Because a star-delta arrangement holds the motor in star only during starting and runs it in delta for the entire operating time. When delta-connected to a 400 V network, every winding receives the whole 400 V; that is why the motor nameplate must state 400 V for delta, in other words it must be rated 400/690 V. With a 230/400 V nameplate, each winding would get more voltage in this arrangement than it was designed for.

DRG-200L2-2 37 kW 3000 rpm cast iron frame DRG three-phase motor – star-delta starter motor voltage

What happens with the wrong rating?

On a motor marked 230/400 V, the winding is built for 230 V. If such a motor is connected to a star-delta panel on a 400 V line, nothing may seem wrong during the starting step; once the changeover to delta takes place, however, the windings are subjected to overvoltage. The iron heads into saturation, current climbs rapidly and the motor soon overheats. Should the protection does not act in time, the winding is damaged; this kind of damage is not covered by the warranty.

And the reverse situation?

When a motor marked 400/690 V is to be started direct-on-line from a panel without a star-delta arrangement, it is delta-connected at 400 V and runs without trouble. So this rating suits both arrangements; the restrictive one is the motor with the low-voltage rating.

Check before replacement

  1. Read the voltage stated for delta on the old motor's nameplate.
  2. Have someone confirm whether the panel has a three-contactor star-delta arrangement.
  3. Check the power group of the new motor: in the DRG range, everything from 4 kW upwards carries 400/690 V, while models up to 3 kW carry 230/400 V.
  4. If a star-delta panel needs a motor no larger than 3 kW, state this before ordering; if necessary, converting the panel to direct starting can be considered.

Changes on the panel side and changeover settings depend on the motor nameplate, the manufacturer's documentation and the judgement of qualified electrical personnel. For a conceptual explanation of the method, see the star-delta starting page, and for models rated 400/690 V, the three-phase induction motor family.