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Capacitive ballasts and PFC; why the transformer vibration?

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GrantX

Fri Dec 02 2011, 06:58AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Hey everyone,

I've been fiddling with various ballasts for an MOT I wish to use for a general purpose AC/DC supply and have come across several issues using capacitors.

Firstly, I simply placed a capacitor in series with the active line, which cause massive vibrations in the transformer (even with very little current flowing - below its designed input) and what seemed to be an almost pulsed output.

Secondly, I had the MOT set up with an inductive ballast that drew 1kVA. I then placed a 40uF capacitor across the 240V input lines for PFC in an attempt to reduce primary heating. According to my clamp meter, the apparent power had decreased, except heating in the primary was WORSE somehow, and the output arc had taken on a much angrier, harsher sound. Most annoying of all, the incredible vibrations in the MOT were back.

I tried capacitive and inductive/PFC ballasting on a separate MOT and the same situation occurred. I also found a youtube video of someone ballasting a PT with capacitors, which visibly shows what I'm ranting about. I thought that the capacitor wouldn't have changed the input waveform much, can anyone explain the theory behind this? Its been bugging me all day!

Dr. Dark Current

Fri Dec 02 2011, 01:43PM

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

Hello,
1. A capacitive ballast in series with a transformer basically forms a series LC circuit; this circuit can resonate close to the mains frequency with the transformer unloaded, but also with the transformer short-circuited (depends on component value of course). This resonance increases core flux and can cause it to vibrate more loudly.

2. A PFC cap is connected in parallel to the mains. It does not decrease your primary current, just the current drawn from the wall.

3. A capacitor cannot be used to ballast an arc.

GrantX

Fri Dec 02 2011, 02:28PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Dr. Kilovolt wrote ...

Hello,
1. A capacitive ballast in series with a transformer basically forms a series LC circuit; this circuit can resonate close to the mains frequency with the transformer unloaded, but also with the transformer short-circuited (depends on component value of course). This resonance increases core flux and can cause it to vibrate more loudly.

2. A PFC cap is connected in parallel to the mains. It does not decrease your primary current, just the current drawn from the wall.

3. A capacitor cannot be used to ballast an arc.

Thanks, that gives me a better understanding of what I'm dealing with.

For series LC resonance on the primary side, is there a 'rule-of-thumb capacitance' for primary resonance on 240V MOTs? 40uF didn't seem to cut it, so at a rough guess (too tired/hungonver to calculate it) it would probably be around >150uF, give or take 90%? :P

Anyway, from what of seen on youtube, primary-resonant MOTs seem to put out some mean-sounding arcs, but in the video I saw he gave no info on the resonant circuit :(

Also, is the increased vibrations damaging at all to the transformers? the MOT I'm using has a welded core, so I'm hoping it won't stress it too much.

Dr. Dark Current

Fri Dec 02 2011, 02:38PM

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

You mean resonance with the secondary shorted, for arc drawing? The leakage inductance of a standard MOT seen from the primary side is around 50 mH, so the capacitor comes to around 200 uF.

However there is a big disadvantage using this configuration - with the resonance on the secondary side, all the VA flows only through the secondary winding and there is just a relatively small current draw on the primary, the circuit is kind of "self power factor correcting". If you put the cap in the primary circuit, it will draw huge currents from mains and the primary winding will overheat very quickly.

The increased vibration shouldn't be damaging, but the higher core flux increases core losses and open-circuit secondary voltage, which could be dangerous especially for HV transformers.

GrantX

Sat Dec 03 2011, 03:47AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Dr. Kilovolt wrote ...

You mean resonance with the secondary shorted, for arc drawing? The leakage inductance of a standard MOT seen from the primary side is around 50 mH, so the capacitor comes to around 200 uF.

However there is a big disadvantage using this configuration - with the resonance on the secondary side, all the VA flows only through the secondary winding and there is just a relatively small current draw on the primary, the circuit is kind of "self power factor correcting". If you put the cap in the primary circuit, it will draw huge currents from mains and the primary winding will overheat very quickly.

The increased vibration shouldn't be damaging, but the higher core flux increases core losses and open-circuit secondary voltage, which could be dangerous especially for HV transformers.

Yes, sorry, I meant primary resonance for arc drawing.

Thank you for the explanation, that post was more informative than the baby-level textbooks they've given us for the first year of my course :D

Looks like I'll start fiddling with caps on the secondary then. My main aim is to build an MOT based variable voltage (variac powered, so 0-2000VAC) HV transformer which can run near-continuously at about 1kVA - 1.4kVA, so I have a sturdy power supply that can be used for a wide variety of applications and experiments.

If heating issues cannot be solved, then expect a thread on the HV board about me winding my own transformer, haha :)

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