In general, three phase motor windings are connected either in a wye or a delta. I say "in general" because some motors are not pre-connected either way. We'll discuss that in a minute. The primary reason for connecting in wye or delta is basically for manufacturing convenience. It DOES NOT have anything to do with the way the upstream transformer is connected. So, all we can do is connect the motor for the correct voltage.
Most motor nameplates have connection data for low or high voltage. If not, there are many charts and publications out there that will show you those connections. One great source is a small handbook put out by our trade association, EASA, that is literally pocket sized and contains a wealth of information. It is called the "Electrical Engineering Pocket Handbook." But don't worry, there's a lot more electrical than engineering. These should be available from your local motor rewind shop. Another common source is George Hart's "Ugly's" book. This has a lot more general electrical information than the EASA book, so it is larger. It sits nicely in a mechanic's toolbox, though. Also, most manufacturer's catalogs have connections in them too. Because of that, this article will not cover which wires to connect for every possible winding you may see. So, let's look at some of the things you will see when you take the cover off the connection box.
In the US, it is not uncommon to see motors up to around 25HP wye connected, and delta connected above that. It is common to find nine leads which locks your connection into either a wye or a delta right out of the box. The thing to remember is that, for either voltage, you always connect the incoming lines to motor leads one, two, and three. The problem is what to do with the other leads.
A low voltage connection is a parallel connection, while a high voltage connection is series. Those of us who like to fish have been doing this with trolling motor batteries forever. Paralleling two twelve volt batteries still gives us twelve volts but with lots of current. Connecting those batteries in series gives us twenty-four volts with less current. But that's OK, because a twenty-four volt motor draws half the current of a twelve volt motor at any given speed.
Many IEC motors will use a delta connection for all ratings. It is also common to see IEC motors with twelve leads, which does not pre-configure the windings into a wye or delta connection. Because motors are made all over the world for shipment all over the world, it is becoming more common to see this on some US motors also. Motors connected like this can be started across-the-line, part-winding, or wye-delta, depending on the final run connection. This allows a "soft-start" of the motor, which becomes more common in larger horsepower motors. We'll cover "soft-start" connections in another article.
Once you are familiar with the numbering system for nine or twelve lead motors, it's much easier to figure out the correct lead numbers if a couple leads have lost their tags. Also remember the jumpers which connect the motor for the correct voltage must be connected before doing any motor testing.