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Seminar Guide:

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ByNaveen Chacko MathewS7 EEE; Roll No.38

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Introduction Airgap Eccentricity -Different types of Airgap Eccentricity -Causes of Airgap Eccentricity -Effects of Airgap Eccentricity The Single Phase Rotation Test (SPRT) Detection of airgap eccentricity (existing methods) Conclusion

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Induction motor drives are the most commonly used drives.

Consume more than 50% of an industrialized country’s generating capability.

Fault detection in induction machines is of great importance.

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Faults in an induction machine

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It is an offline test Change in current due to rotor impedance

variation is observed as the rotor is manually rotated under single-phase pulsating field excitation

Using the Single phase rotation test, airgap eccentricity as well as other faults in an induction motors can be detected.

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The condition where there is a non uniformity in the air gap between the stator and the rotor.

The geometrical centres of the stator and the rotor do not coincide.

Mainly two types:- i) Static eccentricity ii) Dynamic eccentricity iii)Mixed eccentricity

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STATIC ECCENTRICITY

Occurs when the centerline of the shaft is at a constant offset from the centerline of the stator. An example is a misaligned end bell.

DYNAMIC ECCENTRICITY

Occurs when the centerline of the shaft is at a variable offset from the centerline of the stator, such as a wiped bearing

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Bearing degradation Shaft flexing Stator/rotor ovality Improper mounting Load which is not balanced

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Increased levels of vibration

Bearing failure Rotor pull over

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Supplied with a pulsating field at mains frequency by applying 1/8–1/4 of the rated voltage between two phases.

Current magnitude is observed as the rotor is manually rotated

A pulsating field can be resolved into forward and backward rotating fields.

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Equivalent circuit of an induction motor excited with a pulsating field

The equivalent input impedance can be derived as:

Zeq = Req + jXeq ≈ (Rs + Rr) + j (Xls + Xlr )

where subscripts l, s, and r represent leakage, stator, and rotorcomponents, respectively

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If the rotor is in healthy condition, the stator current is constant independent of rotor position θr since the rotor is symmetrical.

However, if a broken bar or end ring is present, the current decreases P times per 360◦ rotor rotation due to the increase in the effective rotor resistance Rr and leakage inductance Llr .

If the current fluctuation exceeds 3–5%, defects in the rotor bar or end ring are suspected

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If airgap eccentricity is present, Zeq changes as a function of rotor position

The change in the pattern of Zeq versus θr can be plotted over time to detect eccentricity.

The zigzag leakage inductance Lzz is influenced by the nonuniform airgap distribution.

At motor standstill under supply frequency excitation, it can be seen that a major portion of the inductive component is due to zigzag

leakage flux.

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An analytic expression for the zigzag leakage inductance Lzz can be derived as a function of the airgap in many cases.

Usually,the airgap-dependent Lzz component increases with the degree of eccentricity

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Analytical expressions for airgap as a function of angle θ under SE and DE conditions at motor standstill can be derived as:

gs (θ) = g0 (1 − as · cos (θ − αs ))

gd (θ) = g0 (1 − ad · cos (θ − αd − θr ))

gm(θ) = g0 (1 − as · cos (θ − αs ) − ad·cos (θ − αd − θr ))

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Measurements of Leq Under Static,Dynamic, and ME Conditions

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Where the variables are as shown:

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For static and dynamic eccentricity,the degree of eccentricity is constant at as and ad respectively, independent of the rotor position θr. Since Leq varies with eccentricity, the value of Leq is constant independent of θr under pure SE or DE conditions.

If the rotor with ME is rotated during the SPRT,the degree of eccentricity changes depending on the rotor position.From the equation gm(θ) = g0 (1 − as · cos (θ − αs ) − ad·cos (θ − αd − θr )),

it can be seen that the degree of eccentricity is maximum at as+ad , when the positions of minimum airgap of SE and DE are aligned

(θr = αs − αd ).

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Variation of Leq with Variation of Leq with θr under SE,DE and ME conditionsunder SE,DE and ME conditions

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It can be seen from the expected pattern of Leq under SE, DE,and ME conditions that eccentricity can be detected if the Leq pattern can be monitored and trended over time with the SPRT.

A device that measures the voltage and current for extraction of Leq can be easily implemented. When a damaged rotor bar or end ring is present, the Leq pattern varies P times per 360 degree rotor rotation.

Thus, two faults can be detected and classified with a single test, which is very economic and convenient.

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Eccentricity fault can be detected without much ambiguity.

Independent of the influence of variations or oscillations in torque, or alignment issues since it is an offline test.

Both airgap eccentricity and rotor bar faults can be detected.

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Benefits of SPRTBenefits of SPRT

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Noise Monitoring

Noise monitoring is accomplished by measuring and analyzing the acoustic noise spectrum

Not practical because of the noisy background from other machines operating in the vicinity

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Detection of airgap Detection of airgap eccentricity (eccentricity (some existing some existing methods and limitationsmethods and limitations))

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FLUX MONITORING

Air gap flux of induction motors contains rich harmonics. A flux monitoring scheme can give information about electrical machine conditions.

Air gap flux can be measured by search coils installed in the stator core,but search coil implementation is difficult and costly.

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VIBRATION MONITORING

Monitoring of unbalanced magnetic pull and vibration.

Different mechanical faults create unique harmonics with different frequencies and power levels in the vibration signal

Vibration sensor is expensive.

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Conclusion Some of the main reasons for the

preference of SPRT over other methods are summarized as follows:

1) Static/dynamic and ME faults can be detected and distinguished without ambiguity from the pattern and change of Xeq over time.

2) Independent of operating conditions such as load variation, oscillations, alignment issues and noise.

3) No information on R or rotor speed (slip) required

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The SPRT can be used for the verification of online fault indications without motor disassembly.

The method is expected to provide valuable information to the maintenance engineers who use the SPRT frequently since airgap eccentricity as well as broken rotor bars can be detected.

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References[1]Doosoo Hyun, Sungho Lee, Jongman Hong, Sang Bin Lee,

Subhasis Nandi, “Detection of Airgap Eccentricity for Induction Motors Using the Single-Phase Rotation Test”, IEEE Transactions on Energy Conversion,Volume.27 Issue.3,2012

[2] G. C. Stone, E.A. Boulter, I. Culbert, and H. Dhirani, “Electrical Insulation for Rotating Machines” ,IEEE Press Series on Power Engineering, New York: Wiley, 2004.

[3]P. Bechard, “Fault zone anlaysis—Airgap,” in Proceedings of Motor Reliability Technical Conference, 2004, pp. 37–46.

[4]W. T. Thomson and A. Barbour, “On-line current monitoring and application of a finite element method to predict the level of static airgap eccentricity in three phase induction motors,” IEEE Transactions on Energy Conversion, volume.13, no. 4, pp. 347–357, Dec. 1998.

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[5] J. R.Cameron, W. T. Thomson, and A. B. Dow, “Vibration and current monitoring for detecting airgap eccentricity in large induction motors,”,Proceedings of The Institution of Electrical Engineers, Part B, volume.133, no. 3, pp. 155–163, May 1986.

[6] A. H. Bonnett and T. Albers, “Squirrel cage rotor options for AC induction motors,”IEEE Transactions on Industrial applications, volume. 37, no. 4, pp. 1197–1209, Jul./Aug. 2001.

[7] The Multi-Technology Approach to Motor Diagnostics, All-Test Pro LLC, 2012. [Online]. Available: http://www.alltestpro.com/articlestestimonials/

[8] “Testing of squirrel cage rotors”, Electrical Apparatus Service Association, Inc., EASA Technical Note 23, 2003.

[9] “Testing the Rotor of a Induction Motor by Measuring the Inductance as a Function of Shaft Position”, GET-8065 rev. 1, GE Industrial Control Systems, September 1997.