If the holes do not line up, it is the base that gets adapted, not the motor feet: either new holes or slots are made in the base, or a rigid adapter plate is fitted between the motor and the base. Enlarging the holes in the motor feet, cutting the feet or welding onto the frame is not acceptable; it reduces strength and voids the warranty.

DRG-100L-6 1.5 kW 1000 rpm cast iron frame DRG three-phase motor – motor foot holes not matching the base

First find the cause of the mismatch

On IEC frames, the transverse (A) and longitudinal (B) distances between the foot holes and the shaft height (H) are standard values that depend on frame size. The mismatch is usually one of the following:

  • The old motor has a different frame size (e.g. 90L instead of 90S, which changes the longitudinal distance).
  • The old motor was built to a non-IEC dimension system (inch-based or specific to the machine).
  • The base was drilled by hand at some point and the holes have drifted.

Writing down the A, B and H values of both motors side by side shows which solution makes sense. You can compare the dimensions using the DRG technical drawings.

Practical options

New holes or slots in the base

If the base is thick steel plate and the new hole does not fall too close to the old one, this is the cleanest route. In belt drives, cutting longitudinal slots also increases the tensioning allowance.

Adapter plate

If the holes overlap or the base is cast, a thick plate with a machined surface is bolted to the old holes and the motor is bolted to the plate. The plate raises the shaft height; in a coupling drive the height must be matched to the machine side, and in a belt drive the pulley alignment must be rechecked.

What to avoid

"Making do" with a single bolt, ovalising a hole and covering it with a washer, or using a thin sheet-metal adapter that flexes will all lead to vibration and loosening. After fitting a plate or cutting slots, check that all four motor feet sit fully.

Keeping the frame size as close as possible to the old motor makes the job easier; in the three-phase asynchronous motor family you can choose by power and speed while seeing the IEC frame size.