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How difficult would it be to make a simple 5kw inverter?

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What is the meaning of "simple 5kw inverter"? Did you find a youtube video that you want to follow? Do you mean using an audio amplifier with a toroid? There is nothing simple at 5 KW. To start with you need to wire your generators/buffers for higher voltage than 50 V.

Regards, Dieter

Here is your basic inverter, simulated in the free LTspice sim programme.

This one is open loop, so only works for this case of fixed load.
In fact, the way to do it is to  first have a sinusoidal reference.
This  represents what you want  your inverter to give as an output voltage. So you increase the PWM duty cycle to the bridge FETs as the sinusoidal reference increases. A big inductor in the bridge will smooth out the current.
You need fast overcurrent limiting  so that the current never goes above your maximum allowable. You also need average current limiting which is obviosuly much slower acting.
In order that your output voltage never goes above 339Vpk....make sure your DC supply is exactly 339VDC. Then enjoy PWM'ing away.

Remember every 10ms to swap from one set of bridge diagonal fets to the other set....then you get your bipolar voltage.

So its easy because you are not grid tied......just increase the pwm duty cycle as the 50Hz sinusoidal reference increases its amplitude, and let the current take care of itself...just have fets overcurrent limiting....and your slow average current limiting

So to get increasing pwm duty cycle as your sine reference  increases.......use the old trick of a repeating  ramp (up and down) into one pin of a comparator...and into the other pin...shove your reference.....then your comparator output will have a duty cycle which increases as your sine reference increases.

Seeing you already have inverters, just a tad too small to run all the loads including the heaters, I would take the simplest and largest load - the resistive heating elements - out of the equation.

You can supply the heaters with a simple boost converter boosting your 50VDC to whatever power level the heaters require, up to 240VDC, but as a side effect, you would get smoother control here, i.e., if the elements currently run with a thermostat turning them on and off, you can instead just boost to lower voltage. Remember though that mechanical switches and fuses rated for 240VAC are not necessarily suitable for 240VDC.

Even simpler. Take the boost out of the circuit and run with DC right from the panels.  You should only be making water when the sun is shining and there is an excess of power.  Panel voltage will be higher which makes heating element simpler.  Then PWM from a capacitor bank to keep the panels at power point.   I do this and it makes the entire solar system efficient by removing the conversion losses.  I love free hot water.  Even my clothes washer has its own 40 gallon tank and all cycles use hot water.  This easily does a KW or more depending on voltage. It shows the simplicity of the circuit.


--- Quote from: Seekonk on June 20, 2021, 11:43:19 am ---Even simpler. Take the boost out of the circuit and run with DC right from the panels.  You should only be making water when the sun is shining and there is an excess of power.

--- End quote ---

This is obviously simplest. You won't get MPPT but the simplicity makes up for it.

But I kinda assumed the need for heating is decoupled from the sun shining and there is some significant battery storage involved. If not, I'm all for the simplest possible solution.

PV voltage being too low for existing heating elements could be a problem though.

More information is needed, i.e., a full schematic of all components in question, including the panel string voltage and battery capacity and voltage, if any.

Maybe I'm misunderstanding your circuit, but I have seen some similar claims before so I'll comment about it even if it doesn't apply to your circuit. "PWM from capacitors to get power point and not have conversion losses" doesn't make any sense. You are likely having significant losses in capacitor ESR although it might kind-of work with parasitic inductance. You really need an inductor in the system for voltage conversion (and voltage conversion is needed to track MPPT while supplying loads). Buck, boost or any other power conversion topology works, and you can't completely get rid of conversion loss but properly designed converter can easily run at 98% efficiency. A ghetto converter which lacks important components like the inductor likely isn't as efficient even if the first instinct is that removing the inductor removes inductor loss.


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