Stator design

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STATOR DESIGN of an

ELECTRIC MOTOR

S.VARUN

M.Tech[EST]

SRM UNIVERSITY

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MOTOR PARTS

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AREAS OF DESIGN

• Stator

• Rotor

• Shaft

• Frame & Bearing

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STATOR• The stator is an static part of the motor which acts

as outer body to house the driven windings on a laminated steel core for creating a rotating magnetic field.

• The stator core is made up of a stack of pre punched laminations assembled into a motor housing that is made of aluminum or cast iron or no separate housing designs.

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STATOR SIZES

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STATOR DESIGN CONSTRAINTS

• Stator Lamination.

• Magnet Wire.

• Stator Insulation.

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STATOR LAMINATION• Lamination Material

• Lamination Pattern

a)One Piece Lamination

b)T-Shaped Segmented Lamination

c)Connected Segmented Lamination

d)Two-Section Stator Lamination

e) Stator Lamination Integrated by Individual Teeth.

f)Slot less Stator core

g)Slinky Lamination Stator Core.

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ONE PIECE LAMINATION• One-piece lamination is fabricated from an undivided piece of steel sheet and continuous in the 360° circumference.• The one-piece laminationmethod offers the advantage of fabrication and assembly simplicity.

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T-SHAPED SEGMENTED LAMINATION

• Segmented stator laminations are frequently encountered in large-size motors.• The recent trend toward using segmented laminations in small motors is a means to not only increase the slot-filling rate and facilitate automated fabrication of electric motors.

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TWO-SECTION STATOR LAMINATION

• A stator lamination is integrated by two lamination pieces: a section of teeth and a section of yoke.• The stator windings are wound on the section of teeth.

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STATOR LAMINATION INTEGRATED BY INDIVIDUAL TEETH AND A YOKE SECTION

• Each tooth is individually positioned in the concave slot made at the inner surface of the yoke.• This design can use the grain-oriented material to make the teeth, arranging the grain orientation in the radialdirection.

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SLOT LESS STATOR CORE• Slot less stator designs have emerged as a solution to zerocogging in conventional PM motors. • The slot less stators totally eliminate the cogging torque, simplify the lamination process, and smooth the motor performance.

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SLINKY LAMINATION STATOR CORE• A stator core is built up from a continuous slotted strip of silicon steel rather than cross-sectional laminations in a conventional manufacturing process.• The strip is wound edgewise in a helical configuration by a coiling machine that consists of three flanged rolls.

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MAGNET WIRE

• Regular Magnet Wire

• Self-Adhesive Magnet Wire

• Litz Wire

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REGULAR MAGNET WIRE

• Regular magnet wire consists of a base metal, commonly copper or aluminum, and coated one or multilayer of insulation materials, such as enamel, fibrous polyester, fiberglass yarn, and polyamide.

• Several cross-sectional shapes of magnetic wires are available in stator windings, including round, square, and rectangular.

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SELF-ADHESIVE MAGNET WIRE

• When activated by heat or solvent, the bond

coating cements the winding turn-to-turn to create a self-supporting coil.

• This type of wires opens up new avenues for some special applications, especially where regular magnetic wires are not suitable.

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SELF-ADHESIVE MAGNET WIRE

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LITZ WIRE• Litz wire is basically used for high-frequency

applications. It contains many thin wire strands that are individually insulated and twisted together.

• Litz wire utilizes the full cross-sectional area of the wire to carry current.

• A size of a Litz wire is often expressed in abbreviated format of N/XX, where N is the number of strands and XX is the AWG size of each strand.

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STATOR INSULATION• Injection Moulded Plastic Insulation

• Slot Liner

• Glass Fibre Reinforced Mica Tape

• Powder Coating on Stator Core

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INJECTION MOULDED PLASTIC INSULATION

• Injection Moulded Plastic insulation provides highly insulating properties to laminated stator stacks.

• The use of moulded plastic insulation prior to winding assures consistent & durable insulation of wound winding to stator cores.

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Main Body & End Cap

Insulator applied to

Stator Windings

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SLOT LINER

• Heat Resistant & mechanically stable insulation papers or thermoplastic materials are inserted in stator slots for preventing coils from shorting from stator core.

• The typical thickness of an insulator material may have a range of 0.1-0.65mm.

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GLASS FIBER REINFORCED MICA TAPE

• For large size motors, stator windings are often made of conducting bars which are continuously wrapped with mica tape.

• The taped winding bars are placed in a vacuum impregnation tank & flooded with a epoxy resin.

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POWDER COATING ON STATOR CORE

• The motor stators are often shaped cylindrically with inwardly facing slots configured to receive stator windings.

• It is required to insulate the copper windings from stator metal surfaces which can also be achieved by applying powder coating techniques to provide uniform insulating coating layers on stator slot surfaces as well as partially on stator end surfaces.

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MANUFACTURING PROCESS OF STATOR

• Stator Lamination Cutting

• Lamination Fabrication Process

• Lamination Annealing

• Lamination Stacking

• Stator Winding

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STATOR LAMINATION CUTTING

• Laser cutting machine is often used for the cutting the laminations with extra large dimensions or complex geometrics.

• The design information is loaded into the machine & a high energized laser beam is focussed in a tiny spot so that the local temperature rises extremely high to melt lamination sheets.

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LAMINATION FABRICATION PROCESS

• This method uses a sequence of stamping operation on strip materials to generate first the rotor sheet piece & then the stator sheet piece.

• The stamping operation starts to punch pilot holes, shaft holes, rotor ventilating holes & rotor teeth to complete the rotor lamination fabrication.

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LAMINATION ANNEALING• During lamination cutting process, residual stresses

are introduced in processed laminations, leading to degradation of material magnetic properties near the edges of the laminations.

• The smaller the rotor size, the larger the cut effected zone relative to whole lamination area.

• So Annealing is necessary step for stress relief & for optimum properties of laminations with temperature of 730-790 deg C

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LAMINATION STACKING• The stator stack is formed by stacking laminations

into a pack. There are several methods to assemble stack laminations into stator cores.

• With the automation, the lamination stacking becomes more important which increases production efficiency & reduces the requirement for storage capacity, especially for rotor stacking.

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LAMINATION STACKING TECHNIQUES

• Welding

• Bonding with Adhesive Materials

• Riveting

• Fastened by pins

• Lamination Interlocking

• Using Slot-Liners

• Using Thin Sleeves

• Bolting

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WELDED STATOR STACK

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SPOT BONDED STATOR WITH EPOXY ADHESIVE

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THIN SLEEVE STATOR CORE

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STATOR WINDING• One of the important parameters in stator winding

is the slot fill ratio, which is defined as the percentage of the space occupied by magnet wires to the total available space of the slot.

• In order to lower the wire resistive loss and increase the power density, it is highly preferable to have the maximum copper fill, that is, maximum slot fill ratio.

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STATOR WINDING Contd..• A winding end turn refers to the amount of the

winding extending beyond each end of the stator’s magnetic core structure.

• Though the end turns are necessary to complete the electrical path within the winding, they contribute little to the motor torque output.

• Motor torque is only generated by the winding that lies within the stator’s magnetic core structure.

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STATOR WINDING Contd..

• So it is highly desired to minimize the length of the winding end turns.

• This can not only save the wiring material and lower the material cost but also reduce the copper loss and increase the motor efficiency.

• The shorter the winding stack length, the greater the impact of the end turn length on motor efficiency.

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WINDING METHODS• Random Winding.

• Distributed Winding by Hand.

• Concentrated Winding.

• Conductor Bar

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Distributed Winding

Concentrated Winding

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STATOR ENCAPSULATION & IMPREGNATION

• Encapsulating and impregnating stators can

strengthen stator winding electrical insulation,

provide reliable protection to chemicals and

harsh environments, enhance thermal

dissipation, promote stator structure integrity,

and stabilize motor operation.

• Partial encapsulation is basically applied to the

stator end windings for integrating them with

other stator components against vibration and

for enhancing heat transfer.

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ENCAPSULATION• Entire encapsulation is applied to the whole stator

assembly for achieving better protection of the stator from moisture, dirt, debris, and erosions caused by chemicals.

• A wide variety of encapsulation materials are available for electric machines.

• Thermoset plastics such as epoxies, phenolics, and thermoset polyesters, have long been applied to electric machines as encapsulation materials.

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ENCAPSULATION MATERIALS

• E88 epoxy.• C89 hardener• Araldite CW 229-3.• Aradur HW 229-1• Araldite XB 2252• Araldite CW 1312• Aratherm CW 2731• EP 234• EP 1282• EP 1285

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VARNISH DIPPING• Varnishing dipping is an effective way of securing

stator windings. In this method, a stator is immersed into an open varnish tank.

• After a certain time, the stator is removed from the varnish tank to allow excess varnish dipping. Then, place the stator in an oven to dry off solvent.

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ENCAPSULATED STATOR

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TRICKLE IMPREGNATION• Trickle impregnation is extensively used in many

motor manufacturers. In practice, a trickle varnish machine is usually used to impregnate the stator winding with a varnish that rigidly secures the wires.

• The varnish improves heat transfer within the winding and between its surrounding magnetic core structure. This improves motor cooling and in turn increases the motor’s continuous torque and power density.

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VACUUM PRESSURE IMPREGNATION

• The characteristic of this technology is to use a VPI tank that is vacuumed first and then pressurized to achieve the best insulation effect on stators.

• This method can provide the highest industrial standards for electric machines. By driving out voids from the electric winding through the VPI process, the thermal conductivity of the winding is remarkably enhanced so that the hot spots are eventually eliminated.

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VPI Contd..•It also reduces the risk of partial discharge in the winding. In fact, the VPI can make the high stator mechanical integration to reduce vibration of the motor.•It can achieve complete penetration of resin throughout turns, coils, slots, and insulation and thus is primarily applied on heavy-duty applications.

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VPI Contd..

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STATOR DESIGN CONSIDERATIONS

• Cogging Torque

• Air Gap

• Stator Cooling

• Robust Design of Stator

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COGGING TORQUE• Cogging torque is one of inherent characteristics

of PM motors, resulted from the interaction of

the PM MMF harmonics and the air gap

permeance harmonics due to slotting.

• As cogging torque can cause speed ripples,

induce motor vibration, and deteriorate motor

performance, it is one of the major design goals

for motor engineers to reduce cogging torque.

Electromagnetic design primarily determines the

level of cogging torque.

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AIR GAP• The radial distance between the rotor and stator in a

motor is defined as the air gap. Normally, a smaller air gap provides a more efficient and powerful motor.

• Hence, it is highly desired to maintain the air gap dimension as small as possible and within a small variation in operation. The control of the air gap dimension involves the design of several components such as the stator, rotor, motor housing, and end bells.

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AIR GAP Contd..• An important factor that affects the air gap

dimension is the accuracy of the coincidence of the stator and rotor axes.

• To provide a motor with a small air gap dimension within only a small tolerance, preciseness in manufacturing of these parts is required.

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STATOR COOLING• An important objective in motor design is to control

the motor temperature below its allowable value. Increased motor temperature often reduces motor efficiency and affects bearing life. The Thermal Engineers focus on cooling for following reasons.

• The stator winding is usually the main heat source in a motor. Test data show that in most applications heat generated in a motor is primarily attributed to the stator.

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STATOR COOLING Contd..• Cooling in the stator end-winding region is particularly

difficult and still remains a challenge due to various factors.

• As a stationary component, the stator is much easier to be cooled compared with the rotor. In fact, the stator often serves as a heat sink for the motor.

• For some electric motors, the pumping effect, which is resulted from the rotor rotation, is strong enough to generate turbulent circulating flows for making the motor self-cooling.

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ROBUST DESIGN OF STATOR• The root causes of motor failure are often related

to mechanical deterioration such as vibration,

static and cyclic loads, insulation fracture, and

bearing lubricant contamination and leakage.

• Because the rotor is supported on bearings

located at the end bells of the machine, the

stator design is significantly impacted by the

dynamic behaviour of the rotor.

• Motor vibration is greatly influenced by its base.

A weak motor base usually results in high

vibration.

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THANKS ALL