WYE motors - why ??

Two part question
1. I have noticed that many motors in USA (if not most ??) are wye connected. Is this simply because the most common (current preferance) power distribution is wye ie 4 wire 480/277 or 208/120?? yielding better uhhhh compatiblity for lack of better term?
2. Dick DV has many times said to only use wye connected motors on VFD output. I have looked thru this site and others seeking more understanding of this and would like some elaboration and better understanding why this is recommended if not demanded by VFD manufacturers.

Is it simply because most industrial VFDs are designed for wye distribution with a grounded neutral (and ground of course) AND by connecting VFD input AC (with just a ground conductor) the VFD internals are all referanced to ground including output
AND with wye motor (properly grounded) the VFD sensing of motor is then referanced to ground - ASSUMING VFD ground fault protection is NOT overriden to OFF (which would only reinforce good practice in grounding all motors)?

Uh Oh one more question
What does US Navy (and others??) do with their VFDs fed on ungrounded distribution?? Are these custom designed for this system only?? We had 5 VFDs (six step 1962 design??) and I do not recall isolation transformers - heck do not even recall if alternators were wye or delta for than matter.
Dan Bentler

All the motors I deal with are Delta connected. We only take the 3 phase leads out to the motor. If they were Wye, then we would have to take the neutral out with them.

They are fed from a wye distribution system, but that is only to get a reference from the phase to ground. Without that, you can only rely on capacitive coupling to get a common refenence point - ie gorund. Without the wye, you would have to go with a corner grounded delta to get that same grounded reference.

Even on VFDs, the output is only 3 phases, not really a wye connection. It is reference to ground, but the VFD does derive a neutral so I don't think you can consider it a Wye system.

I have worked on both ungrounded and grounded 480 (and higher) systems. I like the grounded reference so you can get a good measurement of the voltage. The corner ground system offers all kinds of challenges, the most common being that most electricians are not familiar with it.

As for the VFDs on ungrounded systems, there is usually a note that you need to disconnect the MOVs (at least in the ones I deal with).

If I am off base on this, I'm sure someone will correct me and I will learn something.

Dan, you've misunderstood me. I have repeated said that using wye-derived AC power is preferable to feed inverters but, the motors may be either configuration internally.

As a general rule, NEMA motors 25hp 460V and smaller are internally connected wye and those larger are connected delta.

That is because the coils in a wye connected motor are slightly more compact (due to the 277V rating) even tho the wire in the coil sees full motor lead current (thus somewhat more heat). In a delta connected motor, the windings have more turns and are physically larger but generate less heat because they only see half of the motor lead current.

In the smaller motors, the compact coils are easier to fit into the frames while the excess heat is manageable. In the larger motors there is room for the larger coils and the less heat the better with the higher kw ratings.

In Europe and elsewhere overseas, the 220/380V availability is in a ratio of 1.73 whereas the North American availability of 230/460 is in a ratio of 2/1. That makes us dual wind our motors for the two voltages whereas the rest of the world can make one motor and simply wye or delta connect the same motor to get the two voltages.

It is on the AC drive input where I hate to see floating or unbalanced voltages to ground.

DickDV said:

Dan, you've misunderstood me. I have repeated said that using wye-derived AC power is preferable to feed inverters but, the motors may be either configuration internally.

Click to expand...

Well it is a good thing I asked. No wonder why I was unable to find any answers elsewhere.

Dan Bentler

Reconnect wye to delta

Attached excel spreadsheet shows my lab type reconnection of a delta to wye motor. Took a delta motor designed for 480 and followed Europe reconnect for wye and dropped wye phase to phase voltage to 277 (not realistic for USA I know).

all calculations ignore power factor.

Delta calcs were no surprise

Was a little surprised at the much higher winding current in wye (line to starpoint) at 77 amp compared to the winding A to B amps at 26.

Also calcd a regular wye motor on 480 line.
Interesting comparison.

For various reasons the motors I mostly replace are 10 HP and less which must be why I mostly see wye motors. The big ones dont seem to burn out that often - but only because at my age I am better off working on smaller machinery and not the great big massive stuff as I did in my vigorous days.

Dan Bentler

Ok i am not sure about the reason of this, but i noticed that if the VFD is single phase input (220 VAC), the motor should be Delta connected and the Wye connection is avoided...