http://fluxtrol.com In induction hardening, thermal fatigue is one of the main failure modes of induction heating coils. There have been papers published that describe this failure mode and others that describe some good design practices [1-3]. The variables previously identified as the sources of thermal fatigue include radiation from the part surface, frequency, current, concentrator losses, water pressure and coil wall thickness. However, there is very little quantitative data on the factors that influence thermal fatigue in induction coils available in the public domain. By using finite element analysis software this study analyzes the effect of common design variables of inductor cooling, and quantifies the relative importance of these variables. A comprehensive case study for a single shot induction coil with Fluxtrol A concentrator applied is used for the analysis.

1. Enhancing Induction Coil Reliability
Robert Goldstein
www.fluxtrol.com

2. Overview
•
•
•
•
•
Demand for Improved Inductor Life
Failure Modes of Induction Coils
Extending Inductor Lifetime
Case Story – Single Shot Heating of Shaft
Conclusions

3. 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

4. Common Failure Modes of
Induction Coils
• Mechanical
Damage
• Electrical Break
• Thermal
Degradation

5. 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)

6. 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

7. 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

8. 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

9. 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

10. Case Story – Single Shot
Heating of a Shaft

11. Variables
• 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

12. Dimensions and Materials
1045 Steel
(Above 800 C Non-Magnetic)
Fluxtrol A
0.02”
5/8”
Variable
Copper
0.355”
1”
1/8”
5/8”
1 3/8”
1/4”
JB Weld
1”
1045 Steel
Only hot conditions considered, to limit the number of variables in the
study

13. Assumptions
• 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.

14. Effect of Radiation
No 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

15. Percent of Power Lost in Coil
10 kHz
• 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
60
50
40
Percent of
30
Total Power
20
0.048
0.062
0.125
10
0
5000 A
1 kHz
7500 A
10000 A
3 kHz
60
60
50
50
40
Percent of
30
Total Power
20
0.048
0.062
0.125
10
0
40
Percent of
30
Total Power
20
0.048
0.062
0.125
10
0
5000 A
7500 A
10000 A
5000 A
*Cases where the induction coil wall reached over 250 C are
dropped from the graphs
7500 A
10000 A

16. Reference Depth and Wall Thickness
Frequency (kHz)
10
3
1
Reference Depth (in)
0.031
0.057
0.099
0.048
0.062
2.00/28.6 1.09/27.6 0.63/30.0
0.125
Wall Thickness
1.55/27.6 0.84/30.0 0.48/35.0
4.03/29.0 2.19/28.6 1.26/24.8
*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
•
•
•
•
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

17. Reference Depth and Wall Thickness
60
55
Power Lost in Coil
(%)
50
45
40
35
30
25
20
0
0.5
1
1.5
2
2.5
3
t/δ
• 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.

18. Power Density in Coil
1kHz, 7,500A,
20psi
0.048 (t/δ = 0.48)
3kHz, 7,500A, 20psi
0.048 (t/δ = 0.84)
0.062 (t/δ = 0.63)
0.062 (t/δ = 1.09)
0.125 (t/δ = 1.26)
0.125 (t/δ = 2.19)

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
0.048
20
15
10
5
Percent
Difference
10 kHz
0
3 kHz
-5
5000 A
1 kHz
-15
-20
0.125
20
20
15
15
10
10
5
10 kHz
0
3 kHz
-5
10000 A
-10
0.062
Percent
Difference
7500 A
5000 A
7500 A
10000 A
1 kHz
5
Percent
Difference
10 kHz
0
3 kHz
-5
-10
-10
-15
-15
-20
5000 A
-20
*Cases where the induction coil wall reached over 250 C are
dropped from the graphs
7500 A
10000 A
1 kHz

20. Corner and Center Temperature Difference
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.
1 kHz
7500 A
40 psi
0.048 in
10 kHz
7500 A
40 psi
0.125 in

21. •
•
Effect of Water Pressure
10 kHz
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
50
45
40
35
30
Decrease in
25
Temperature
20
15
10
5
0
0.125
0.062
0.048
5000 A
1 kHz
0.125
0.062
0.048
7500 A
10000 A
3 kHz
50
45
40
35
30
Decrease in
25
Temperature
20
15
10
5
0
5000 A
7500 A
10000 A
50
45
40
35
30
Decrease in
25
Temperature
20
15
10
5
0
0.125
0.062
0.048
5000 A
*Cases where the induction coil wall reached over 250 C are
dropped from the graphs
7500 A
10000 A

22. 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.
3 kHz
50
40
30
Decrease in
Temperature 20
0.125
10
0.048
0.062
0
5000 A
7500 A
10000 A

23. Effect of Increasing Water Pressure
• 3 kHz, 7,500A
0.048”
40 to 20 psi
pressure increase
0.048”
0.125”
40 to 20 psi
pressure increase
0.125”

24. Thermal Cycling
• 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

25. Cycling Results
10kHz 7500A 0.062 40psi
200
Center Temperature
Corner Temperature
Concentrator Corner
Concentrator Backside
150
Temperature
( C)
100
50
0
0
20
40
60
80
Time (s)
100
120
140
160

26. Thermal Profile During Cycling
10kHz,
7,500A
40psi,
0.062
10 s
70 s
25 s
85 s
40 s
100 s
55 s
115 s

27. Cycling Results
• 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

28. Conclusions
• 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

29. Conclusions Continued
• 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

30. Thank You!
www.fluxtrol.com
Booth #1623