gradually to failure. Drive system components that progress to failure abruptly with short indication such as the switching device, driver circuit, or stator turn insulation failures cannot be monitored with this method since it is not a continuous on-line method. To be able to apply this method, the condition of the component being monitored must also be observable at motor standstill. The proposed strategy cannot be applied for bearing monitoring since most of the problems in the bearing can only be observed when the motor is operating.

Test methods based on the proposed strategy can be devised for most of the ASD system components listed above in I. Although this method provides off-line testing, the test can be performed more frequently than the conventional off-line tests performed during regular maintenance since the motor can be tested whenever it is stopped. In addition, since the existing inverter hardware is used for testing, the additional hardware or equipment requirement is minimal. Maintenance can also be performed in an efficient manner since the motor can be tested automatically without motor disassembly. Another advantage is the remote monitoring capability for machines that are difficult to access. The new method cannot provide continuous on-line monitoring, but it is sufficient to monitor the component whenever the motor is stopped, since the faults do not progress rapidly into failure and do not require immediate shutdown. The new method also has many potential advantages over existing on-line monitoring methods. As mentioned above, the hardware and computational requirements are minimal compared to on-line methods that require specialized test equipment or sensors such as partial discharge (PD), infrared thermography, or core monitors. In addition, unlike on-line techniques that require spectrum analysis or the mathematical model, it is independent of operating conditions such as the input frequency, load, or noise, and does not require motor parameters or speed.

In this paper, inverter-based off-line test methods for monitoring the quality of the 1) dc link aluminum electrolytic capacitor, 2) electrical connections, 3) cable and stator winding insulation, 4) stator core, and 5) rotor bar are presented. A summary of failure mechanism, existing off-line and on-line test methods, and experimental study will be given for each component being monitored.

III. DC LINK ELECTROLYTIC CAPACITOR MONITORING

A. Failure Mechanism and Existing Test Methods Aluminum electrolytic capacitors are employed as the DC

link capacitor for power converters due to their low cost per capacitance and large capacitance per volume. The main limitation of electrolytic capacitors is its finite lifetime and high failure rate as witnessed by failure statistics that show that 60% and 72% of power supply failures were caused by electrolytic capacitor breakdown [7]. The primary failure mechanism is evaporation and loss of the electrolyte solution in the capacitor with temperature rise due to high ripple currents, ambient temperature, or over-voltage. This results in increase in the ESR, Rc, and decrease in equivalent capacitance, C, of the DC link capacitor, shown in Fig. 1. This increases the heating and further accelerates the degradation process and eventually causes drastic changes in the capacitor characteristics.

The traditional method for preventive maintenance of electrolytic capacitors is to replace them periodically to prevent failure or remove the capacitor and measure and monitor the C or ESR. Since this is inconvenient and/or not economical, many advanced off-line and on-line techniques for capacitor assessment based on ESR, C, or capacitor voltage ripple monitoring have been proposed in [9-13]. However, the methods in [9-10] are influenced by variations in operating conditions such as the load, capacitor temperature, which can cause false indications. The methods proposed in [11-13] require a priori capacitor data, additional hardware/measurements, or intensive computation for implementation, which limits its application to low cost drive systems.

B. Proposed Test Method and Experimental Results The equivalent values of ESR and C can be calculated

automatically whenever the motor is stopped from the dc link voltage, ec, and stator current, ias, measurements available in the inverter while the capacitor voltage is discharged to the motor stator winding [14]. The dc link voltage is discharged through the phase a and b windings, by applying a constant duty cycle, constant frequency unipolar PWM to switch S1 with switch S6 is turned on. The equivalent circuit of the inverter operated under unipolar PWM can be derived as shown in Fig. 2. There is no torque induced in the motor since unidirectional current is injected through the phase a and b windings. Since the ESR and C values are sensitive to temperature, the stator resistance is also calculated from ec and ias under thermal equilibrium for temperature compensation.

An experimental study was performed on a 250W AC motor adjustable speed drive system with healthy and aged capacitor samples where the ESR and C are 0.148 and 628 F, and 2.43 and 432 F, respectively, at 26oC and 360Hz. The experimental measurements of ec, ias, and ic

with the two capacitors are shown in Fig. 3(a)-(b), respectively. The values of ESR and C are calculated from ec and ias when the source current, isrc, is zero using,

dcasavgcc IeRESR ,, /ˆ , (1)

cdcass vIDTC /ˆ, . (2)

It can be seen in Fig. 3 that ec is small when ESR is small, but increases significantly for the aged capacitor with large ESR. The values of ESR and C estimated from (1)-(2) for the two capacitors at 26oC and 50oC are summarized in Table I. It can be seen from the experimental study that

Fig. 2. Equivalent circuit under unipolar PWM for capacitance monitoring

Table I. Measured and estimated values of C and ESR at 26oC and 50oC

ESR and C can be estimated with sufficiently high accuracy for condition assessment purposes. The proposed method is independent of motor operating conditions and noise since it is a standstill test, and also does not require any a prioridata or additional hardware/measurements for implementation.

IV. ELECTRICAL CONTACT QUALITY ASSESSMENT

A. Failure Mechanism and Existing Test Methods High-resistance(R) electrical connections are common

problems that occur due to poor contact at the joint of any connection between the inverter and motor. They are usually initiated due to a combination of poor workmanship (mainly under-tightening of connectors), loose connections, or corrosion/oxidation/contamination/damage in the contact surface [2,8,15]. Once a high-R contact is created, repeated thermal cycling (expansion and contraction of components at the connection) and vibration deteriorates the contact quality at an elevated rate and increases the contact resistance. Heating at the contact accelerates surface oxidation and increases the resistance and temperature further. If the contact resistance increases to an unsafe level, this can result in localized overheating, voltage unbalance, and/or sparking [15]. Local thermal overloading at the contact causes open or short circuit failures in the electric circuit, and increases the losses. Voltage unbalance causes negative sequence current flow, which results in increase in losses, and accelerated degradation of the motor due to thermal overheating and vibration. Overheating and sparking at the contact surface can initiate electric fires (significant portion of fires are caused by poor contacts [16]). Therefore, many off-line and on-line test methods have been developed to monitor and correct high-R connections for reliable, efficient, and safe operation of the drive system.

The resistive imbalance test is a very effective and widely accepted test that measures the percent difference between the phase to phase resistances of the motor off-line under DC excitation for identifying high-R contacts. The on-line voltage drop test is a simple and low cost voltmeter test that compares the voltage drop in the circuit between phases

while the motor is operating. Infrared (IR) thermography monitors the temperature distribution in the circuit using a thermal imaging camera to identify hot spots due to poor contacts. IR thermography is safer, faster, and more accurate compared to the voltage drop test, but requires good line of sight to the problem and expensive equipment or service. Since both on-line tests require the MCC to be opened during operation for physical or visual access to the terminals, there is a potential safety hazard due to arc flash or electric shock. It can be seen that the existing tests available for monitoring high-R connections are inconvenient since they are either off-line or walk-around type tests. Recently, on-line test methods based on monitoring the negative sequence current have been proposed in [17-18], but require motor parameters and/or intensive computation.

B. Proposed Test Method and Experimental Results The resistive imbalance test can be performed

automatically using the inverter whenever the motor is at standstill from the ec and iabcs measurements available in the drive. A similar unipolar PWM switching method used in III.B for electrolytic capacitors can be applied for measuring the three phase resistances Rab, Rbc, and Rca. The equation for calculating the phase resistance can be derived from Fig. 2, based on the dc components of ec, Ec,dc, and iabcs, Iabcs,dc,obtained over an integer number of electrical cycles, as shown in Fig. 3, as

dcasonakonceoncedccab IDVVDVER ,,,,, 2/)]1)(()2[(ˆ , (3) where, Vce,on and Vak,on represent the forward voltage drop across the IGBT and diode at the test current level. If a high-R contact is present between the phase a terminals of the inverter and motor, as shown in Fig. 1, the calculated values of Rab, Rbc, and Rca would be 2Rs+RHR, 2Rs, and 2Rs+RHR, respectively. The difference between the three resistances can be monitored using the inverter whenever the motor is stopped to provide automated and frequent monitoring of high-R contact problems without additional hardware requirements.

V. STATOR WINDING INSULATION AND CABLE CONDITIONMONITORING

A. Failure Mechanism and Existing Test Methods Surveys on motor reliability show that stator winding

insulation is one of the weakest electric machine components (30-40% of motor failures are caused by stator insulation problems) [1-2]. Electrical power cable insulation failure is also known to be one of the main root causes of failure in electrical distribution systems along with high-R connections. Since electrical insulation is continuously exposed to a combination of thermal, electrical, mechanical, and environmental stresses during operation, the insulation material deteriorates gradually over time. The failure rate of motors operated using PWM adjustable speed drives is higher due to increased electrical and thermal stresses on the insulation material [1-4]. The high dv/dtrepetitive voltage surges due to PWM operation results in elevated motor terminal voltages (up to 3 times inverter output voltage due to reflection) and uneven voltage distribution across the stator winding, where most of the voltage is applied across the first terminal end coil. This significantly increases the electrical stress on the turn and

Fig. 3. Experimental measurements of ec, ias, and ic waveforms for capacitormonitoring technique with (a) healthy capacitor (C=628 F,ESR=0.148 ), (b) aged capacitor (C=432 F, ESR=2.43 )

groundwall, GW, insulation of the motor and cable, and can result in partial discharge, PD, even for random wound motors rated as low as 440V. In addition, thermal stress on the stator insulation is increased due to reduced cooling ability of the shaft fan at low speed and increased copper, core, and dielectric losses with harmonics and high dv/dt.Corrective measures such as filters or adjustments in the insulation, cable, or grounding have been studied and implemented to mitigate the influence of high dv/dt and fs,but cannot completely eliminate the stresses. Motor and cable insulation problems are taken seriously since they not only lead to forced outages due to system failure, but can also cause casualties or serious injuries due to electric shock, fire, or arc flash hazards.

Off-line insulation tests such as the insulation resistance (IR), capacitance (C), dissipation factor (DF, tan ), and/or surge tests have been extensively used throughout industry over a long period of time for assessment of cable and motor insulation condition [1-2,19-20]. Although the tests cannot be performed frequently, these tests have been proven to detect insulation problems due to thermal deterioration, moisture absorption, contamination, or any other type of dielectric losses, when used in conjunction and trended over time. On-line PD monitoring is the only on-line method commercially available for insulation condition assessment. PD monitoring is effective for detecting and potentially preventing failures caused by corona activity driven insulation aging [2]. However, stator insulation failure is caused by a synergistic aging effect due to a combination of multiple stresses, and corona is not the only root cause of insulation failure. In addition, the cost involved in installing the specialized equipment required for PD monitoring is a limitation for most applications.

B. Proposed Test Method and Experimental Results Standard off-line tests such as the IR, C, and DF tests can

be performed at motor standstill by applying DC or AC voltage to the cable and motor insulation using the inverter,

as shown in Fig. 4-5 [21]. If the test voltage is applied across the insulation in one phase, the leakage current through the cable and motor insulation can be measured using a sensing resistor (100s of k ~1M ) installed at the inverter output (Fig. 4-5). DC voltage, vPg, can be applied to the GW insulation for performing the IR test by opening MCs 2-3 and closing MC1, shown in Fig. 5. Opening of mechanical contacts 2 and 3 are required for preventing ground leakage current through the power devices in the inverter. AC voltage can be applied to the GW insulation for C and DF testing by opening MCs 1,3 and closing MC2 or MC2’. The 60hz source voltage can be applied to the insulation with MC2 opened and a variable frequency AC voltage can be applied with MC2’ opened, if S1 and S4 are turned on and off alternately at the desired frequency. The values of IR, C, and DF can be calculated from known voltages and the leakage current measurement, as shown in (4)-(6).

),(/)( ' leakleaksensePgga iavgiRvvavgIR (4)

leaksensegaAgaleak IRVVIC~~~/)cos(~

)(' (5)

)cos(~/)sin(~)( leakleak IITanDF . (6)

where is the phase angle between Ileak and the capacitive component of Ileak. It should be noted that the values of IR,C, and DF represent the average condition of the cable and motor insulation combined (Fig. 4).

An experimental study was performed on a 380V, 7.5hp random wound induction machine with external resistors (Rext= 0.5M ~500M ) connected in parallel to the insulation, as shown in Fig. 5, for simulating insulation degradation. The experimental measurements of vPg and ieak (Rsense=1M ) are shown in Fig. 6 for four different values of Rext (1, 5, 50, 250 M ) under IR testing. It can be seen that the DC component of ileak increases as Rext is decreased, which implies that IR is decreasing. The waveforms of va’g, vag, and ileak under C and DF testing are shown in Fig. 7 for the case when Rext is not connected ( M ) with MC2’ closed and S1 and S4 switched at 60hz. The phase angle between va’g and ileak is close to 90o ( is very small) for healthy insulation (Fig. 7), and decreases (increases) for degraded insulation. The calculated values of IR for each value of Rext are summarized in Table II, and

Fig. 4. Electrical equivalent circuit of insulation of inverter-cable-motorsystem under proposed insulation testing

Fig. 5. Inverter configuration for IR testing with rectified DC voltage, and Cand DF testing with AC voltage; Rext for experimental verification

0 0.0056 0.0111 0.0167 0.0222 0.0278 0.033-50

0

50

100

150

200

250

300

350

Time (sec)

VPg

(V),

I leak

( A

)

vPg

ileak (Rext=1M )

ileak (Rext=5M )

ileak (Rext=250M )

ileak (Rext=50M )

Fig. 6. Insulation testing with rectified DC voltage – waveforms ofmeasured vPg & ileak for Rext = 1, 5, 50, and 250 M

Table II. Insulation testing with rectified DC voltage – calculated values ofIR for Rext = 0.5, 1, 5, 10, 50, 100, and 250 M

the calculated values of , DF, C, and R for all the values of Rext summarized in Table III. It can be seen in Tables II-III that the precision of the IR, C and DF estimates is sufficient for early detection of insulation problems for failure prevention purposes. The proposed technique provides a low cost solution for frequent and automated monitoring of cable and stator winding insulation quality for improved reliability and efficient maintenance.

VI. STATOR CORE QUALITY ASSESSMENT

A. Failure Mechanism and Existing Test Methods Any type of defect in the stator core caused by excessive

thermal, electrical, mechanical, or environmental stress can increase the losses and/or can result in core damage [1-2, 22-23]. Exposure to excessive thermal or mechanical (physical abuse or compressive stress) stresses alters the magnetic properties of the lamination (decrease in permeability), and results in an increase in hysteresis losses. Shorting between laminations due to inter-laminar insulation failure results in large circulating eddy currents and increase in eddy current losses. Inter-laminar insulation damage can be introduced due to stator-rotor contact, vibration induced abrasion, core overheating during rewind (burnout oven), ground current, or defects/foreign materials introduced during inspection, repair, manufacturing. Increase in the core loss decreases the efficiency of the machine and can result in localized heating, which can damage the stator winding insulation leading to ground failure. For large machines, inter-laminar core fault gradually progresses in severity and can result in melting of the stator core. When operated with PWM inverters, the increased thermal and electrical stress accelerates the degradation of the stator core and inter-laminar insulation, increasing the chances of failure [1].

Many core test methods have been developed over the years to monitor the core quality (losses) for reliable and efficient motor operation. On-line chemical monitoring (core monitors or tagging compounds), and off-line core ring or low energy core tests are the most effective methods for detecting local damage in the stator core [1-2]. Although

these tests can detect local core problems with high sensitivity, they require specialized test equipment and/or motor disassembly, and are considered cost effective for generators or large motors rated above tens of MWs. For motors rated up to several MWs, which fall in the range of inverter-fed induction machines, the core loss test [23-24] is the most commonly used test. In this test, the yoke of the stator core is excited at near rated flux after rotor removal, and the power input (stator core loss) is measured to monitor problems in the stator core that result in increase in core losses. The core loss can also be measured without motor disassembly if the loss segregation test procedure in IEEE STD. 112B is used [23, 25]. In this test, the core loss can be separated from the no load loss measurement, but requires the motor to be operated at no load with a variable sinusoidal voltage source.

B. Proposed Test Method and Experimental Results The stator core loss can be measured at motor standstill

with the inverter operated to excite the machine with a pulsating AC magnetic field at a set of fixed locations, as shown in Fig. 1 [26]. If the flux vector is alternated between and + to produce a pulsating field in the motor, as shown in Fig. 7(a)-(b) ( = 0o and 90o), current is induced in the rotor but the motor does not rotate since the induced torque is zero. The input power, Pin, can be calculated from the voltage and current measurements as a function of

with the motor excited at multiple positions, to monitor the increase in the overall and local core loss due to core problems, as shown in Fig. 8. If the stator core of the motor is healthy and symmetrical, the pattern of Pin is constant independent of the flux vector position, (Fig. 8). If the laminations at the = 90o location is shorted as shown in Fig. 7, it can be seen in Fig. 7(a) that the increase in eddy current loss would be maximum if the yoke flux that induces the circulating eddy current is maximum (pulsating flux at = 0o). The eddy current loss does not change if the pulsating field is at = 90o as shown in Fig. 7(b) since the yoke flux that links the circulating fault current path is zero. Therefore, the pattern of Pin is maximum at = 0o, 180o and minimum at = 90o for a motor with stator core fault located at = 90o, as shown in Fig. 8. Since eddy current loss is proportional to fe

2 and hysteresis loss is proportional to fe,the sensitivity of the method can be improved with high excitation frequency, fe.

The proposed technique was tested on a 10hp aluminum die-cast squirrel cage rotor induction motor. To simulate

0 0.0056 0.0111 0.0167 0.0222 0.0278 0.033

-400

-300

-200

-100

0

100

200

300

400

Time (sec)

VPg

(V),

I leak

( A

)

ileak

va'g

vag

Fig. 7. Insulation testing with 60 Hz AC voltage – waveforms of measuredva’g, vag & ileak with no Rext (= M )

Table III. Insulation testing with AC voltage – calculated values of , DF,Req, and Ceq for Rext = 0.5, 1, 5, 10, 50, 100, 500, and M

(a) (b) Fig. 7. Representation of the pulsating flux vector location for =0o (a) and

90o (b) and stator inter-laminar core fault and broken rotor barlocation in the stator and rotor cross section

inter-laminar core faults, half of the axial length of 2 and 4 slots was welded at the tooth tip at the =120o location. According to the core los test, the maximum increase in core loss and temperature for 2, 4 slot faults were 53%, 98% and 20oC, 28oC (after 30 minutes of excitation), respectively, and the increase in the no load loss was 7.2%, 34.3%, which show that the fault is severe enough to increase the core loss. The measured input power, Pin, and % increase in Pin, are shown in Figs. 9-10 for the healthy and faulty stator cores as a function of when the excitation frequency is 1kHz. It can be seen that there is a noticeable increase in the power loss after fault insertion, and the pattern of the loss is as predicted in Fig. 8. It can be clearly seen that local core problems that increase the core loss can be detected with high sensitivity by observing the magnitude and pattern of Pin. Although the proposed method is not as sensitive as the existing off-line test, it can provide automated and frequent monitoring of core problems without motor disassembly using existing inverter hardware.

VII. DETECTION OF BROKEN ROTOR BARS

A. Failure Mechanism and Existing Test Methods It has been shown in many resources that rotor faults due

to cracked/broken bars or end rings account for 5~10% of induction motor failures, and that they are caused by a combination of motor operating stresses or manufacturing imperfections [1-2]. A broken rotor bar results in re-distribution of the current in the bars and increases the current and stress in the adjacent bars further degrading the motor performance, efficiency, and reliability.

On-line monitoring for rotor faults relies on analyzing the frequency spectrum of the speed, vibration, torque, flux, or current measurements [2, 27]. MCSA is the most popular approach since it provides sensorless monitoring of rotor problems by observing the fbrb = (1-2s) fs component, where fs is the operating frequency and s is the slip. However, it requires rotor speed information and manipulation of lengthy steady state data for obtaining a high resolution assessment. In addition, MCSA is difficult to apply for applications where the load constantly changes since fbrb is not fixed or cases where the load produces sidebands close to fbrb. MCSA is not very effective for closed loop inverter-fed due to variable operating frequency and the masking effect of the feedback controller [28]. A number of methods have recently been proposed in [27-31] for rotor fault monitoring of closed loop drives or monitoring under transient conditions; however, there are limitations such as being applicable only for a specific control scheme, requirement of motor parameters, complexity, or heavy computation requirement, etc.

Many off-line tests for rotor faults are also available for quality assurance at manufacturing facilities or regular inspection at service shops or end user sites [1-2, 32]. Some of the traditional off-line tests for detecting rotor bar damage are visual inspection, the dye penetrant test, and the low resistance ohmmeter test. The growler test and high current excitation test are more commonly used off-line tests, where the current distribution in the rotor bars is indirectly observed; however, all of these off-line tests are inconvenient since they require rotor disassembly. In the single phase rotation test, an alternating field is produced by exciting two phases, and the change in current due to a broken bar is observed while slowly turning the rotor [2, 32]. This test does not require motor disassembly, but rotating the rotor is difficult for large motors and under certain operating environments. Standstill testing for broken bar detection has been proposed in [33-35] for quality assurance and/or inverter-fed machines; however, there are limitations such as the requirement of a flux sensor or locking the rotor.

B. Proposed Test Method and Experimental Results The proposed method for detecting broken rotor bars is

also based on operating the inverter to excite the machine with a pulsating AC magnetic field at a set of fixed locations, as in stator core fault detection presented in section VI. For detection of broken bars, the equivalent impedance is calculated from the voltage and current measurements as a function of with the motor excited at multiple positions, to monitor the variation in the impedance pattern. The equivalent circuit of the induction machine under pulsating field excitation can be derived as shown in Fig. 11 according to the double revolving field theory at s=1. If the rotor is

Fig. 8. Pattern of Pin (or Req, Xeq) measurement as a function of pulsatingflux vector electrical angle, , for a healthy motor and motor withfaulty stator core (or broken rotor bar) at =90o location

Fig. 9. Experimental measurement of Pin as a function of with 1kHz excitation for motor with healthy and faulty stator core

Fig. 10. Experimental measurement of % increase in Pin as a function of with 1kHz excitation for motor with faulty stator core

healthy, the equivalent circuit impedance, Zeq( ), is constant independent of , since the motor is symmetrical; however, if a broken bar is present, Zeq( ) changes with . If the fault is located at =90o and the pulsating field is at =0o, as shown in Fig. 7(a), the equivalent resistance, Req, increases since the bar where the maximum voltage is induced is broken (increase in Rr). The equivalent reactance, Xeq, also increases since the reduced current in the broken bar makes it easier for the rotor flux produced by the adjacent bars to leak (increase in Xlr). If the pulsating field is located at

=90o, as shown in Fig. 7(b), Req or Xeq do not increase significantly since the induced voltage or current in the broken bar(s) or adjacent bars is small. The calculated value of Req and Xeq will therefore be maximum at = 0o,180o and minimum at = 90o for a motor with broken bar(s) located at = 90o, as shown in Fig. 8.

The proposed technique was tested on a 4P, 7.5 hp induction motor with a 44 bar aluminum die-cast rotor, and bars were broken by drilling holes at the contact between the rotor bars and end rings (2~3 bars broken at =90o). The experimental results of the single phase rotation test, where the stator current magnitude measurement is plotted as a function of rotor mechanical position under 60hz AC

excitation at 1/8 rated voltage is shown in Fig. 12. The decrease in stator current can be clearly observed when the field is 90 electrical degrees apart from the broken bar due to increase in Zeq. The Req and Xeq vs. measurements with 50hz pulsating vector excitation (25% rated current) are shown in Fig 13 (proposed technique). It can be clearly seen that there is a noticeable increase in Req and Xeq

proportional to the number of broken bars near 0o and 180o

after fault insertion, as predicted. The results clearly show that the proposed method can automatically detect broken bars with high sensitivity without motor disassembly or manual motor rotation. It cannot provide continuous on-line monitoring, but it is sufficient to monitor the rotor condition whenever the motor is stopped since rotor faults are not rapidly progressing faults that require immediate motor shutdown. The proposed method is also independent of motor operating conditions and load, since it is a standstill test.

VIII. CONCLUSION

A new strategy for predictive maintenance of adjustable speed induction machine drive systems was presented in this paper. In the new method, condition monitoring algorithms are built-in to the inverter to perform off-line tests for quality assessment of the vulnerable components in the inverter, cable, and induction motor automatically, whenever the motor is stopped. Off-line test methods for monitoring the quality of the 1) dc link aluminum electrolytic capacitor, 2) electrical connections, 3) cable and stator winding insulation, 4) stator core, and 5) rotor bar were proposed and verified experimentally. The experimental results have shown that the proposed techniques provide indications of problems with sensitivity comparable to existing test methods with minimal or no additional hardware requirements. The proposed maintenance strategy also allows prioritization and scheduling of machine maintenance to be performed in an efficient manner without a scheduled outage.

In addition to using the proposed methods for diagnosis of drive systems, it can also be developed as an instrument for quality assurance at a manufacturing facility or a portable equipment for inspection at a machine shop or motor end-user site. The proposed method has many advantages over existing off-line and on-line tests listed below, whether it is implemented in an inverter or developed as a stand-alone portable equipment.

Automated testing without motor disassembly, operation, or locking Frequent monitoring (compared to off-line testing) Minimal or no additional test hardware requirements Proximity to motor not required for testing Independent of motor operating conditions (load, variable frequency, noise) Independent of load influence Independent of motor speed or parameters

The new strategy is expected to help improve the reliability, efficiency, and safety of the adjustable speed drive system and save maintenance costs.

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Fig. 11. Induction motor equivalent circuit with pulsating field excitation at standstill (s=1)

Fig. 12. Off-line single phase rotation test results: stator current magnitude vs. for healthy motor and motor with 2-3 of 44 broken bars

Fig. 13. Experimental results of proposed method with 50hz excitation: Req

& Xeq vs. for healthy motor and motor with 2-3 of 44 broken bars

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[22] Can energy efficient motors be wound satisfactorily?, EASA tech. note no. 26, Electrical Apparatus Service Association, Inc, 1999.

[23] The effect of repair/rewinding on motor efficiency: EASA/AEMT rewind study and good practice guide to maintain motor efficiency,EASA/AEMT, 2003.

[24] Stator core testing, EASA tech. note no. 17, Electrical Apparatus Service Association, Inc, 2002.

[25] IEEE standard test procedure for polyphase induction motors and generators, IEEE Standard 112-2004, Nov. 2004.

[26] K. Lee, J. Hong, K. Lee, S.B. Lee, and E.J. Wiedenbrug, “A Stator Core Quality Assessment Technique for Inverter-fed Induction Machines,” Proc. of IEEE IAS Ann. Meet., Oct. 2008.

[27] A. Bellini, F. Filippetti, C. Tassoni, & G.A. Capolino, “Advances in Diagnostics Techniques for Induction Machines,” IEEE Trans. on Ind. Elec., vol. 55, no. 12, pp. 4109-4126, Dec. 2008.

[28] A. Bellini, F. Filippetti, G. Franceschini, & C. Tassoni, “Closed-loop control impact on the diagnosis of induction motors faults,” IEEETrans. on Ind. Appl., vol. 36, no. 5, pp. 1318-1329, Sept./Oct. 2000.

[29] S.M.A. Cruz, & A.J.M. Cardoso, “Diagnosis of rotor faults in closed-loop induction motors drives,” proceedings of IEEE IAS Annual Meeting, vol. 5, pp. 2346-2353, 2006.

[30] C. Kral, R. Wieser, F. Pirker, & M. Schagginger, “Sequences of field-oriented control for the detection of faulty rotor bars in induction machines—The Vienna monitoring method,” IEEE Trans. on Ind. Elec., vol. 47, no. 5, pp. 1042-1050, Oct. 2000.

[31] F. Briz, M.W. Degner, A.B. Diez, & J.M. Guerrero, “On-line diagnosis of inverter-fed induction machines using high-frequency signal injection,” IEEE Trans. on Ind. Appl., vol. 40, no. 4, pp. 1153-1161, July/Aug. 2004.

[32] T. Bishop, “Squirrel cage rotor testing,” EASA Convention, 2003. [33] C. Kral, F. Pirker, & G. Pascoli, “Detection of rotor faults in squirrel-

cage induction machines at standstill for batch tests by means of Vienna monitoring method,” IEEE Trans. on Ind. Appl., vol. 38, no. 3, pp. 618-624, May/June 2002.

[34] B. Akin, B. Ozturk, H. Toliyat, & M. Rayner, “DSP-based sensorless electric motor fault diagnosis tools for electric and hybrid electric vehicle powertrain applications,” IEEE Trans. on Veh. Tech., vol. 58, no. 1, 2009.

[35] C. Demian, A. Mpanda-Mabwe, H. Henao, & G.A. Capolino, “Detection of induction machines rotor faults at standstill using signals injection,” IEEE Trans. on Ind. Appl., vol. 40, no. 6, pp. 1550-1559, Nov./Dec. 2004.

X. BIOGRAPHIES

Sang Bin Lee (S'95-M'01-SM' 07) received the B.S. and M.S. degrees from Korea University, Seoul, Korea in 1995 and 1997, respectively, and his Ph.D. degree from Georgia Institute of Technology, Atlanta, GA in 2001, all in Electrical Engineering.

He is currently a professor of Electrical Engineering at Korea University, Seoul, Korea. From 2001-2004, he was with the Electric Machines and Drives Laboratory, General Electric Global Research Center, Schenectady, NY. At GE, he developed an inter-laminar core fault detector for generator stator cores, and worked on insulation quality assessment for electric machines. His research interests are in protection, monitoring and diagnostics, and control of electric machines and drives.

Dr. Lee was the recipient of the 2005 PES Prize Paper Award from the IEEE Power Engineering Society and the 2008 Second Prize, 2005 First Prize, and 2001 Second Prize Paper Awards from the Electric Machines Committee of the IEEE Industry Applications Society. He serves as an Associate Editor for the IEEE Transactions on Industry Applications for the IAS Electric Machines Committee.

Jinkyu Yang (S’05) received his B.S. degree in Electrical Engineering from Korea University, Seoul, Korea, in 2005. He is currently pursuing his Ph. D. degree in electrical engineering at Korea University, Seoul, Korea.

In 2007, he was an intern at the Korea Electric Power Research Institute (KEPRI) in Taejon, Korea. He worked at Baker Instrument Company – An SKF Group Company, Fort Collins, CO, on evaluation of stator insulation tests in 2008 and 2009. His research interests are in insulation quality assessment techniques for electrical equipment.

Mr. Yang was the recipient of the 2005 First Prize Paper Award from the Electric Machines Committee of the IEEE Industry Applications Society.

Jongman Hong (S’09) received his B.S. degree in Electrical Engineering from Korea University, Seoul, Korea, in 2008, where he is currently pursuing his Ph.D. degree.

In 2009, he worked at Baker Instrument Company – An SKF Group Company, Fort Collins, CO, on development of condition monitoring tools for electric machines, as a summer intern. His research interests are in condition monitoring, diagnostics, and control of electric machinery.

Mr. Hong was the recipient of the 2008 Second Prize Paper Award from the Electric Machines Committee of the IEEE Industry Applications Society.

Byunghwan Kim (S’09) received his B.S. degree in Electrical Engineering from Korea University, Seoul, Korea, in 2008, where he is currently pursuing his M.S. degree.

In 2008, he worked at Baker Instrument Company – An SKF Group Company, Fort Collins, CO, on evaluation of stator insulation tests in 2008 as a summer intern. His research interests are in condition monitoring, diagnostics, and control of electric machinery.

Ji-Yoon Yoo received the B.S. and M.S. degrees in Electrical Engineering from Korea University, Seoul, Korea, in 1977 and 1983, respectively, and his Ph.D. degree in Electrical Engineering from Waseda University, Tokyo, Japan, in 1987.

From 1987 to 1991, he was an Assistant Professor in the Department of Electrical Engineering, Changwon National University. He joined the Department of Electrical Engineering of Korea University in 1991, where he has performed research on Control of Electric Machines and Drives and

Power Electronics Converters. His current research interests include Modeling, Analysis, and Control of Hybrid Electric Vehicle Systems.

Kwanghwan Lee (S’08) received his B.S. and M.S. degrees in Electrical Engineering from Korea University, Seoul, Korea, in 2007 and 2009, respectively.

He is currently a researcher at the Power Conversion Research Laboratory of Hyundai Heavy Industries, Yongin, Korea. His research interests are in design of electric power machinery.

Mr. Lee was the recipient of the 2008 Second Prize Paper Award from the Electric Machines Committee of the IEEE Industry Applications Society.

Jangho Yun (S’07) received his B.S. and M.S. degrees in Electrical Engineering from Korea University, Seoul, Korea, in 2005 and 2007, respectively.

He is currently a researcher at Hyundai Heavy Industries, Yongin, Korea, where he is participating in the development of motor designs for Electric Vehicles and Crude Oil Pump Motors. His research interests are in design of electric power machinery.

Myungchul Kim received his B.S. degree from Dankook University, Seoul, in 2000, and his M.S. degree from Korea University, Seoul, Korea, in 2008, respectively, all in Electrical Engineering.

Since 2000, he was a Senior Engineer at Samsung Electronics, Suwon, where he participated in developing motor controllers for home appliances. His current research interests are in inverter design, and control & diagnostics of electric machines and drives.

Kwang-Woon Lee (M’04) received the B.S., M.S., and Ph.D. degrees in Electrical Engineering from Korea University, Seoul, Korea, in 1993, 1995, and 1999, respectively.

He is currently a full-time lecturer at the Division of Marine Electronics and Communication, Mokpo National Maritime University, Mokpo, Korea. From 2000 to 2002, he was with the Samsung Advanced Institute of Technology, Younggin-si Gyunggi-do, Korea, where he was worked on developing micro-electromechanical systems (MEMS) sensor applications. From 2002 to 2007, he was a Senior Research Engineer at the Samsung Living Appliance R&D Center, Samsung Electronics, Suwon, Korea, where he was engaged in research on sensorless motor drive systems for refrigerators and air conditioners. His current research interests are in the area of power electronics and control, which includes ac machine drives, digital signal processing (DSP)-based control applications, and fault diagnosis of electrical machines.

Dr. Lee was the recipient of the 2008 Second Prize Paper Award from the Electric Machines Committee of the IEEE Industry Applications Society.

Ernesto J. Wiedenbrug (S'94-M'00-SM'01) was born in Buenos Aires, Argentina. He received his Dipl. Ing. from RWTH Aachen, Germany and his Ph.D. in electrical engineering from Oregon State University, Corvallis, Oregon, in 1995 and 1998, respectively.

Since 1998, he is with Baker Instrument Company – an SKF Group Company, Fort Collins, CO, where he is currently the Technology Manager. He worked for Siemens SA in Buenos Aires, Argentina in 1984, and as a power engineer for ISCOR, Republic of South Africa from 1994 to 1995. During his doctoral degree he had a fellowship with Volkswagen AG Germany, and was employed as the general manager of the Motor Systems Resource Facility, an EPRI funded center in Corvallis, Oregon, USA. In 2006, he worked as an Adjunct Professor for electrical machines at Colorado State University, Fort Collins, CO. His main area of interest is predictive and preventive maintenance of electrical motors.

Dr. Wiedenbrug was the recipient of the 2008 Second Prize Paper Award from the Electric Machines Committee of the IEEE Industry Applications Society.

Subhasis Nandi (S’97-M’00-SM'06) received the B.E. degree from Jadavpur University, Calcutta, India, in 1985, the M.E. degree from the Indian Institute of Science, Bangalore, India, in 1988 and the PhD degree from the Texas A&M University, College Station, USA in 2000, all in electrical engineering.

He joined the Department of Electrical and Computer Engineering, University of Victoria, Victoria, Canada where he is currently employed as an Associate Professor. He is currently on sabbatical leave at Korea University. Between 1988 and 1996, he was with TVS Electronics and the Central Power Research Institute, Bangalore, India, working in the areas of power electronics and drives. His main research interests are power electronics and drives and analysis and design of electrical machines, with special emphasis on fault diagnosis.

He received the Power Engineering Society Prize Paper Award in 2007. He is a reviewer of many journals and conferences and currently serves as an Associate Editor for the IEEE Transactions on Industry Applications for the IAS Electric Machines Committee.