CONTINUOUS, DYNAMIC & INTERMITTENT THERMAL OPERATION IN ELECTRIC MOTORS.

INSTRUCTOR: Richard Welch Jr. - Senior Member IEEE

WHO SHOULD ATTEND: Design, System & Application Engineers - Motor Users

SCOPE: This Tutorial is a must for everyone using electric motors. Every motor has a maximum continuous or rated operating temperature. Exceed this rated temperature and the motor can suffer permanent and irreversible thermal damage. To prevent this from happening we first derive and discuss the motor's two-parameter thermal model. This "standard" model is still used extensively by motor manufacturers and motor users alike to calculate the motor's dynamic winding temperature during all possible modes of motor operation. Unfortunately, recent research has proven that this simple two-parameter model incapable of accurately predicting the motor's dynamic winding temperature during power overload operation. Next, the new four-parameter thermal model is derived and discussed. Using this four-parameter model you will learn why the motor's winding heats up much faster than calculated by the two-parameter model and also learn why a temperature sensor mounted inside the motor may not be able to protect its winding from overheating or even burning up. Using measured temperature data the improved accuracy of the four-parameter model is compared against the two-parameter model. Finally, we will also discuss practical ways to reduce
power dissipation inside the motor along with improving its heat transfer efficiency.

TUTORIAL OUTLINE:

"Standard" Two-Parameter Thermal Model

A. Maximum continuous operating temperature-maximum winding temperature
B. Total ambient condition
C. Dynamic Power dissipation and loss mechanisms inside motor
D. Winding's maximum "Hot-Spot" temperature - UL 1446
E. Winding to ambient thermal resistance
F. Thermal Resistance
G. Thermal capacitance
H. Thermal time constant
I. Maximum continuous current
J. Motor's Safe Operating Area Curve (SOAC)
K. Motor's Peak operating curve
L. Dynamic and Intermittent thermal operation
M. Time averaged power dissipation - RMS operating point
N. Calculating Dynamic winding temperature
O. Duty Cycle calculation
P. Limitations and Inaccuracy of two-parameter model

Four-Parameter Thermal Model

A. Winding's separate operating temperature
B. Winding's separate thermal resistance and thermal capacitance
C. Thermal resistance and capacitance for rest of motor (Case)
D. Derive dynamic equations for both winding and case heat-up and cool-down
E. Theoretical temperature curves for heat-up and cool-down
F. Comparison with two-parameter model
G. Power overload heat-up
H. Duty Cycle calculation
I. Why a temperature sensor may not protect winding from overheating
J. Verification with experimental data

Practical Ways to Reduce Motor Power Dissipation and Improve Heat Transfer Efficiency

A. Use of a Heat Sink
B. Forced Cooling
C. Use of Thermally Conductive Epoxy to Pot Stator Winding
D. Reduced Winding Resistance - increased slot-fill
E. " Cut-Core Stator"
F. Lower Core loss - Improved Lamination Materials - Powdered Metal
G. PWM loss from Drive
H. Examine Actual Motor Hardware - Questions and Answers

Biography for Richard Welch Jr

Richard Welch Jr. is a Senior Member of the IEEE Electric Machines Standards Committee and Adjunct Faculty at the University of Saint Thomas. He received his BEE and MSEE degrees from the University of Minnesota specializing in electro-magnetic field theory. In 1970 he began his professional career in the Electric Motor industry by becoming a Senior Design Engineer for the former Electro-Craft Corporation. At Electro-Craft he designed, built, tested and modeled high performance brush and brushless dc servomotors along with conducting extensive research on motor related issues such as mechanical resonance, dynamic braking, heat transfer, and power efficiency. In 1980 he changed focus and became Manager of Research for Despatch Industries Inc. At Despatch, he acquired a hands-on, in-depth, understanding of heat transfer and thermal issues by conducting numerous experiments on different products and materials using microwave, infrared, dielectric, and convection air ovens. Since 1985 he has been an independent consultant to both the electric motor and industrial heat processing industries. As a consultant he has worked on numerous design and development, manufacturing, cost saving, and application projects for several clients. In 1993 he became the Certified Motor's Instructor for the Association of International Motion Engineers (AIME). In this capacity he created several tutorials on different aspects of electric motor technology and taught these tutorials at numerous conferences and trade shows as well as to private industry. Richard's students appreciate his enthusiastic, hands-on, "real world" teaching style along with his ability to make complex motor theory understandable to both college students and practicing engineers and technicians. Richard has also published more than 30 technical papers and articles in both IEEE and Industry publications.

Tutorial includes 52-page Tutorial Book