I'll add my comment on your first video to this one, for reference: The transformer portion of that inverter is a ferroresonant power supply. Same a Sola, except the primary is driven from the inverter instead of a 120 volt winging. The immediate giveaway was the 600 volt output, which will go to the large oil filled cap forward of the transformer. The core is run in saturation all of the time as a means of voltage regulation, regardless of load or input voltage (within reason). That's where most of the 125 watts is lost. The 600 volt winding and the oil filled cap are a tank circuit, resonant at 60 cycles. The output voltage should rise when the frequency is raised up to spec. What a super cool piece. Would be neat to see a scope on the output. Edit: now that we've seen it with a scope, you can see that the tank circuit would be a sine wave, but, being driven with a square wave, it results in that "in between" shape. Very, very cool. ;) To add some more, they're loud and the transformers run warm specifically as a result of running in saturation all of the time. The reason for using such a high voltage for the tank circuit is that it requires exponentially less capacitance. Even though the cap has to be larger for the higher voltage, it can be much smaller due to the exponentially smaller required capacity. 600 volts is common in these, probably as that is a common oil filled cap voltage. If you can read the capacitance, you can calculate the circulating VARs. Or could just measure the current with clamp on ammeter. The amount would probably surprise you. ;)
It is a "ferroresonant" transformer circuit, also often used by AC voltage stabilizers. The output voltage becomes stabilized and close to sine shaped as long as the drive frequency is kept constant and correct. The capacitor is used together with the special transformer to shape the output voltage and regulate it. The high voltage is used for the capacitor because it is/was easier to make high power AC capacitors for high voltages.
Oh I see! I was wondering why my vintage voltage stabilizer actually improved the sinewave output on my modified sine wave UPS. I noticed this simply when I was testing an AC induction motor fan on it and it was way quieter and reaches full speed when the voltage stabilizer was connected in between. I wonder if this is a easy way to reliably make modified sine inverters more pure sine wave! For now I'm using the vintage voltage stabilizer on a modified sine wave UPS as an inverter for a solar setup and it works really well!
I did some research on the ferroresonant transformer and schematic and that appears to be exactly what is used here. Thanks for the info you shared. In particular I found that ferroresonant transformers dissipate more heat than conventional transformers and produce more audible noise at resonance, exactly what this inverter exhibits. Its ability to regulate the output explains how the inverter voltage remains quite stable. It's important the frequency be adjusted to 60 Hz for the resonance to work correctly.
Ferroresonant transformers also have quite high losses both under load and when idle. Wasn't the consumption of the inverter in the video over 100 W with unloaded output? That's 20% of rated power in no-load losses :)
I agree the cap is for taking the edge off the square-wave output. This is important because if you put square-waves into a power supply, there will be large current spikes into the filter caps due to high dv/dt, and that will damage those caps. Loads such as motors and incandescent lamps dont care what the waveshape is as long as the RMS value is correct. The smoothed square-wave is probably an acceptable tradeoff. So, why is the cap running at 600V, rather than 120V ? Recall that capacitors store energy (E = 1/2 C V-squared). If the capacitor is charged to 5X the voltage, it stores 25X the amount of energy. I would need to run SPICE simulations, but I'm certain the amount of filtering capacitance at 120 VRMS is substantially higher than at 600VRMS. While we're discussing RMS voltage, the waveform from the inverter isn't sinusoidal so the RMS value of 120V is being obtained from a peak-to-peak source that is less than the usual 2*sqrt(2) . It would be interesting to put a very small load, say 10-20 watts on the inverter and compare the waveforms with and without the 600V capacitor. I suspect you will see the latter case being more like a square-wave, and likely with some ringing. I would also suggest running the DC input at a lower value to reduce the amplitude of the ringing so it doesn't damage or false-trigger the large SCRs. Inverters are difficult to design so that they operate reliably under all load conditions; they often work very well at low or zero load, and fall apart at high loads.
Only universal brushed motors won't care about supply waveform. Induction motors care a lot since the harmonics can either result in a stator magnetic field that tries to stop the rotor or spin it the wrong way.
The "600 volt" tap is a resonant winding on the transformer to commutate the SCRs on and off to prevent latchup where it is possible they could both switch on together.. This is also the reason there is a short delay in startup to allow the signals to settle.. The high voltage is simply a by product of using a high impedance winding allowing a "smallish" resonating capacitor at these very low frequencies.
I noticed a bank of 8 large plastic film capacitors between the power transformer and heatsink. I suspect these caps are connected across the 120 volt output to smooth out the SCR pulses, which are probably filled with voltage spikes. I am not sure why SCRs were chosen for the power switching, possibly a spec was driving that design. Power bipolar switching transistors were available when this inverter was designed, making for a better switching device. That power transformer is huge, probably good for 1,000 watts if you run a pure 60 Hz AC sinewave through it. The smaller winding bobbin that is separated from the other bobbin is most likely the 600 volt winding.
The bank of yellow capacitors are on the SCR's and primary transformer side but probably for spike and noise suppression. SCR's are more rugged than transistors and given this was built for the military in harsh use I'm sure that dictated circuit design. It would take many many paralleled transistors to handle the current that just these two SCR's switch. The 600 volt winding is in the large upper part of the windings, that is where the tap is placed and you can see the wire go into the windings.
Looks like a sinewave with lots of even-order harmonics, not exactly a thing you'd want on an inverter, but a sweet waveform for a guitar amp or overdrive pedal.
I'll add my comment on your first video to this one, for reference:
The transformer portion of that inverter is a ferroresonant power supply. Same a Sola, except the primary is driven from the inverter instead of a 120 volt winging. The immediate giveaway was the 600 volt output, which will go to the large oil filled cap forward of the transformer. The core is run in saturation all of the time as a means of voltage regulation, regardless of load or input voltage (within reason). That's where most of the 125 watts is lost. The 600 volt winding and the oil filled cap are a tank circuit, resonant at 60 cycles. The output voltage should rise when the frequency is raised up to spec. What a super cool piece. Would be neat to see a scope on the output.
Edit: now that we've seen it with a scope, you can see that the tank circuit would be a sine wave, but, being driven with a square wave, it results in that "in between" shape. Very, very cool. ;)
To add some more, they're loud and the transformers run warm specifically as a result of running in saturation all of the time. The reason for using such a high voltage for the tank circuit is that it requires exponentially less capacitance. Even though the cap has to be larger for the higher voltage, it can be much smaller due to the exponentially smaller required capacity. 600 volts is common in these, probably as that is a common oil filled cap voltage. If you can read the capacitance, you can calculate the circulating VARs. Or could just measure the current with clamp on ammeter. The amount would probably surprise you. ;)
Thanks for sharing this information. I may revisit this inverter in a future video to look at the ferroresonant transformer in more detail.
It is a "ferroresonant" transformer circuit, also often used by AC voltage stabilizers. The output voltage becomes stabilized and close to sine shaped as long as the drive frequency is kept constant and correct. The capacitor is used together with the special transformer to shape the output voltage and regulate it. The high voltage is used for the capacitor because it is/was easier to make high power AC capacitors for high voltages.
Thank you for the information.
Oh I see! I was wondering why my vintage voltage stabilizer actually improved the sinewave output on my modified sine wave UPS. I noticed this simply when I was testing an AC induction motor fan on it and it was way quieter and reaches full speed when the voltage stabilizer was connected in between. I wonder if this is a easy way to reliably make modified sine inverters more pure sine wave! For now I'm using the vintage voltage stabilizer on a modified sine wave UPS as an inverter for a solar setup and it works really well!
I did some research on the ferroresonant transformer and schematic and that appears to be exactly what is used here. Thanks for the info you shared. In particular I found that ferroresonant transformers dissipate more heat than conventional transformers and produce more audible noise at resonance, exactly what this inverter exhibits. Its ability to regulate the output explains how the inverter voltage remains quite stable. It's important the frequency be adjusted to 60 Hz for the resonance to work correctly.
A major downside of ferroresonant regulators is that they distort the sine wave, particularly when operated below the designed maximum load.
Ferroresonant transformers also have quite high losses both under load and when idle. Wasn't the consumption of the inverter in the video over 100 W with unloaded output? That's 20% of rated power in no-load losses :)
I agree the cap is for taking the edge off the square-wave output. This is important because if you put square-waves into a power supply, there will be large current spikes into the filter caps due to high dv/dt, and that will damage those caps. Loads such as motors and incandescent lamps dont care what the waveshape is as long as the RMS value is correct. The smoothed square-wave is probably an acceptable tradeoff.
So, why is the cap running at 600V, rather than 120V ? Recall that capacitors store energy (E = 1/2 C V-squared). If the capacitor is charged to 5X the voltage, it stores 25X the amount of energy. I would need to run SPICE simulations, but I'm certain the amount of filtering capacitance at 120 VRMS is substantially higher than at 600VRMS.
While we're discussing RMS voltage, the waveform from the inverter isn't sinusoidal so the RMS value of 120V is being obtained from a peak-to-peak source that is less than the usual 2*sqrt(2) .
It would be interesting to put a very small load, say 10-20 watts on the inverter and compare the waveforms with and without the 600V capacitor. I suspect you will see the latter case being more like a square-wave, and likely with some ringing. I would also suggest running the DC input at a lower value to reduce the amplitude of the ringing so it doesn't damage or false-trigger the large SCRs.
Inverters are difficult to design so that they operate reliably under all load conditions; they often work very well at low or zero load, and fall apart at high loads.
Thanks for sharing this information. It now makes sense why the capacitor voltage is so high.
Only universal brushed motors won't care about supply waveform. Induction motors care a lot since the harmonics can either result in a stator magnetic field that tries to stop the rotor or spin it the wrong way.
The "600 volt" tap is a resonant winding on the transformer to commutate the SCRs on and off to prevent latchup where it is possible they could both switch on together.. This is also the reason there is a short delay in startup to allow the signals to settle.. The high voltage is simply a by product of using a high impedance winding allowing a "smallish" resonating capacitor at these very low frequencies.
600 Vac filter cap takes out harmonics from the output signal i.e. shape signal towards sinus ;)
Sounds right to me. Does it make the transformer pull double duty as a saturable reactor to shape the waveform?
I noticed a bank of 8 large plastic film capacitors between the power transformer and heatsink. I suspect these caps are connected across the 120 volt output to smooth out the SCR pulses, which are probably filled with voltage spikes.
I am not sure why SCRs were chosen for the power switching, possibly a spec was driving that design. Power bipolar switching transistors were available when this inverter was designed, making for a better switching device.
That power transformer is huge, probably good for 1,000 watts if you run a pure 60 Hz AC sinewave through it.
The smaller winding bobbin that is separated from the other bobbin is most likely the 600 volt winding.
The bank of yellow capacitors are on the SCR's and primary transformer side but probably for spike and noise suppression. SCR's are more rugged than transistors and given this was built for the military in harsh use I'm sure that dictated circuit design. It would take many many paralleled transistors to handle the current that just these two SCR's switch. The 600 volt winding is in the large upper part of the windings, that is where the tap is placed and you can see the wire go into the windings.
I agree with other answers, to shape the waveform. I would check the value of it to see if it is still OK.
Looks like a sinewave with lots of even-order harmonics, not exactly a thing you'd want on an inverter, but a sweet waveform for a guitar amp or overdrive pedal.
Odd order harmonics, not even. The transformer would sound a _lot_ angrier if there were even harmonics due to the saturation effects thereof.
Power Factor.
Why not correct PF on the main AC output? Why would it be on a separate high voltage winding?