Intermittent vibration on large electric motor 1

[podobing]

Hello,
Underground coal mines use large electric motors to operate their ventilation fans. The number of fans required depends on the size and depth of the coal mine. One fan, in particular is a 2000 HP, 4kV, 890 rpm motor that is started directly across the line. The motor has two vibration sensors on it, one on the drive end (DE), and one on the ODE. The fan is a 10 foot diameter axi-vane fan. Both fan bearings are monitored with three axis accelerometers mounted to the fan pillar block bearings. The fan also turns at 890 rpm. This fan operates 24/7/365. If the fan is stopped, and the ventilation in the coal mine is disrupted, the mine must be evacuated. All sensors are monitored at a remote site 24/7/365.

I have not witnessed this personally, but from what I am told, occasionally, for no apparent reason, the fan motor "growls" for 3-4 seconds. Originally, when the motor growled,the motor vibration sensors tripped off and de-energized the motor, resulting in a mine-wide evacuation. The sensitivity on the motor vibration sensors has since been decreased, and now the motor growls, but the fan continues to operate. The acclerometers on the fan bearings do not pick up anything unusual. I was told that the "growl" sounds like a large transformer being energized.

Currently, we have a power system analyzer connected to the fan motor starter. The power system consists of a 138:69 kV substation, 4 miles of 556.5 ACSR line and a 5 MVA 69:4.16 kV transformer. The line is lightly loaded with other mining loads at this time. Since we installed the power system analyzer, we have had two vibration trips and no events being recorded on the power analyzer.

Any ideas?

Regards,
Podobing

 

[waross]

A Multi-Lin has a wide selection of accessories. We need the model number to see what features you have available.
You want your delay to be shorter then the duration of the vibration and to warn and record rather than shutdown.
You may be missing events shorter than your time delay setting.

Bill
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"Why not the best?"
Jimmy Carter

 

[podobing]

Guys,
Sorry for taking so long to respond, but I have been gathering information. When I changed the power system analyzer to look for voltage imbalance (thank you for that recommendation), the voltage imbalance "spikes" corresponded with the motor vibrations. We re-located the analyzer to other points in the distribution system to see if the voltage imbalance were "visible" at other locations. We finally ended up at the utility 138 kV feed to the customer main substation. There was no imbalance on the 138 kV side, but we could detect it on the 69 kV side. We then were able to download data from a different power system analyzer at the 69 kV feed.

Long story short - it appears that the fan motor vibration is caused by energizing transformers that are located underground. These transformers range in size from 2 MVA to 8 MVA and can be thought of as "mobile load centers" for those not familiar with underground mining. As the mining cycle progresses, these load centers are moved to keep up with the mining activities. They are 12.47 kV primary and secondaries of either 4.16 kV, 1 kV or 480 volts, depending on the purpose of the transformer.

I understand that there are inrush currents and various harmonics associated with energizing a transformer, but moving these transformers has been happening since forever in the mining industry. Some are moved as frequently as once a week. Why are these motor vibrations just now showing up now at this site? Could it be a mechanical resonance issue excited by the harmonics caused by transformer energization? What are the predominant harmonics to be concerned about?

As I said, these motor vibrations occur when the transformers are energized and they disappear in 3 to 5 seconds. I set the fan motor Multi-lin current unbalance alarm down to 4% for 4 seconds, and it goes into alarm during these events.
Jraef - how much "braking" would be caused by 5% negative sequence current, and could that make a "growling" noise? People that have heard the noise said it sounds like the fan is starting up. If you have not heard a mine fan start, it is loud and takes about 15 seconds to accelerate to full speed. Growling is a good description of the noise.

Thanks to all for all of you help and suggestions.

Regards,
Dave

 

[Skogsgurra]

This sounds very much like the "sympathetic inrush current" described by H. Bronzeado. Have a look at

http://www.eng-tips.com/viewthread.cfm?qid=317135

The decay time for such an inrush current is usually a few seconds and there are high levels of even harmonics, which create the growling sound.

I do not think that it is very bad for the motors or transformers. But they may trip unexpectedly if you do not have certain (even hermonics) constraints in your ptotection.

Gunnar Englund

http://www.gke.org

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