Loud shaking/buzzing motor starting

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MD84

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Stow, Ohio, USA
I have a 125 HP 3 phase motor 480v service. On startup the motor pulls about 800A on each line per my fluke inrush current measurement. At the panel and in the conduit one can hear the conductors/OCPD shaking and buzzing during startup. The sound lasts for a fraction of a second.

How normal is this? Should the magnetic fields cancel out and mitigate this?

One theory is that multiple circuits are ran in a common 4" conduit for about 100' of the run. I did read about 6A of unbalanced current in the sum of these conductors.

Thoughts?
 
MD84 said:
I have a 125 HP 3 phase motor 480v service. On startup the motor pulls about 800A on each line per my fluke inrush current measurement. At the panel and in the conduit one can hear the conductors/OCPD shaking and buzzing during startup. The sound lasts for a fraction of a second.

How normal is this? Should the magnetic fields cancel out and mitigate this?

One theory is that multiple circuits are ran in a common 4" conduit for about 100' of the run. I did read about 6A of unbalanced current in the sum of these conductors.

Thoughts?
Click to expand...

Even though the net current may be small, the rattling is coming from the magnetic forces the different conductors are exerting on each other within the raceway.
It happens when the current is high and the wires are not individually restrained.
If the current carrying wires were twisted as they were pulled the forces on the wires would be at a maximum (because of the reduced average distance) but the wires would not be able to move as far separately under those forces.
 
MD84 said:
I have a 125 HP 3 phase motor 480v service. On startup the motor pulls about 800A on each line per my fluke inrush current measurement. At the panel and in the conduit one can hear the conductors/OCPD shaking and buzzing during startup. The sound lasts for a fraction of a second.

How normal is this? Should the magnetic fields cancel out and mitigate this?

One theory is that multiple circuits are ran in a common 4" conduit for about 100' of the run. I did read about 6A of unbalanced current in the sum of these conductors.

Thoughts?
Click to expand...
6A of current imbalance on a 125HP motor is less than 4%, should be fine. But that's not the cause of the sound.

Although the phase magnetics will cancel out the effects of CURRENT on each other, it's still magnetic force and the cables will rattle in the conduits because they push away from each other when the current is really high. If you don't like it, you would have had to "triplex" the cables; twist them together before pulling them, however that makes it a lot harder to pull. Not much you can do about it after the fact though. If you hear a sound in the BREAKER, that might be concerning as that could result in the conductors pulling out of the lugs over time. I'd make sure that sound was not just coming into the compartment via the conduit and if it is making noise at the breaker, you need to strap the conductors better.
 
MD84 said:
I have a 125 HP 3 phase motor 480v service. On startup the motor pulls about 800A on each line per my fluke inrush current measurement. At the panel and in the conduit one can hear the conductors/OCPD shaking and buzzing during startup. The sound lasts for a fraction of a second.

How normal is this? Should the magnetic fields cancel out and mitigate this?

One theory is that multiple circuits are ran in a common 4" conduit for about 100' of the run. I did read about 6A of unbalanced current in the sum of these conductors.

Thoughts?
Click to expand...
Did you measure the 6A at the time of inrush or is it a steady state value?
 
ATSman said:
Did you measure the 6A at the time of inrush or is it a steady state value?
Click to expand...

To clarify, the motor has a measured 0A imbalance. There are some other circuits sharing a part of the raceway that I measured anywhere from 2A to 6A imbalance.

Thanks for the input. I understand that although phase currents should cancel, the magnetic forces do not. Since the cables were not triplexed the shaking is normal.

My concern is that over time the conductors may wear through the insulation from the heavy starting vibrations. This motor gets started and stopped frequently. I will look into some mitigation and or monitoring.
 
MD84 said:
To clarify, the motor has a measured 0A imbalance. There are some other circuits sharing a part of the raceway that I measured anywhere from 2A to 6A imbalance.

Thanks for the input. I understand that although phase currents should cancel, the magnetic forces do not. Since the cables were not triplexed the shaking is normal.

My concern is that over time the conductors may wear through the insulation from the heavy starting vibrations. This motor gets started and stopped frequently. I will look into some mitigation and or monitoring.
Click to expand...
You could put a soft starter on that motor instead of starting it across the line.

But did you use THHN cable? If so it will take years for that little bit of movement to wear through the nylon jacket AND the insulation against the walls of RGC or IMC, even longer if you used EMT, and maybe never if you used PVC.
 
Normal.
Own house has 3 #6 THHN in 1" EMT to 4T heat pump, pulls 150A at startup on cold days, have heard the 'thump' at start for over 20 years, wife says it just 'reassures' her the heat is coming on!.
 
Jraef said:
You could put a soft starter on that motor instead of starting it across the line.

But did you use THHN cable? If so it will take years for that little bit of movement to wear through the nylon jacket AND the insulation against the walls of RGC or IMC, even longer if you used EMT, and maybe never if you used PVC.
Click to expand...

Yes THHN in EMT.

Thanks for the help in this.
 
Magnaflux machine

Magnaflux machine

I used to work on a machine called a "magnaflux" for welding inspections. It would pass an enormous amount of current for a short time ( 6000amps for 100ms) I think. It made a loud hum, and would make the cables dance when pulsed. It used something like 20 - 750MCM fine strand cables in parallel, and they would move about 2 feet.
 
MD84 said:
Thanks for the input. I understand that although phase currents should cancel, the magnetic forces do not. Since the cables were not triplexed the shaking is normal.

My concern is that over time the conductors may wear through the insulation from the heavy starting vibrations. This motor gets started and stopped frequently. I will look into some mitigation and or monitoring.
Click to expand...

I used to work at a 14 or 15 story building all the time, all 6 or 8 elevator feeders went up through the core electric room. Every one of them vibrated every elevator start.

The building was at least 25 years old at that point.
 
Jraef said:
This is a VERY exaggerated version of this issue, but I like to share it because I've had people not understand just how strong the magnetic forces are.


Click to expand...


Not many electricians do know about magnetic repulsion under heavy load or short circuit.

I got involved with adding extra load to a very old open switchboard, the busbars warped enough to touch. The aftermath was not a pretty sight.
 
Once upon a time we mechanically broke a 250 MCM cable due to a 4 megamp current pulse.
(Nuclear EMP testing event)
 
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