Re: Calculating DC Output Current From Unregulated AC Transformer

On Thu, 17 Jul 2008 19:56:15 +1000, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:


"The Phantom"

I have a transformer with a nominal 24 volt secondary, rated at 8 amps.
It
has a measured series resistance (secondary plus reflected primary) of
about .125 ohms.


** This is a fabricated falsehood - the numbers simply do not add up.

A 192VA rated transformer does not have 4% regulation - correctly rated
it has 8%.

This one does.

With 121.7 VAC applied to the primary, the secondary voltage measures 26.9
VAC, unloaded.

With 121.7 VAC applied to the primary, and rated load of 8 amps drawn from
the secondary with a pure resistive load, the secondary voltage measures
25.85 VAC.

This is a change of 1.05 volts for a load of 8 amps; 1.05/8 = .13125 ohms.


Look up makers data if you doubt this.

This transformer has no maker's identification, so I can't look it up.

However, the primary (it's a 60 Hz transformer) says 120 VAC and it
measures .961 ohms, cold.

The secondary (24 volts, 8 amps nominal) measures .080 ohms.

With 121.7 VAC applied to the primary and no load, the secondary voltage
measures 26.9 VAC, for an approximate turns ratio of 4.524:1. The .961 ohm
primary, divided by the turns ratio squared, gives .04695 ohms reflected to
the secondary. Adding the measured .080 ohm secondary gives a total .12695
ohms, which is *about* .125 ohms.

This is what I would expect to measure at the secondary if the primary were
shorted. What I actually measure is .131 ohms; the transformer is still a
little warm from having been under load a few hours ago. See the picture
of the impedance meter's display over on ABSE.


The oft quoted ratio of 1.6 for AC amps to DC amps applies ONLY at full
load and for a correctly rated transformer.

BTW:

Percent voltage regulation and percent power loss with resistive load are
virtually the same numbers (ignoring I mag loss).


..... Phil



.

Relevant Pages
Loading