Demagnetizing Unit

[racobb]

I have been getting info from a customer as follows. They have a unit that is supposed to draw 17 to 18 amps at single phase 480 volts. The facility is served 4 wire 480/277 Y 2500 amp service. Service voltage is about 490 and 480+ at the unit. The current measurement on each of the lines connected to the unit are about 54 and 47 amps consistently. That I do not understand since it is 480 single phase, no neutral. I had them shut down everything in the plant except for the Demag unit and they tell me that the current climbed to about 64 and 56 amps. Don't know how much the voltage climbed, but I am sure it probably did to an extent.

The unit was installed in May 09 and I am told that it worked just fine until about October when it started blowing 50 amp fuses that were installed by the Mfg for this 17 to 18 amp load. Appears that no one ever checked the amps before it began blowing the fuses.

Since October, the unit has been sent back to the factory and checks out ok. The customer also has taken it to another plant and it draws 10 and 23 amps on a single phase 480 connection? They also added a temporary isolation transformer and the amps dropped about 6 to 8 on each leg.

I am told that the amp measurements were taken with a clip on ammeter, but I do not know what type.

We have installed a recorder on the service and will be talking with the Mfg tomorrow.

Am waiting on drawings, but I am told it has a 70 kVAR capacitor and a "demag" coil in parallel internally. If that is correct, I am having some heartburn with the 17 to 18 amps.

Any of you guys have experience with the demag equipment?

Thanks

Alan

 

[Skogsgurra]

Not so sure what you mean by demag unit. There is a company named Demag that produces lots of different things and then there's a demagnetization unit. I assume the latter.

You say it is a single phase unit and still connects to 480 V, which usually means two phases. OK, that dependes on how you define 'single phase'. Assuming again ('scuse me Jeff) that it is a device with two terminals that are connected to two phases and run from 480+ V it seems to me that he coil part of it draws more current than it is expected to and that the 70 kvar capacitor doesn't compensate as it should.

Works in other places, no problem when manufacturer checked it. Weird, huh? There could be an explanation to all this, some units that are built with an iron core running close to saturation will increase current drastically when voltage is increased. Simply because when the 'knee' in the iron is reached, the current increases 'überproportionally'

If your grid is at a higher voltage than usual, this could easily happen. If it is 490, and 480+ at the device's terminals, I think that you should do something about the voltage. Rated voltage is usually 460 V and I think you should keep it that way.

One fact that supports this thinking is that current climbed to circa 60 A when all other loads were shut down. That increases voltage and takes unit into deeper saturation. Running with an insulation transformer probably reduced voltage somewhat and that is why current dropped when doing so.

Was voltage increased on this site in May 2009? New transformer, perhaps? Tap changed? PFC installed?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

To test Gunnar's saturation hypothesis - look for harmonic content.

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[waross]

As Gunnar points out, your voltage is a little high. But the unequal currents are a concern also. An internal ground near the electrical neutral of the device would easily raise the voltage on each side of the ground into saturation levels. Look for 40 or 50 amps returning on a ground or neutral conductor. A quick test without all connections made up may have missed this.
If this equipment has provisions for connection at either 240 V or 460 V, there is probably provision for connection of two sections either in series or parallel. If a well meaning electrician has connected a neutral conductor to the center point that may explain the high currents. All connections would have been removed when the unit was sent out for testing and if it was connected properly (no neutral connection) no problem would be seen when testing.
An erroneous connection to neutral would explain both the high currents and the unequal currents.

Bill
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"Why not the best?"
Jimmy Carter

 

[racobb]

Thanks guys, I have asked most of these. I had them shut it down and disconnect the ground and check and there was no change. No ground current when connected. Since then it has occurred to me that it is probably connected to metal conduit all the way as well.

It has no neutral connection.

Gunnar...it is a demagnetization unit for a bearing plant. They tell me the nameplate is marked 480 volts.

I am also suspecting it could be the voltage level since the current increased when the plant shut down.

I am most confused by the difference in amps on the 2 legs.

Alan

 

[waross]

Metering errors are not uncommon when using a clamp CT type meter in close quarters. There may be interference from the fields of nearby conductors and the conductors may not be in the same position in the jaws.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racobb]

We are recording to check the harmonics today. They did install an isolation tx but no help. I have considered the interaction of the strong magnetic field with the clip on meter and plan to have them check at the disconnect which is about 40 feet away.

I think the field just might explain the differences in the amps on the 2 legs, but not the total amps as they are melting out the 50 amp fuses on occasion.

Please keep the ideas coming!
Thanks

Alan

 

[racobb]

And Thanks Gunnar. So far as I can tell for now, the unit is rated 480 volts and +/- 10%.

Alan