Design and Testingof Insulation

for Adjustable Speed DrivesAlfredo Contin

DIA University of Trieste(Italy)

contin@units.it

Alfredo ContinDIA University of Trieste

(Italy)contin@units.it

In cooperation with

Andrea Cavalliniandrea.cavallini@unibo.it

Davide FabianiDavide.fabiani@unibo.itUniversity of Bologna(Italy)

Germano RabachUniversity of Trieste(Italy)

Purpose: to provide information on moderntechniques adopted to design and test insulation foradjustable speed drives (ASD).

Reasons: ASD dramatically increases electrical stress due tothe significant harmonic content of the power supplyand can promote premature breakdown. Design and testing criteria are quite different withrespect AC applications. Most of the procedures are still underinvestigationStandards are still under discussion

Purpose: to provide information on moderntechniques adopted to design and test insulation foradjustable speed drives (ASD).

Reasons: ASD dramatically increases electrical stress due tothe significant harmonic content of the power supplyand can promote premature breakdown. Design and testing criteria are quite different withrespect AC applications. Most of the procedures are still underinvestigationStandards are still under discussion

High slew rate Overshoots High switching frequency Uneven voltage distribution

PD activity Space charge accumulation Localized overheating Increased electrical losses Electromechanical fatigue

ProblemsProblems

DesignDesign New constrains Multi-objective design Accelerated life tests

SolutionsSolutions Filters (high cost) New insulating materials

(nano-tech)

EvaluationEvaluation Space charge measurements PD measurements

New constrains Multi-objective design Accelerated life tests

Summary:

1. Design of Insulation Systems for AC Applications

2. Additional Stresses

3. Degradation Processes in ASD Applications

4. Tests to Evaluate the Degradation Processes

5. New Materials and Systems

6. Design of Insulation Systems for ASDApplications

7. New Standards

Summary:

1. Design of Insulation Systems for AC Applications

2. Additional Stresses

3. Degradation Processes in ASD Applications

4. Tests to Evaluate the Degradation Processes

5. New Materials and Systems

6. Design of Insulation Systems for ASDApplications

7. New Standards

Summary:

Insulation technologies

Stresses and Aging

Design of insulation systems

for LV-MV-HV rotating machines

Reasons: current design criteria for insulation of ASDare derived from those adopted in AC considering additional stress typologies different impact of typical AC stresses

Design of Insulation Systemsfor AC Applications

Summary:

Insulation technologies

Stresses and Aging

Design of insulation systems

for LV-MV-HV rotating machines

Reasons: current design criteria for insulation of ASDare derived from those adopted in AC considering additional stress typologies different impact of typical AC stresses

1

2

31

a

c

b

Random Wound Machines

Typical solution for LV, LP rotating machines

Voltage

1) phase-to-phase

2) phase-to-ground

3) turn-to-turn 1

2

31

a

c

bVoltage

1) phase-to-phase

2) phase-to-ground

3) turn-to-turn

Insulation

a) phase-to-phase insulationwithin the slot and on theoverhang

b) ground insulation

c) turn insulation

dForm WoundCoils

Random WoundCoils

Wire Insulation

Modern magnet wire typically uses 1-4 layers ofpolymer film insulation.Increasing the temperature range:

Polyvinyl Polyurethane Polyamide Polyester

Polyester-polyimide Polyamide-polyimide

(or amide-imide) Polyimide (up to 250°C)

To improve the insulation strength and the long-term reliability, the insulation is often augmented by

wrapping it with fiberglass or mica tapes

using VPI technology

Polyvinyl Polyurethane Polyamide Polyester

Polyester-polyimide Polyamide-polyimide

(or amide-imide) Polyimide (up to 250°C)

Classification

magnet wire is classified by

diameter (AWG number or SWG)

area (square millimeters)

thermal class

insulation class.

The thermal class indicates the temperature of thewire corresponding to 20,000 hour service life

Common temperature classes are 105° C, 130° C,155° C, 180° C and 220° C (IEC 60085)

Classification

magnet wire is classified by

diameter (AWG number or SWG)

area (square millimeters)

thermal class

insulation class.

The thermal class indicates the temperature of thewire corresponding to 20,000 hour service life

Common temperature classes are 105° C, 130° C,155° C, 180° C and 220° C (IEC 60085)

Ground-Wall & Phase/Overhang Insulation

Wide use

Nomex : synthetic aramid paper (with high porosityfor VPI applications).

Mylar : Polyester film (Polyethylene Terephthalate(PET)).

Less adopted

Kapton : polyimide film higher performances butexpensive

Imp Obs: most of LV LP rotating machines are insulatedusing only organic materials

Ground-Wall & Phase/Overhang Insulation

Wide use

Nomex : synthetic aramid paper (with high porosityfor VPI applications).

Mylar : Polyester film (Polyethylene Terephthalate(PET)).

Less adopted

Kapton : polyimide film higher performances butexpensive

Imp Obs: most of LV LP rotating machines are insulatedusing only organic materials

Materials for MV & HV Motor Coils

Turn Insulation: is designed according to the ratedvoltage and the thermal class of the machine

Volt/turn 10<Vt<100 V

<3kV: 1-4 layers of polymer film insulation

3<Vn<6 kV: enamel ins.+1 layer of fiber-glass tape

5<Vn<7.5 kV: enamel ins.+2 layers of fiber-glasstape

7<Vn< 9 kV: enamel ins.+1 layer of paper/micatape

> 9 kV: enamel ins.+ layers of paper/mica tape

Turn Insulation: is designed according to the ratedvoltage and the thermal class of the machine

Volt/turn 10<Vt<100 V

<3kV: 1-4 layers of polymer film insulation

3<Vn<6 kV: enamel ins.+1 layer of fiber-glass tape

5<Vn<7.5 kV: enamel ins.+2 layers of fiber-glasstape

7<Vn< 9 kV: enamel ins.+1 layer of paper/micatape

> 9 kV: enamel ins.+ layers of paper/mica tape

Stack Insulation: mechanicalreinforcement for HV and HP rotatingmachines

Ground-wall or Main-wall Insulation: single ormultilayer tapes are currently adopted to form theground-wall insulation depending on:

the rated voltage

the impregnation technology

the severity of the application

Ground-wall or Main-wall Insulation: single ormultilayer tapes are currently adopted to form theground-wall insulation depending on:

the rated voltage

the impregnation technology

the severity of the application

Paper-Mylar-FiberGlass/Mica Tapes: high-grademuscovite/phlogopite sheets glued with a polymer andpressed on a tape (mechanical support of paper, Mylar or fiberglass)

Mechanical support Mica flakes Impregnation resin

Inorganic materials withstand to the discharge growthInorganic materials withstand to the discharge growth

Less the tangential discharges

Fiber-Glass Tapes: different texture of fiber-glassimpregnated with a polymer and pressed

Fiber-glass Impregnation resin

High mechanical and thermal properties. Withstand thetangential discharges, less the longitudinal discharges

Impregnation Technology

1) Resin Rich (HP turbo and hydro generators)

2) Single bar/coil VPI: improved testing, applicable forlarge machines, high costs

3) Global VPI: hermetic sealing, low costs, difficultrepair

Tape Typologies

A) 2 layers: paper/mica+fiber-glass

B) 3 layers: mylar+paper/mica+fiber-glass

C) 4 layers: mylar+paper/mica+fiber-glass+mylar

Taping half overlapped in a number of layers thatdepend on the rated voltage

Impregnation Technology

1) Resin Rich (HP turbo and hydro generators)

2) Single bar/coil VPI: improved testing, applicable forlarge machines, high costs

3) Global VPI: hermetic sealing, low costs, difficultrepair

Tape Typologies

A) 2 layers: paper/mica+fiber-glass

B) 3 layers: mylar+paper/mica+fiber-glass

C) 4 layers: mylar+paper/mica+fiber-glass+mylar

Taping half overlapped in a number of layers thatdepend on the rated voltage

Coils are inserted within the slotsto form the complete winding

The complete machine will beimpregnated using a properresin depending from the• rated voltage• insulation thermal class• costs

The complete machine will beimpregnated using a properresin depending from the• rated voltage• insulation thermal class• costs

Approx. Prices of VPI Resins

Insulation System for Higher Voltages

The insulation system for MV and HV machines iscompleted using slot and end-arm stress grading tapes

Slot Conductive Tape (Vn>4 kV)

Conductive tapes (polyester tapeimpregnated with conductive fillers)are provided on the surface ofthe coils to provide a uniformcontact with the laminatedmagnetic core to avoid tangentialdischarges on the coil surface

Conductive tapes (polyester tapeimpregnated with conductive fillers)are provided on the surface ofthe coils to provide a uniformcontact with the laminatedmagnetic core to avoid tangentialdischarges on the coil surface

End-Arm Stress Grading (Vn>6 kV)

Due to material discontinuity, high values of electricgradient affect the surface of the coil at the edge ofthe slot grading tape thus generating tangentialsurface discharges

End-Arm Stress Grading (Vn>6 kV)Semi-conductive tapes (polyester+SiC) are provided atthe edge of the slot grading tape to reduce the electricgradient below the inception of the discharges

Design of Inter-Turn and Ground-Wall Insulation

The insulation thickness must be subjected to anelectric stress << breakdown strength

The dielectric materials must be selected according tothe temperature class of the machine

nVKKd 21 nVKKd 21

whered [mm] is the insulation thicknessVn [kV] the rated voltageK1 minimum thicknessK2(V) parameter related to the applied voltage

Statistical Design of the Insulation

Due to small imperfections, two insulatedconductors/coils build up with the same materials andusing the same procedure, are not equal

Example: bubbles on the surface of an enameled wire

The insulation design is currently performed using theso called “life curves”, experimentally derived onstatistical basis

Ageing Models and Life Curves(summary from: L.Simoni, “Fundamentals of Voltage Endurance ofElectrical Insulation Materials”, Ed.Pitagora, Bologna (Italy), 2000)

Due to the applied stresses, the electrical insulation issubjected to changes in time

Ageing is referred to irreversible changes and can beevaluated by measuring some significant properties

Since strength is related to the material structure, itsmodification involves structural changes of dielectrics

The time modification progresses up to the insulationproperties decay to values where the insulation is unable towork satisfactorily or to breakdown (the insulation strengthis below the applied stresses)

Ageing Models and Life Curves(summary from: L.Simoni, “Fundamentals of Voltage Endurance ofElectrical Insulation Materials”, Ed.Pitagora, Bologna (Italy), 2000)

Due to the applied stresses, the electrical insulation issubjected to changes in time

Ageing is referred to irreversible changes and can beevaluated by measuring some significant properties

Since strength is related to the material structure, itsmodification involves structural changes of dielectrics

The time modification progresses up to the insulationproperties decay to values where the insulation is unable towork satisfactorily or to breakdown (the insulation strengthis below the applied stresses)

Let P the selected property and

Pi its initial value (Pi=P(t=0))

PL a threshold level assumed as end-of-life criterion(PL=P(t=L)) where L is the time duration or “life” of the material

S is the value of the applied stress

The dielectric ageing can be described by a function:

f(S, P, t)=0

If S=const=k f(P, t)S(k)=0 are the ageing curve

while f(S, t)P=PL=0 defines the “life curves”

Let P the selected property and

Pi its initial value (Pi=P(t=0))

PL a threshold level assumed as end-of-life criterion(PL=P(t=L)) where L is the time duration or “life” of the material

S is the value of the applied stress

The dielectric ageing can be described by a function:

f(S, P, t)=0

If S=const=k f(P, t)S(k)=0 are the ageing curve

while f(S, t)P=PL=0 defines the “life curves”

Pi

PL

S1 S2 S3 S4

S 4S 3

S 2S 1

Assuming L asthe expected lifeof the insulation,the stress value,S, to be appliedto obtain aduration of L, isderived from thelife curve

L1 L2 L3 L4

S 4S 3

S 2S 1

L

S

Assuming L asthe expected lifeof the insulation,the stress value,S, to be appliedto obtain aduration of L, isderived from thelife curve

Thermal Ageing

Thermal ageing is due to acceleration of chemical reactionsmainly in organic materials (depolarization, oxidation, hydrolysis…)caused by the temperature rise

In the presence of a dominant reaction, the ageing rate isequal to the reaction rate given by the Arrhenius equation

kT

E

rr eATK)( kT

E

rr eATK)(

where T is the absolute temperature [°K]E the activation energyK the Boltzman constantAr a constant that depends to the material

Since the time-to-failure (life) is inversely proportional tothe aging rate, it follows that:

where Lt is the thermal life and At=1/Ar

Considering a log transformation the equation becomes:

kT

E

tt eATL )(

Tk

EATL tt

1)ln())(ln(

Tk

EATL tt

1)ln())(ln(

that is a straight line in the(ln(Lt), 1/T) plot with a slope ofE/k (approximation)

The life curve is experimentally obtained

considering more than 4 samples of more than 5specimens each

fixing the temperature levels in a range of 200 hours(higher temperature) 5000 hours (lower temperature)

selecting a specific property to be monitored (the electricstrength and the weight loss are the most common prop)

fixing a threshold level for the property, e.g., PL=X%Pi

reporting the experimental data in the (ln(Lti), 1/Ti) plot

deriving the life curve using e.g., a linear regression

The life curve is experimentally obtained

considering more than 4 samples of more than 5specimens each

fixing the temperature levels in a range of 200 hours(higher temperature) 5000 hours (lower temperature)

selecting a specific property to be monitored (the electricstrength and the weight loss are the most common prop)

fixing a threshold level for the property, e.g., PL=X%Pi

reporting the experimental data in the (ln(Lti), 1/Ti) plot

deriving the life curve using e.g., a linear regression

Significant information can be derived extrapolating the lifecurve

The thermal index is defined as the temperature thatcorresponds to 20000 hours of duration

The different dielectric materials are classified intemperature classes and the insulation class is defined

The thermal profile (HIC Index)is defined by a couple of temperaturevalues evaluated at 10000 and 20000hours

Example: if the given material has aTI =133°C, its IC=B (B 130°-155°)

t=133°C

The thermal profile (HIC Index)is defined by a couple of temperaturevalues evaluated at 10000 and 20000hours

Example: if the given material has aTI =133°C, its IC=B (B 130°-155°)

Voltage Endurance

The interaction between different degradation processes(mechanical fatigue, dielectric losses, partial discharges, electro-chemicalprocesses) causes the electrical ageing of the dielectric atroom temperature. This quite complex phenomenon is stillnot completely understood.

Two simple models are very often considered on empiricalbasis, that is:

the inverse power law (IPL)

the exponential law EL)

nEAEL )(

Voltage Endurance

The interaction between different degradation processes(mechanical fatigue, dielectric losses, partial discharges, electro-chemicalprocesses) causes the electrical ageing of the dielectric atroom temperature. This quite complex phenomenon is stillnot completely understood.

Two simple models are very often considered on empiricalbasis, that is:

the inverse power law (IPL)

the exponential law EL)

nEAEL )(

where E is the electric stress [kV/mm]L the electric lifeA, n two constants for IPLK, h two constants for ELConstants are characteristics of the material

)exp()( hEkEL

Both the equations represent a straight line in a log-log andlog papers

IPL

EL

)ln()ln())(ln( EnAEL

Among the two, the IPL is often adopted for its ability torepresent the time behavior of dielectric materials having athreshold level for the electric stress below of whichelectrical ageing is negligible

hEkEL )ln())(ln(Among the two, the IPL is often adopted for its ability torepresent the time behavior of dielectric materials having athreshold level for the electric stress below of whichelectrical ageing is negligible

The time to breakdownand the electricstrength reduction arethe two commonproperties adopted involtage endurancetests

)ln()ln( ent

If E0 is the electrical stress at t=t0, two variables defined ast=L/t0 and e=E/E0 can be adopted and the IPL becomes

Using these variables,all the life curves relevant to the different materials showan e(t=t0)=1 and they differ only by the slope value n=n(e)

n is defined as theVoltage EnduranceCoefficient (VEC)Higher is the n value,longer is the durationof the material

Life Tests and Experimental Data Processing

Let us consider a sample of N specimens subjected to aconstant stress (e.g., constant electric stress)

An end-of-life criterium is selected according to the testtypology (e.g., the time to breakdown at roomtemperature)

Due to the small differences betweenthe different specimens, the end oflife is reached after a time of t1, t2,……, tN for the different specimens

Their average, tav, represents the“life” of the sample

Due to the small differences betweenthe different specimens, the end oflife is reached after a time of t1, t2,……, tN for the different specimens

Their average, tav, represents the“life” of the sample

t0 t2t3 ti tN

tav

t1

The accuracy of the estimation can be improved fitting theexperimental data with a probability distribution

The Weibull function is widely adopted in voltage endurancetests

)exp(1)( F

F

ttF

The cumulative probability that asingle specimen fails before a fixedtime tF is given by

where and are the scale and the shape parameters ofthe Weibull function

(63.2% of failure probability) is assumed as thecharacteristic time-to-failure of the sample under test

The Weibull plot where the Weibull function is representedby a straight line, can be obtained with a log-logtransformation

where and are the scale and the shape parameters ofthe Weibull function

(63.2% of failure probability) is assumed as thecharacteristic time-to-failure of the sample under test

The Weibull plot where the Weibull function is representedby a straight line, can be obtained with a log-logtransformation

exp)(

exp1)(

1

xxxf

xxF

0

63.2%F(x) xif0

63,2%

x

f(x)

ln

)(1lnln

)ln(

C

xFY

xX

YCX

0

63.2%F(x) xif0

x

x

The linear regression or the maximum likelihood methodsand goodness-of-fit test (2 , F,..) can be adopted to fit theexperimental data

The confidence intervals are also evaluated

The procedure is repeated more that 3 times and theexperimental data are fitted using a suitable aging model

The linear regression is typically adopted obtain thecharacteristic parameters of the aging model

)exp(1)( Fi

Fi

ttF

tFi= failure time= scale parameter(63.2% of failureprobability)= shape parameter

Life test results: failure timesWeibull Probability Plot

tF = k V –N

tF= failure time (relevant to x% failure prob.)k = model parameterN= Voltage Endurance Coefficient (VEC)

tFi= failure time= scale parameter(63.2% of failureprobability)= shape parameter

Life model

IPL

Warning

The duration of the endurance tests must be shortened withrespect to the service conditions (accelerated ageing tests)

The long term performances of the materials areextrapolated by the aging model

Accelerated aging test are useful to compare differentmaterials

Multifactor aging is a quite complex topic not treated here

Warning

The duration of the endurance tests must be shortened withrespect to the service conditions (accelerated ageing tests)

The long term performances of the materials areextrapolated by the aging model

Accelerated aging test are useful to compare differentmaterials

Multifactor aging is a quite complex topic not treated here

Additional Stresses in ASD Applications

Both ground-wall, enamel wire and turn insulationare designed to operate, mostly, at power frequencyvoltages (50/60 Hz).

In ASD applications, the fast switching producescomplex transients that severely stress the motorinsulation and can cause premature failures

Transients generated in ASD were investigatedconsidering mainly the PWM technique, with the aimto evaluate their impact on the insulation

Additional Stresses in ASD Applications

Both ground-wall, enamel wire and turn insulationare designed to operate, mostly, at power frequencyvoltages (50/60 Hz).

In ASD applications, the fast switching producescomplex transients that severely stress the motorinsulation and can cause premature failures

Transients generated in ASD were investigatedconsidering mainly the PWM technique, with the aimto evaluate their impact on the insulation

InputLine Filter

Capacitor

Typical PWM VFD

Speed Hz

Torque V/Hz

DC Bus

InputLine Filter

Capacitor

InverterRectifier

AC Motor

AC to DC DC to AC

PWMOutput

PWM VFD ComponentsNumerous advancements over the 25 yrs

• Power devices evolved:• Thyristor (SCR) GTO Bipolar

Transistor to present day IGBT devices• IGBT (Insulated Gate Bipolar Transistor):

• Faster Switching / Higher performance• Lower losses / Higher efficiency• Smaller packaging• Robust / Increased reliability

PWM VFD ComponentsNumerous advancements over the 25 yrs

• Power devices evolved:• Thyristor (SCR) GTO Bipolar

Transistor to present day IGBT devices• IGBT (Insulated Gate Bipolar Transistor):

• Faster Switching / Higher performance• Lower losses / Higher efficiency• Smaller packaging• Robust / Increased reliability

faster the switching (lower rise time)higher the failure rate is

Typical Wave Shapes

MM50 Hz P.S.50 Hz P.S.

+V

0

-V

+V

V

+V

0

-V

+V

V

Inverter outputwave shapes

Over voltage peaksat the motor

terminals

Impact of the Cable Length

Over-voltages at the motor terminals are due to:

rise/fall time of the rectangular supply voltage

impedance mismatch at the connection cable/motor

cable lengthSee e.g.,:

A. H. Bonnett, “Analysis of the Impact of Pulse-Width ModulatedInverter Voltage Waveforms on AC induction Motors”, IEEE Trans. Ind.Applicat., Vol. 32, No. 2, pp. 386-392, March 1996.

B. Wu, and F. A. DeWinter, “Voltage Stress on Induction Motors in Medium-Voltage (2300-6900V) PWM GTO CSI Drives”, IEEE Trans. Ind. Applicat., Vol.12, No. 2, pp. 213-220, March 1997.

J. P. Bellomo, P. Castelan and T. Lebey, “The effect of PulseVoltages on Dielectric Material Properties”, IEEE Transaction on Dielectricsand Electrical Insulation, Vol. 6, No. 1, pp. 20-26, February 1999.

Impact of the Cable Length

Over-voltages at the motor terminals are due to:

rise/fall time of the rectangular supply voltage

impedance mismatch at the connection cable/motor

cable lengthSee e.g.,:

A. H. Bonnett, “Analysis of the Impact of Pulse-Width ModulatedInverter Voltage Waveforms on AC induction Motors”, IEEE Trans. Ind.Applicat., Vol. 32, No. 2, pp. 386-392, March 1996.

B. Wu, and F. A. DeWinter, “Voltage Stress on Induction Motors in Medium-Voltage (2300-6900V) PWM GTO CSI Drives”, IEEE Trans. Ind. Applicat., Vol.12, No. 2, pp. 213-220, March 1997.

J. P. Bellomo, P. Castelan and T. Lebey, “The effect of PulseVoltages on Dielectric Material Properties”, IEEE Transaction on Dielectricsand Electrical Insulation, Vol. 6, No. 1, pp. 20-26, February 1999.

The connection cablesbehave as transmission linefor the inverter output pulses

Distributed cable leakageinductances and couplingcapacitances (L-C)

mismatch impedancecable/motor connections

determines a damped highfrequency ringing at themotor terminals, resulting inover voltages whoseamplitude depends both by

The connection cablesbehave as transmission linefor the inverter output pulses

Distributed cable leakageinductances and couplingcapacitances (L-C)

mismatch impedancecable/motor connections

determines a damped highfrequency ringing at themotor terminals, resulting inover voltages whoseamplitude depends both by

the cable length

its resonant frequency

A) Inverter output

B) 5m cable length

C) 20m cable length

Cable length [m]

1 10 100O

verv

olta

gefa

ctor

(rela

tive

valu

e)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

tr = 50 nstr = 100 nstr = 200 nstr = 1000 ns

Lettr = rise timelC= cable lengthf = resonant frequency (MHz)ν = wave propagation speed

Each cable shows a properresonant frequency

that is explored varyingthe cable length

Cable length [m]

1 10 100O

verv

olta

gefa

ctor

(rela

tive

valu

e)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

tr = 50 nstr = 100 nstr = 200 nstr = 1000 ns

MPCr

MPCr

VVmnslt

VVmnslt

25.12

25.12

cc llf /404/

f = resonant frequency (MHz)ν = wave propagation speed

VP = peak voltageVM= DC bus voltageVP /VM = overvoltage ratio

Warning

At higher commutationfrequencies (short T) thesecond transition can occurduring the transient of thefirst one thus determining:

Vp>2VM in unipolar pulses

Vp>4VM in bipolar pulses

Warning

At higher commutationfrequencies (short T) thesecond transition can occurduring the transient of thefirst one thus determining:

Vp>2VM in unipolar pulses

Vp>4VM in bipolar pulses

Due to impedance mismatch

part of the signal is reflected

part is absorbed by thewinding

0 0.5 1 1.5 2 2.5 3

x 10-7

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

VV11--VV22:: turnturn--toto--turnturnVV11:: turnturn--toto--groundground (first(first turn)turn)VV22:: turnturn--toto--groundground (last(last turn)turn)

V1

PhasePhase--toto--phasephase

+ ∆ V

Impact on the Motor Winding

TurnTurn--toto--TurnTurn

0 0.5 1 1.5 2 2.5 3

x 10-7

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

V1

V2V1-V2PhasePhase--toto--groundground

+V

0

-V

−∆ V

Phase-to-Phase Voltage Wave-Shape

• Due to the phase shift fptp=3fsp

• Unipolar rectangular pulses in a half cycle• High over-voltages (Vpp) that can double in the

transition between two half cycles• Higher voltage stresses with respect the rated

voltage (derivative effect)

Phase-to-Ground Voltage Wave-Shape

• Bipolar rectangular pulses in a half cycle

• The same fundamental of the PWM modulation

• High over-voltages (Vpp) in the presence of shortduty

• Higher electrical stress with respect the ratedvoltage (derivative effect)

Turn-to-Turn Voltage Wave-Shape

Mag

netic

Cor

e

V terminal V turn-to-turn

• Each winding is composed by M coils composed by N turns

• Each turn is characterized by a turn-to-ground and turn-to-turn capacitances

• In the presence of sinusoidalsupply, the voltage dropalong the winding is linear

Vt=V/MN

• Using PWM supply, thevoltage drop is not linear :

• exponential-like in form-wound windings

• Dependent by the relativeposition of the turns inrandom-wound windings

Mag

netic

Cor

eCenter star V sin V impulse

• In the presence of sinusoidalsupply, the voltage dropalong the winding is linear

Vt=V/MN

• Using PWM supply, thevoltage drop is not linear :

• exponential-like in form-wound windings

• Dependent by the relativeposition of the turns inrandom-wound windings

V 1

V 2

V 1-V 2

Voltage Drop in Random-Wound Windings

The time behavior of the voltage stress between twoadjacent turns are due by the difference between their turn-to-ground voltages V1-V2

The max amplitude is highest if the first and the last turn ofthe coil are in contactIf V2 is negligible (fast rise-time)the turn insulation is stressed bythe whole phase voltage:

Vt>>V/MN

V 1

V 2

V 1-V 2

If V2 is negligible (fast rise-time)the turn insulation is stressed bythe whole phase voltage:

Vt>>V/MN

Voltage Drop in Random-Wound Windings

V

tr [μs]

Voltage Drop in Random-Wound Windings

Worst case voltage stressing the turn/turninsulation in a variety of random woundstators as a function of the rise time of theimpulse. 1.0 is the peak phase/ground jumpvoltage at the machine terminals (V –Voltage, tr - Impulse risetime).

V

tr [μs]

Neutral-to-Ground Voltage Wave-Shape

Due to the voltage imbalance, the instantaneous neutral-to-ground voltage is not null but impulsive

The neutral point becomes a ”pulse generator” and theconnected coils are subjected to impulsive stress like thefirst coil

Even the turn insulation ofthe last coil is stressed byan higher level of voltage

Vt>>V/MN

Even the turn insulation ofthe last coil is stressed byan higher level of voltage

Vt>>V/MN

Simulations

Transients effects in ASD applications are studiedresorting to simulation of the complete system in order to predict the distribution of the voltage stresses to obtain information for insulation design

The signal transmission theory is adopted to simulate theconnection cables instead the more complex distributedparameter model

SSZR

ZRS

o

o

1 SZR

RS

oo 0

Transients effects in ASD applications are studiedresorting to simulation of the complete system in order to predict the distribution of the voltage stresses to obtain information for insulation design

The signal transmission theory is adopted to simulate theconnection cables instead the more complex distributedparameter model

SSZR

ZRS

o

o

1 SZR

RS

oo 0

Where S is the input signalZ characteristic impedance of the cableRo output impedance of the cableS1 reflected signalSo output signal (So=S+S1)

Coil modelThe random nature of the coil with unknown position ofthe turns inside the slot can be approached consideringthe coupling between two turns:

where Lt, Rt are the turn self inductance and resistanceMij the magnetic coupling between turnsCtt, Ctg the capacitive couplings between adjacent turnsand external turns and grounded stator, respectively

After selecting the number of turns, all the possibleconfigurations of turn connections are explored (e.g.,7turns)

Two different simulations arecompared with the experimentalplot: coil impedance vs frequencyZin-gnd shows two resonantfrequencies. These resonanceschange with the turnarrangement but not in largedomain

Two different simulations arecompared with the experimentalplot: coil impedance vs frequencyZin-gnd shows two resonantfrequencies. These resonanceschange with the turnarrangement but not in largedomain

Winding modelA simplified electrical model (derived from the coil model) canbe adopted to simulate the whole winding where

Lw, Rw are the turn self inductanceand resistanceRp, Cp the equivalent parallelresistance and capacitanceRgnd, Cgnd the equivalent phase-to-ground resistance and capacitanceThe experimental validation shows a good agreement

Lw, Rw are the turn self inductanceand resistanceRp, Cp the equivalent parallelresistance and capacitanceRgnd, Cgnd the equivalent phase-to-ground resistance and capacitance

I° Solution: Multi-Level Converters

• Reduction of the Jump voltage 0.7(Vdc/(n-1) + Vb)

• Reduction of the ph-to-ph Vpp Vdc/(n-1) + 2Vb• Effective but expensive• Valid if adopted for other purposes

0 500 1000 1500 2000 2500 3000 3500-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

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-1

-0.5

0

0.5

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1.5

2

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1.5

2

0 500 1000 1500 2000 2500 3000-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2 Levels

3 Levels turn-to-turnstressdecreases0 500 1000 1500 2000 2500 3000

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 500 1000 1500 2000 2500 3000-2

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-0.5

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-0.5

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0.5

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1.5

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-1.5

-1

-0.5

0

0.5

1

1.5

2

5 Levels

Phase-to-Ground V

turn-to-turnstressdecreases

Turn-to-Turn V

Harmonic Filters

Two kinds of analog filters can be connected at theinverter output: to reduce the harmonic content of the supply voltage(rise-time increase, overvoltages reduction, delay timeincreased, voltage stress reduction at bearings) to transform the PWM in sinusoidal wave-shape (avoidthe use of shielded cables and EMC problems)

inverter output filter output

Which quantities associated to the harmonicWhich quantities associated to the harmonicdistortion affect accelerated degradation?distortion affect accelerated degradation?

VVslopeslope(dV/dt)(dV/dt)

The selection of the materials and the design ofinsulation systems for ASD are here considered

But

VVpeakpeak VVrmsrms VVslopeslope(dV/dt)(dV/dt)

Freq.Freq.

Do we know the voltage waveforms affectingDo we know the voltage waveforms affectinginsulation systems & electrical apparatusinsulation systems & electrical apparatus(rotating machines)?(rotating machines)?

Do we know the degradation mechanism?Do we know the degradation mechanism?

Rep.RateRep.Rate

Voltage waveforms for ASD are characterized by: High slew rate Overshoots High switching frequency Uneven voltage distribution

Stresses in ASD Applications

The effects are: PD activity Space charge accumulation Localized overheating Increased electrical losses Electromechanical fatigue

The effects are: PD activity Space charge accumulation Localized overheating Increased electrical losses Electromechanical fatigue

Many investigations have been performed to evaluate the impact of typical ASD stresses (Vpp,Vrms, dV/dt, f, rep.rate) on the ageing processes to identify the most important factors that affects theinsulation life-time.This was done by conducting accelerated ageing tests(mainly electrical and thermal ageing) using different voltagewave-forms and repetition rate (see e.g.)

S.Grzybowski et al. “Accelerated Ageing Tests on Magnet Wires Under HighFrequency Pulsating Voltage and High Temperatures”, Proc. of CEIDP, pp.555-558, 1999

M.Kaufhold, et al. “failure Mechanisms of the Interturn Insulation of LowVoltage Electric Machines Fed by Pulse-Controlled Inverters”, IEEE El.Ins.Mag.,Vol.12, pp.9-16, Sept./Oct. 1996.

A.Mbaye et al. “Existence of PD in Low-Voltage Induction Machines Suppliedby PWM Drives”, IEEE Trans. on Diel., El. Ins., Vol.3, pp.555-560, August1996

Many investigations have been performed to evaluate the impact of typical ASD stresses (Vpp,Vrms, dV/dt, f, rep.rate) on the ageing processes to identify the most important factors that affects theinsulation life-time.This was done by conducting accelerated ageing tests(mainly electrical and thermal ageing) using different voltagewave-forms and repetition rate (see e.g.)

S.Grzybowski et al. “Accelerated Ageing Tests on Magnet Wires Under HighFrequency Pulsating Voltage and High Temperatures”, Proc. of CEIDP, pp.555-558, 1999

M.Kaufhold, et al. “failure Mechanisms of the Interturn Insulation of LowVoltage Electric Machines Fed by Pulse-Controlled Inverters”, IEEE El.Ins.Mag.,Vol.12, pp.9-16, Sept./Oct. 1996.

A.Mbaye et al. “Existence of PD in Low-Voltage Induction Machines Suppliedby PWM Drives”, IEEE Trans. on Diel., El. Ins., Vol.3, pp.555-560, August1996

Effect of Duty CycleThe test was conducted on twistedpairs with polyurethane resin, atVp0=950 Vtr=200 nsRR=15 kHzT= 100°C.

100T

tD p

tp: duration of positive pulsesT: periodthe lifetime of the insulationdecreases with increasing the dutycycleDuty is proportional to the rmsvoltage

Effect of the Slew RateThe average ttb under differentrise times were recorded usingtwisted pairs with polyurethaneresin, atVp0=950 VDC=16%RR=15 kHzT= 100°C.

Effect of the Slew RateThe average ttb under differentrise times were recorded usingtwisted pairs with polyurethaneresin, atVp0=950 VDC=16%RR=15 kHzT= 100°C.

fast rising voltage pulses create high capacitive impulsivecurrents. RT causes spikes, over-voltages and uneven voltage distributionalong the winding local dielectric heating space charge formation

Repetition Rate EffectThe TTB were evaluate usingtwisted pairs with polyurethaneresin, atVp0= increased in steps of 50V/sDC=16%RT=200 nsT= 100°, 155° 180°C.

Samples aged at 40 kHz (shorterduration) endured the life testlonger than those which wereaged at 25 kHz.The dielectric losses are smaller athigher pulsating frequencies dueto the prevailing polarizationmechanism under pulsatingfrequencies

Samples aged at 40 kHz (shorterduration) endured the life testlonger than those which wereaged at 25 kHz.The dielectric losses are smaller athigher pulsating frequencies dueto the prevailing polarizationmechanism under pulsatingfrequencies

Thermal EffectsBecause of the short pulse duration and fast RT at highfrequencies, voltage waveforms are affected by high orderharmonic components

high values of the capacitive current (proportional to ) polarization processes insulation conductivitySignificant increaseg the insulation temperature (thermaldegradation)

tgCVPd2

n

iiiid tgVCiP

1

2

1

1

P

PDP

n

ii

Kf= ageing acceleration factor

Thermal EffectsBecause of the short pulse duration and fast RT at highfrequencies, voltage waveforms are affected by high orderharmonic components

high values of the capacitive current (proportional to ) polarization processes insulation conductivitySignificant increaseg the insulation temperature (thermaldegradation)

1V

V ii

Thermal EffectsThe TTB=f(T, RR) atVp0= increased in steps of 50V/sDC=16%RT=200 nsRR= 15, 25 and 40 kHz

the dielectric loss increases with: pulsating frequency rise timeof the voltage pulses

Voltage StressesThe impact of the voltagestress must be evaluatedconsidering: Square unipolar (phase-to-phase voltage) Square bipolar (phase-to-ground turn-to-turn voltage) Sinusoidal at differentfrequencies (dV/dt negligible) With and without over-voltages

+V

0

-V

+ ∆ V

−∆ V

Voltage StressesThe impact of the voltagestress must be evaluatedconsidering: Square unipolar (phase-to-phase voltage) Square bipolar (phase-to-ground turn-to-turn voltage) Sinusoidal at differentfrequencies (dV/dt negligible) With and without over-voltages

Sinusoidal Voltages at Different FrequenciesAt constant voltage, the time-to-breakdown is reducedincreasing the frequency of the voltage mainly in thepresence of organic materials

2D Graph 1

0.01 0.1 1 10 100 1000

Vol

tage

(rm

s va

lue)

[V

]

1000

10000

50 Hz OIL10 kHz OILVEC = 11.7

VEC = 9.2

Failure time [h]

Comparison of life curvesobtained testing twisted pairssamples immersed in oil toavoid surface discharges.Tests performed at 50 Hz and10 kHz

2D Graph 1

0.01 0.1 1 10 100 1000

Vol

tage

(rm

s va

lue)

[V

]

1000

10000

50 Hz OIL10 kHz OILVEC = 11.7

VEC = 9.2

Failure time [h]

VEC is evaluated for the comparison:VEC=11.7 indicates a longer life with respect to VEC=9.2

Comparison of life curvesobtained testing twisted pairssamples immersed in oil toavoid surface discharges.Tests performed at 50 Hz and10 kHz

With and Without Over-Voltages

Over-voltages are due to R/F-T, cable length, mismatchimpedance, converter topology

Causes an uneven distribution of the stress along thewinding

In random wound windings, the voltage stress between thedifferent turns depend by their relative position

It is highest when the first and thelast turn of the first coil are incontact

The overvoltage can be consideradiabatic (negligible heating).Rectangular wave shapes heat theinsulation in the same way

It is highest when the first and thelast turn of the first coil are incontact

The overvoltage can be consideradiabatic (negligible heating).Rectangular wave shapes heat theinsulation in the same way

Unipolar and Bipolar Wave-Shapes

Rectangular wave shapes are characterizedby:

V0p: 0-peak and

Vpp: peak-to-peak amplitude

The uneven voltage distribution along thewinding cause an impulsive stress whoseamplitude is related to the time behavior ofthe turns in contact

A voltage jump is determined (derivativeeffect of the winding) whose amplitude isrelated to V0p (unipolar) and Vpp (bipolar)voltages

Unipolar and Bipolar Wave-Shapes

Rectangular wave shapes are characterizedby:

V0p: 0-peak and

Vpp: peak-to-peak amplitude

The uneven voltage distribution along thewinding cause an impulsive stress whoseamplitude is related to the time behavior ofthe turns in contact

A voltage jump is determined (derivativeeffect of the winding) whose amplitude isrelated to V0p (unipolar) and Vpp (bipolar)voltages 0 0.5 1 1.5 2 2.5 3

x 10-7

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V2V1-V2

Partial DischargesTypically, the insulation thickness is designed at ratedvoltageHigh values of the voltage jump can incept PartialDischarges (PD) even in low voltage machines

IEC 60270: Partial Discharge (PD) - localized electricaldischarge that only partially bridges the insulation betweenconductors or the adjacent area of a conductor

Life Curves With and Without Partial DischargesExperimental evidence show the huge impact of PD onthe life of organic materials (turn insulation)Tests were performed at 50 Hz and 10 kHz, sinusoidalvoltage with specimens in air (with PD) and in oil (no PD)

2D Graph 1

Failure time [h]

0.01 0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

10000

50 Hz OIL50 Hz AIR10 kHz OIL10 kHz AIR

VEC = 11.7

VEC = 6.4

VEC = 4.5

VEC = 8.7

NO PD200

5010

HzkHz

LL

VEC values clearly indicate the insulation life is stronglyshortened in the presence of PD when f=10 kHz

2D Graph 1

Failure time [h]

0.01 0.1 1 10 100 1000

Vol

tage

(rm

sva

lue)

[V]

1000

10000

50 Hz OIL50 Hz AIR10 kHz OIL10 kHz AIR

VEC = 11.7

VEC = 6.4

VEC = 4.5

VEC = 8.7

PD14000

5010

HzkHz

LL

– Electrons on the surface anode– Positive ions on the surface catode• These charges induce a local

electric field Eq

• In opposition with that induced bythe external supply, E0.

• After discharge, the local field is– Ei=E0-Eq

• Charges move inside the solidmaterial and the intensity Eq

decrease thus increasing Eo

• Next discharge occurs when E0 >Eif (PD inception field)

Anode, +Catode, -

Anode, +

E0

PD is a localized discharge, resulting from transientgaseous ionization where the voltage stress levels exceeda critical value. PD transfer

– Electrons on the surface anode– Positive ions on the surface catode• These charges induce a local

electric field Eq

• In opposition with that induced bythe external supply, E0.

• After discharge, the local field is– Ei=E0-Eq

• Charges move inside the solidmaterial and the intensity Eq

decrease thus increasing Eo

• Next discharge occurs when E0 >Eif (PD inception field)

Catode, -

Anode, +

E0Eq

Catode, -

3- the charge impact erodes thevoid surface

2- charge cloud hits thevoid surface

ElectronsPos. Ions

The degradation of stator insulation that is exposed to acontinuous voltage stress above the PDIV is a physicalerosion of the insulation due to the PD attack: in voids

80

3- the charge impact erodes thevoid surface

5- both the erosion andcarbonization processesenhance the local field

6- tree formation andgrowth until breakdown

2- charge cloud hits thevoid surface1- charge avalance

4- discharges carbonizethe polymer

On the surface

The Paschen CurvesPaschen’s Curve defines the relationship betweenbreakdown voltage, pressure and airgap

PD transform a part of the capacitively stored energy in theinsulation into heat and radiation as well as mechanical andchemical energies, which can degrade insulation materials

The insulation progressively reduces its breakdown voltage,until the breakdown and failure of the whole drive occurs

The Paschen CurvesPaschen’s Curve defines the relationship betweenbreakdown voltage, pressure and airgap

PD and Stress Typologies

The major factors that affect the PD or corona are voltage

frequency

temperature

Since PD is the worst ageing factor, in organic materialsthe PDIV is very often assumed as the end-of-lifeparameter

The ability of materials to withstand PD is a fundamentalcondition but is not the only parameter to be considered

The effects of ASD on ageing acceleration is quite wide

The problem is to verify whether fixing the voltage belowthe PDIV, PD occurs due to other parameter modification

pulsation

humidity

geometry

dielectric thickness

pulse rise time

PD and Stress Typologies

The major factors that affect the PD or corona are voltage

frequency

temperature

Since PD is the worst ageing factor, in organic materialsthe PDIV is very often assumed as the end-of-lifeparameter

The ability of materials to withstand PD is a fundamentalcondition but is not the only parameter to be considered

The effects of ASD on ageing acceleration is quite wide

The problem is to verify whether fixing the voltage belowthe PDIV, PD occurs due to other parameter modification

Voltage Amplitude and PD Erosion

The PDIV for the inter-turn insulation can be evaluatedusing a twisted pair model

PD occurs in the air gap (see the electric-field intensitycurves around the magnet wires

Voltages below PDIV do not lead to any PD

Breakdown strength, PDIV and pulse repetition rate arealso related to the dielectric material and its thickness)

Unipolar vs Bipolar Voltages

Life curves of the inter-turn insulation obtained applyingimpulse voltages of different polarity and different pulseamplitude (fixing the RT=100ns, PW=5s, RR=5kHz) arecompared. Considering the IPL, letVb: the pulse amplitudetb: time to breakdown (at 63.2%)nb: number of voltage pulses to breakdownkb: a constantn: VEC

These results were drawn as an area of possible log-normal distribution functions considering the scatter ofspecimen and PD ignition

Unipolar vs Bipolar Voltages

Life curves of the inter-turn insulation obtained applyingimpulse voltages of different polarity and different pulseamplitude (fixing the RT=100ns, PW=5s, RR=5kHz) arecompared. Considering the IPL, letVb: the pulse amplitudetb: time to breakdown (at 63.2%)nb: number of voltage pulses to breakdownkb: a constantn: VEC

These results were drawn as an area of possible log-normal distribution functions considering the scatter ofspecimen and PD ignition

nbbb nkV

1

Assuming

as the probability to PD inception (ratio of number of pulseswhere PD occur and the total number of voltage pulses)

)(

)(

VP

Vnpn

PD

bb

3 different rangescan be derived:1: each pulse is ableto trigger at least 1PD per pulse (itfollows the IPL)2: the number ofpulses to breakdownis larger due to thereduction of pnb

3: no PD inception

3 different rangescan be derived:1: each pulse is ableto trigger at least 1PD per pulse (itfollows the IPL)2: the number ofpulses to breakdownis larger due to thereduction of pnb

3: no PD inception

Contrasting results were obtained by comparing thesame materials subjected by unipolar and bipolaresquare wavesCharges accumulated by PD on the surface generates alocal electric fieldIf the charge diffusion rate is lower than the polarityreversal speed, the two fields can be added thusincreasing the local electric stress (W.Yin, “Failure Mechanismsof Winding Insulation in Inverter-Feed Motors”, IEEE El.Ins.Mag.,Vol.13, pp.18-23, November 1997)

Contrasting results were obtained by comparing thesame materials subjected by unipolar and bipolaresquare wavesCharges accumulated by PD on the surface generates alocal electric fieldIf the charge diffusion rate is lower than the polarityreversal speed, the two fields can be added thusincreasing the local electric stress (W.Yin, “Failure Mechanismsof Winding Insulation in Inverter-Feed Motors”, IEEE El.Ins.Mag.,Vol.13, pp.18-23, November 1997)

Different aging phenomenaat high and low stressesmust be considered

Further considerations afterPD and space chargemeasurements will be drawn

PD and Slew RateBesides over-voltages, dielectric heating, uneven voltagedistribution between turns, high slew rate affects also thespace charge formationIf the RT is shorter than the time constant of the surface-charge build up, the max electric field in air can beenhanced thus reducing the PD inception voltage

PD and Repetition RateThe number of pulses to breakdown: is independent of their repetitionrate if PD occur less that 1xpulse(voltage range type 2) is linearly dependent to theinverse of the RR in voltage range 1

With bipolar square waves, up about5 kHz, the charge accumulationaffects the PD intensity thusdecreasing the number of pulse-to-failure(W.Yin et al., “Critical Factors for EarlyFailures of Magnet Wires in Inverter FedMotors”, Proc. of IEEE CEIDP, pp.258-261,October 1995)

PD and Repetition RateThe number of pulses to breakdown: is independent of their repetitionrate if PD occur less that 1xpulse(voltage range type 2) is linearly dependent to theinverse of the RR in voltage range 1

With bipolar square waves, up about5 kHz, the charge accumulationaffects the PD intensity thusdecreasing the number of pulse-to-failure(W.Yin et al., “Critical Factors for EarlyFailures of Magnet Wires in Inverter FedMotors”, Proc. of IEEE CEIDP, pp.258-261,October 1995)

PD and Thermal and Mechanical StressesHigher values of temperature can promote PD thusreducing the PDIV due to an increased permittivity of the polymer (the specimencapacitance increases leading to higher electric field intensity in the air-gap) the decreased breakdownstrength of air because of its lowerdensity

PD and Thermal and Mechanical StressesHigher values of temperature can promote PD thusreducing the PDIV due to an increased permittivity of the polymer (the specimencapacitance increases leading to higher electric field intensity in the air-gap) the decreased breakdownstrength of air because of its lowerdensity

Besides the thermal ageing, highertemperatures lead to a thermallyaccelerated electrical ageing of thelow voltage interturn insulation

Form-Wound Windings (MV, HV)Enamelled wires are protected by strand insulation basedon mica tapesInorganic materials withstand PD and the life of theinsulation system when subjected by rectangular waveshapes, is almost comparable with that supplied bysinusoidal voltages

Enamelledwire

StrandInsulation

Ground WallInsulation

ConductiveTape

Form-Wound Windings: End-Arm Stress Grading

The performance of the end-armstress grading decreasesincreasing the frequency contentof the supply voltage

0

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V [V]

GEN.3.1 GEN.3.2 GEN.3.3 GEN.3.4

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250 kHz50 Hz

30 kVf variabile

20 kHz

1 kHz

The stress grading forASD applications mustbe designed properlyto avoid the inceptionof PD and its rapiddeterioration

Discussion PD is the dominant ageing factor in organic insulationwhile longer life if found when organic/inorganic materialswere consideredType 1: insulation based only on organic materials thatdoes not withstand PDType 2: insulation based on combination oforganic/inorganic materials able to withstand PD

Inter-turn insulation breakdown is the most importantfailure in type-1 random wound motors due to PD in theair-gaps of enameled wires that are touching

High frequencies, short rise times and fast oscillatingpulses shorten the lifetime. However, if no PDs occurredno premature breakdown was observed

Discussion PD is the dominant ageing factor in organic insulationwhile longer life if found when organic/inorganic materialswere consideredType 1: insulation based only on organic materials thatdoes not withstand PDType 2: insulation based on combination oforganic/inorganic materials able to withstand PD

Inter-turn insulation breakdown is the most importantfailure in type-1 random wound motors due to PD in theair-gaps of enameled wires that are touching

High frequencies, short rise times and fast oscillatingpulses shorten the lifetime. However, if no PDs occurredno premature breakdown was observed

Satisfactory lifetime of inverter-fed low voltage motorscan be achieved if PDs in the winding insulation areavoided

This can be done by an appropriate limitation of the risetime and amplitude of the terminal voltage using shortcables, appropriate filters, or lower dc voltages

Great care should be taken with proper insulation designto avoid a low PD inception voltage

A short description of PD and Space Chargemeasurements is provided before to discuss new solutionsfor insulation systems for ASD applications

Satisfactory lifetime of inverter-fed low voltage motorscan be achieved if PDs in the winding insulation areavoided

This can be done by an appropriate limitation of the risetime and amplitude of the terminal voltage using shortcables, appropriate filters, or lower dc voltages

Great care should be taken with proper insulation designto avoid a low PD inception voltage

A short description of PD and Space Chargemeasurements is provided before to discuss new solutionsfor insulation systems for ASD applications