Guide
The motor rewind process: what happens in the workshop
Updated
Knowing the sequence tells you which questions to ask, and which stages a cheap quote has quietly skipped.
Stage by stage
- Incoming inspection and test. Insulation resistance, winding resistance and a visual check, recorded before anything is dismantled. This is what tells you the fault was where you thought it was.
- Strip-down and winding data recording. The old winding is measured and documented: turns per coil, wire size, coil pitch, connection and slot arrangement. If this is not recorded properly, the rebuilt motor is a guess.
- Core loss test, before. A baseline for the condition of the stator core while the old winding is still in place.
- Removal of the old winding. Usually a controlled burn-off in a temperature-monitored oven, then mechanical removal of the copper. Temperature control at this stage is what protects the core.
- Core loss test, after. Compared against the before reading. A significant increase means the core has been damaged and the workshop should tell you rather than wind onto it.
- Slot cleaning and insulation. Slots are cleaned out and new slot liners, wedges and phase separators fitted.
- Winding and connection. New coils wound and inserted, connections made and tested, then the winding braced so it does not move in service.
- Impregnation and cure. Varnish or resin applied by dip and bake, trickle or vacuum pressure impregnation depending on the machine and the duty, then cured.
- Mechanical overhaul. New bearings, seals and any shaft or housing repair, plus balancing where the rotor has been disturbed.
- Final test and report. Insulation resistance, winding resistance balance across phases, no-load run for current and vibration, and the paperwork that proves it.
Where quotes differ without saying so
- Whether core loss testing is done at all. It is the check that protects you from a rewound motor that runs hot forever.
- Which impregnation method is used. Vacuum pressure impregnation costs more and suits demanding duties; a single dip and bake does not suit them all.
- Whether the original winding data is reproduced or redesigned. Redesigns are sometimes an improvement and sometimes a shortcut. Ask which, and why.
- Whether bearings, seals and balancing are included or added once the motor is apart.
- What test evidence you get back. Readings, not just a pass label.
Motor repair workshops commonly work to recognised repair standards for rotating electrical machines, and standards exist covering both repair practice and good-practice guidance for maintaining efficiency. Ask which standard a workshop works to and what their quality documentation looks like, rather than taking a general claim of compliance at face value.
Do not skip the incoming test
It is tempting to send a motor away with "it has burnt out" and take the quote that comes back. The incoming test costs almost nothing and occasionally saves the whole job: motors are sent for rewind every week that turn out to have a failed cable, a loose terminal, a failed bearing or a supply problem rather than a failed winding. Ask what the incoming test found before you authorise a rewind.