Electrical problem

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mikegoetz

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Northern Kentucky
We are experiencing alot of vibration and humming on our main switchgear. I installed a new 4,000 amp Siemens switchgear with a 2500 amp main breaker. The switchgear contains (5) 600 amp breakers that feed (2) VFD's and (3) soft starters for compressors. The compressors are 250 and 300 horsepower motors @ 3 phase, 480 volts. The two VFD's are in bypass mode on the controllers, so the motors are starting up @ 60 hertz off the breakers. The breakers are hotter than normal and are reading a thermal temperature of a 115 degree's. The refrigeration company said when the compressors run off the VFD's at low frequency startup they are experiencing to much vibration on the compressor screws, so for right now they are over the line startup off the breakers. The voltage on the switchgear is fluctuating between 462 to 471 volts a.c. and the amperage on the meter is reading 1200 to 1300 amps. on each phase. The compressors are alternating startup off the refrigeration system. When one of the compressors starts up the gear surges and vibrates heavily. Once the compressors are fully started the gear vibrates and humms like a transformer. When I turn the compressors off, the vibrating goes away. There are other breakers in the switchgear that feed evaporator panels, lights, and motors, but when they are all on and the compressors are turned off we aren't experiencing any vibration. The vibration in the switchgear is in the first (3) sections of gear and not in the last (4) sections were the breakers are that feed the equipment. The 3rd section of gear is where the bussing splits with Kirk-key breakers. We are scheduling a shutdown to re-torque all connections in the gear and starters. Does anyone know what else might be the problem causing the switchgear to do this?
 
I would expect the gear to hum on across the line start with this size of motor. The coils for the starters themselves will make enough noise after start but that doesn't explain the noise in the first three sections. Check the bus bracing while you are in there. Maybel a loose insulator or two?
 
when we use to test 6 & 12 hundred a frame breakers , the cts for trip settings
you could get to hum pretty good at 50% or so of load....
also check covers,handles,maybe the rack in screw. is there steel plates on the floor or grating?
 
emf?

emf?

Sounds like EMF from phase cables that fan out when they get to the gear, and don't have the opportunity to cancel out each other's EMF.
It might be something you have to live with, and can minimize with insulatory damping material between panel surfaces tightened to prevent vibrations. Or you might have opportunity to route all three phases of a given load together until just before their point of termination inside the gear - if it looks like there is significant separation between the phase wires.
Or, you might be able to install a mu metal screen between phases to mimimize the EMF.

Of course this explanation applies to the gear, and not the vibrating compressor motor. That might be improved with the addition of an appropriately sized line reactor.

John M
 
mikegoetz said:
We are experiencing alot of vibration and humming on our main switchgear. I installed a new 4,000 amp Siemens switchgear with a 2500 amp main breaker. The switchgear contains (5) 600 amp breakers that feed (2) VFD's and (3) soft starters for compressors. The compressors are 250 and 300 horsepower motors @ 3 phase, 480 volts. The two VFD's are in bypass mode on the controllers, so the motors are starting up @ 60 hertz off the breakers. The breakers are hotter than normal and are reading a thermal temperature of a 115 degree's. The refrigeration company said when the compressors run off the VFD's at low frequency startup they are experiencing to much vibration on the compressor screws, so for right now they are over the line startup off the breakers. The voltage on the switchgear is fluctuating between 462 to 471 volts a.c. and the amperage on the meter is reading 1200 to 1300 amps. on each phase. The compressors are alternating startup off the refrigeration system. When one of the compressors starts up the gear surges and vibrates heavily. Once the compressors are fully started the gear vibrates and humms like a transformer. When I turn the compressors off, the vibrating goes away. There are other breakers in the switchgear that feed evaporator panels, lights, and motors, but when they are all on and the compressors are turned off we aren't experiencing any vibration. The vibration in the switchgear is in the first (3) sections of gear and not in the last (4) sections were the breakers are that feed the equipment. The 3rd section of gear is where the bussing splits with Kirk-key breakers. We are scheduling a shutdown to re-torque all connections in the gear and starters. Does anyone know what else might be the problem causing the switchgear to do this?
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Was this switchgear tested after installation?
 
Is it possible that there's ferrous metal running somewhere between the phases or neutral? I've seen guys install pipe or strut in the crown boxes and tops and back of switchgear to use for cable supports.
 
What percentage of load?

Number one issue is a byproduct of load and how the covers fit.

This is assuming you have ruled out any electrical issues.

Perform an IR.

Try to isolate where the sound is coming from.

Loosen and/or tighten any covers press on components.

Look at the starters is it the coils?
 
There are some pretty good ideas here posted above.
To add to the list;
Was the bus that is bolted together from section to section properly tightened (torqued)? Sometimes this is missed during installation.

I am curious about the separation of the conductors from one another.
 
humming in SWG

humming in SWG

Simple question. Is the cable from the VFD to the compressor a shielded cable? How far away from the Compressor is the VFD?
 
I'd be looking for something loose. Could be anything from a wire moving around to a loose bolt that holds a bus bar in place.

Could be a loose piece of conduit. I once heard a horrible noise coming from an MCC that was eventually traced to a piece of conduit that was run into the top of the MCC through a drilled hole with no hub, not even a double locknut and it was vibrating against the sheet metal of the MCC.
 
line and load conductors for motors

line and load conductors for motors

We have a junction box with line side conductors feeding through to a VFD and the load side conductors come to the same j-box and feed through to a 250 hp motor. Do we need seperation in the j-box by some type of barrier? Line and Load conductors are installed in different conduit systems, but are installed in the same j-box. Does that cause harmonic distortion? Need to know what to do, because the compressor motor vibrates heavily and don't know if it is an electrical problem or an unbalanced coupler on the motor or bad bearings.
 
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