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Forums
4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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ISEF help

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klugesmith
Wed Apr 28 2010, 11:48PM
klugesmith Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
It sounds like some of your disks are thicker than the skin depth associated with your current pulse,
so their upper layers will not contribute much to the hoop conductance.
With similar induced current amplitude and duration, a heavier disk receives a similar impulse of momentum (newton seconds) but less energy (newton meters).

If you have an oscilloscope, it is instructional to look at the waveforms of low-energy
(non launching) pulse discharges, with and without flyer disks.
From the unloaded circuit frequency and damping, you can figure the effective L and R.
When you add a disk, effective L goes down and R goes up due to the inductively coupled, shorted turn.

Good luck.
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ramses
Thu Apr 29 2010, 12:27AM
ramses Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
Yes, all disks are at least 5 times thicker than the skin depth. Unfortunately, the project shipped yesterday, so additional oscilloscope traces are not available. How much do you think this would effect the timing? without a disk, I got a rise time of 140us.

I don't have a proper oscilloscope; I am using a Vernier LabPro with a voltage probe and a homemade rogowski coil. Max frequency is 50khz... but the software has amazing tools.
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Steve Conner
Thu Apr 29 2010, 12:02PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
ramses wrote ...

Okay, I just finished my tests and I found some surprising results. The Copper disks performed much worse than the aluminum ones, and the thick disks performed much worse than the thinner ones. I can honestly say I'm stumped.

It's a well known fact that the starting torque of an induction motor can be increased by using thinner, higher resistance conductors for the rotor circuit. Maximum starting torque is when the rotor resistance is equal to its reactance.

A ring launcher is a linear induction motor, so I bet the same effect is at work here.
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ramses
Thu Apr 29 2010, 04:24PM
ramses Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
I figured as much. How does this effect efficiency at higher RPM's?

I am speaking with several engineers, but another dilemma is whether to use momentum or energy in application to motors and transformers. I'm pretty sure momentum is "equivalent" to torque, and energy to power in motors. I think Energy would be applicable to transformers. A major limitation of my project is lack of a formal physics and calculus education.

Thanks
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Steve Conner
Fri Apr 30 2010, 08:54AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Of course it ruins the efficiency at normal operating speed, so induction motor design is a tradeoff in that respect. You can have high starting torque or high efficiency.

Since inverter drives came in, the problem is solved. You just make the rotor as conductive as possible, and the inverter gives you high torque at any speed by changing the frequency.
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