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.)