The first thing to examine is not the motor but the pump itself and whatever caused it to lose water. When a centrifugal pump is not delivering water, its shaft power usually drops, so the motor is rarely overloaded; the real damage occurs in the sealing element, the bearings and the impeller. Replacing the motor is only necessary if it shows traces of that damage.

DRG-100L2-4 3 kW 1500 rpm DRG three-phase motor with cast iron frame – motor replacement after pump dry running

Marks left on the pump by running dry

The mechanical seal cracks from frictional heat and starts to leak. The leaking water can reach the motor's front end shield and from there the bearing. An overheated impeller may rub against the casing, and the shaft may bend slightly. In that case a new motor fitted to a damaged pump will soon experience the same vibration and heating.

Checks on the motor side, in order

  1. With the coupling or close-coupled connection released, does the shaft turn freely by hand, and is there any grinding noise from the bearing?
  2. Are there traces of water or rust on the front end shield, the bearing housing or in the terminal box?
  3. Has the winding insulation been measured by qualified electrical personnel?
  4. Is there any heat-related deterioration visible on the frame paint, in the smell or on the nameplate?

You can find the reasoning behind insulation measurement in the insulation resistance test article; interpreting the result remains the job of the specialist on site.

Ordering information once replacement is decided

Note the kW and speed from the old nameplate, the frame size, the mounting arrangement (foot or flange) and the shaft diameter × length. If the pump is likely to run dry again, replacing the motor without adding a level switch, a flow switch or a protection that detects underload only postpones the problem.

Information that may still be missing for a decision

Without knowing how long the pump ran without water, what type of pump it is and whether the seal has been replaced, a firm verdict is difficult. Once these points are clear, you can review the three-phase motor family suited to pump applications and compare the model with the same power and speed as the old nameplate in the online motor list.