Testing Area - Accelerator
on a 15 cell (~32V) Lead Acid Battery Source -
A low voltage optical
triggering circuit was constructed for the purpose of investigating some basic
multi-coil behaviour. Figs 1-4 show this circuitry mounted on the coilgun accelerator.
Click on the images to see a high resolution version.
The optical triggering
circuit for each coil is virtually identical to that used in the Optical
Trigger Module. Each coil is switched by a set of three IRFZ44A mosfets connected
in parallel. These mosfets are only rated to 60V so they wont feature in the final
capacitor testing. If you have read the details of the commutating
experiments then you'll have an idea as to why there are large diode arrays
connected across the coils. The gun was originally fired with only a single diode
on each coil resulting in a poor muzzle speed of about 13m/s - not much better
than a single stage coil! Running with modified diode commutation gives a huge
improvement in muzzle speed - the gun now fires at just over 18m/s.
Now let's look
at the current, voltage and power curves for this accelerator. Fig 5 shows the
supply current and voltage, fig 6 illustrates the source and supply power.
5. Current and voltage traces for 3 stage accelerator.
6. Power curves showing large internal loss in the source.
first thing which is striking is the much shorter current pulses in the second
and third stages. The 'spikes' in the voltage and supply power curves are due
to turn off transients. If we calculate the energy gained by each stage and compare
this to the energy lost in each stage then we can determine the efficiencies of
each stage. The source energy transfer refers to the total energy generated within
the source. Supply energy transfer means the energy which is delivered to the
||Energy Gain (J)
||Source Energy Transfer (J)
||Supply Energy Transfer (J)
Coilgun Efficiency (%)
|Overall Efficiency (%)
As expected, the
efficiency of the first coil is comparable to coil D. This isn't surprising since
each coil in this accelerator is dimensionally similar to coil D. What is interesting
is that the second and third coils have a much better efficiency rating. The reason
for this is that the second and third pulses are much shorter than the first.
Also, the current never reaches its maximum steady state value so the peak I2R
losses will be less. In this condition the current is said to be induced voltage
to note the distinct 'knee' in gradient of the leading edges of pulses two and
three. This is caused by induced voltage as the projectile penetrates the coil.
would appear that the limiting factor in a multistage accelerator is going to
be the rise time of the current. As the projectile accelerates it takes less time
to travel into the next coil, so, there comes a point when the frictional and
electromagnetic losses equal the limited accelerating force available from the
reduced current magnitude, the coilgun has then reached its terminal muzzle velocity.
For any given coil configuration the peak current density available in the time
window can be increased by using a higher source voltage.
source for the pistol accelerator setup was originally going to be a 50V capacitor
on stage one and 80V capacitors on stages two and three. However this was changed
after some testing with different capacitor setups.