[RE-wrenches] Efficient step up, down transformers

[Exeltech]

Focusing on VA ratings would not be advisable.

Here's why.

Watts are real power.  This would be actual DC volts
times actual DC amps.  Measuring DC volts and DC amps
as you're presently doing is a good approach.  It
yields a valid result.

On the other hand, volt-amps ("VA") are nothing more
than real AC watts at some unspecified power factor.
Without knowing the associated power factor, a VA
value is meaningless.


For example:
Let's say I'm a customer, and I tell you I need a
1250 VA inverter.

You offer to sell me a 625 watt inverter.  Will it work?

Answer: Only if it's a true sine-wave inverter and my load
power factor is 0.5 or worse (<0.5).  I'm ignoring the
possibility the inverter may not be designed to work with
a load power factor that low .. but that's another topic.


Example #2: a 1 kW-rated sine-wave inverter operating with
a resistive load that has a PF of 1.0 is truly generating
1,000 watts.

At a load PF of 0.8, it becomes a 1250 VA inverter.

At a PF of 0.5, it's magically a 2000 VA inverter.

Unfortunately, "VA" is used by marketing departments to
inflate the actual output wattage and make an inverter
(or other device) seem more capable than it really is.

In the end, the inverter won't generate more than 1,000
watts, but incorporating a power factor of less than 1.0
into the spec makes it seem to the untrained eye like
they're getting more bang for the buck.

Real watts = real watts.  Period.

---

Lars mentioned at the onset of this thread he measured a
no-load current of 15.6 amps flowing into the transformer.

It's very possible there could be that many amps flowing
through the transformer primary to build, then reverse the
magnetic field in the core.  This would mean, however, the
current is 90 degrees out of phase with the voltage, hence
no real power being consumed.

The majority loss at that point would be the resistive
loss in the copper, which would be on the order of a few
watts - not kilowatts.

If he's in fact measuring real power somehow, and there's
no load on the transformer secondary, he's got a defective
transformer.


Dan



--- On Mon, 7/25/11, R Ray Walters <ray at solarray.com> wrote:

From: R Ray Walters <ray at solarray.com>
Subject: Re: [RE-wrenches] Efficient step up, down transformers
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Monday, July 25, 2011, 12:20 PM

I thought (based a previous thread) that we needed to be
concerned with VA output of inverters, not wattage, at
least on the transformer based machines like the SW. I
realize this wouldn't be an actual load if on grid. In
these situations, I just measure DC amps into the
inverter to determine the actual consumption, irregardless
of power factor, AC efficiency, etc.

R. Waltersray at solarray.com
Solar Engineer





On Jul 25, 2011, at 9:29 AM, Exeltech wrote:
Measuring current without the phase relationship
between the current and voltage isn't indicative
of the actual power consumed.  Likely what you were
seeing is the eddy current in the transformer core.
If so, it's 90 degrees out of phase with the voltage,
and the real wattage consumed with no load connected
is minimal.

Dan
Sr. Engineer
Exeltech