SW Efficiency/ PWM Angle Adjustment [RE-wrenches]

[Bill Brooks]

Wrenches,

Just a quick word about what this about. I think many are completely
misunderstanding what this setting change addresses. It would be easiest to
show you on an oscilliscope, but since I can't do that in an email, let me
try to explain.

This problem was discovered when looking at the current waveform coming out
of the SW inverter. At each zero crossing (when utility voltage goes to zero
120 per second), the Trace SW would inject a substantial amount of current
(from the battery into the grid at zero volts). Since the current injection
happens at nearly zero volts, no power is transferred to the grid. It was
injecting current because it was not expecting to see zero volts so it tries
to inject current to keep the voltage from going to zero (trying to fix the
utility voltage). Well, the utility voltage ain't broke--it's trying to fix
something that ain't broke (technically speaking).

However, when an inverter flows current, there is heat lost in all the wire
in the transformers in the inverter (a lot of wire). We often refer to these
losses as I-squared-R losses (current squared times resistance in wire equal
the power lost). These are the very same losses we have when we talk about
voltage drop in wires on the DC and AC sides of the inverter. Heat is
generated, as power is lost.

It's a lot like jumping rope. If you get in sync with the rope, you can jump
faster, better and more efficiently. If you are out of sync slightly, you
have to jump too early and expend a lot of energy just staying up in the air
longer than you need to. Try it sometime.

There is nothing magic or mysterious about this in the least. It is simple
physics. Joel and Don are simply testifying that their inverters cooled off
(less heat loss due to unnecessary current flow). Most of the wrenches
community is extremely cognisant of the slightest ineffeciencies in loads.
We are simply talking about the efficient operation of the inverter.

You can debate me offline if you like and I will be happy to enlighten those
who enjoy learning. I learn new things every day. That's what makes life
interesting. I learned volumes from this one incident.

Bill.



-----Original Message-----
From: Don Loweburg, Offline [mailto:i2p at aol.com]
Sent: Wednesday, September 19, 2001 2:06 PM
To: RE-wrenches at topica.com
Subject: Re: SW Efficiency/ PWM Angle Adjustment [RE-wrenches]


In a message dated 09/19/2001 8:47:15 AM Pacific Daylight Time,
joeldavidson at earthlink.net writes:



I changed the PWM on my SW4048 from 129 to 126 the day after Bill Brooks'
post.



Joel and Bill,
I just happened to be at one of my grid sites last week and did notice the
inverters fan running in sel mode with only about 900w input. Since this
procedure was fresh in my mind I made the change. Within 2 minutes the fan
turned off.

Seems like a good thing to me. As I have occasion I plan to change the
setting on all ETL grid SW systems I maintain.

Don
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