The 230 V and 400 V currents on a motor nameplate belong to two different connections of the same motor: at 230 V the windings are connected in delta (Δ), at 400 V in star (Y). The motor delivers the same power in both cases; because the voltage is √3 (about 1.73) times lower, the line current at 230 V is also about 1.73 times higher.

Why is that?

Each winding must see the same voltage. In star connection the 400 V line voltage falls across the winding as 400 / √3 ≈ 230 V; in delta connection the 230 V line voltage is applied directly to the winding. The current through the winding is the same in both cases; only the current drawn from the line changes. The power formula P = √3 × U × I × cos φ × η shows this too: for the same P, when U falls, I rises.

Worked example

The 2.2 kW 1,500 rpm 100L1-4 motor in the HEM Motor catalogue is rated 230/400 V and draws 4.52 A in star at 400 V. Assumption: the same motor is delta-connected on a 230 V three-phase supply. The line current is approximately 4.52 × 1.73 ≈ 7.8 A. This is an approximate value calculated from the ratio; the exact figure is on the motor nameplate.

4 kW and above: 400/690 V

In the HEM Motor catalogue, models of 4 kW and above are rated 400/690 V. These motors are connected in delta on a 400 V supply; the 690 V star value is only for networks at that voltage. Connecting such a motor in star at 400 V gives the winding too low a voltage; the motor lacks power and heats up under load. For details see the question on whether a 380 V motor is connected in star or delta.

When setting the thermal relay, always use the current line for the connection actually in use. Models for your supply voltage are listed on the three-phase electric motors page; send your nameplate on WhatsApp and we will determine the correct connection and the equivalent model together.