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StevenCaton

Mon Mar 16 2009, 12:08AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I have quick question about transformers.

Normally a MOT is fed 117VAC and gives 2100VAC on the secondary. (with a peak value of 3000V on the secondary)

If I fed an MOT pulsed DC Half wave (directly out of a full bridge rectifier) (117VAC input) would this work, and provide 3000V peaks on the secondary.

the_anomaly

Mon Mar 16 2009, 02:43AM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

I think you will just saturate the core. I belive the other half of the cycle is needed to first unmagnetize and then reverse the magnetic field of the core.

rp181

Mon Mar 16 2009, 12:21PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I tried it a while back. The core does saturate, and you need a half-wave (1 diode) rectifier for a halfwave ouit.

KLH

Mon Mar 16 2009, 06:01PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

The core WILL saturate within a few cycles. Transformers (except ones that are meant to store energy) are meant to be driven with equal volt-seconds for each half of the cycle. If the magnetic path is closed and there is a miniscule DC component in the drive signal, then the flux will eventually "walk" into saturation. Of course, small imperfections in E core construction (gaps) and winding resistance will allow these transformers to tolerate a little DC offset. Toroids, however, are absolutely intolerant of DC offset if it is not controlled by winding resistance. If you really need to use this type of drive waveform, then you might be able to wind a reset winding onto the transformer, like in a forward topology transformer. This does reduce core utilization, which in turn reduces max power output.

Sulaiman

Mon Mar 16 2009, 06:45PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Just to add to the above;
- ANY dc in the primary will reduce available output
- In practice if you put a half- or full-wave rectifier before the primary you will blow a fuse/breaker in under a second !
More importantly,
- No matter what waveform you put into the primary,
the secondary output voltage MUST have an average output of 0 volts,
the area under the volts vs. time for the positive part of the waveform WILL/MUST equal the area under the volts vs. time for the negative part of the waveform.
In short - there is no way you can do better than ac in / ac out,
unless you can shorten the time of each half-cycle,
e.g. operate at a higher frequency or short pulse-width.
Neither of which are of much benefit when you only have normal ac supply.

In theory you could put a triac-type phase-angle controller (light dimmer etc.)
before the primary and operate on 230 Vac for example,
but as soon as you get too 'brave' something will 'pop'

If you need higher dc output voltage you could use a Greinacher (Cockroft-Walton) voltage multiplier.

StevenCaton

Mon Mar 16 2009, 06:53PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Thanks for clearing that up for me guys.

Dr. Dark Current

Mon Mar 16 2009, 08:23PM

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

Sulaiman wrote ...

- No matter what waveform you put into the primary,
the secondary output voltage MUST have an average output of 0 volts,
the area under the volts vs. time for the positive part of the waveform WILL/MUST equal the area under the volts vs. time for the negative part of the waveform.

Actually, I believe this is true only for the primary waveform, we all know that it is perfectly possible to drive halfwave rectified TV transformers from a bridge and draw arcs from them (which is basically a short circuit for every other halfwave on the secondary).

KLH

Mon Mar 16 2009, 09:55PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Dr. Kilovolt wrote ...

The transformers you are referring to are flyback transformers. These store energy, and so are designed to support some DC magnetizing force in their air gap / gaps. The volt-seconds for ALL waveforms / windings must average zero in the core for transformers not designed to store energy, or the flux will saturate.

Dr. Dark Current

Mon Mar 16 2009, 10:09PM

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

killah573 wrote ...

Dr. Kilovolt wrote ...

I thought that too, until I removed the air gap and it worked still! I heard some sub-harmonic resonances though, but the performance didn't seemed to be hurt much. However this was a long time ago and some details are a bit "blurry"...

KLH

Wed Mar 18 2009, 10:07PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Dr. Kilovolt wrote ...

killah573 wrote ...

Dr. Kilovolt wrote ...

Tunnels on 4HV? Let the fun begin.

Anyway, all transformers store some (usually undesired) energy. I'm thinking you were using either an E-core or a U-core set, since these tend to store more energy. I don't believe that this would be possible with ferrite toroidal cores, because these have very low leakage inductance.

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