© Copyright 2012 Wiedenbrug, LLC

Advanced Rotorbar Analysis – Cage Damage vs. Porosity

Ernesto J. Wiedenbrug, Ph.D., SM IEEE

Manufactured vs. Die-Cast Rotors Squirrel Cage Induction Motors are of either a manufactured, or a die-cast design. Larger motors (above 300 to

500hp) will tend to be of the manufactured kind, being produced by inserting the rotorbars into the rotor slots

first, and then connecting the bars to the end-rings. Copper is the most common material for such squirrel cages,

but they can also be made of Aluminum, or even Brass in the case of the outer cage of a Dual-Cage rotor design.

The squirrel cage of die-cast motors is typically aluminum, and in a few cases made of copper. Fig. 1 shows an

example of a manufactured rotor and a die-cast rotor.

Figure 1: Manufactured Rotor (left) [1], Die-Cast Rotor (right) [2]

Porosity in Die-Cast Rotors The die-casting process injects high pressure molten aluminum into

the stacked rotor laminations, filling rotor bars and end-rings. Voids

in the aluminum are a common imperfection of the die-casting

process, and are called porosity. Manufacturers take many

measures to avoid these imperfections, but slight amounts are

frequently inevitable. Minor amounts of porosity will cause small

cage damage signatures in vibration and MCSA, but will barely show

a negative effect in the motor’s capabilities, operation, or its

longevity. A little porosity shouldn’t cause concern for maintenance.

The endring of a 150hp motor in Fig. 2 has extreme porosity. Such

cases are very rare, showing poor motor performance under loaded

conditions. Some motor manufacturers’ quality control test for such

defects [3].

Figure 2: Excessive voids in endring

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Managing Die-Cast Rotors showing Positive Cage Damage Results Vibration and MCSA react equally to porosity as to damaged cages: Vibration and MCSA are the most

sensitive diagnostic technologies for detection of cage damage of induction motors operated under loaded

conditions. Both technologies are unable to distinguish the difference between porosity and broken or cracked

bars or endrings. From a maintenance perspective, however, these cases are fundamentally different, since

porosity requires no maintenance action, but a motor with a damaged cage does.

Advantage of knowing that a rotor is die-cast and

not manufactured: Manufactured rotors have an

immediate catastrophic failure mode where parts of

broken bars can loosen themselves into the airgap,

causing stator windings to short against the stator core.

This failure mode is so rare for die-cast designs, that it

needn’t be considered.

Differentiating Rotor Porosity from Damaged Cage

Rotors: A rotor’s porosity has been there since

manufacturing; it cannot worsen over time. Motors that

have cage damage have developed that damage during

operation, because their cage’s sturdiness doesn’t match

the severity of the application in which they are being

used. After the first damage, they are less capable of

operating than before, and are more likely to develop

further damage. This resembles a “slow avalanche” condition. Conclusion: When encountering Positive Results on

die-cast rotor designs, it is best to put that motor on a “watch” list, increasing the frequency of diagnosis. If over

time there is no worsening of the condition, it is safe to assume that there is no broken cage damage, just

imperfections due to porosity. At this point it is safe to put the motor back into the typical maintenance

monitoring cycle.

Lesson Know Thy Rotor!!! The management of cage damage signatures of die-cast vs. manufactured rotor differ. It is

of critical importance for maintenance programs to be aware which of their motors are die-cast, and which ones

are manufactured. Have your motorshop document particulars of the rotor for every motor that is reconditioned!

If possible, demand documentation from motor suppliers as well.

Further Reading:

Advanced Rotorbar Analysis: False Positives – Spiders / Axial Cooling Vents

Advanced Rotorbar Analysis: False Negatives – Dual Cage Rotors

References:

[1] http://www.aemdessau.de/produkte/english/Squirrel%20cage%20rotor.html

[2] http://en.wikipedia.org/wiki/File:Wirnik_by_Zureks.jpg

[3] http://www.deltatronic.com/html_english/rqa300_its.htm

[4] “Squirrel Cage Rotor Testing”, Tom Bishop, EASA Convention 2003

Figure 3: Broken Bars expelled into Airgap [4]