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Diagnosis of Induction Motor faults using Motor Current Signature

Analysis at VIZAG STEEL

By

C.Pavan Sameer Kumar, M anagement Tr ainee (Techni cal), Visakhapatnam Steel Plant.

Abstract

Induction motors are the most widely used electrical drive systems. Hence it is of immense

importance to reduce breakdowns of induction motors and prevent unscheduled downtimes. Motor

current signature analysis (MCSA) is a condition monitoring technique that is now being widely

used to diagnose problems such as broken rotor bars, abnormal levels of air gap eccentricity,

shorted turns in low voltage stator windings and certain mechanical problems in an induction

motor using stator current on no load. Using MCSA at VIZAG STEEL problems at production

units like Coke Oven have been detected at an early stage, thus avoided secondary damage and

complete failure of the motor. Detailed case study at VIZAG STEEL showing the complete

interpretation of MCSA is presented to aid practicing engineers .

1. INTRODUCTION

Induction Motors are a critical component of many industrial processes. Hence it is of

immense importance to reduce maintenance costs of induction motors and prevent

unscheduled downtimes that result in lost production and financial income. Condition

monitoring of the dynamic performance of induction drives received considerable

attention in recent years as it is crucial to effective monitoring and increased reliability of

the plant. The Motor Current Signature Analysis (MCSA) is considered the most popular

fault detection method now a days because it can easily detect the common machine faults

such as turn to turn short circuit, cracked /broken rotor bars, bearing deterioration etc..

The present paper discusses the fundamentals of Motor Current Signature Analysis

(MCSA) plus condition monitoring of the induction motor using MCSA. It also presents

an industrial case study wherein a catastrophic failure of an induction motor (Mill Fan 9

of Coke Oven) at VIZAG STEEL has been avoided by early detection using MCSA and

by initiating corrective measures.



2. METHODOLOGY

An Industrial case study of condition monitoring using MCSA is presented in the

following sections. Section 3 introduces a typical MCSA system, Section 4 deals with

problem description, Section 5 proceeds with the effects of cracked rotor bars and

collection of data using MCSA, Section 6 deals with the analysis of MCSA data and the

results found on dismantling the motor. It also mentions the corrective action initiated.

Section 7 proceeds to Conclusion and section 8 include References.

3. MOTOR CURRENT SIGNATURE ANALYSIS (MCSA)

Motor current signature analysis is the online analysis of current to detect faults in a three- phase induction motor drive. MCSA provides a non-intrusive method for detecting

mechanical and electrical problems in motor driven rotating equipment. Motor current

analyzer applies techniques to capture frequency signature of electric line current to

measure the variation in the flux of the electric motors. The basis for MCSA is that an

electric motor driving a mechanical load acts as an efficient, continuously available

transducer. The motor senses mechanical load variations and converts them into electrical

current variations which can be monitored and recorded at convenient location away from

the operating equipment. Periodic MCSA can provide a more subtle indication of rotor

condition, distortion, coupling or gear wear etc. Analysis of these variations can provide

an indication of machine condition, which may be trended over time to provide an early

warning of machine deterioration or process alteration.

Figure1. MCSA System Configuration



3.1.1 HOW MCSA IS USED?

Motor current analyzer consists of one current collector clamp probe and one analyzer.

Probe collects the data and analyzer analysis the motor input current data with display.

Clamps on probe are placed around supply cable of LT motor (CT secondary wire, if available) and for HT motors around CT secondary wire in the MCC panel. A basic

MCSA instrumentation system will consist of the following:

A current transformer (CT) to sense the signal.

A resistive shunt across the output of the CT-note that CTs are available with

internal shunts.

An MCSA instrument (or spectrum analyzer) to produce the current spectrum or

signature.

The various faults that may occur in an induction motor, the probable causes,

consequences and remedial measures to be taken are indicate below.

Faults in an Induction Motor

Component

Abnormality or

deviation Possible Causes ConsequencesAction Required

Stator

Winding

Increase in Stator

Current Winding Short

Stator burn-out

Winding Repair Over Heating

Unbalance

Currents

More Leakage Current Insulation DamageUn-symmetrical

currentsRe-insulation

More Stator Temperature

Winding Short

Early Winding

burn-outWinding Repair

Core Damage Reduced Efficiency Core Repair

Unbalanced winding

Reduced Efficiency

Winding Repair Reduced TorqueAbnormal Noise

Rotor side Less Torque

Rotor Bar break Stator-Rotor Rub Motor Repair

End Ring break Fluctuating torque Rotor Repair

Non-Uniform air-gap More slip Air-gap correction

Table1. Faults in an Induction Motor



4. PROBLEM DESCRIPTION

At VIZAG STEEL, all HT motors are monitored on a monthly basis to check for any

abnormality. The Condition Monitoring Method used is MCSA. MCSA gives information

such as percentage of current unbalance, distortion index and delta db value of theinduction motor. MCSA conducted on CDCP (Coke Dry Cooling Plant) Mill Fan-9 of

Coke Oven(CO) indicated that the delta db value (35.62) is much less than the required

norm i.e. 40 db. The Technical Specifications of CDCP Mill Fan-9 are tabulated below.

VSP CO Mill Fan Equipment Details

Voltage 6.6 KV

Current 71 A

Speed 1500 rpm

Rating 630 Kw

No of Poles 6

Operating Frequency 50 Hz

No of Rotor Bars 58

Table2. Coke Oven Mill Fan-9 Equipment details

5. CRACKED ROTOR BARS IN INDUCTION MOTOR

A broken or cracked rotor bar in the motor create heating & thermal expansion,

especially in the core while motor is running under over load or subject to frequent

start with longer start time. This will cause

Adjacent rotor bars to break

Loosening of the broken bar

If the broken bars work out of the motor, then it rubs the stator winding insulation and

damage the stator -- that results catastrophic failure. When a squirrel cage induction

motor is running with cracked rotor bar, the broken rotor bars will cause localized

heating and also cause marginal increase in current in adjacent bars.

The motor current signature waveform appearance will have presence of side

band peak. The side band peak will appear at a frequency off set from the line



frequency equal to the no. of poles times the slip frequency (N P X S F). The side band

peak appearance will help for diagnosing the faulty rotor bars. The amplitude of the

side band will increase with the severity of the no. of rotor bar cracks or opened.

5.1 SEVERITY OF CRACKED ROTOR BARS

A commonly used indicator of the presence and severity of rotor bar faults is the dB

amplitude difference between the amplitude of side band peak and the amplitude of

the line frequency

5.1.1 CALCULATION OF SIDE BAND FREQUENCY

Let us take an example of a blower motor running with the following parameters.

Fr = rotational frequency, L f = Line frequency, N P = No. of poles, S F = slip frequency

LSSB = Lower slip side band of Line frequency.

SF = (2* L f /NP) - F r

Then LSSB = L f – (NP*S F).

5.2 FAULT CRITERIA

A general rule of fault criteria of rotor bar crack for Sq. cage motor running with more

than 50 % load is

The measured delta dB difference between the amplitude of LSSB peak and line

frequency peak is of greater than 40 then motor does not have rotor problem

If the delta dB is less than 40, indicates starting of degradation of rotor bars,

which required to be closed monitoring for trending.

If the delta dB value continues to be decreasing, then it is preferred to dismantle

the rotor for inspection and do repair work.

6. ANALYSIS OF MCSA DATA

The data collected using MCSA technique using current analyzer is as follows. The

frequency spectrum is also shown below.



Table3. Current Analysis of Mill Fan 9 using MCSA

Rotor Bar AnalysisParameter Frequency(Hz) Amplitude(db) Delta(db)

LF 49.67 21.4335.62

LSSB 49.22 4.19

Table4. Rotor Bar Analysis of Mill Fan-9 using MCSA

Figure2. The Frequency spectrum of CO Mill Fan-9 collected using MCSA

Current Balance Analysis

R-Phase 42.27 A

Y-Phase 40.44 AB-Phase 40.66 A

% Current Unbalance 2.80%

CT Ratio 15

Clip-On CT(V/amp) 0.005



The current unbalance and distortion index are found to be within limits. After

analyzing the measured data it has been inferred that the deviation may be due to

cracked rotor bars. It was recommended to check for the rotor bars and rectify

6.1 RESULTS

Following the recommendation, the motor was dismantled at CME (Central

Maintenance Electrical) of VIZAG STEEL. The results are in sound agreement with

the MCSA analysis. On dismantling the motor, cracked rotor bars have been

identified. The Mill Fan Rotor has developed 14 damaged rotor bars with 8 rotor bars

having severe cracks. The Mill fan rotor has 72 stator slots and 52 rotor bars. The

cracked rotor bars have been brazed and the motor was restored to operation. Thus,

using MCSA technique the catastrophic failure of the motor was averted and normaloperation restored.

Figures 3, 4 showing cracked rotor bars of CO Mill Fan-9



Figure5. Cracked Rotor bars of Mill Fan-9

7. CONCLUSION

Industrial case histories have clearly demonstrated that MCSA is a powerful online

monitoring technique for assessing the operational condition of three-phase induction

motors. The avoidance of catastrophic failures can be achieved via MCSA and other

major benefits include the prevention of lost downtime, avoidance of major motor

repair, or replacement costs. Thus, it provides a highly sensitive, selective and cost-

effective means for online monitoring of a wide variety of heavy industrial machinery

thus contributing to increased reliability of the industry.

8. REFERENCES

[1] .‘Motor Current Signature Analysis and its Applications In Induction Motor Fault

Diagnosis” by Neelam Mehala, Ratna Dahiya.

[2].A Review of Induction Motors Signature Analysis as Medium for Faults Detection. IEEE

Transactions on Industrial Electronics, VOL. 47, NO. 5, October 2000.

[3].Motor Current Signature Analysis to detect faults in Induction Motor drives-Fundamentals,

Data Interpretation and Industrial Case Histories by William T.Thomson and Ronald

J.Gilmore.

[4].Reliability Assurance of Large Electric Motors in a Process Plant by D.Edwin Vijay Kumar

and Shyam Sundar A , Vizag Steel, India.

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