Enhancing Induction Coil Reliability

Robert Goldstein

www.fluxtrol.com

Overview

• Demand for Improved Inductor Life

• Failure Modes of Induction Coils

• Extending Inductor Lifetime

• Case Story – Single Shot Heating of Shaft

• Conclusions

Demand for Improved Inductor Life

• Increased competition has led to

increased pressure to maximize

manufacturing efficiency and equipment

utilization

• Machine downtime is extremely costly,

especially if it is unplanned

• Inductor failure is one of the leading

causes of machine downtime

Common Failure Modes of

Induction Coils

• Mechanical Damage

• Electrical Break

• Thermal Degradation

Mechanical Damage

• Coil to part impact

– Inaccurate coil set-up

– Improper part installation

– Incoming part defect

• Electrodynamic forces

– Distortion of winding shape

– Elongation of winding (copper creep)

Electrical Break

• Insufficient insulation between turns

– Poor design

– Insulation displaced during shipping/installation

– Wearing of insulation over time

• Process Debris

– Scale from the part

– Magnetic chips from prior machining

Thermal Degradation• Total overheating of inductor

– Insufficient water flow

• Local overheating of inductor

component

– Copper cracking due to

thermal ratcheting (intermittent

processes)

– Gradual coil deformation

(continuous processes)

– Intergranular oxidation

• Failure of braze joint

Extending Inductor Lifetime

• Failures due to mechanical damage and

electrical break can be prevented

– Good machine design

– Proper coil manufacturing procedures

– Proper maintenance

• Failures due to thermal degradation more

complicated

Thermal Degradation Prevention Methods

• Good brazing practices– Nearly all braze joint failures preventable with good

manufacturing practices and proper material selection

• Primarily done in response to failures based upon experience – Increase water flow

– Add booster pumps

– Change water pockets in windings

– Split concentrator into multiple sections

– Change winding design

– Improve water quality

• Changes made on trial and error basis, no scientific method

Case Story – Single Shot

Heating of a Shaft

• Combinations of the following variables are used in

the simulations

– Frequency: 10 KHz, 3 kHz, 1 kHz

– Current: 10,000 A, 7,500 A, 5,000 A

– Water Pressure: 40 psi, 20 psi across inlet and outlet of

inductor leg

– Wall Thickness: 0.125 in, 0.062 in, 0.048 in

• Heating lasts for 10 seconds

Variables

Dimensions and Materials

1 3/8”

1/4”

5/8”

5/8”

1/8”1”

Variable

0.02”0.355”

1”

Fluxtrol A

1045 Steel

(Above 800 C Non-Magnetic) Copper

1045 Steel

JB Weld

Only hot conditions considered, to limit the number of variables in the

study

• The heat transfer coefficients used are calculated

at a constant temperature when in reality they will

change with temperature

• When the temperature of the inductor wall is 250 C

or higher, the correlations used for heat transfer

coefficient are no longer valid

– Above this temperature, the heat transfer coefficient will

initially rise rapidly then drop dramatically. The specifics

of these changes are case dependent.

– Therefore, the results from these cases will be dropped

from the study.

Assumptions

Effect of RadiationNo Radiant Heat Transfer

During the entire cycle 1000 C radiation

from part considered

• 3 kHz 10000A 40psi 0.125in

• With radiation accounted for the copper temperature increases 2 C and the

concentrator temperature increases 10 C

• Since the influence is not very strong, radiation can be neglected

Percent of Power Lost in Coil• The percent of power in the coil out

of the total power is plotted

• For the data shown here, the water

pressure is 40 psi

*Cases where the induction coil wall reached over 250 C are

dropped from the graphs

0

10

20

30

40

50

60

5000 A 7500 A 10000 A

Percent of Total Power

10 kHz

0.048

0.062

0.125

0

10

20

30

40

50

60

5000 A 7500 A 10000 A

Percent of Total Power

1 kHz

0.048

0.062

0.125

0

10

20

30

40

50

60

5000 A 7500 A 10000 A

Percent of Total Power

3 kHz

0.048

0.062

0.125

Frequency (kHz) 10 3 1

Reference Depth (in) 0.031 0.057 0.099

Wall Thickness

0.048 1.55/27.6 0.84/30.0 0.48/35.0

0.062 2.00/28.6 1.09/27.6 0.63/30.0

0.125 4.03/29.0 2.19/28.6 1.26/24.8

Reference Depth and Wall Thickness

• The ratio between the wall thickness and reference depth can be used to

minimize coil losses

• Theoretically, it has been found that electromagnetic losses will be at their

minimum when the ratio is π/2δ (≈1.6), but these calculations were made for an

infinitely long heat face of the coil turns and uniform proximity effect

• Taking into account the effects of the sidewall of the coil turns for real inductor

and varying coupling gap, the optimal wall thickness will be influenced. The

sidewalls influence both the electromagnetic losses and the heat removal.

• The authors are not aware of any other published studies that look at the effects

for short coils, such as those used for heat treating

*The first value is t/δ, the second is the percent of power lost in the coil

*For the values shown, current is 5000A the water pressure is 40psi

• Shown here is a curve of multiple wall thicknesses for the 3kHz,

5000A case

• Coil losses are highest when the coil wall thickness to reference

depth ratio falls below 1, there is a slight minima around 1.2 and

essentially flat above 2

• The interaction of all of the variables is complex and this curve will

look different for different inductors with different parts.

Reference Depth and Wall Thickness

20

25

30

35

40

45

50

55

60

0 0.5 1 1.5 2 2.5 3

Po

we

r Lo

st in

Co

il(%

)

t/δ

Power Density in Coil

1kHz, 7,500A,

20psi

0.048 (t/δ = 0.48) 0.062 (t/δ = 0.63) 0.125 (t/δ = 1.26)

3kHz, 7,500A, 20psi 0.048 (t/δ = 0.84) 0.062 (t/δ = 1.09) 0.125 (t/δ = 2.19)

Corner and Center Temperature Difference• The percent difference between

the temperature of the corner and center of the copper tubing is plotted

• A positive difference correlates to the corner being hotter

• For the data shown here, the water pressure is 40 psi

*Cases where the induction coil wall reached over 250 C are

dropped from the graphs

-20

-15

-10

-5

0

5

10

15

20

5000 A 7500 A 10000 A

Percent Difference

0.048

10 kHz

3 kHz

1 kHz

-20

-15

-10

-5

0

5

10

15

20

5000 A 7500 A 10000 A

Percent Difference

0.062

10 kHz

3 kHz

1 kHz

-20

-15

-10

-5

0

5

10

15

20

5000 A 7500 A 10000 A

Percent Difference

0.125

10 kHz

3 kHz

1 kHz

Corner and Center Temperature Difference

10 kHz

7500 A

40 psi

0.125 in

1 kHz

7500 A

40 psi

0.048 in

t/δ = 0.48 t/δ = 4.03

• The reference depth is

shown to influence the

thermal profile in the coil

• As shown here, when

the wall thickness to

reference depth ratio is

small the temperature is

higher in the center, but

when the ratio is large it

is higher in the corners.

Effect of Water Pressure• The percent decrease in temperature

when water pressure across the leg of

the inductor is dropped from 40 psi to

20 psi is plotted

• The temperature of the center of the

copper tubing is analyzed here

*Cases where the induction coil wall reached over 250 C are

dropped from the graphs

0

5

10

15

20

25

30

35

40

45

50

5000 A 7500 A 10000 A

Decrease in Temperature

10 kHz

0.125

0.062

0.048

0

5

10

15

20

25

30

35

40

45

50

5000 A 7500 A 10000 A

Decrease in Temperature

3 kHz

0.125

0.062

0.048

0

5

10

15

20

25

30

35

40

45

50

5000 A 7500 A 10000 A

Decrease in Temperature

1 kHz

0.125

0.062

0.048

Effect of Increasing Water Pressure• With increasing current the percent temperature

drop is greater. This is due to the higher

temperature gradient.

• The percent temperature drop is higher for

thinner wall thicknesses. The water cooled

surface is in closer proximity to the hottest

points on the copper for thin walled tubing.

0

10

20

30

40

50

5000 A 7500 A 10000 A

Decrease in Temperature

3 kHz

0.125

0.062

0.048

40 to 20 psi

pressure increase

40 to 20 psi

pressure increase

Effect of Increasing Water Pressure

0.048” 0.048”

0.125”0.125”

• 3 kHz, 7,500A

• A cycling process is modeled with intervals of 10

seconds of heating following by 5 seconds with

no current

• Analyses of different points on the inductor are

done to determine if and when a steady state is

reached

Thermal Cycling

Cycling Results

0

50

100

150

200

0 20 40 60 80 100 120 140 160

Temperature( C)

Time (s)

10kHz 7500A 0.062 40psi

Center Temperature

Corner Temperature

Concentrator Corner

Concentrator Backside

Thermal Profile During Cycling

10 s 25 s 40 s 55 s

70 s 85 s 100 s 115 s

10kHz,

7,500A

40psi,

0.062

• The copper reaches steady state 1-2 seconds into the first

cycle, since it has a high thermal conductivity and is in

contact with the cooling source

• The corner of the concentrator closest to the copper

reaches steady state after 4-5 cycles. The Layer of epoxy

causes it to reach a much lower temperature than the

corner of the copper tube adjacent to it.

• The backside of the concentrator is slow to reach steady

state, but the fact that it did within a reasonable amount of

time shows that the whole inductor reaches a steady state

during continuous cycling

Cycling Results

• Heat loss from radiation has little effect compared to

the heat generated from coil losses in single shot

coils

• Coil losses are higher when the reference depth is

greater than the wall thickness

• There is a optimal wall thickness that will result in a

minimum copper temperature for a given case

• Coil losses are higher when the temperature of the

copper is greater, since the resistivity of copper

increases with temperature

Conclusions

• When the reference depth is greater than the wall

thickness, the temperature tends to be higher in the

center of the tubing, and vice versa

• Thin walled tubing cools more efficiently and has a

higher response to an increase in water pressure

• During cycling the copper tubing reaches steady

state immediately, while the concentrator is slow to

reach it on the backside.

• Coupling of electromagnetic and thermal results

with deformation and stress simulation would

provide additional insight into the coil lifetime

Conclusions Continued

Thank You!

www.fluxtrol.com

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