ECN Forum Forums Technical Discussion Electrical Theory and Applications Missing phase start up

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.

Yes, the running motors generate the third phase like a phase converter.

Was there a power factor correction capacitor in the system?

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.

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.

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.

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.

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! )