On Feb 21, 10:18*am, Robert Macy wrote:
On Feb 17, 6:25*pm, "Phil Allison" wrote:





"Robert Macy" = *one stubborn ****er

A cap does improve PF...

** But only with theoretical transformers - *NOT real ones.

First, use a Linear Model to represent the isolation transformer.

** Waste of ****ing time and effort *- *as it only repeats the same ****WIT
error you have been sprouting here all along.

The primaries of *REAL *commercially made E-core transformers are *NOT
linear inductors !!!!!!!!!!!

The off load primary current at rated voltage is *dominated by the third
harmonic * of the AC supply frequency.

The laminated iron core is then saturating, quite heavily.

JW's 1kVA iso tranny is a very typical example of this fact.

You will *NOT *find this information on webs sites that merely discuss
transformer basics.

You WILL *find this if you test a cross section of commercial E-core
transformers with the aid of a variac, RMS current meter and a scope
monitoring the current waveform.

This has NOTHING do with badly or well made transformers - *all makers do it
to save weight and cost.

BTW:

I happen to own a 1kVA transformer very similar to that described by the OP.

Tested as above, these are the figures:

VAC * A rms * *I peak

30 * * * 0.08 * * * 0.11
50 * * * 0.11 * * * 0.14
70 * * * 0.20 * * * 0.35

90 * * * 0.45 * * * 1.0
110 * * 1.0 * * * * 2.0
120 * * 1.4 * * * * 2.9
130 * * 2.2 * * * * 4.6

Up to 70 volts AC, the tranny is approximately linear with an effective
inductance of about 1.1 H.

At and above 90 volts AC it suddenly changes - *current starts to increase
exponentially and the wave becomes very peaky with a 1:2 ratio between rms
and peak values.

At 130 VAC input, effective primary inductance ( based on simplistic
calculations) *has dropped to less than 0.2H due to core saturation.

I must have tested hundreds of E-core trannys this way in the last 20 years
or so and ALL do much the same thing.

Toroidal and C- core types are different.

.... *Phil

For various reasons, it took MUCH longer than I anticipated to post
back here!

After more accurately modeling a REAL transformer based upon PA's
data, I am convinced that adding a cap in parallel will NOT improve
the PF.

,,,snip...!!!

What a WRONG STATEMENT! Adding a cap in parallel to the isolation
transformer 'should' improve PF, but not as much as one would like.,
but does improve it, without makinganything worse, except cost.

The error was caused by monitoring the current through Rpri, not
through Racmains, so OF COURSE PF never changed!!!

Didn't discover the error until went to a three inductor model which
matches the data fairly closely.

Note: the behavioural model should be fairly accurate, because the
transformer has a constant voltage across it, so it is possible to
approximate the hysteresis loss with a fixed resistor. The transformer
has enough voltage that each cycle the same amount of power is lost.
I would NOT use this model tor represent an audio transformer, where
the drive can vary a great deal.

The 'three' inductor model starts very closely matching the data,
results and data are at the end of this posting. Same conclusion,
adding a cap helps PF.


For the single inductor model, I added a residual amount of AIR
inductance [about 30mH] and got the following:

The model very closely models observation, at low voltages current is
fairly sinusoidal, near 70+ Vac, the thrid harmonic starts to dominate
the wave shape, DRAMATICALLY. We're talking major 'pointy' waveforms.
I can't post the plot, but you can copy the model below and see for
yourself.

Checking the model's match to the data:
LTspice PA's Data
Vac Arms Apk Arms Apk
30 0.068 0.098 0.08 0.11
50 0.118 0.172 0.11 0.14
70 0.180 0.270 0.20 0.35
90 0.275 0.447 0.45 1.0
110 0.744 1.68 1.00 2.0
120 1.38 2.96 1.4 2.9
130 2.08 4.22 2.2 4.6

simulating the circuit with the model:
NO LOAD [add Rcore = 350]
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 1.408 119.9 41.54 0.25
28uF 0.762 120 41.57 0.45
56uF 1.542 120 41.61 0.22

FULL LOAD 14.4 ohm [add Rcore = 350]
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 8.594 119.9 1,020 0.99
29uF 8.533 119.9 1,020 1.00

CONCLUSION: adding a cap in parallel to an isolation transformer,
whether loaded or unloaded, is expected to improve PF

copy and name something ending in .cir:
TEST_ModelBehaviour - nonlinear inductor using behavioural Model
* Voltage is V(in), current is I(Racmains)
* Real power is average of V(in)*I(Racmains)
* apparent power is V(in)rms times I(Racmains)rms
* Plot these three: I(Racmains), V(in), V(in)*I(Racmains)
..tran .1 10 9.95 .1m
..param k0=sqrt(2)
..param k1=120
Vac 1 0 AC={k1} SIN(0 {k0*k1} 60)
Racmains 1 IN 0.01
*Cpf IN 0 28uF
Rpri IN 3 0.26
*
..param kAIR=.03 ;set to 'residual' inductance in Henries
..param kk0=1.2 ;set to 'initial' inductance very low current
..param A0=0.274 ;set to current where var. ind. drops to half
..param kk=1.13
..param kk1={A0*kk} ;set to current for half saturation
..param kk2={kk0-kAIR}
*
Lcore 3 0 Flux={ {kk2*kk1}*tanh(x/{kk1})+{kAIR}*x }
*
*Rcore 3 0 350
*Rload 3 0 14.4
..end



= = = =
The three inductor model:

LTspice PA's Data
Vac Arms Apk Arms Apk
30 0.071 0.114 0.08 0.11
50 0.130 0.210 0.11 0.14
70 0.231 0.383 0.20 0.35
90 0.464 0.830 0.45 1.0
110 0.922 1.74 1.00 2.0
120 1.40 2.83 1.4 2.9
130 2.21 4.58 2.2 4.6

NO LOAD
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 1.441 119.9 41.30 0.24
29uF 0.642 120 41.31 0.54
64uF 1.728 120 41.32 0.20

FULL LOAD 14.4 ohm
120Vac Arms Vinrms Pwr(Rcore) PF
no cap 8.708 1,032 119.9 0.99
29uF 8.622 1,032 119.9 1.00


TEST_MB three - nonlinear inductors using behavioural Model
* for use on LTspice
*
..tran .1 20 19.95 .1m
..param k0=sqrt(2)
..param k1=120
Vac 1 0 AC={k1} SIN(0 {k0*k1} 60)
Racmains 1 IN 0.01
Cpf IN 0 29uF
*
Rpri IN 3 0.1
*
* first inductor
..param kk0=1.2
..param kk1={kk0-mm0}
..param kk2=.2
Lcore01 3 4 Flux={kk2*kk1}*tanh(x/{kk2})
* second inductor
..param mm0=0.185
..param mm1={mm0-nn0}
..param mm2=1.17
Lcore02 4 5 Flux={mm2*mm1}*tanh(x/{mm2})
* third inductor == air core
..param nn0=.02
Lcore03 5 0 {nn0}
*
Rcore 3 0 350
*Rload 3 0 14.4
..end