Post on 06-May-2017
transcript
Austin BonnettEASA Education and
Technology ConsultantGallatin, MO
2008 EASAConventionDallas, TX
Stator & Rotor Design Considerations for Integral HP Motors
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
Introduction
1. Influence of Application Requirements2. Basic Motor Fundamentals3. Stator Core Design Factors4. Winding Elements5. Stator and Frame Construction6. Rotor Design and Construction7. Motor Noise and Vibration8. Questions and Answers9. Appendix
The presentation is divided into the following components:
WINDING OF A LARGEHIGH-VOLTAGE STATOR
Typical cast rotor assembly
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.
1. Influence of application requirements
MOTOR POWERING ELECTRIC UTILITY INDUCTED-DRAFT FAN
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
The following are examples of the choices that need to be made.
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
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.
2. Basic Motor Fundamentals
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
Power engineers from all aspects of industry can benefit from a basic understanding of the following items.
BASIC MOTOR FUNDAMENTALS
15
Basic Motor Equations
Stator Stresses• Thremal stresses
Thermal agingVoltage variationCyclingLoadingVentilationAmbient
• Electrical stressesDielectric agingTrackingCoronaTransients
• Mechanical stressesCoil movementRotor strikesDefective rotorFlying objectsLugging of leads
• Environmental stressesMoistureChemicalAbrasionDamaged partsExcessive ambientRestricted ventilation
Rotor Assembly Stresses• Thremal
Thermal overloadThermal unbalanceExcessive rotor lossesHot spotsSparking
• MagneticRotor pulloverNoiseVibrationOff magnetic centerSaturation of lamintationsCirculating currents
• ResidualStress concentrationsUneven bar stresses
• DynamicVibrationRotor rubOverspeedingCyclic stressesCentrifugal force
• EnvironmentalContaminationAbrasionForeign particlesExcessive ambientRestricted ventilation
Rotor Assembly Stresses-cont.• Mechanical
Casting variationsLoose laminationsIncorrect shaft/core fitFatigue or broken partPoor rotor-to-stator geometryMaterial deviations
• OtherMisapplicationsPoor design practicesManufacturing variationLoose bars, coreTransient torquesWring direction of rotation
TYPICAL SPEED-TORQUE/CURRENT CURVE
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.
AC Squirrel Cage Induction Motor Temperature Considerations
Sources of Heat and Air Flow Within Motors
Tube-Cooled Air-to-Air Heat Exchanger
• Oxidation
• Loss of Volatile Product
• Molecular Polymerization
• Reaction to Moisture
• Chemical Breakdown
• Vulnerable to Other Stresses
Terminal Aging Processes
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
1. Laminations2. Electrical Steel3. Magnetic Circuit Design4. Winding Configuration5. Loss Distribution6. Slot Combination7. L/D Ratio
The seven key elements of stator core design can be summarized as follows:
3. Stator Core Design Factors
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.
The Stacked Stator Core
Ring Laminations
Segmented Laminations
Stacked and Welded Stator Core
GENERAL CLASSES• Non-Oriented (AISI Grades M15 - M47)• Grain Oriented (AISI Grades M2 - M6)• Fully Processed• Semi-Processed
SURFACE INSULATION• Oxidation
• Core Plate
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)
Electrical Steel Characteristics
• 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)
Criteria for Motor Efficiency
Typical Motor Grade Electrical Steels
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 110-130 kilolines per square inch in the lamination teeth, and 80-110 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.
Effects of Magnetic Flux Density
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
Bg =E n P m 105
13.95 f T N1 Di Lg Kp Kd
KILOLINES
INCHES2
Form-Wound Stator
Stator Winding
4. Winding Elements
• Types of windingsRandom-wound lapRandom-wound concentricForm-wound
• Types of varnishPolyester100% solid epoxy or 100% polyester
• Magnet wireRoundRectangular
• CoilsRandom woundForm wound
• Slot insulationSlot linerBottom sticksCenter sticksTopsticksGroundwall
4. Winding Elements-cont.
• Group insulationPhase paperSleevingCenter sticks
• ConnectionLead cableSleevingTie cord
• Coil bracingTie cordDacron feltSurge ropeTape
• TreatmentVacuum pressure impregnationDip varnishAbrasion-resistant coating
Random wound
Form wound
• 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 aretypically random wound with some exceptions.
• Form wound is available on many ratings that arenormally random wound.
Criteria for Random-Woundand Form-Wound Stators
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.
Form Wound vs Random Wound
INSERTION OF STATOR COILS
WINDING OF A LARGEHIGH-VOLTAGE STATOR
Terminal Markings and ConnectionsThree-Phase Motors - Single Speed
FORM-WOUND STATOR
Grouping, Pitch and Connection
POSSIBLE NUMBER OF CIRCUITS
Grouping, Pitch and Connection
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 Movementand End Turn Bracing
Straight Line Blocking
Straight Line Blocking
Winding Movementand 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.
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
5. Stator and Frame Construction
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
The ten key elements of rotor design can be summarized as follows:
1. The Rotor Forces and Stresses2. Cast vs. Fabricated3. Bar Shapes and Fits4. Aluminum vs. Copper Cages and Other Alloys5. Rotor Skew and Air Gap6. The Cooling Circuit7. Length to Diameter Ratios8. Speed Torque Characteristics and Slip9. End Ring Forces10. Unbalance Magnetic Forces and Noise
6. Rotor Design and Construction
Typical Cast Rotor Assembly
Typical Squirrel Cage
The majority of rotor failures are caused by a combinationof various stresses which act on the rotor.In general terms, these stresses can be broken down as follows:
• Thermal
• Residual
• Environmental
• Electromagnetic
• Dynamic
• Mechanical
Rotor Forces
POTENTIAL ROTOR FORCES
Calculating Slip
Construction of Cast Rotors
A. Semi-processed, no insulationB. Fully processed, core plate insulationC. Semi-processed and annealed
B
A
C
Typical Rotor Laminations
A. Aluminum with hard anodizeB. Aluminum without insulationC. Aluminum with light anodizeD. Copper without insulation
A
B
C
D
Sample Fabricated Rotor Bars
A Variety of Rotor Bar Shapes(Courtesy of Darby Electric)
Explanation of Skin Effect
Stator
Air Gap
Rotor
Shaft
Typical lamination set showing relationship between stator and rotor teeth and air gap.
Typical Lamination
Large Motor, Slow SpeedSpider Shaft
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.
Rotor Bar Material
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 Forcesand 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
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.
Skewing
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 componentwhich develops a small axial force which imposes an additional loadon bearings.
• Non-uniform air gap flux increases core and stray losses.• Improved speed-torque characteristics, including elimination oflocking torque at zero speed and cusps at various speeds.
• Reduced likelihood of noise problems.
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 of Rotor Bars
Swaging Rotor Bars
Beforeswaging
Afterswaging
Example of a rotor where bars have been swaged.
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.
Aluminum vs Copper Construction Preference
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 (windage) noise is created in the air streamused to cool the motor. Windage noise is generated by theair 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- throughsix-pole motors, the ventilation noise must be reduced.
Ventilation Noise
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
Major Electrical Noise/Vibration Considerations
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.
Stator/Rotor Slot Combinations
Be sure you have a copy of the2008 Select Presentations
CD-ROMIt contains most of the handouts plus many complete
technical papers from this year’s quality lineup of speakers!
(If you did not receive a CD in your packet as part of your registration, you may purchase a copy for only $30. Visit www.easa.com for more information.)