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Does resonance in a MOT actually boost power output?

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Hazmatt_(The Underdog)

Tue Mar 14 2017, 04:33PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Check out Power Factor on Wikipedia.

As mentioned before, voltage and current are out of phase without the capacitor in parallel with the MOT secondary.

Theoretically you could measure that phase angle between the current and voltage on your scope, but it's dangerous.

The Power Factor is the cosine of the phase angle between voltage and current, PF = cos(phase).

Then Power in Watts = VA (for DC) and P = VA cos(phase) (for AC)

When we have a purely resistive circuit, power factor becomes 1 because there is no reactive component (no phase component)

For your transformer without the capacitor, voltage and current are out of phase by some amount (probably a power factor of .86, or cos(30 degrees) )

So the power in Watts delivered to your load becomes P = VA cos (phase)

But you are wondering WHY is there more power drawn without the cap then with?

That's because there is your Real Power (cosine) and your Reactive power VAR (Volt Amps Reactive) (sine).

So real power = VA cos(phase), and Reactive power = VA sine(phase), and the actual power being delivered is the vector sum.

The reactive power is power consumed but not being transferred to the load.

Power = VA cos(30) = VA * .8660
Reactive = VA sin(30) = VA * .5 < SEE! this is not insignificant loss!

With the capacitor correcting the phase angle to 0 degrees, you have the current and voltage now in phase with each other

Real power W = VA cos (0) = VA, and Reactive power VAR = VA sin(0) = 0

ScottH

Wed Mar 15 2017, 01:37PM

Registered Member #61373 Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 87

Hazmatt_(The Underdog) wrote ...

Check out Power Factor on Wikipedia.

As mentioned before, voltage and current are out of phase without the capacitor in parallel with the MOT secondary.

Theoretically you could measure that phase angle between the current and voltage on your scope, but it's dangerous.

The Power Factor is the cosine of the phase angle between voltage and current, PF = cos(phase).

Then Power in Watts = VA (for DC) and P = VA cos(phase) (for AC)

When we have a purely resistive circuit, power factor becomes 1 because there is no reactive component (no phase component)

For your transformer without the capacitor, voltage and current are out of phase by some amount (probably a power factor of .86, or cos(30 degrees) )

So the power in Watts delivered to your load becomes P = VA cos (phase)

But you are wondering WHY is there more power drawn without the cap then with?

That's because there is your Real Power (cosine) and your Reactive power VAR (Volt Amps Reactive) (sine).

So real power = VA cos(phase), and Reactive power = VA sine(phase), and the actual power being delivered is the vector sum.

The reactive power is power consumed but not being transferred to the load.

Power = VA cos(30) = VA * .8660
Reactive = VA sin(30) = VA * .5 < SEE! this is not insignificant loss!

With the capacitor correcting the phase angle to 0 degrees, you have the current and voltage now in phase with each other

Real power W = VA cos (0) = VA, and Reactive power VAR = VA sin(0) = 0

So if I just include a good PF correction cap parallel to the primarys input wires, that would give me the results of the resonant caps on the secondary, since I'm just making the current and voltage in phase?

Dr. Slack wrote ...

You've left out some connections. You draw the output arc (a) in series with the capacitor, to ground or (b) the capacitor is to ground, and you draw the arc across the capacitor?

Here's a link

Its wired just like this Schematic, except I have 2 Mots (each wired the same way) and no VARIAC.

The 2 HV outputs after the caps are wired together into a single hot, then arced to ground for the parallel setup. For the Anti-parallel setup, the 2 HV outputs after the caps are arced together with primaries fed 180 deg out of phase.

Is it bad that I have a 80uf PF cap in parallel with the wires going into the primary, since the Mocs correct PF? Would the PF cap make the PF better, or worse in this scenario?

Dr. Slack

Wed Mar 15 2017, 05:41PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

In that case, you're resonating out the leakage inductance, not resonating with the secondary inductance. I don't have an estimate for the leakage inductance, so don't know how near those caps are to resonance.

You could make some measurements from which you could estimate the the leakage inductance. The safest way to do this would be to drive the HT side from the mains, putting your test capacitors in series with the HT, and measuring the AC current output by simply shorting the LV output across with an AC ammeter. You can use both caps in series, or parallel, or one of them, to get three distinct values, and no cap will give you a 4th.

ScottH

Wed Mar 15 2017, 11:22PM

Registered Member #61373 Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 87

Dr. Slack wrote ...

In that case, you're resonating out the leakage inductance, not resonating with the secondary inductance. I don't have an estimate for the leakage inductance, so don't know how near those caps are to resonance.

You could make some measurements from which you could estimate the the leakage inductance. The safest way to do this would be to drive the HT side from the mains, putting your test capacitors in series with the HT, and measuring the AC current output by simply shorting the LV output across with an AC ammeter. You can use both caps in series, or parallel, or one of them, to get three distinct values, and no cap will give you a 4th.

I could set the resonance at different frequencies (for the secondary) and still get max power output, right? I was tinkering with some resonant online calcs, and the cap values are based on the resonant frequencies you set. Should I set the resonance of the secondary to 60hz if that is the primary input frequency and secondary frequency before caps?

And you're saying my PF cap only affects the leakage inductance part, and the resonant caps are to resonate my secondary?

Dr. Slack

Thu Mar 16 2017, 07:10AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

No, the PF cap resonates the primary inductance, the output caps resonate the leakage inductance.

'Resonance' only has a signaifiant effect when the resonant freuqency is equal tot he driving frequency. If the driving frequency is off resonance, then the effect is reduced.

So a small PF cap will reduce the magnetising current that the mains has to supply, as it becomes larger to resonance, all of the mag current is supplied by the cap, and it it gets too large, the mains supply has to supply excess capacitive current. This affects only the supply side current, and doesn't affect the output at all.

The leakage inductance limits the output current (to some extent, the winding resistance does as well). Using a large series output C reduces the effective leakage inductance, increasing the output current. As the output C becomes smaller, to resonance, the outpu current is maximum and determined only the by the winding resistance. As it becomes yet smaller, the outout current is limited by the capacitor.

The output winding inductance is not resonanted, but as it's big and in parallel with the output voltage, does not significantly affect the output.

Kolas

Thu May 11 2017, 10:16PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

I have experimented with this using a much more robust and electrically insulated device. I found the output to be greatly increased. This did; however, come at the cost of much higher stresses on the secondary of the transformer ultimately lead to it's demise.
it also caused the pole pig to draw significantly more power when loaded.

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