Re: Coil Efficiency (and true wattmeter)
From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Thursday, June 26, 1997 7:36 PM
Subject: Re: Coil Efficiency (and true wattmeter)
HI John (Freau), all,
> From: FutureT-at-aol-dot-com[SMTP:FutureT-at-aol-dot-com]
> Sent: Thursday, June 26, 1997 4:01 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: Coil Efficiency (and true wattmeter)
> > Agreed. I think that's where a lot of false figures are coming from.
> >In many cases the waveforms are terrible. However, I have scoped
> > transformer primary currents using a very low value resistor in series
> > and probing across it with an isolated scope which not only shows
> > waveform but peak current as well. For an RMS figure, a bit of
> > calculus can be used. A valve scope is a must for a running coil.
> > I like your idea better though. The thought of a blown up scope
> > makes me blanch.
> > Comments?
> > Malcolm
> Thanks, I'll try the EHT probe with scope. By the way, I've been using a
> solid state scope to measure my quench times, and it hasn't blown out
> yet, so I guess I'll use it for the cap voltage measurement too (maybe
> lady luck will continue to be kind to me) :^)
> Can you give any idea of the results you've obtained using calculus to
> figure the input power compared with an ordinary wattmeter, and how
> different the results were with different break-rates? For instance,
> (I'll just make this up) suppose wattmeter read 1000 watts at a low
> break-rate, did calculus results show (let's say) 700 watts, in other
> words a lower figure? And if wattmeter read 1000 watts at a high
> break rate, did calculus method show (let's say) 500 watts, or an
> even lower figure? Intuition tells me to expect these kinds of general
> results...but since I don't trust my intuition very much, I'd be interested
> in your general findings...ballpark of course.
OK, confession time. I haven't done the maths on it but could if you
would like to see some figures. I first cottoned on to this after
examining the primary current waveforms present in a couple of
microwave transformers. The waveform hardly changed when loading was
applied to the transformer but the phase angle between that and the
primary voltage did somewhat (not a huge amount from memory).
You could approximate the current envelope using piecewise
approximation which gives a triangular waveform with around a 60:40
duty cycle for these transformers. Applied voltage was sinusoidal of
course. The product of RMS conversions x power factor (cos PHI) will
give true power. I'll try and find time to do some this weekend but
no promises. I already have too many hares running.
I think simplest would be to build Dave's meter since its
accuracy has been verified by Richard. I think that would be a good
way to go for wallplug power. Thanks for the reference. I'll go to
the library shortly.