This was the first and largest of the coils tested.
Yes, I realize all these numbers and calculations are boring, but this engineering notebook
includes all my raw data to permit an independent check of the calculations and conclusions later.
Coil of 97 Turns
See the Projectiles page for details of these projectiles.
Measurements
The raw measurements are shown in red in the table
below, using horizontal ballistics measurements.
Projectile:
A
C
D
F
Length:
3.5"
2.5"
2"
1.5"
Mass:
4.301 g
2.163 g
1.464 g
0.7345 g
Potential
energy
(joules)
Charge
(volts)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
Horiz
Distance
(inches)
0.6 J
10 v
23"



2.4
20
51
58


5.4
30
75
98
104
114
9.6
40
97
108
121
132
15.0
50
107
125
137
138
33.8
75
140



60.0
100
155



Velocity
Using the equations for horizontal ballistics and the raw data, the velocity is calculated
for each of the measurements above. The computed velocity is shown in red.
Projectile:
A
C
D
F
Potential
energy
(joules)
Charge
(volts)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
Velocity
(m/s)
0.6 J
10 v
1.316



2.4
20
2.919
3.319


5.4
30
4.292
5.609
5.952
6.524
9.6
40
5.551
6.181
6.925
7.554
15.0
50
6.124
7.154
7.840
7.898
33.8
75
8.012



60.0
100
8.871



Efficiency
Calculate efficiency as shown in red.
Projectile:
A
C
D
F
Potential
energy
(joules)
Charge
(volts)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
Efficiency
(%)
0.6 J
10 v
0.6 %



2.4
20
0.8
0.5


5.4
30
0.7
0.6
0.5
0.3
9.6
40
0.7
0.4
0.4
0.2
15.0
50
0.5
0.4
0.3
0.2
33.8
75
0.4



60.0
100
0.3



Graphical Results
Coil Analysis
This coil with 97 turns measured 8 ms halfcycle waveform. The image below
indicates peak currents of 350 A at 100v charge.
How Does This Coil Need to Change?
The speeds and efficiencies are very low. We need to shorten the time and increase the peak
current.
From the oscilloscope, the halfcycle time is 8 ms, and we desire our next coil to be 25% faster,
which means a 6 ms discharge time.
Using a Java RLC simulator, we find the estimated inductance
is L = 525 uH for an 8 ms discharge with a 12,000 uF capacitor.
To reduce this to 6 ms, a Java LC Time Simulator finds that
inductance should be reduced to a value of 304 uH.
We don’t want to model an ironcore coil in FEM to derive the inductance, so let’s
apply the same ratio change to an aircore model. This is much easier to do, and the next bench
test will tell us if this method is accurate.
We need to reduce inductance L by a factor of (304 uH) / (525 uH) = 0.5790
The 97turn coil in air has inductance L = 186 uH, according to a Java
Inductor Sim (using 12 AWG, ID=6mm, OD=69mm, length=16mm).
The new coil in air should have L = (186 uH) * (0.5790) = 107.7 uH
Using the Java Inductor Sim again, we find an 84turn coil will have the desired inductance.
Therefore, removing 13 turns should result in the desired 6 ms discharge
time.