41 Motor Thermal Model Protection Applications 1. Abstract This paper discusses the fundamentals of a motor thermal model and its mathematical interpretation and physics for the different stages of motor operation. (overload, locked rotor, too frequent or prolonged acceleration, duty cycling applications). It explains Thermal Model Time Constants and other technical parameters that cause the biasing of the thermal model algorithm. Other topics covered in this paper show that (a) detailed motor data sheet information, and (b) coordination between the protection engineer and the motor supplier, can lead to proper selection of motor thermal protection parameters. This paper presents a closer look at motor stall, acceleration and running thermal limit curves. It also explains the concept of thermal capacity and elaborates on how thermal capacity is evaluated in motor protection devices. The following points are also covered in this paper: • Discusses some additional methods, such as voltage- dependant and slip-dependant motor overload curves, employed to evaluate thermal capacity in nonstandard motor applications, • Presents the concept of matching thermal time constants for motor cyclic loads cases. In addition, the response of a thermal model algorithm in practical applications is demonstrated. • Describes a real case example showing how to apply and fine-tune the thermal model in high-inertia load application. • Explores in this context, some of the key topics that will ensure safe operation of the motor while promoting satisfactory motor design characteristics. 2. Introduction Induction motors are the workhorses of any industrial plant. Typical motor applications include pumps, fans, compressors, mills, shredders, extruders, de-barkers, refiners, cranes, conveyors, chillers, crushers, and blowers. Statistics have shown that despite their reliability and simplicity of construction, annual motor failure rate is conservatively estimated at 3-5% per year, and in extreme cases, up to 12%, as in the Pulp and Paper industry. Downtime in a factory can be very expensive and, in some instances, may exceed the cost of motor replacement. Proper machine protection is required to minimize the motor failure rate, prevent damage to associated equipment and to ensure both personnel safety and production targets. The document “Report of Large Motor Reliability Survey of Industrial and Commercial Installations”, published by the IEEE Motor Reliability Working Group [3] contains the results of IEEE and EPRI surveys on motor reliability and major causes of motor failure. The summary of these results is shown in Table I. In spite of different approaches and criteria (IEEE failure groups are formed according to “cause of failure” and EPRI according to “failed component”) both studies indicate a very similar failure percentage associated with mechanical- and electrical-related machine problems. Analyzing the data from this table we can conclude that many failures are directly or indirectly related to, or caused B.Venkataraman, B.Godsey Black & Veatch Corporation W. Premerlani GE Global Research Niskayuna, New York E.Shulman, M.Thakur, R.Midence GE Multilin Markham, Ontario Fundamentals of a Motor Thermal Model and its Applications in Motor Protection Table 1. Summary of IEEE and EPRI Motor Reliability Surveys. IEEE Study EPRI Study Average Failure Contributor % Failed Component % % Persistent Overload 4.2% Stator Ground Insulation 23.00 Electrical Related Failures 33% Normal Deterioration 26.40% Turn Insulation 4.00 Bracing 3.00 Core 1.00 Cage 5.00 Electrical Related Total 30.60% Electrical Related Total 36.00% High Vibration 15.50% Sleeve Bearings 16.00 Mechanical Related Failures 31% Poor Lubrication 15.20% Antifriction Bearings 8.00 Trust Bearings 5.00 Rotar Shaft 2.00 Rotor Core 1.00 Mechanical Related Total 30.70% Mechanical Related Total 32.00% High Ambient Temp. 3 Bearing Seals 6.00 Environmental Maintenance & Other Reasons Related Failures 35% Abnormal Moisture 5.8 Oil Leakege 3.00 Abnormal Voltage 1.5 Frame 1.00 Abnormal Frequency 0.6 Wedges 1.00 Abrasive Chemicals 4.2 Poor Ventilation Cooling 3.9 Other Reasons 19.7 Other Components 21.00 Environmental Reasons & Other Reasons Total 38.70% Maintenance Related & Other Parts Total 32.00%