Sparking of SC Induction Motors on Starting

[KenAlmon]

I have heard that in the oil and gas industry, there is a tendancy to specify XP motors (EEx 'd' or Class I, Div 1) even though the installation may be in a Class 1, Zone 2 (or Div 2) area.

The reason given is due to several "incidents" where upon a start given to a medium voltage (3.3 or 4 kV) motor caused an explosion. the subsequent analysis suggested that under certain conditions, these normally innocuous motors can spark.

Has anyone ever heard of this phenomenon and know why it occurs? How about any references?

Thanks

Ken

 

[busbar]

Isn't the intent of class-1 enclosures not to prevent internal ignition, {or even exclusion of flammable materials inside the enclosure} but to contain it, preventing flame propagation to ignitable materials outside the enclosure?

Does "SC" mean self-contained?

 

[electricpete]

SC usually means squirrel cage.

I have heard it is very common for motors to spark during starting for various reasons... most of them harmless (unless you are in explosive environment). But the limited ability to see through vent openings toward rotor prevents us from observing them.

The common harmless causes include:

the intense heating of the rotor due to high frequency (higher than during running) currents on the surface of the rotor. This can cause tiny particles to fly off.

Rotor bars may form intermittent contact with the slot during starting due to very high forces from starting currents. This intermittent contact may interrupt some current which flows in parelle between copper and iron during starting. Interruption of current may cause sparking. Movement of bars within slot may also help to dislodge the hot particles.

Broken bar or bar-to-end-ring connection is a more severe cause of sparking during starting which can cause further damage to the rotor over time.

 

[KenAlmon]

Thanks for your comments.

SC means (in my case) squirrel cage.

I subsequently checked in the IEEE Xplore area for followup and found many articles related to this area. Of significance is a paper by GE Canada's J.H. Dymond entitled "Sparking, electrical discharge, and heating in synchronous and induction machines: can it be controlled?"

The summary is that you can't be guaranteed that a motor will not arc when started and that some are more prone than others. The tendency in the oil and gas field is to go to XP motors to guard against this condition.

Thanks again for your interests and posts.

 

[jbartos]

Suggestion: Visit

http://www.geocities.com/Pentagon/Bunker/2206/coral/history2.html

https://hosta.atsc.eustis.army.mil/cgi-bin/atdl.dll/accp/in0545/ch2.htm

etc. for more info.
The motor can produce sparking or arcing similarly to any other electrical equipment, e.g. transformer, battery, cables, wires, etc.