IEC Delta-Wye Motors

In the US, we use two windings to provide dual-voltage motors. Those motors are either connected in parallel for low voltage or in series for high voltage. Therefore, US motor voltages have a ratio of 2 to 1, such as 230/460 volts. Foreign motors, with the exception of the ones built specifically for sale in the US, aren't wound that way.

IEC motors typically use one winding that is connected in a delta for low voltage or in a wye for high voltage. In a three-phase electrical system, the ratio of a wye voltage to a delta voltage is 1.732 to 1, such as 220/380 volts. The higher voltage is ALWAYS a wye connection. Unless you are interested in square roots, I won't go into that further. It does, however, make the connections easier because there are usually only six motor leads instead of nine, like on American motors. You can have twelve leads in either version if the motor is large enough to need reduced-voltage starting.

Also, most of the time those IEC voltages are given at 50 Hz, not our 60 Hz US power systems. So, you have to increase the IEC voltage by twenty percent to get the equivalent 60 Hz rating. Let's look at an example:

Take the 220/380 volt rating we discussed above. Those are 50 Hz voltages, and they will be shown that way on the motor nameplate. If we convert them to 60 Hz equivalent voltages, we get 264/456 volts. Since 264 volts does not exist in the US, the 456 (very close to our 460) volt connection is the one we can use. And since we know that the higher voltage is a wye connection, we know how to connect this motor and use it on the proper voltage.

Now, when you see a 220 volt delta/380 volt wye voltage rating on a foreign motor, you'll know to convert those voltages to their 60 Hz ratings and connect them properly. You won't go looking for a reduced voltage starter by mistake.

Speaking of reduced voltage starters, don't mistake an IEC delta-wye winding for a wye-delta reduced voltage starter. This is a starter that temporarily connects a 460 volt, delta connected winding in a  wye configuration. When you do this, you would need almost 800 volts to run the motor. Since you are applying your typical industrial 480 volt power to this 800 volt winding, you get a reduced voltage "soft" start. Once the motor has reached its maximum speed with the wye connection, the starter quickly reconnects the windings for the proper delta connection. Now with the correct voltage on the properly connected winding, the motor will accelerate to its full nameplate RPM. Of course, today we see many variable frequency drives being used instead of two-stage reduced voltage starters. But that is the subject for another discussion.