The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
230 or 345 kV transmission lines?
by Vlado
Yesterday at 09:33 AM
breaker meltdown
by crselectric
Yesterday at 12:42 AM
Electrical mast flashing product
by ThomasWinfrey
09/22/16 12:14 AM
What estimating software do you recommend?
by sparky
09/21/16 07:20 PM
"Dry Run" Inspection goes awry
by HotLine1
09/20/16 07:39 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
HotLine1 17
sparky 9
gfretwell 9
sparky66wv 8
Vlado 6
Who's Online
0 registered (), 237 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#196654 - 10/19/10 09:57 PM Missing phase start up
Merlin Offline
Member

Registered: 01/27/04
Posts: 170
Loc: NW Indiana
I had a unique situation happen recently that has me puzzled. I have a location that has a 480 ungrounded delta bank that runs some water pumps,conveyors,and other equipment. When I arrived on site, I found a fuse blown on a primary phase conductor feeding this bank. However, one pump was running. Other 480 3-phase equipment on this service was also started with the primary fuse blown prior to noticing the fuse blown.

Now I understand that a motor will run on 2 phases but not start......right. But how did the other equipment start on only 2 phases? When I metered the voltage, I read 480 volts across all three phase. Does the running motor cause a "false" phase allowing other motors to start? Similar to a phase converter?

Thanks for the info.

Top
#196655 - 10/19/10 11:21 PM Re: Missing phase start up [Re: Merlin]
twh Offline
Member

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
Yes, the running motors generate the third phase like a phase converter.

Was there a power factor correction capacitor in the system?

Top
#196656 - 10/19/10 11:25 PM Re: Missing phase start up [Re: Merlin]
wewire2 Offline
Member

Registered: 11/14/02
Posts: 246
Loc: California
Sounds to me like your presumption is correct. The other motors
are acting as an idler motor and are inducing voltage on the missing
leg.

Top
#196658 - 10/20/10 12:43 AM Re: Missing phase start up [Re: wewire2]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
This was common in the old computer equipment with a lot of 3p motors. They run but they run slow and hot. If you can actually get a load on them you will be tripping breakers.
_________________________
Greg Fretwell

Top
#196659 - 10/20/10 05:31 AM Re: Missing phase start up [Re: gfretwell]
Merlin Offline
Member

Registered: 01/27/04
Posts: 170
Loc: NW Indiana
Thanks for the answers. twh - no,there is no a power factor correction capacitor in this system yet. Although, we are in the process of getting that done.

Top
#196670 - 10/20/10 06:15 PM Re: Missing phase start up [Re: Merlin]
twh Offline
Member

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
I asked about the capacitors because the systems that I saw running had them. I assumed they played a part in keeping everything going. Now I know they didn't. Thanks.

Top
#197040 - 11/05/10 09:09 PM Re: Missing phase start up [Re: twh]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
Losing a single fuse results in "Single Phasing" -- that's the term of art.

Let's say it's leg C that blew... Then the only leg pair that can provide true energy is A to B. B to C is open. A to C is open.

The rotating field of a running 3-phase machine will induce apparent power in the other windings. However, it's pretty much a phantom voltage since any attempt to really load it down causes the load to re-direct back to the sole surviving leg pair.

It's the attempt by the induction motor to get all of its needs via one leg-pair instead of three that cooks the windings/motor feeders.

So you NEVER want single-phasing to continue. It's brutal.

Some years back a janitor noticed something weird about the system in the middle of the night. He left a note to have everything checked. Two out of three lights were acting really weird. By 9:AM all of the critical freezer motors were ruined by single-phasing just 8 hours. All of the ice-cream was lost. The facility took months to fix. Medium voltage compressors are NOT off the shelf items and rebuilding the smoked ones was not acceptable to the insurance company. All for want of a single fuse --- and a lack of a single phasing alarm. ( You can't expect a janitor to recognize single-phasing. I've even seen thirty-year men puzzled by it! )
_________________________
Tesla

Top



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals