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Re: [TCML] MOT Measurements



Hi Finn,

Yes, the change in leakage inductance is what caught my attention. But as I think about how a normal transformer without shunts are designed (very low leakage) and then think about the shunts themselves and how they are designed to force leakage, I can understand why (and how) the change occurs. For coilers, this is more than academic and maybe aids overall understanding of the components were using to drive the coil and how they react.

Some coils do kick in as you stated, others don't. Depends on C and the transformer. Something I've realized with Microsim is that to truly model the transformer for a given input voltage, it helps to know the inductances and k at that input. Sure, we could adjust k to achieve a measured current, but I think it's better to model the transformers k and inductances at that input (which is how I go about it now). Yes Microsim equates the leakage inductance. In some simulators, that leakage may need to be modeled separately. It's also a great exercise for people. It shows how current through the windings change the inductance of the windings. Then they can dig into "why" this occurs which leads to even a better understanding of induced voltages.

It's not so much the low voltage situation I'm interested in (but interesting to see), rather the ranges we run our coils. For example, my 12/200+ NST (1/2 shunt gizmo). I usually run about the 100V range and get excellent sparks with that power. When I increase power beyond that things get unmanageable.

Here is a list of Cres values for this particular NST as it is now (far from the original unmodified 12/60 case). With this coil, I'm using a low 18.8nF cap size. So I'm very STR at Cres / 2.1 at 100V input. There's no resonant charging occurring. I have also measured bps a while back and noted that there was a slight change in bps, but nothing as significant as others were seeing (this sort of baffled me). Now looking at the transformer form this vantage point, I can understand why, and I can maybe understand why their bps was higher. Wish I had their transformers in my hands to do some measurements with!

Anyway, this is the Cres expectation for the modified NST. This NST might be a decent replacement for my pig since it's fitted with 64nF and 6 Mark Dunn boards... I should look at the voltages at the NST output first (and this is where load impedance fits in).

Vin        Cres
-----      -------
10.08      25.0 nF
20.27      25.9 nF
29.92      26.5 nF
40.60      27.3 nF
50.30      28.1 nF
60.10      29.3 nF
70.00      31.6 nF
80.10      34.3 nF
90.00      37.8 nF
100.5      41.6 nF
110.4      46.0 nF
120.4      55.6 nF
130.1      58.5 nF
140.1      61.1 nF

Take care,
Bart


Finn Hammer wrote:
This is getting more and more interesting.
To me, the news is the variation in leakage inductance with input voltage. Back when I ran simulations with these transformers, I measured the inductance of the primary, then calculated the secondary inductance and plugged them in microsim. After that, I would adjust coupling until the model delivered the same current into a short, as the real thing did. Of course, Microsim supplied a leakage inductance that fits in the process, so resonance at full input voltage wouls be modelled faithfully. I never interested in the low voltage situation, but maby this explains why the coils seem to "kick in" when the variac gets turned right up?

Cheers, Finn Hammer

bartb skrev:
Hi All,

I hope I'm not boring you with this (I'm sure some are). But I think the next step is to look at a 15/30 NST which has not been modified. I have a couple on hand (Franceformers). I have a "gut feeling" that we really don't know where shunted transformers (NST's, MOT's, etc.) are truly at. Some resonant charging may or may not be occurring with our tank cap values. All the programs (including my own Javatc) are looking at Cres of non-shunted transformers. The shunted transformer is a different beast and without measurements, only guesses.

These transformers force leakage inductance via the shunts to apply regulation. This changes the transformer as we assume it to be as far as currents, voltages, Cres, etc. The load also changes the output. NST's will begin to suffer a voltage drop even with a small load. MOT's are current beasts, and they will require larger loads before significant change in output voltage is seen. Load impedance of shunted transformers is yet another set of tests that should be performed at some point.

I'll run through my normal 10V increment routine with one of my 15/30's. I'll set a variac to as near 10V as I can get, then measure the open and short circuit primary currents as well as open secondary voltage. Then head up to 20V and do all again. This is my routine as it keeps the source voltage unchanging, and thus, the numbers for Ioc, Isc, Vin, and Vout are all in very good agreement. My plan is to first do this unloaded (basic transformer testing). But loading is yet another issue.

If anyone has any additional thoughts, please let me know. For the NST, considering it's low power, I can will also measure true power as well as VA.

Best regards,
Bart

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