Stator and Rotor Design Considerations for Integral HP Machines - Austin Bonnett

Stator & Rotor Design Considerations for Integral HP Motors Austin Bonnett EASA Education and Technology Consultant Gallatin, MO

2008 EASA Convention Dallas, TX

Introduction Stator design can not be discussed in a vacuum because it is inseparably connected to the rotor through mutual inductance. Another critical component is the motor enclosure which houses the stator and rotor and facilitates the critical cooling circuit. Equivalent Circuit

The presentation is divided into the following components: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Influence of Application Requirements Basic Motor Fundamentals Stator Core Design Factors Winding Elements Stator and Frame Construction Rotor Design and Construction Motor Noise and Vibration Questions and Answers Appendix

WINDING OF A LARGE HIGH-VOLTAGE STATOR

Typical cast rotor assembly

1. Influence of application requirements In order to achieve the desired motor performance over a wide range of operation, it is critical for the designer to have a clear definition of the application requirements. Unnecessary, contradictory, or confusing information can adversely effect the desired outcome. There are numerous compromises in the design of an electric motor.

MOTOR POWERING ELECTRIC UTILITY INDUCTED-DRAFT FAN

The following are examples of the choices that need to be made.

1. Efficiency vs. Power Factor 2. Current-Torque Characteristics 3. Noise vs. Efficiency 4. Size vs. Operating Temperature 5. Insulation Quality vs Operating Temperature 6. Cost vs. Performance 7. Reliability vs. Enclosure

BOILER FEED PUMP

In summary the major application requirements are:

1. Power and Speed Ratings 2. Power Source 3. Enclosure and Frame Selection 4. Speed-Torque Issues 5. Duty Cycle 6. Environmental Factors

2. Basic Motor Fundamentals The electrical motor is still the work horse of industry. 2/3 of all generated electricity is used to drive these motors which are converted of electrical energy into mechanical energy.

Power engineers from all aspects of industry can benefit from a basic understanding of the following items. 1. Motor Nomenclatures 2. The Motor as a Converter of Energy 3. Power Equations 4. Efficiency and Loss Management 5. The MMF Forces 6. Using Simple Design Ratios 7. Containment of Motor Forces and Stresses

BASIC MOTOR FUNDAMENTALS

Basic Motor Equations

15

Stator Stresses • Thremal stresses ƒ Thermal aging ƒ Voltage variation ƒ Cycling ƒ Loading ƒ Ventilation ƒ Ambient

• Electrical stresses ƒ Dielectric aging ƒ Tracking ƒ Corona ƒ Transients

• Mechanical stresses ƒ Coil movement ƒ Rotor strikes ƒ Defective rotor ƒ Flying objects ƒ Lugging of leads

• Environmental stresses ƒ Moisture ƒ Chemical ƒ Abrasion ƒ Damaged parts ƒ Excessive ambient ƒ Restricted ventilation

Rotor Assembly Stresses • Thremal ƒ Thermal overload ƒ Thermal unbalance ƒ Excessive rotor losses ƒ Hot spots ƒ Sparking

• Magnetic ƒ Rotor pullover ƒ Noise ƒ Vibration ƒ Off magnetic center ƒ Saturation of lamintations ƒ Circulating currents

• Residual ƒ Stress concentrations ƒ Uneven bar stresses

• Dynamic ƒ Vibration ƒ Rotor rub ƒ Overspeeding ƒ Cyclic stresses ƒ Centrifugal force

• Environmental ƒ Contamination ƒ Abrasion ƒ Foreign particles ƒ Excessive ambient ƒ Restricted ventilation

Rotor Assembly Stresses-cont. • Mechanical ƒ Casting variations ƒ Loose laminations ƒ Incorrect shaft/core fit ƒ Fatigue or broken part ƒ Poor rotor-to-stator geometry ƒ Material deviations

• Other ƒ Misapplications ƒ Poor design practices ƒ Manufacturing variation ƒ Loose bars, core ƒ Transient torques ƒ Wring direction of rotation

TYPICAL SPEED-TORQUE/ CURRENT CURVE

AC Squirrel Cage Induction Motor Temperature Considerations 1. Basic elements of thermal circuit. a. Stator. b. Rotor. c. Bearing and lubrication system. 2. The thermal aging process and insulation life. 3. NEMA/IEEE insulation classifications and temperature rise. 4. Impact of service factor. 5. Altitude considerations. 6. Usual and unusual service considerations. 7. Special ambient considerations. 8. Voltage variation. 9. The effect of unbalanced voltage. 10. Harmonic impact including variable frequency.

Sources of Heat and Air Flow Within Motors

Tube-Cooled Air-to-Air Heat Exchanger

Terminal Aging Processes • Oxidation • Loss of Volatile Product • Molecular Polymerization • Reaction to Moisture • Chemical Breakdown • Vulnerable to Other Stresses

Temp. vs. Life Curves for Insulation Systems by AIEE 510 Method

*Assumes life doubles for a 10° C decrease in temperature.

Allowable Winding Thermal Load vs. Ambient (Class F System)

WINDING TEMPERATURE ALLOCATIONS

Typical Stator-Rotor Cross-section

3. Stator Core Design Factors The seven key elements of stator core design can be summarized as follows: 1. 2. 3. 4. 5. 6. 7.

Laminations Electrical Steel Magnetic Circuit Design Winding Configuration Loss Distribution Slot Combination L/D Ratio

The Stacked Stator Core The stacked stator core (SSC) can be defined as the stator laminations, air ducts (if needed), and any clamping plate or fingers needed to hold the assembly together prior to insertion into the stator frame. In these cases, this assembly is usually stacked on some sort of arbor to control the stator slot geometry. In larger sizes, the lamination may be stacked directly into the frame, which serves as tooling to control the geometry. These laminations normally are ring laminations made as a full circle. On stators larger than 45” in diameter, a segmented laminations are used.

Ring Laminations

Segmented Laminations

Stacked and Welded Stator Core

Electrical Steel Characteristics GENERAL CLASSES

SURFACE INSULATION

• Non-Oriented (AISI Grades M15 - M47)

• Oxidation

• Grain Oriented (AISI Grades M2 - M6)

• Core Plate

• Fully Processed • Semi-Processed

STEEL LOSSES • Core Loss (1.6 w/# - 3.1 w/# Range) • Hysteresis Loss • Eddy Current Loss

ANNEALING • Simple Stress Relief • Stress Relief Plus Decarburization (Grain Growth)

Criteria for Motor Efficiency • Watts Loss Per Pound • Permeability - Amount of Flux Density Without Saturation • Thermal Conductivity - Ability to Dissipate Heat • Steel Thickness - Loss vs. Strength (.018” x .025” Range)

Typical Motor Grade Electrical Steels

Effects of Magnetic Flux Density Low magnetic flux densities often indicate inefficient use of the magnetic materials. However, low is relative, and there are legitimate reasons for using low flux densities than ODP motors simply because there is more magnetic material per horsepower in the TEFC motors, and to use air gap flux density equivalent to that of ODP motors would create starting current problems. Also, low flux density is often inherent in the slow speed of two-speed winding motors. The magnetization of the steel core is not a linear function. The core steel will saturate with flux. When the densities reach saturation, the ampere-turns required to magnetize the steel will increase rapidly, causing high magnetizing current and low motor power factor. Also, high magnetic flux densities in the steel will cause high eddy current and hysteresis losses in the steel, thus lowering the efficiency of the motor and possibly causing it to run too hot.

Effects of Magnetic Flux Density What are the limits for flux densities? Well, isn’t a fixed answer. Design target limits should be in the range of 110130 kilolines per square inch in the lamination teeth, and 80110 kilolines per square inch in the backiron. Maximum should not exceed 138 in the teeth and 120 in the core, and even this is too high for most small motors, and for high speed large motors. The usually larger teeth and backiron of high speed motors simply cannot handle the losses associated with using high flux densities. (Backiron densities on the rotors of solid core rotors is usually low because the shaft carries part of the flux. On large rotors with air passages in the rotor, the rotor backiron density must be considered).

Effects of Magnetic Flux Density The magnetization and loss characteristics of the particular steel being used also must be considered. The magnetization curves of various steels will vary only slightly, affecting power factor to the extent that more or less ampere-turns are required to establish the design flux density. However, loss characteristics of lamination steels will vary greatly, and certain steel may not be usable for a particular rating. Good slot design is the key to optimum utilization of the magnetic materials in the motor. Don’t look just at the flux densities, but also at the ampere-turns required to magnetize the various parts of the motor. High ampere-turn requirements for any part (except air gap) is indicative of poor slot design.

Mutual (Coupling) Flux (4-Pole)

Flux Linkage of Stator to Rotor

Leakage Flux

Development of Average Air Gap Flux Density in Kiloline/Inches2

Development of Average Air Gap Flux Density in Kiloline/Inches2

E n P m 105

KILOLINES

13.95 f T N1 Di Lg Kp Kd

INCHES2

Bg =

Form-Wound Stator

Stator Winding

4. Winding Elements • Types of windings ƒRandom-wound lap ƒRandom-wound concentric ƒForm-wound

• Types of varnish ƒPolyester ƒ100% solid epoxy or 100% polyester

• Magnet wire ƒRound ƒRectangular

• Coils ƒRandom wound ƒForm wound

• Slot insulation ƒSlot liner ƒBottom sticks ƒCenter sticks ƒTopsticks ƒGroundwall

4. Winding Elements-cont. • Group insulation ƒPhase paper ƒSleeving ƒCenter sticks

• Connection ƒLead cable ƒSleeving ƒTie cord

• Coil bracing ƒTie cord ƒDacron felt ƒSurge rope ƒTape

• Treatment ƒVacuum pressure impregnation ƒDip varnish ƒAbrasion-resistant coating

Random wound

Form wound

Criteria for Random-Wound and Form-Wound Stators • All ratings over 700 hp should be form wound. • All ratings over 600 volts should be form wound. • Ratings 700 hp and less or 600 volts and less are typically random wound with some exceptions. • Form wound is available on many ratings that are normally random wound.

Form Wound vs Random Wound In comparing the two processes, keep in mind the basic differences in coil construction and the objectives of treating the coils. The form wound coils are completely wrapped with many layers of non-porous tape. It is voids which can result in hot spots or corona. The random wound coil has no tape in the slot portion and is not susceptible to corona. Because of these differences, several of the steps critical to the form wound stator are not required on the random wound stator. On the form wound construction, a pre-heat is necessary to remove moisture due to the many layers of tape. On the random wound construction, the moisture can be easily removed during the dry vacuum cycle. Again, because of the form wound coil construction, the pressure cycle is required to force the resin into small voids within the coil, whereas on the random wound the coils are more directly exposed to the resin and complete wetting and satisfactory slot fill is obtained during the wet vacuum cycle.

INSERTION OF STATOR COILS

WINDING OF A LARGE HIGH-VOLTAGE STATOR

Terminal Markings and Connections Three-Phase Motors - Single Speed

FORM-WOUND STATOR

Grouping, Pitch and Connection

Grouping, Pitch and Connection POSSIBLE NUMBER OF CIRCUITS

Winding Movement and Bracing

Blocking and Tying

Insulation Extension at Slot Edge

Slot insulation should protrude at least 3/8” beyond the end of the slot.

Phase Insulation

Phase insulation should protrude past the phase coils.

Winding Movement and End Turn Bracing

Straight Line Blocking

Straight Line Blocking

Winding Movement and Coil Bracing

Two examples of alternative bracing on a random winding (left) and a form winding (right). These examples use epoxy to simulate a surge.

5. Stator and Frame Construction The six key elements of stator frame design can be summarized as follows: 1. Motor Enclosure Options for Horizontal and Vertical Positions 2. Enclosure Impact on Motor Performance 3. Cast Iron vs. Fabricated Steel Materials 4. Noise and Vibration Issues 5. The Cooling Circuit 6. Environmental Considerations

Motor Nomenclature for Horizontal Motors

Typical Weather Protected I Enclosure

Typical Weather Protected II Enclosure

Typical Tube-Cooled (Air-to-Air) Enclosure

Motor Air Flow

450 hp, open dripproof, 5000 frame, 8-pole motor.

WPI Base Air Flow

WPII Air Flow

8000 frame, WPII, 6-pole motor.

Stator/Rotor Cooling

Air Ducts

6. Rotor Design and Construction The ten key elements of rotor design can be summarized as follows: 1. The Rotor Forces and Stresses 2.

Cast vs. Fabricated

3.

Bar Shapes and Fits

4.

Aluminum vs. Copper Cages and Other Alloys

5.

Rotor Skew and Air Gap

6.

The Cooling Circuit

7.

Length to Diameter Ratios

8.

Speed Torque Characteristics and Slip

9.

End Ring Forces

10. Unbalance Magnetic Forces and Noise

Typical Cast Rotor Assembly

Typical Squirrel Cage

Rotor Forces The majority of rotor failures are caused by a combination of various stresses which act on the rotor. In general terms, these stresses can be broken down as follows: • Thermal • Residual • Environmental • Electromagnetic • Dynamic • Mechanical

POTENTIAL ROTOR FORCES

Calculating Slip

Construction of Cast Rotors

Typical Rotor Laminations B C

A

A. Semi-processed, no insulation B. Fully processed, core plate insulation C. Semi-processed and annealed

Sample Fabricated Rotor Bars D C B A

A. B. C. D.

Aluminum with hard anodize Aluminum without insulation Aluminum with light anodize Copper without insulation

A Variety of Rotor Bar Shapes (Courtesy of Darby Electric)

Explanation of Skin Effect

Typical Lamination Stator Shaft Air Gap

Rotor

Typical lamination set showing relationship between stator and rotor teeth and air gap.

Large Motor, Slow Speed Spider Shaft

Rotor Bar Material The two common rotor bar materials are copper and aluminum. Traditionally, cast rotors have been aluminum; fabricated rotors can be aluminum or copper. Aluminum alloys and copper alloys have been available for special purposes such as high slip (NEMA type C & D ). In recent years a number of manufacturers have changed from copper to aluminum fabricated rotors. Although the higher conductivity of copper usually gives it a slight advantage in running loss, this can be largely overcome by the optimum shaping available in extruded aluminum bars. Extruded shapes are also available in copper but are very expensive.

Fabricated Rotors

Cast Rotor With Air Ducts

Side view of a large cast rotor showing the position of the air ducts.

Fabricated Rotors With Various Numbers of Air Ducts

Magnetic Centering Forces and Air Gap

Air Gap

This photo illustrates the air gap between the stator inside diameter and the rotor outside diameter.

Skewed Rotor Cage

Rotor With Skewed Bars

Skewing Skew is the angular twist of a slot away from the axial direction. Typical skew is one stator slot pitch. The purpose of the skew is to reduce special harmonics in the air gap flux that are introduced by a finite number of slots and the slotting combination. The results of skewing are: • Reduction of induced E.M.F. in the rotor bar. • Decrease in rotor leakage reactance. • A non-uniform axial distribution of the air gap flux. • Skewed bars have a current that has a circumferential component which develops a small axial force which imposes an additional load on bearings. • Non-uniform air gap flux increases core and stray losses. • Improved speed-torque characteristics, including elimination of locking torque at zero speed and cusps at various speeds. • Reduced likelihood of noise problems.

Swaging of Rotor Bars Sometimes it is necessary to tighten rotor bars during the manufacturing process or during repair and maintenance. Swaging is a relatively easy process which has been used for years. Swaging can also be used to tighten bars that have loosened in service and minimize propagation of bar cracking. The following slide shows a rotor bar before and after swaging.

Swaging Rotor Bars

Before swaging

After swaging

Example of a rotor where bars have been swaged.

Aluminum vs Copper Construction Preference Currently, the rotors of large induction motors are constructed of either aluminum or copper and their associated alloys. It is interesting that many people exhibit a preference of one or other of these materials in the construction of the rotor, when it is the construction itself that is important when considering rotor life. In fact, both have their advantages and are justified depending upon the specific application.

Aluminum vs Copper Construction Preference Supporters of copper will argue that aluminum melts at 1250° F as compared to copper’s 1980° F melting point, and therefore has greater stall capacity. While true, this disregards that most copper rotors are brazed to the end rings with a brazing alloy that melts at 1100° F. The results of a stall are no less disastrous with either material once the temperature to obtain molten metal is achieved. Extensive testing shown that either material, as normally applied, can be designed to exhibit comparable thermal, electrical and physical characteristics, including fatigue life as related to motor design.

End Ring Construction for a Typical Aluminum Bar Rotor Rotor bar

End Ring Construction for a Typical Copper Bar Rotor

7. Motor Noise & Vibration As Influenced by the Stator and Rotor Design

Ventilation Noise Ventilation (windage) noise is created in the air stream used to cool the motor. Windage noise is generated by the air flowing in and around the motor, as follows: 1. Fan blades rotating in close proximity to mounting bolts or other mechanical parts. 2. Restrictions in the air stream. 3. Abrupt changes in the direction of air flow. 4. Rotor air duct vent spacers passing by stationary stator vent spacers. Generally, the predominant noise source for six-pole and faster motors (two- through eight-pole speeds for TEFC motors) is ventilation noise. This is due to the higher fan speeds and greater CFM. Thus, to reduce noise levels on two- through six-pole motors, the ventilation noise must be reduced.

Major Electrical Noise/Vibration Considerations 1. Stator/Rotor Slot Combination (N1/N2) 2. Rotor Length to Diameter Ratio ( L/D) 3. Flux Density Saturation (Bg) 4. Air Gap Geometry 5. Stator Core Stability and Frame Structure 6. Speed Options 7. Shaft Stiffness 8. Miscellaneous Factors

Stator/Rotor Slot Combinations The magnetic flux in a motor is composed of the rotating fundamental sine-wave and the harmonic components. Only the fundamental wave or field actually provides useful tangential forces and usable rotating torque. Whereas the harmonic components of the wave only produce Parasitic torques which distort the accelerating speed-torque characteristic of the motor. The presence of these non-sinusoidal fields in the air gap of the motor can result in any of the following detrimental effects; 1. Starting or running noise 2. Synchronous locking torques 3. Dead points at zero speed 4. Torque dips 5. Stray losses Although it is not practical to eliminate all of these parasitic torques, the proper selection of the stator and rotor slots can minimize these influences. The proper selection of the stator winding span and rotor skew can further reduce these influences.

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