4 kV Motor Shoots Fire and Sparks on Startup but Still Runs 3

[joepower]

On several occassions it was noticed by operators that a 1750 HP compressor motor shot "sparks" or "fire" out the back when it started. The motor is open frame. After starting the motor would continue to run, at times for over a week before being shut down based on system demand. As a result of the concerns of the operators the motor was sent out to a repair shop. No significant problems were found besides that the motor was extremely contaminated with coal dust, oil, etc, and was difficult to clean. I have heard that motors shooting sparks on startup is a phenomenon which has occurred previosuly with large motors, but I have no documentation to quote. Any ideas?

 

[Marke]

Hello joepower

This can be the result of a cracked or broken rotor bar. Check the run current. If it swings at a low frequency that varies with load, look to the rotor.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

Not a simple question. There are several possible causes:
Here is an excerpt from
EASA Motor Root Cause Analsis:

"ROTOR SPARKING
There are several potential causes of rotor sparking on
fabricated rotors. Some are of a nondestructive nature, and
some can lead to rotor failure. (See Figure 5.)
Nondestructive sparking can and probably does occur
during normal motor operation. Such sparking is seldom
observed due to its low intensity and/or the motor enclosure
prohibits its observance. Normal operation can be defined
as any condition that could subject the motor to voltage dips,
load fluctuation, switching disturbances, etc. Sparking usu-ally
is not observed while running at full load. The centrifugal
force at full-load speed is usually greater than the electro-magnetic
forces acting on the bar, due to rated load current,
and tends to displace and hold the bar radially in the slot.
Furthermore, the frequency within the rotor circuit is very
low (equal to the slip frequency). This low frequency corre-

sponds to a low impedance of the rotor cage circuit, essen-tially
confining all rotor current to the cage itself. Therefore,
while possible, sparking is not normally observed during
operation at full load and speed.
During across-the-line starting, however, the current in
the rotor cage can be 5 to 7 times normal. This high current
combined with the higher cage impedance, due to the
frequency of the rotor current initially decaying from line
frequency at standstill, will cause a voltage drop along the
length of the bar in excess of 6 times the normal running
value. This voltage tends to send current through the
laminations. In effect, during start-up, there are actually two
parallel circuits—one through the rotor bar, and the other
through the laminations.
The magnetic forces created by the high current flow
during start-up cause the rotor bars to vibrate at a decaying
frequency, starting at line frequency, which produces a
force at twice line frequency. This tangential vibration within
the confines of the rotor slot causes intermittent interrup-tions
of the current flow between the bars and various
portions of the laminations with resultant visible arcing.
The rotor design and manufacturing processes include
measures intended to reduce sparking. However, material
and manufacturing tolerances, together with the effects of
differential thermal expansion and thermal cycling, pre-clude
any motor from “sparkless” operation. Even identical
or duplicate motors can and will exhibit differing levels of
spark intensity, since all component parts have tolerances
and are thermally cycled during operation.
The sparks observed in the air gap are actually very small
particles of bar and/or core iron, heated to incandescence
by current passing through the iron-bar boundary. Initial
punching burrs and/or particles of bar material removed
during installation can generally be expected to decrease
after several starts. However, particles generated by inter-mittent
sparking due to bar motion will not decrease during
the life of the motor.
The brief period of intensified sparking that can occur
during starting is not detrimental to motor life. Motors with
more than 20 years of operation have shown only slight
etching of the rotor bars at areas of contact with the core iron
when disassembled.
Destructive sparking can occur under several circum-stances,
the most common being a broken bar or a defective
bar-to-end ring connection.
Bars usually break near where the bar connects to the
end ring. Breakage is preceded by radial cracks starting
either in the top or bottom of the bar. While sparking caused
by fatigue failure of the rotor bar is usually greater in
intensity than that previously mentioned, it is still difficult to
visually detect since the majority of motor enclosures pre-vent
“line of sight” visual observation of the air gap.
Common methods of determining whether sparking is
caused by broken bars or end ring connections are:
• Visual inspection of the rotor assembly.
• Tapping the bars with a small hammer. Broken bars
have a dull sound, like a cracked bell. For loose bars,
tap one end of bar while feeling the opposite end for
movement.
• Current pulsation when the motor is under load.
• Single-phase rotational test.
• Growler test.
• Motor current signature analysis.
• Observed noise (rattling sound) during starting cycle.
• Audible cyclical noise.
Proper design, manufacture and operation of the motor
can prevent advanced levels of rotor sparking."

 

[jbartos]

Suggestion: EASA motor root cause analysis does not address the motor power supply quality. This can also be cause of motor sparking due to damages in the rotor and or stator caused by various shortcomings in the motor power supply.

 

[DougMSOE]

I have seen this all too often, even in new motors.
Broken/cracked rotor bars and or shorting rings.
This can be tested for by single phasing the stator with a low voltage 120 or 240 AC and watching the current of the single phase current. If the current wiggles when the rotor is rotated there is a cage circuit problem (Broken/cracked rotor bars and or shorting rings). This test will get about 70% of the failing motors.
Also try to capture the motor starting current during startup using the protective relaying CT secondary current to a Hall effect clamp on amp meter to an ocilloscope. If in the first 10 cycles you see wiggle in the single phase current wave formm, you have cage problems. this test works everytime!

 

[jraef]

electricpete, as usual you are a veritable fountain of usefull information! I know you didn't write it but thanks for sharing it.



Quando Omni Flunkus Moritati

 

[Mendit]

You did not mention the speed of your motor ?
As electricpete states so nicely, current in the rotor is very high at start as much as five or seven times. However, depending on the rotor design, particularly slow speed rotors used on recip drive compressors, these often have rotor bars made of brass or copper nickle alloys. These rotors are even more prone to sparking at start up. I have even observed this on motors installed in Class 1 Div II locations and believe me arcs and sparks in the presence of gas mixtures is pretty scary.

 

[jbartos]

Suggestion: It is probably better to post a link to a good reference, rather than to copy it with divided words, etc. which are hard to read.

 

[joepower]

Thanks for the input - even though the rotor passed a growler test and all other routine tests and was ready to ship from teh repair shop, I had them run it up to about 1100 HP load and watch it with their MCE tester. An anomoly did show up in the rotor and I am waiting for details.