Motor Circuit Analysis – Principles & Case Studies

1. Motor Circuit Analysis – Principles & Case Studies Treadstone Rubber Pvt. Ltd.

2. MCA Capabilities • Test directly from the MCC • Check connections, cable & motor • Detect broken bars, inter-turn shorts, winding contamination, insulation problems & air-gap eccentricity • Check AC/DC motors, generators, servos & transformers

3. Motor basics - stator construction

4. Motor basics - stator failure modes • Turn to Turn • Coil to Coil • Open Circuit • Phase to Phase • Coil to Ground

5. What is Motor Circuit Analysis? Measurement & analysis of the following parameters: • Resistance • Inductance • Impedance • Phase Angle • Current/Frequency Response (I/F) • Capacitance

6. What are Phase Angle & Current/ Frequency Response? • Phase Angle • The relative angle between the two vectors of the applied test voltage & drawn current • Current/Frequency Response • Voltage is applied to the winding at different frequencies & the change in the current drawn is assessed

7. Evaluation of Windings • Winding Shorts • Phase Angle and Current/Frequency Response • Loose Connections • Resistance • Winding Contamination/Overheating • Inductance, Impedance & Capacitance • Rotor Condition and Severity • Inductance and Impedance Waveforms

8. Setting the Rules • Phase Angle and I/F readings • Both Fi and I/F > +/- 2 – Shorted Turns • Fi > +/- 1, I/F Balanced – Shorted Coils in the same phase • Fi Balanced, I/F > +/- 2 – Shorted Phase to Phase • Rules stand regardless of motor size • Resistance ~ +/- 5%

9. Setting the Rules (Cont.) • Impedance, Inductance & Capacitance • If impedance and inductance are parallel, winding is clean. • If impedance and inductance are not parallel, winding contamination or overheating has occurred. • If capacitance changes over time, winding contamination &/or partial discharges are indicated.

10. Setting the Rules (Cont.) • Inductance and Impedance Rotor Tests • Measurements of inductance at different rotor positions will indicate the condition of the rotor bars. • Rotor tests should be symmetrical and not necessarily ‘perfect’ sine-waves

11. Insulation Diagram of Motor Phase A Ground Circuit capacitance changes due to charge effects of molecules in an insulation medium. Dipoles are created as electric field crosses the molecules. As they align, capacitance increases. Phase B Phase C

12. - - The Dipole + Neg Potential Pos Potential

13. Dipolar Motion in Operation Capacitance Wire High Low GRND High Wire MegOhms High Ground Insulation Low High Voltage

14. Dipolar Motion in DC Tests Capacitance Neg High Low GRND Pos Neg MegOhms High Low Meg Ohms Time

15. Dipolar Motion in Surge Test Conductor 1 Conductor 2 Overcomes Dipolar Spin. Requires Higher Voltage in order to cross air gap. Potentially Destructive!!! Impulse Voltage Time

16. Dipolar Motion in MCA - 1 wire wire wire wire wire wire Good Phase Phase Angle: 77 degrees Current/Frequency: -44%

17. Capacitive Defect Dipolar Motion in MCA - 2 wire wire wire wire wire wire Bad Phase Phase Angle: 73 degrees Current/Frequency: -40%

18. Insulation Test Method Overview • Electrical Insulation is a dielectric. • Electric fields cause atoms to polarize with a change to capacitance as they polarize. • DC insulation tests measure change to capacitance as charge crosses in one direction. • Surge test uses a high voltage impulse in order to detect defects by causing an arc. Voltages too high to detect small changes in capacitive reactance due to defects in insulation. • MCA fully excites the insulation at low voltage. Small changes will effect circuit capacitance, which will not be ‘masked’ by high voltage. • MCA method also allows early detection of faults and, as a result, can trend winding faults over time.

19. Case Study 1 – 650 KW motor with interturn short in stator Motor is not grounded but trips on start-up ! Data courtesy – Rajashree Cement, Malkhed

20. Case Study 2 – 900 KW motor with winding contamination & loose overhang Motor had significant contamination, also the overhang lacing was completely broken ! Data courtesy – Reliance Industries, Hazira

21. Case Study 3 – 6000 KW synchronous motor with interturn short in rotor • 6000 KW, 200 RPM Synchronous Motor – 36 rotor coils and 244 stator coils. • Failed on start-up but unable to determine cause over two days with surge testing and other winding test technologies. • Motor circuit analysis applied, which identified faults within the rotor. • Four rotor coils found to be directly shorted, causing a fault in the motor secondary circuit. • Repaired and returned to service. Data courtesy – BJM Corporation, USA

22. Case Study 4 – 300 KW motor with repetitive bearing failures • 300 KW, 3000 RPM motor with continuous bearing faults • MCA rotor test shows combination of casting voids (resulting in severe rotor heating), and eccentric rotor • Fault detected on inspection - combined casting void and eccentric end-shield bearing fit. Data courtesy – BJM Corporation, USA

23. Case Study 5 – Noisy 150 KVA transformer Data courtesy – BJM Corporation, USA

24. Software Evaluation: EMCAT EMCAT software for MCA automatic data interpretation from direct data access from ALL-TEST IV PRO Data collector or manual data entry of combined technologies.

25. EMCAT: Automated analysis of AC motors

26. EMCAT: Automated analysis of AC rotors

27. EMCAT: Trending charts

28. TFE Software – estimation of residual life!

29. All-Test 31 Impedance Phase Angle Current/Frequency Response Insulation Resistance Automated Rotor Test All-Test IV Pro Impedance Phase Angle Current/Frequency Response Insulation Resistance Manual Rotor Test Resistance Inductance Capacitance Data logging Routes All-Test: Which one suits your needs?

30. For more details, contact: TREADSTONE RUBBER PVT. LTD. COMMERCE HOUSE ROOM#307 SHARDA BABU STREET, RANCHI-834001 JHARKHAND Tel: 91-651-3290691 Fax: 91-651-2305307 E-mail: treadsto@rediffmail.com