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Motor Root Cause Analysis by Electrical Apparatus Services Association, 2002
"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. 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 usually is not observed while running at full load. The centrifugal force at full-load speed is usually greater than the electromagnetic 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 corresponds to a low impedance of the rotor cage circuit, essentially 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 interruptions 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, preclude 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 intermittent 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 circumstances, 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 prevent “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."

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Reference 2 - " Electrical Insulation For Rotating Machines - Design, Evaluation, Aging, Testing, And Repair", Greg C. Stone Edward A. Boulter Ian Culbert Hussein Dhirani - ISBN 0-471-44506-1
"The only time that significant voltage can appear on the rotor conductors is during motor starting. This is also the time that extremely heavy currents will flow in the rotor windings. Under some conditions during starting, the conductors make and break contact with the rotor core, leading to sparking. This is normally easily tolerated. However, some SCI motors operate in a flammable environment, and this rotor sparking may ignite an explosion."

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It has been shown that insulated rotor bars (Fig. 16) eliminate sparking due to inter-bar currents. It is considered by GEC ALSTHOM Large Machines that insulating is preferable to trying to ensure tight bars as the differential thermal expansion of the copper and laminations will cause a deterioration of the bar tightness and natural oxidation of the copper will increase the contact retsistance. "S" type machines (Fig. 17) have such insulated bars.