A low insulation reading on a shipboard motor tends to trigger one of two wrong responses: ignore it because the motor still runs, or condemn the motor and order a rewind. Both cost money. The reading is a data point, not a verdict.
Why marine motors fail differently
The same motor lasts longer ashore. On a vessel it lives in salt-laden, humid air, with constant vibration, temperature swings and duty cycling that a factory motor never sees. Moisture and salt migrate into the windings, and insulation resistance falls gradually rather than suddenly. That gradual slope is exactly why a single reading tells you so little.
What the insulation test actually measures
The insulation resistance (megger) test measures the dielectric condition of the winding at one moment. Two reference points are worth knowing:
- ■ The IEEE 43 formula: minimum IR = rated voltage in kV + 1 MΩ.
- ■ The common rule of thumb: a 10 MΩ floor for most Class B and Class F insulated motors at 600 V and below.
Neither is a pass mark on its own. A motor sitting just above the threshold and falling year on year is in worse shape than one sitting just below it and stable.
A low reading is not automatically a rewind
Before a motor is pulled and sent ashore, the first step is usually cleaning and controlled drying or stoving, following the EASA AR100 approach, then retesting. A great many low readings on ships are moisture, not failed insulation, and they recover. Only if the reading stays low after cleaning and drying does the motor genuinely need rewinding or varnish treatment.
Skipping the drying step is the most common way a serviceable motor gets rewound unnecessarily.
Log the things that make the reading meaningful
A bare megohm figure with no context cannot be trended. Record, every time:
- ■ Motor identification and date
- ■ Winding temperature and ambient temperature
- ■ Test voltage applied
- ■ The one-minute and ten-minute readings
- ■ Polarisation index (PI), and the one-minute reading corrected to 40 °C
Temperature correction is the step most often skipped, and it is the one that makes readings taken in a Gulf summer comparable with readings taken in a North Atlantic winter. Without it you are comparing weather, not insulation.
Rewind or replace
Once a rewind is genuinely indicated, the decision against replacement usually turns on availability and lead time rather than pure cost. A rewind keeps the original frame, mounting and shaft dimensions, which matters when the motor sits in a space built around it. A replacement can be faster if the unit is a standard frame size and in stock regionally — and slower if it is not.
For a vessel with a scheduled call, the question worth asking early is not “which is cheaper” but “which can be completed inside the port stay”.
How MARSEA supports this
Rewinding of electrical motors is part of our marine engineering scope, and our engineers are available 24/7 for afloat repairs in port. If you send the megger history rather than a single reading, along with the motor plate details and the port and ETA, we can advise whether the unit is a drying candidate or a genuine rewind before anything is dismantled.