DC electric motor armature being rewound.
Many people make the assumption that the rewind is done by machines. Whilst this is usually true when it’s made at the manufacturer’s factory, when it comes to a rewind during the repair process, there’s a considerable amount of labour involved.
Automatic machines are used to wind the copper coils, however they still need to be inserted into the armature slots by hand. The ends of the coils, the individual conductors, then need to be slotted in a specific order and pattern into the commutator slots, known as the risers. The commutator is the copper orange part on one end of the armature. Carbon brushes slide over this surface when the motor is spinning and this is where the DC current powers the motor.
This commutator has 150 segments and therefore 150 slots/risers. Each slot/riser has 6 conductors in it. For those of you quick on the maths here, you’re right, that’s 900 individual wires that need to be fitted in the right order.
Dudley, one of this year’s new staters, is just over half-way through fitting the coils in this armature. After this is done there are still plenty more processes needed. The conductors need to be cut back flush to the commutator slots/risers and then carefully welded or soldered. There needs to be two bands of glass tape wound around the visible copper to stop the coils moving due to centrifugal force when the armature spins. The coils need to be varnished (We use vacuum & pressure impregnation – VPI) To hold them solid too.
The commutator is then skimmed, undercut and chamfered. That means that it is machined round so the run-out is less than 35 microns. The insulating sections between the segments are cut down so they are below the segment height. and the segments are chamfered. All those processes give an easier ride-path for the carbon brushes. The whole armature is then balanced in a process similar to how your car wheels are balanced. This is so it doesn’t add vibration to the motor when it spins.
Then it’s ready for fitting back into the motor.
So, whenever you have a DC motor armature rewound or hear of an armature rewind, you’ll hopefully have more of an understanding of the skills and hand-crafting goes into it.