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PV Panels $.98/Watt? - Page 4

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Posted by wmbjkREMOVE on September 27, 2009, 2:08 pm


Not that I've heard of, and it would seem to be a niche too small.
It's the same with welding at my place - 24VDC gets converted to
230VAC, and then the welders turn it mostly back into 20 some volts


Posted by daestrom on September 27, 2009, 8:43 pm

Mark F wrote:

I think you'll find it's a bit more complicated than that.

Most variable speed drives are actually VVVF (variable-voltage,
variable-frequency) drives.

A standard motor for a compressor or blower can be run on variable
frequency *if* the volts/hertz ratio is kept nearly constant.  If the
voltage is kept constant while the frequency is varied over a wide
range, you burn out the motor.

Of course if the inverter section was a pulse-width modulated type of
unit, you could get something similar to VVVF, but it would still be
hard on the motor (high harmonics in PWM).  AFAIK, most hi-power VVVF
use a simple H-bridge for the inverter section and vary the voltage
output of the converter section.

So the AC-DC converter section is a variable DC voltage.  If you have a
homepower system with DC, you'd have to find a way to vary the voltage
by a wide range.  One common way to do that is with an
inverter-converter arrangement, but now you're right back where you
would be if you ran the VSD off of a typical inverter.


Posted by N9WOS on September 29, 2009, 12:38 am

Most VFD's use three phase to drive the motor. how inductive current is
controlled depends on the output of the inverter. If it's a sine wave using
high frequency PWM through the motor inductor, like large VFDs use, then it
will vary the output voltage, and current, by adjusting the modulation. It's
called sine wave approximation. Supply voltage to the inverter remains
constant. But most consumer grade VFD's, and older large VFD's, don't use
sine wave output, that would cost too much money.

They use square wave output direct to the motor windings. They modulate the
winding current by shifting the pulse width. Remember, there is two ways of
controlling the current in the inductor, that is voltage, and duty cycle "on
time". It large units, that method will cause problems with harmonics, like
you said. But in small systems, it doesn't cause enough of a problem to
worry about.

When you slow down the motor, you either have to reduce the driving voltage
into each winding. Or you have to reduce the "on time" of each winding.

At full speed, The supply to each phase goes directly from +170V to -170V in
a square wave pattern. At reduced speed, the output goes from 0V to +170V
back to 0V then to -170V basically a modified sine wave.

If you have ever messed with a modified sine wave inverter, you will notice
than when you increase voltage input, the duty cycle of the output wave
drops, while the peak increases. That is the inverter's method of regulating
the true RMS output of the inverter.

VFD's do the same thing. As it slows down, the peak voltage remains the
same, but the duty cycle drops to reduce the true RMS voltage going to the
winding, and thus, the current. With large VFD's they basically do the same
thing, but very quickly so that the winding current appears sine wave like.

Remember, inductors have a time lag. If you apply a voltage to an inductor,
to generate a current, then you drop the input to zero, the current will
stay circulating in the inductor, and slowly decay.

Posted by daestrom on October 1, 2009, 9:35 pm

N9WOS wrote:

 From what I've seen, modified-sine wave inverters and PWM all require
FET's of one form or another.  If you're just using FET's or such, you
can turn them off whenever you want in the cycle to reduce the pulse
width.  But highly inductive loads that can act as induction generators
will have a current that is out of phase with the applied voltage.  This
requires some snubbers (filters) to absorb this reactive current.

In large VVVF drives, those suitable for several horsepower and up, the
reactive load can be more than what simple caps and resistors can handle
in any realistic way.

The large units I've worked on (50 to 500 hp) opt for SCR's in an
H-bridge circuit instead (well, the three-phase equivalent of an 'H').
To turn off an SCR in one leg of the bridge, the corresponding SCR
above/below it is turned on.  This would be a direct short if not for a
center-tapped inductor placed between the SCRs that commutate the SCR
that was previously conducting current through the inductor when the
non-conducting SCR is turned on.

Using SCRs like this doesn't allow for any sort of PWM, so the supply
voltage to the bridge has to be controlled.  Typically this is done with
a three-phase rectifier with the diodes replaced with more SCRs and
these are phase-controlled to control the average output of the converter.

I suppose the small motors in a furnace (fractional hp) could use PWM
and power FET's, didn't think about that much.  I'm used to thinking in
larger numbers :-)


Posted by z on September 20, 2009, 7:07 pm

Wow thats a good price and they are actually in stock looks like.  Every
time I tried to buy some panels from sunelec.com they were always out of
stock so I was thinking it was a bit bogus with the price.  But I suspect
they just sell out fast.

Man if it was beginning of summer i'd get a few of those bad boys.  As
we're heading into to winter its probably not worth it now.


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