Simple 3-wire

[JBender]

Hi,

I'm a young EE for a small systems integration company working up the learning curve of industrial power distribution. I have a solid understanding of electronics and residential wiring, but am new to the industrial end of power distribution.

When I asked if the 480 Volt, 3 phase power was 4-wire, I was told that no, the 4-wire is only used for overhead lighting and the available 480 VAC power for motors (for which I'm interested) is "simply 3-wire". This is coming from a seasoned professional. Does this mean it's an ungrounded delta? For someone who is new to this terminology, could you please help me understand what he means.

Thanks in advance,
Jason Bender
Process Logic

 

[wareagle]

A 4 wire 277/480 service is used for industrial installation. However some industrials will use the delta
480 volt service when most of the load is 3 phase. If you have a large load of single phase such as lighting, I would
suggest you take the 277/480 volt service.

 

[stevenal]

A 480V 3 wire service may be obtained from a 277/480V grounded wye transformer; or it may be obtained from an ungrounded or corner grounded delta.

 

[JBender]

So from the sounds of it, his answer of "simple 3-wire" still leaves some ambiguity as to the type of power that is available.

Like you said, stevenal, it could be ungrounded or grounded delta...this I can see. How could it be from a grounded wye? by just neglecting the common?

If it is in fact an ungrounded delta, what impact does this have on the motors. Does a motor rated for 480 VAC 3Ph. work on such a system, or does it need to be rated for this. I know that it does in fact impact the VFD's, specifically the MOV's. (says Allen Bradley) Are there any other practical considerations here?

 

[DRWeig]

JBender,

Even if we have a 4-wire Y 480/277 system, we typically only run the three phase wires to motor loads. There isn't a need for a neutral, and the neutral costs money too. Your ground is the system ground (bonded to the neutral) if there.

If it's truly a delta service, you can have one of the phases grounded or not. That's a question you'll have to ask.

To sum up, if you are connecting to a motor and you have three phases in your hand (480/3), it doesn't matter if they came from a Y (leaving out the neutral) or a delta transformer. Does that make some sense?

Tell us more about your application if you can...

Old Dave

 

[JBender]

That does make some sense, Dave, thanks.

My application is to power a dozen or so motors, most all of them being controlled by VFD's. Most of these motors are small (fractional horsepower). Where I get uneasy is not so much whether or not the 480 3Ph. comes from a Delta or Wye but whether or not it is grounded. The AB literature on VFD's says that "grounding the transformer secondary is essential to the safety of personnel and safe operation of the drive" So I'm wondering why my client wouldn't make it perfectly clear weather or not its grounded. Instead just telling me its simple 3-wire.

btw, this is my first post on this website and I'm very impressed, thanks again.

-Jason

 

[DRWeig]

Ah, Jason, that makes it more clear. The safety issue involves quite a few factors. The goal is to keep potential differences within save limits during faults and lightning intrusions.

I don't usually do ungrounded delta unless there's a reliability issue. With an ungrounded delta, the first ground fault that happens doesn't shut you down. No fault current flows. It just turns it into a grounded delta! A second fault will result in current. Usually found in the industrial environment where uptime is critical and the general public isn't around.

Some good discussion can be found at thread242-75592, especially as it progresses.

Have a look!

Best to ya,

Old Dave

 

[stevenal]

Yes, the neutral of the wye may just be grounded and not extended to the load. If the transformer is on the utility side of the service, the neutral is usually extended to the service panel per NEC. Got a multi-meter?

 

[advidana]

clients will alway say it is a simple 3 wire system . That is not alway true. I would advise you to hire a experienced electrical power engineer to look over your project.

People see electrical equipment and power an think it is simple because they do not understand how complicated these systems are. I work with a lot of electronics people that understand the PLC and VFD way more than I do, but the electrical power system is really unknown to them.

Nobody knows everything. They do thier thing and I do what I do best. It really works out fine that way. Team efford usually work out well.

 

[mc5w]

You can have up to 2,000 volts to ground on a 480 volt ungrounded system unless the ground detector uses 200 watt light bulbs.

If they only have 1 480 volt system then they are just not bringing the neutral out to the motor control centers.

 

[JBender]

I finally got in contact with my client and have been informed that I am in fact working with an ungrounded delta system. Why would somebody in his position glaze over that fact like it was no big deal! This is a serious design consideration, and all he tells me is it's "simple 3-wire" wtf. So, enough venting from me. Thanks for all your insight. It has been truly helpful.

I know the VFD's will be an issue with the ungrouded delta. Are there any other foreseeable pitfalls awaiting me? As far as hiring an experienced power engineer, that would be nice, but for now that professional is me. I do love a challenge, just not challenging people...

Jason Bender
Process Logic

 

[DRWeig]

Jason,

The next thing you should do is explain to the drive manufacturer that your system is ungrounded delta. He (or she) will be able to explain to you the potential hazards so you can make an informed decision...

Let us know how it all goes for you!!!

Old Dave

 

[mc5w]

In 2005 National Electrical Code ground detectors are MANDANTORY on ungrounded systems. In addition to the ground detector you should have some 1 watt 1 megohm resistors going phase to ground to get rid of static electricity in the event that the ground detector is broken of being maintained.

 

[Shray]

What is the NEC 2005 article that specifies you must have ground detectors on ungrounded delta power systems ?
was not aware that existed. Thanks

 

[mc5w]

Right in the first few pages of article 250. This is doing people a favor.

Ungrounded 3-wire 3-phase is NOT as simple as some people think that it is. Keepers Of Old Knowledge think that it is simple. A first ground fault can be very difficult to trace - high frequency current tracers only get rid of 90% of the guesswork when trying to find the right circuit breaker to turn off on 120/240 volts single phase.