Short Course on:

Adjustable speed ac motor drives:

APPLICATION PROBLEMS AND SOLUTIONS

by

Drs. P. Enjeti and H. Toliyat

Electric Power & Power Electronics Institute (EPPEI)
Department of Electrical Engineering

Texas A&M University

College Station, TX-77843

Tel: 409-845-7466

Fax: 409-845-6259

Email: enjeti@tamu.edu
http://enjeti.tamu.edu

Summary

Adjustable speed AC drives have become the preferred choice in many industrial applications where controlled speed is required. At the same time, the maturing of the technology and the availability of fast and efficient solid state power semiconductor switches (IGBTs) has resulted in voltage source, PWM controlled inverters becoming a standard configuration in the power range to 500kW. While high frequency PWM control represents the most advanced drive concept, when inappropriately applied, it also generates side effects, some which have been recognized only recently.

This course presents a comprehensive coverage of application issues of PWM inverter controlled ac motor drives which include: damage to motor insulation due to reflected voltages caused by long motor leads; the mechanism of motor bearing failures due to excessive common mode dv/dt and leakage currents to ground.

Following a description of the problems, several practical solutions are also presented and analyzed. Finally, line harmonic problems caused by the input diode rectifier are also described and several low cost solutions selected to meet IEEE 519-1992 are presented.

PWM Inverter/motor manufacturers, system designers as well as adjustable speed drive application engineers and users will find this course informative and beneficial.

Tentative Short Course Outline

Adjustable Speed AC Motor Drives: Application Problems And Solutions

 

9.00 to 10.00

Section I Introduction to ASDs and Application Problems

  1. ASDs – Status Overview
  2. ASDs application requirements
  3. AC ASD Family
  4. PWM Control of output voltages
  5. ASD Manufacturers & their products
  6. Advantages of PWM drive technology
  7. Why the migration to IGBTs
  8. Application issues: motor heating, dv/dt problems, power quality

10.00 to 11.00

Section II Introduction: Application Problems Related to high dv/dt and Long Motor Leads

  1. Effect of using long cables between the inverter and the motor
  2. Voltage reflection analysis and calculation of motor terminal peak voltage as a function of cable length and dv/dt of the PWM pulses
  3. Simulations, example calculations and experimental waveforms on 460V, PWM inverter fed induction motor drive system for 10ft, 50ft, and 100ft cable length.
  4. Inverter and motor currents with long leads
  5. Factors contributing to motor voltage transient > 2pu dc-bus voltage, example calculation and results
  6. Pspice simulation example - Demo

11.00 to 11.15 – Break

11.15 to 12.00

Section III Over voltage stress on motor windings due to high dv/dt

  1. Introduction: types of failure modes
  2. Voltage distribution
  3. Experimental evaluation of turn / coil voltage distribution
  4. Predicting transient effect of PWM voltage waveform on stator windings – SABER model
  5. Failure mechanism of interturn insulation of low voltage electric machines fed by PWM Inverter
  6. NEMA MG-31 - Definite purpose inverter-fed motor specification
  7. IEC-60034-17 (1998) "Guide for the application of cage induction motors when fed from converters"

12.00 to 1.30 – Lunch

1.30 to 2.15

Section IV Effect of high common mode dv/dt at the terminals of an inverter fed induction motor

  1. Introduction
  2. Why there is common mode voltage at the inverter output ?
  3. Test Results
  4. Common mode impedance plots of typical induction motors & equivalent circuits
  5. Common mode dv/dt at the motor terminals is a function of Converter topology: Example motor powered by a PWM rectifier/PWM inverter system
  6. Shaft voltages, bearing currents and leakage currents to ground
  7. Equivalent circuit model
  8. Example experimental shaft voltages and bearing currents

2.15 to 2.30 – Break

2.30 to 3.45

Section V Solutions to Minimize motor failures

A – Filters:

  1. Introduction
  2. Application of series reactors
  3. Motor terminal filter to reduce dv/dt
  4. Conventional Inverter output filter
  5. A New Inverter Output Filter
  6. Comparison of various filtering methods
  7. Application of common mode choke to reduce leakage currents/EMI
  8. Software modifications in PWM generation to avoid >2pu overvoltages at the motor terminals
  9. B – Possible Motor design modifications
  10. In-slot winding method by Lincoln electric motor div., Cleveland, OH
  11. New magnet wire – Thermaleze QS by Phelps Dodge – Pulse endurance index
  12. Solutions to enhance bearing life in PWM drives

3.45 to 4.15

Section VI Other ASD Related Issues & Emerging trends

  1. Harmonics generated by ASD equipment
  2. Low cost clean power rectifiers for ASDs
  3. Nuisance tripping of ASDs for voltage sags
  4. Smart motors – motor mounted converters
  5. Emerging medium voltage (2300V, 3300V and 4160V) drive systems

References

4.15 to 5.00

Discussion & Conclusion

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