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.


Single Phasing Three Phase Motors

When a three phase motor is "single phased", it is a power system problem, not a motor problem. A three phase motor needs three EQUAL phases in order to operate properly. When the symmetry of the motor is interrupted by the loss of a phase, the motor will die quickly unless the controls have single phase protection. Many heater type overload relays do not have this.

Single phasing occurs as a result of several possibilities. A loose wire, a bad connection, bad starter contacts, overload relay problems, a bad breaker, a blown fuse, and other things can cause this destructive condition. Obvious signs are a louder than normal humming from the motor and/or a shaft that vibrates rather than rotating.

Testing for this possibility needs to be done quickly since motors are not happy with this condition at all. The obvious test is to look at the current in each phase. This is where multiple meters will help so you can see all three phases at once. You can also look at the voltage, again with multiple meters if possible. I look at the phase to ground readings first. The phase to ground voltage will equal the phase to phase voltage divided by 1.7; thus 480 volts phase to phase will be 277 volts phase to ground. The advantage of taking phase to ground measurements is that each reading is independent of whatever is happening in the other phases. However, you can read phase to phase if you want. You would see an unbalance there too. The phase to ground reading will show you the bad phase, though; this will make troubleshooting easier.

These tests need to be made as close to the motor as possible, preferably in the motor's connection box while the motor is driving the load. If the motor is not connected, or you take your readings at the starter or breaker with the motor off, you can get fooled. A bad set of contacts in a contactor or breaker can just barely touch and still tell you that you have good voltage. Ask those same contacts to deliver enough current to run a loaded motor, and the voltage will take a dive.

You could continue to test at various stages of the power system upstream of the motor, but that keeps subjecting the motor to the stress of running in the single phase condition. Otherwise, make sure the circuit is off and locked out, and then start taking things apart.

The first place to look is at those suspect contactor contacts. But, Bo Diddley said "you can't judge a book by looking at the cover." Contacts can be like me - real ugly but still functional. Contacts that are gone don't work very well, though. Also look at the connections in and out of the contactor. Loose or burned wires or terminals are probably the second most frequent offenders.

If the contactor looks good, take continuity readings from the line to the load side of each phase of the overload relay. It should look like a short circuit. A word about overload relays here. With today's motors being smaller than their U-Frame predcessors, you need a fast overload relay with single phase protection. An IEC Class 10 thermal overload relay works very well. You can get more expensive, solid state models, but the Class 10 thermal relays work well for all but the most sensitive applications. Be careful with replaceable element versions, though. They are usually Class 30 (slower) and don't have single phase protection.

If everything is good at the starter, check wires, connections, and devices ahead of the starter until the problem is found and corrected. Once you have three good phases again, you should see a voltage balance of within two or three percent. Your motor will be happier, healthier, and have a shiny coat.