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Issues with running a large air coil using a full bridge

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si2030

Mon Jan 16 2012, 10:12AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi all,

Help.

I am having some trouble with this. I have just finished building a large air coil.

The picture shows a secondary of 20 turns however after doing some checks I verified my inductance meter was not able to read the inductance of the primary correctly so I have rewound it to be a 5+5 arrangement. The inductance of each of the two coils is 20uH. The inductance of the secondary is 2 Henry.

I have placed this in a oil bath and attached a full bridge driver to it. in the circuit is a globe - 60 watts to reduce the current. However all that happens is that the globe starts to glow and the primary pretty much works as a full short. I can not get more than a volt across the bridge... If I take out the globe then I just get very little increase in volts for a large increase in current... a short.

This is going to be the system to drive a CW rig at resonance however I seem not be be able to run the coil at any voltage because the impedance is too low...

I have tested this configuration on LT spice and it does work well there however I seem not be able to run this without drawing way to much power...

I have an isolation transformer from which a globe is attached in series (can be switched out if required) and then a variac and then a step down transformer and then my bridge which includes a rectifier. (see pictures below).

My question is this... is there something I am not taking into account or is this what I am to expect from this configuration... say a flyback with snubbing?

Wondering if someone might tell me whether thats what I should expect or what I can do to get this to work... would have thought I could have run it at low voltage and still get it to spark but it seems like I cant use the globe as ballast and I am in real trouble of blowing the IGBTs in the bridge...

Kind Regards

Simon

Thomas W

Mon Jan 16 2012, 10:25AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

i THINK (i dont know much on the topic) but arnt Air Core transforemers
HF?

if so are your IGBTs fast enough

Just my 2cents

Patrick

Mon Jan 16 2012, 10:27AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well first maybe youll need to add some impedance, air cores always have large currents, since air cant couple the field from one place to another.... how did you get 2 henerys from an air core?

MAybe a ballast resistor...?

si2030

Mon Jan 16 2012, 11:20AM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi there,

OK.. I am using irgp50b60pd1 which are warp drive IGBTs which should be fine up to 150kHz... I am running the system at around 18kHz at the moment.

Now a ballast resistor... I do have a globe in series with the electronics which is 60 watt... am in Australia so 240AC mains...

Problem here is that all the power is being dissipated in the globe.. (the primary is acting as a short)...

Simon

Patrick

Mon Jan 16 2012, 11:30AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I need a schematic, otherwise i cant help you.... do you have any ferrite anywhere in the power train?

si2030

Mon Jan 16 2012, 12:00PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi there,

No ferrite anywhere.

1326714729 1571 FT132436 Basic Schematic

At short notice this is a screen print of the Spice schematic which represents the basic bridge etc of the system... just that the secondary has a large cap across it instead.

I have also included a zip of the LT spice sim if you have LTâ€¦

Simon
]full_bridge_v1.zip[/file]

Patrick

Mon Jan 16 2012, 12:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

How are you getting 2 heneries with an air core? how many turns and what diameter/hieght is that coil..?

Steve Conner

Mon Jan 16 2012, 12:19PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When you drive an inductor with a square wave from an H-bridge inverter, the magnetising current is a triangle wave. The peak current is V/(4fL) - V being the DC bus voltage of your inverter. With 320V, 20uH and 18kHz, you can expect 222 amps peak.

In other words you have way too few primary turns.

Dr. Dark Current

Mon Jan 16 2012, 12:41PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes, your primary impedance at the drive frequency is way too low.
One way to get rid of the reactive component of the current is to tune your primary with a capacitor to form a parallel resonant circuit. You can then use a current fed inverter to drive it at the resonant frequency and ideally it will see just the real power.

si2030

Mon Jan 16 2012, 01:32PM

Registered Member #1571 Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44

Hi Guys,

just over 2000 turns...

Can I put this to you. The coil needs to produce 100k peak to peak and the frequency needs to be tuned to the CW tower so as to run at resonance... If I increase the turns on the primary and thus the inductance to an acceptable level how do I still produce 100k on the secondary?

The CW has an equivalent capacitance of about 50pF so if I increase the inductance on the secondary then the resonant frequency between the secondary and the CW will drop which then increases the peak current...

Is there a way to produce 100k peak to peak without such a huge current?

I am open to new topologies/coil design here... any suggestions would be much appreciated.

Simon

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