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chad44 (Electrical) (OP)
25 Aug 03 13:30
The elevators in a 50 year old building were recnetly upgraded by Otis elevator to new  6-pulse scr drive units. The building is 18 stories.  Whenever a new elevator is running, a singing or humming sound can be heard.  We traced the source to the elevator feeder conduit that runs from the basement to the elevator penthouse, up through the freight elevator shaft.  The noise seems to originate from the conduit between the 3rd and fourth floor.  There are six passenger elevators on this feeder.  Three have been converted to the new drives.  A little experimentation confirmed that the noise only accurs during operation of one of the new drives.  Also, whenever one of these new elevators starts up, there is a banging noise coming from the same conduit, between the basement and the first floor.  It sounds to me like is the conductors banging around inside the conduit.  The elevator company doesn't even acknowledge that there is a problem.  Any thoughts??
advidana (Electrical)
25 Aug 03 14:06
1. If one of the phases got mix into a separate conduit then the magentic force will cause the conduits to bang when engergized.

2. check for harmonics for the humm.
BJC (Electrical)
25 Aug 03 14:46
Was the cable installed with proper intermediate supports?  The normal magnet forces may make the cables viberate like  giutar strings if they are under tension. The forces produced by normal operating current may be just enough to "strum" the strings.
If you don't have supports then it is not the elevator companies problem. The NEC section 300.19 gives the minimum vertical supports required.  Note that the operative word is minimum.
Helpful Member!  busbar (Electrical)
25 Aug 03 15:38

Conductor-in-raceway noise during motor starting has been around for a century, but characteristics of the raceway and cable have to be "just right."  Current harmonics can increaser the 'pitch' {and likely incidence} of rattling.  The process of cable-insulation deterioration will likely be accelerated by the new conditions, given the usual starting duty of the subject motor(s).

The elevator-service firm may be playing "dummy up" to the complaint.  A solution MAY be to replace ferrous raceways with non-ferrous material.
  
Helpful Member!  DanDel (Electrical)
25 Aug 03 21:07
As already mentioned, check for current harmonics during an elevator run which are probably causing the humming noise(5th and 7th, to be specific, if you are using a 6-pulse drive).

The conductors of each phase inside one conduit will tend to push away from each other upon current flow. This is probably causing the banging as the conductors hit the pipe from inside during a motor start.
busbar (Electrical)
25 Aug 03 22:18

Given adequate raceway size, replacing the single conductors with jacketed 3- or 4-conductor tray cable would limit conductor movement and may also reduce reactance in the circuit.
  

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