Author Topic: Energy and Efficiency  (Read 1498 times)

Offline RobertL

  • High Voltage Enthusiast
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
Energy and Efficiency
« on: January 16, 2020, 11:51:51 PM »
I'm having fun building a multistage coil gun.   Using low voltage, and high current.

In reading some of the canon websites (eg. Barry's Coil Gun size - which is excellent, btw).  theres a lot of talk about determining energy from the capacitor, and comparing with the kinetic energy of the projectile to determine efficiency.

However, it occurred to me that it was more useful to look at the energy stored in the inductor, rather than the capacitor.  As its the device directly affecting the projectile.

Energy in an Inductor is

    EL = 1/2 L I2

The inductance (L) can be measured, calculated etc.  and has a direct bearing on the construction of the coil.  For example, in a solenoid type coil:

    L = u N2A / l

Where A is the cross section area of the inductor, and l is the length.  Other formula exist for disk/pancake coils.

The current (I) can be measured too, and is more "important" as the energy is proportional to the square.  The field strength is proportional to current also:

    B = u N I / L

So my plan is to use super high current, several hundred to kilo Amps, in relatively low value inductors.   Switched with ganged MOSFETs.

I'm hoping this will prompt a discussion.
cheers!


*edited, based on Twospoons comments
« Last Edit: January 17, 2020, 04:20:16 AM by RobertL »

Offline Twospoons

  • High Voltage Technician
  • ***
  • Posts: 107
  • Karma: +1/-0
    • View Profile
Re: Energy and Efficiency
« Reply #1 on: January 17, 2020, 01:37:37 AM »


The inductance (L) can be measured, calculated etc.  and has a direct bearing on the construction of the coil.

    L = u N2A / l

Where A / l represents the volume of the inductor.


Eh ... its a bit more complicated than that. Your equation is based on the long solenoid approximation  -  the length must be longer than the diameter of the hole.  Other solenoid shapes are possible, and may be more efficient in terms of  copper use, and inductance vs size.  This is why finite element field solvers exist.  Have a play with FEMM 4.2 - its reasonably easy to understand , and its free.

"Where A / l represents the volume of the inductor."  Not even close. It has units of length, not volume.

Offline RobertL

  • High Voltage Enthusiast
  • *
  • Posts: 3
  • Karma: +0/-0
    • View Profile
Re: Energy and Efficiency
« Reply #2 on: January 17, 2020, 04:22:25 AM »

Eh ... its a bit more complicated than that. Your equation is based on the long solenoid approximation  -  the length must be longer than the diameter of the hole.  Other solenoid shapes are possible, and may be more efficient in terms of  copper use, and inductance vs size.
thanks, I've edited the head post.

Quote
This is why finite element field solvers exist.  Have a play with FEMM 4.2 - its reasonably easy to understand , and its free.

thanks I'll take a look

Offline T3sl4co1l

  • High Voltage Technician
  • ***
  • Posts: 172
  • Karma: +9/-0
    • View Profile
Re: Energy and Efficiency
« Reply #3 on: January 17, 2020, 04:01:23 PM »
You will find that the number of turns is not actually important to the energy storage of an inductor.  After all, you cannot get as many turns in the same space.  The limiting factor is actually the resistivity of copper, which limits you to a certain total amp-turns, in a given winding volume, for a given power dissipation.

Under pulsed conditions, this can be quite high, but the efficiency drops substantially as most of the inductor's voltage drop is due to I^2 R losses rather than L dI/dt electromotive force.

The efficiency drops further when considering the lack of pole pieces (in a typical coilgun design), so that the amount of field seen by the armature is a tiny fraction of the total field.  You're wasting a lot of electrical energy building fields that will never perform mechanical work.

If this is a pulling type coilgun, there's the added limit that the armature and pole pieces saturate at 1.5T or so.  This corresponds to a best-case pressure of about 10 atm -- a respectable "potato cannon" level of performance, but embarrassingly pitiful compared to powder propellant types.

This limit can be overcome by pushing a conductive slug instead, in which case the flux density is unlimited, but material resistivity dominates quite seriously.  On top of coil efficiency, you're also induction heating the slug, which increases its resistance, which increases its heating...  At some point, it will be so hot that acceleration can't overcome drag, and it'll be at top speed.

One bonus to the pushing type launcher is, if the barrel is filled with ferrofluid, the inductance of coils away from the armature can be made relatively high, and we can wire them in parallel, in sequence.  The barrel would not be cylindrical, but a channel which guides the armature and which otherwise allows free flow of the ferrofluid around it.  Thus we can make a brushless linear motor.  And as an induction type motor, the frequency isn't too important, as long as the force is adequate (high slip condition).

The full build of such a thing would look like, probably a stack of pancake coils alternating with wedge shaped ferrite pole pieces, soaked in a bucket of ferrofluid, with a guide channel in the middle.  The armature is copper, and the drive signal is a specialty VFD (voltage-frequency drive), a rising voltage and frequency (chirp) shaped to match the armature's acceleration (the coils can also be spaced differently towards the muzzle end to ease the frequency chirp rate).

This is also a ferrofluid fountain or splasher, so have fun with cleanup. :P

Tim

Offline klugesmith

  • High Voltage Engineer
  • ****
  • Posts: 379
  • Karma: +12/-0
    • View Profile
Re: Energy and Efficiency
« Reply #4 on: January 17, 2020, 08:55:37 PM »
>> This is also a ferrofluid fountain or splasher, so have fun with cleanup.

How 'bout a mercury fountain?   Cross between disc launcher and "repulsion" coil gun.  Or, with electrical contacts to the mercury, a cross between MHD pump and rail gun.


On the original topic of energy: 
If you don't count losses from coil resistance, and losses in the projectile,
the peak energy in inductance (L*I^2/2) is automatically the same as initial energy in capacitor (C*V^2/2).  You can check that using Barry's "simulator" by making R very small.

[edit] For given coil dimensions, as you trade off the wire size and number of turns, the L/R time constant is invariant.  But thicker wire & fewer turns reduces the oscillation period, which is proportional to sqrt(LC). That means a larger fraction of initial C energy will get to be L energy at the end of first 1/4 cycle (if we ignore the fixed L and R of capacitor bank and interconnections).  Not necessarily better for coilgun efficiency, which depends on matching between pulse width and projectile moving time. 

As Tim said, most of the energy in typical coil guns goes into I^2 R heating of the coils.
Whether the damping is heavy or light (but finite), you can figure the temperature rise per shot by dividing capacitor energy by copper mass and heat capacity.
« Last Edit: January 18, 2020, 02:12:27 AM by klugesmith »

High Voltage Forum

Re: Energy and Efficiency
« Reply #4 on: January 17, 2020, 08:55:37 PM »

 


* Recent Topics and Posts

post Re: CW Help -- too much info to digest
[Voltage Multipliers]
MRMILSTAR
Today at 05:16:39 AM
post Re: CW Help -- too much info to digest
[Voltage Multipliers]
abstruse1
Today at 03:38:23 AM
post Re: Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
MRMILSTAR
July 28, 2021, 08:31:50 PM
post Re: Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
SalinsLV
July 28, 2021, 08:07:10 PM
post Re: Tesla Coil Music
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Max
July 28, 2021, 05:17:06 PM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
klugesmith
July 28, 2021, 05:15:48 PM
post Tesla Coil Music
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Transgarp
July 28, 2021, 02:12:35 PM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
Twospoons
July 28, 2021, 01:39:46 AM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
ResonantEMF
July 28, 2021, 01:02:30 AM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
davekni
July 27, 2021, 11:26:15 PM
post Archival magnifier information
[Spark Gap Tesla Coils (SGTC)]
MRMILSTAR
July 27, 2021, 11:10:19 PM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
ResonantEMF
July 27, 2021, 09:12:05 PM
post Re: Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
MRMILSTAR
July 27, 2021, 08:47:49 PM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
MRMILSTAR
July 27, 2021, 08:45:26 PM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
ResonantEMF
July 27, 2021, 07:38:47 PM
post Re: Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
MrScrOggs
July 27, 2021, 07:12:06 PM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
davekni
July 27, 2021, 06:48:04 PM
post Re: Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
MRMILSTAR
July 27, 2021, 05:27:50 PM
post Grounding VTTC
[Vacuum Tube Tesla Coils (VTTC)]
MrScrOggs
July 27, 2021, 05:14:34 PM
post Re: Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
Felix B.
July 27, 2021, 04:58:46 PM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
petespaco
July 27, 2021, 04:36:37 PM
post Need second opinion on tesla coil capacitors
[Spark Gap Tesla Coils (SGTC)]
ResonantEMF
July 27, 2021, 09:14:01 AM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
johnf
July 27, 2021, 06:19:19 AM
post Re: Skm flyback destroyer 🤗
[Transformer (Ferrite Core)]
alan sailer
July 27, 2021, 02:50:30 AM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
July 26, 2021, 10:13:23 PM
post Re: How to completely remove oil from MOTs?
[Transformer (Iron Core)]
SalinsLV
July 26, 2021, 03:04:47 PM
post Re: How to completely remove oil from MOTs?
[Transformer (Iron Core)]
MRMILSTAR
July 26, 2021, 02:42:57 PM
post Re: Skm flyback destroyer 🤗
[Transformer (Ferrite Core)]
oneKone
July 26, 2021, 12:37:52 PM
post Re: How to completely remove oil from MOTs?
[Transformer (Iron Core)]
rikkitikkitavi
July 26, 2021, 12:21:54 PM
post How to completely remove oil from MOTs?
[Transformer (Iron Core)]
SalinsLV
July 26, 2021, 10:47:22 AM
post Re: HFSSTC design help and questions
[Solid State Tesla Coils (SSTC)]
Weston
July 26, 2021, 10:15:43 AM
post Re: CW Help -- too much info to digest
[Voltage Multipliers]
MRMILSTAR
July 26, 2021, 06:37:46 AM
post Re: HFSSTC design help and questions
[Solid State Tesla Coils (SSTC)]
davekni
July 26, 2021, 04:25:03 AM
post Re: HFSSTC design help and questions
[Solid State Tesla Coils (SSTC)]
Power-Max
July 26, 2021, 03:29:32 AM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
July 26, 2021, 02:18:39 AM
post Re: BrOdin coil - High power Big sparks!
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
July 26, 2021, 01:23:42 AM
post Re: Gate driver for big IGBTs
[Electronic Circuits]
TMaxElectronics
July 26, 2021, 12:58:41 AM
post Re: Proper phase lead adjustment at the input
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
July 25, 2021, 10:15:30 PM
post CW Help -- too much info to digest
[Voltage Multipliers]
abstruse1
July 25, 2021, 06:16:53 PM
post Re: Proper phase lead adjustment at the input
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Chrisader
July 25, 2021, 04:16:11 PM
post Re: Gate driver for big IGBTs
[Electronic Circuits]
davekni
July 25, 2021, 05:38:42 AM
post Gate driver for big IGBTs
[Electronic Circuits]
TMaxElectronics
July 25, 2021, 02:15:25 AM
post Re: Modified VTTC for induction heating
[Vacuum Tube Tesla Coils (VTTC)]
dbach
July 24, 2021, 11:38:33 PM
post Re: Modified VTTC for induction heating
[Vacuum Tube Tesla Coils (VTTC)]
Mads Barnkob
July 24, 2021, 11:29:15 PM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
Twospoons
July 24, 2021, 10:41:38 PM
post Modified VTTC for induction heating
[Vacuum Tube Tesla Coils (VTTC)]
dbach
July 24, 2021, 09:18:42 PM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
Mads Barnkob
July 24, 2021, 08:08:36 PM
post Re: Tesla Coil Show Controller Project
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Max
July 24, 2021, 11:13:18 AM
post Re: repair of Panasonic microwave inverter
[Electronic Circuits]
Weston
July 24, 2021, 10:08:34 AM
post Re: Horrible waveforms advice--just bad test setup?
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
July 24, 2021, 03:20:42 AM

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 
SimplePortal 2.3.6 © 2008-2014, SimplePortal