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Grid tie inverter

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Ash Small

Sat Nov 22 2014, 09:29AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

Ash Small wrote ...

Here's a link to the Wikipedia page on class D amplifiers.

You don't actually need to bother with the bit that says

"After amplification, the output pulse train can be converted back to an analog signal by passing through a passive low pass filter consisting of inductors and capacitors."

With class d there has to be a low pass filter somewhere. I mean, you're PWMing, you have to filter off the harmonics.

If you're driving a speaker, the speaker coil is an LR filter.

Driving the mains, presumably you'd need an inductor/transformer or something to smooth it out wouldn't you? You don't want huge currents flowing in and out of your inverter as it switches.

You don't want to filter the signal between the class D amp and the bridge driver, you need the 'chopped' signal to feed to the MOSFETS. The place to put the filter/transformer would be after the bridge, not on the class D output.

EDIT: Actually, I don't think any of the 'representations' of the waveforms are brilliant.

You want the pulses from the positive half of the cycle fed to one side of the bridge, and the pulses from the negative half of the cycle to be inverted, and fed to the other side, if that makes sense. (Maybe I should sketch it?)

EDIT: Or could you use a 50-60 Hz xformer driven by the bridge. Would the transformer 'smooth' the pulses, and give a sine wave output?

Shrad

Sat Nov 22 2014, 01:40PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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an irons core transformer will not let much 20+kHz pass to the extend of creating huge parasitics

the transformer will behave in itself like a filter with its parasitic inductance plus the iron properties... if there are some parasitics just put a RC filter to shunt high frequency components, and you could even design it so the RLC filter results in a resonance at 50Hz and a low pass filter

hen918

Sat Nov 22 2014, 08:54PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

For efficiency I would recommend an LC filter, not an LR filter. You need an inductor in series with the output and a capacitor on the load side of the inductor going to ground/neutral.

furnace

Mon Nov 24 2014, 06:50PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Is there really no simple way of producing a low frequincy sine wave of 50 to 60 hz?
I tried using filters on the 555 and the best I get is an almost sine wave but not pure. I can't seem to find a simple schematic of this.

Daedronus

Mon Nov 24 2014, 07:05PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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using a uC with a look up table and a cheap DAC should be pretty straight forward, unless you are bent on using only analog circuitry.

Sulaiman

Mon Nov 24 2014, 07:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

There are several methods of implementing a grid-tie-inverter,
you will see some as 'pure sine wave' .. others are quasi-sinewave, others nothing like a sinewave,
I think that utility companies are not keen on other than 'pure sine wave'.

Some found in industrial (motor) regenarative drives just use a 3-phase bridge
with the switching transistors enabled by the incomming line frequency
this basically stuffs (near) constant current back up the mains,
(I hated testing regenarative drives ... scary, and nowadays I do almost no work on drives)
works with single-phase or 3-phase motors in regeneration mode.

Even for single phase I doubt that a half-bridge would be easy, a full-bridge would be better.

The type of design depends a lot upon the power source;
. solar cells produce a maximum current proportional to illumination so are quite easy
. batteries or generator/alternator/turbine do not have constant current so need complex designs

What is your power source?

I think (but I'm not sure) that 'upgrading' the power section of a cheap inverter is do-able
(probably just current-sense scaling and higher current switches/transistors)

You may find more info. by researching UPS design,
the inverter part is conceptually similar to some grid-tie inverters.

WARNING; I am not a grid-tie inverter expert, the above are just ideas for you to ponder.

I truly think that unless you intend a carreer in inverter technology you should just buy one.

furnace

Mon Nov 24 2014, 08:02PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

I'm sorry for being a bit of topic but I was thinking of first trying to build a normal 12v sine wave inverter before getting to a grid tie. Now it seems that it is very difficult to create a sine wave from your generator even to start with or you need huge filters on your primary, I thought there would be a simple IC for this.

Shrad

Tue Nov 25 2014, 09:54AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

I really have to insist on a simple class D amp which would amplify the output of a line transformer (3VAC or something), and the output would have no filter but a 50/60 hertz transformer with primary rating of the volt/amperes of the DC bus you use (be it solar panels or a battery or whatever)... a small transformer will be enough for testing purposes

There are dozens of designs on the net, and you can easily scale up later on

furnace

Tue Nov 25 2014, 04:58PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

I tried using a class d amp and you don't get a pure sine wave at the low frequencies like 50 or 60 hz it only gets pure sine at 150 hz and up and than you have to use large caps for filtering and adding the inductor does not make much of a difference to the wave, so your inverter would be very bulky and inefficient.
To my experience the best way would be to feed the mosfets a pure sine wave at the gate to achieve a perfect sine wave at the output?
Please correct me if I'm wrong but after testing this seems to be the only way for efficiency?

hen918

Tue Nov 25 2014, 06:47PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

nope feeding the MOSFETs with a varying voltage will make it less efficient because the MOSFETs will work in the high resistance region between on and off dissipating a huge amount of heat (remember changing the voltage on the input of a MOSFET will not change the voltage on the output. It will only change the MOSFET's resistance) MOSFETs, unlike bipolar transistors, are designed to spend most of their time on or off.

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