Both mistakes damage the motor, but in different ways. If a motor marked 230/400 V is connected in delta on a 400 V supply, each winding receives about √3 times (≈1.73) its design voltage. The magnetic circuit saturates, the current exceeds the rated value even at no load and the winding overheats within a short time; if protection is not set correctly, the insulation is damaged.

In the opposite case, if a motor marked 400/690 V is connected in star on a 400 V supply, the winding voltage falls by a factor of √3. Since torque varies with the square of voltage, the motor can deliver only about one third of its rated torque. It may appear to run normally at no load; once loaded, speed drops, slip increases, current rises and the motor heats up. The same picture appears in a star-delta panel when the delta contactor never closes.

Symptoms: in the first case, rapid heating and overload trips even at no load; in the second, weak pulling under load, long starts and heating. Checking the links against the nameplate and measuring the three phase currents before the first start catches both errors. For a step-by-step check, see our motor commissioning checklist.