# Re: Ammeter readings with MOT power supply

```Original poster: David Dean <deano@xxxxxxxxxxxxxxxx>

Hi

On Thursday 29 March 2007 18:19, you wrote:
> Original poster: "Breneman, Chris" <brenemanc@xxxxxxxxxxxxxx>
>
> Thanks, but I'm still somewhat confused.  If the problem actually is
> the circuit breaker, that would mean that the 15A breaker wasn't
> tripping when more than 20A were flowing through it for several
> minutes.  That link says that they can take a while to trip if the
> current is only slightly over the limit, but I would think that over
> 5A over the limit of a 15A breaker should be enough to trip it
> quickly.  Also, I'm using a circuit breaker that's newly bought, and
> I don't think it's a FPE breaker.  It says "Type-Q" on it if that's any
> help.
>
Probably GE or one of the clones.

Brand is not much of an issue, there is some variation between brands, but
there is quite a bit of variation between breakers of the same brand and even
the same lot.

In the real world 150 percent of rated current might hold for 5 minutes  or
more. They have to be able to take surges like when a motor starts. A safe
rule of thumb is six times the FLA or full load running amps to start.  A 20A
breaker needs to be able to hold 80 percent or 16 amps indefinitely. An array
of light bulbs that pull 16 amps when they are burning might pull 160 amps
when they are switched on. The breaker needs to be able to hold 100 percent
rated current for three hours or so to prevent nuisance tripping.  So now we
have some points, we can plot a curve.  Should not trip at time = infinity
and load = 80 percent, instantaneous trip at time = 0 and load  > 10 x 100
percent, trip at time = 3 hours and load = 100 percent,  and finally trip at
time = 5 minutes and load = 150 percent.  Draw it and you get a kind of knee.
Of course you can't really draw infinity, but you can draw a curve through
three points. Just as long as the never trip time is off the graph before you
get down to 80 percent load.

load    80 90 100 200 500 1000 1500
t=0                     ____________x
t=5 min           /
t=10 min        /
t=20 min       x
t=50 min       x
t=120 min     x
t=270 min     x
t=360 min     x

Your power supply likely is pulling more than 20 amps. If your measurements
are close you could be pulling I would guess 20 to 25 amps.

I don't know about power factor correction. When you power a Tesla Coil you
are charging a capacitor and the circuit will behave differently than when
the output is short circuited.  If you want to lower the amp draw, open the
secondary on the ballast.  Short circuit current will drop. No PFC required.
Not the best way to do things for a Tesla Coil.  In a TC you are charging a
capacitor so current is drawn from the source in a nonlinear fashion. With
the secondaries short circuited the current is drawn in a more linear
fashion. In the case of a motor or something which looks to the source like
an inductance in parallel  with a resistance, PFC can help you  by canceling
some of the inductance leaving you with mostly resistance. As long as you
don't get too close to resonance (power factor = 1 ) at which point weird
things will happen. In the case of the TC the source sees a capacitive load
and so you add inductance to cancel that in the form of ballast. Until you
get to resonance. Maximum power will be transfered from source to load and
weird things can happen. A DC power supply is a whole other case.

As to whether your ammeter will work out for you will depend on your selection
of tank cap, spark gap, brake rate, etc. which will determine the current
drawn and the power factor, as well as whether PFC should be used or avoided.

later
deano

> Thanks,
> Chris
>
>
>
> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Wed 3/28/2007 8:59 PM
> To: tesla@xxxxxxxxxx
> Subject: Re: Ammeter readings with MOT power supply
>
> Original poster: David Dean <deano@xxxxxxxxxxxxxxxx>
>
> Hi
>
> Unless you are using old FPE breakers that were popular in the 1960's and
> 1970's, which have a bad reputation for never tripping, it may take a while
> for the breaker to trip. See
> http://www.swgr.com/CircuitBreakersTech_Home.asp for a better explanation
> than I can give.
>
> later
> deano
>
> On Tuesday 27 March 2007 19:40, you wrote:
>   > Original poster: "Breneman, Chris" <brenemanc@xxxxxxxxxxxxxx>
>   >
>   > Hello,
>   >
>   > I've almost finished the MOT power supply for my first Tesla coil,
>   > but have a few questions about its operation.  The supply consists of
>   > two MOTs with the primaries in parallel and secondaries in series and
>   > a third MOT in series with the parallel arrangement of the first two
>   > acting as a ballast.  The ballast has its high voltage winding
>   > shorted.  All of them run on 120V.  In addition to the transformers,
>   > I also have a 15A circuit breaker in series with the circuit, and the
>   > house has its own 15A breaker on this circuit.
>   > I recently received a 15A analog panel ammeter which I wanted to use
>   > in the circuit to monitor current drawn.  It is placed in series
>   > between the breaker and the ballast.  When I run the power supply
>   > with the hv end shorted, no breakers trip, but the ammeter reads off
>   > the scale.  When I replaced the analog meter with a digital
>   > don't know if if more than 15A is really flowing, and if it is, why
>   > it isn't tripping the breakers, and if it isn't, why do the meters
>   > show that it is?
>   > The only possible cause I can think of for this discrepancy is that
>   > the power factor is probably greatly offset by the highly inductive
>   > load, but why wouldn't the breakers trip in this case?  I also tried
>   > the circuit with a 10uF PFC capacitor, and saw no difference in
>   > behavior.  If the problem is the power factor, do you think a 180uF
>   > PFC capacitor would be reasonable for such a supply?  The open
>   > circuit voltage is slightly over 5kV and the short circuit current is
>   > slightly over 500mA (that is, assuming these readings from the
>   > digital multimeter are correct; I determined these values indirectly
>   > by measuring current through different resistances on the hv side).
>   >
>   > Thanks a lot,
>   > Chris

```