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Old 03-12-2014, 09:35   #31
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Re: Inverter for 4 ring Induction Domestic Hob?

All the above still isn't addressing voltage sag and peurket effect of lead acid batteries.
Assuming your house bank is 24 volts, 400 ahr, at the inverter loads your talking about, you can derate that 400 ahr to about 180 ahr. Now on to voltage sag even with stout cables, the battery could drop from 25.6 volts no load to 21 volts under load. This 21 volts is at the battery and there will be more drop in the cable run on the DC side to the inverter.

I just don't think this will work on a daily basis with lead acid batteries without having to replace those batteries in less than a year. When that time come, go to lithium LiFePO4 cells and voltage sag and peukert effect will be a thing of the past.
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Old 03-12-2014, 09:41   #32
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Re: Inverter for 4 ring Induction Domestic Hob?

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Originally Posted by ryon View Post
I could go on with my list of reservations, but it seems to be working for you and that's what's important. You make a point about pure sine wave inverters, and those shouldn't be necessary in this application at all. A dirty square wave or whatever should work just as well.
You might want to check this with a tech. A modified (stepped) square wave would work fine on a heating element type but an induction is like a big transformer where the metal pot or pan is basically the secondary winding. Have you ever heard how transformers can buzz from the modified sine wave inverters?
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Old 03-12-2014, 09:53   #33
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Re: Inverter for 4 ring Induction Domestic Hob?

More than slightly off topic, but all induction cooktops/hobs that I know of actually use DC input at essentially line voltage. 50/60Hz is simply not enough frequency to drive the eddy currents, most cooktops run in the 20-50kHz range.

So, inside the cooker you have an AC-DC conversion through a rectifier, then a DC-AC conversion at high frequency through an inverter.

For installation on the boat then we have DC battery - AC Inverter - AC Cooker input - internal cooker DC rectification - internal cooker AC high frequency inverter.

While I love induction cooktops, seems like lots of room for efficiency improvement when installed on the boat. Now if I could only come up with a 200-300VDC source on the boat....gut the cooktop, remove the internal rectifier, and away you go with the only the high frequency inverter.
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Old 03-12-2014, 09:58   #34
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Re: Inverter for 4 ring Induction Domestic Hob?

I do know of induction cook tops being used on a number of high end boats running off the inverter but all have had a LiFePO4 house bank.
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Old 04-12-2014, 12:04   #35
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Re: Inverter for 4 ring Induction Domestic Hob?

Quote:
Originally Posted by Dsanduril View Post
More than slightly off topic, but all induction cooktops/hobs that I know of actually use DC input at essentially line voltage. 50/60Hz is simply not enough frequency to drive the eddy currents, most cooktops run in the 20-50kHz range.

So, inside the cooker you have an AC-DC conversion through a rectifier, then a DC-AC conversion at high frequency through an inverter.

For installation on the boat then we have DC battery - AC Inverter - AC Cooker input - internal cooker DC rectification - internal cooker AC high frequency inverter.

While I love induction cooktops, seems like lots of room for efficiency improvement when installed on the boat. Now if I could only come up with a 200-300VDC source on the boat....gut the cooktop, remove the internal rectifier, and away you go with the only the high frequency inverter.
Good point! Several appliances use what is called a switching power supply that rectifies to DC then chops it into a square wave which is then run thru a transformer to convert to whatever voltage needed. The reason? Higher efficiency, closer regulation of current at higher frequencies, plus much less iron needed in the transformer. Most desktop computers have this sort of power supply. Something like your "hob" might do this in order to achieve a better coupling with your cookware. Didn't think of that possibility.

In this case, since the line current is rectified, the shape of the wave is not important. In the case of 60 cycle AC being sent directly to the coil, or maybe it's 50 cycle there, all that is required is the oscillation, the power transferred in both cases will be whatever is under the waveform.
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Old 04-12-2014, 13:43   #36
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Re: Inverter for 4 ring Induction Domestic Hob?

Quote:
Originally Posted by Dsanduril View Post
More than slightly off topic, but all induction cooktops/hobs that I know of actually use DC input at essentially line voltage. 50/60Hz is simply not enough frequency to drive the eddy currents, most cooktops run in the 20-50kHz range.

So, inside the cooker you have an AC-DC conversion through a rectifier, then a DC-AC conversion at high frequency through an inverter.

For installation on the boat then we have DC battery - AC Inverter - AC Cooker input - internal cooker DC rectification - internal cooker AC high frequency inverter.

While I love induction cooktops, seems like lots of room for efficiency improvement when installed on the boat. Now if I could only come up with a 200-300VDC source on the boat....gut the cooktop, remove the internal rectifier, and away you go with the only the high frequency inverter.
For someone willing to hack their induction hob/cooktop and bypass the rectifier, a DC-DC converter could be used to go from the boat's 24V or 48V battery bank to the 230V DC needed by the hob/cooktop after bypassing the rectifier. That would be significantly more efficient than using an inverter to supply 230V AC to the hob/cooktop.
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Old 04-12-2014, 15:25   #37
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Re: Inverter for 4 ring Induction Domestic Hob?

I would hazard a guess that the induction cookers are more complex than they might appear to be. Certainly more so than old fashioned hot plates.


I doubt they convert AC to DC and back to AC again. What would seem more efficient and flexible, is to convert AC to pulse width modulated (PWM) DC directly. That in turn can be treated similarly AC in the sense that transformers and all will work as well as the fancier DC-to-DC equipment. And PWM would give the oven a very simple way to "control the flame" by just varying the pulse frequency. Which also opens other options, like alternating the pulses between two rings, so the actual wattage drawn is "just" 100% of the rating, even though two fully rated rings are being powered.


There's never a marine induction cooktop electronics engineer around when you need one, is there?
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Old 04-12-2014, 15:40   #38
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Re: Inverter for 4 ring Induction Domestic Hob?

Actually they do convert AC to DC then back to AC again at another frequency. They usually control the heating power by adjusting output frequency which is a very efficient method. The electronics are pretty simple and PWM would just add another level of complexity they don't really need. Very little heat is lost that does not go into the bottom of the pan. The really cool part is that when you remove the pan the power draw goes nearly to zero until you put the pan back on the coil.
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Old 04-12-2014, 20:36   #39
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Re: Inverter for 4 ring Induction Domestic Hob?

is your house battery 12v or 24v? at 40' I'm guessing 12v.


if so, and you just bought a 24v inverter. you're in for some complications...
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Old 04-12-2014, 21:01   #40
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Re: Inverter for 4 ring Induction Domestic Hob?

hmm, I don't see that there is any efficiency to be gained from a re-engineering an induction cooktop for a boat. The conversion of AC to DC is not particularly inefficient and is required since the stove uses high frequency AC, not 60hz.
The conversion to high-voltage AC from the battery is going to be needed too.

So I don't see anywhere there is inefficiency to eliminate.

I will have an induction cooktop on my boat. I did a little looking,
and 2 burner units sell for a few hundred dollars, not particularly expensive at all.
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Old 05-12-2014, 12:13   #41
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Re: Inverter for 4 ring Induction Domestic Hob?

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There's never a marine induction cooktop electronics engineer around when you need one, is there?
Probably not, but these guys hacked an induction cooktop:
1800W Induction Cooktop Teardown | Openschemes
Circuit Analysis of the 1.8kW Induction Hotplate | Openschemes
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Old 05-12-2014, 12:36   #42
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Re: Inverter for 4 ring Induction Domestic Hob?

And from the teardown URL's fifth page, it appears as if that cooker is indeed using PWM-DC.


"The full-wave rectified current ...
Check it out folks – GROUND referenced resonant switching!! "


At 20KHz, which is lower than some alternators in cars and boats used. I suppose there's some "sweet spot" for resonating ferrous metals and turning the energy into heat.


Now, how about a nice spreadsheet showing how much weight an entire induction cooker system (power supply, batts, pots, stovetop) would WEIGH, as compared to a nice Korean butane single-burner, and some proper titanium cookware? (WEG)
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Old 05-12-2014, 16:40   #43
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Re: Inverter for 4 ring Induction Domestic Hob?

You all wrote while I slept!

Quote:
Originally Posted by transmitterdan View Post
Jwing,
Chris is talking about battery amp draw when he mentions 100A. You have to be careful when discussing amps to know whether it is the AC side or the DC side. As a general rule of thumb in 24V systems the DC current is 6 times higher than the AC. So your 15 amps AC becomes 90A DC. For 12V systems the DC current is 12 times more than AC.
Indeed it is 12V current.

Quote:
Originally Posted by transmitterdan View Post
Atoll,
.... So DC current would be 12X (24V) and 24X (12V) as a better rule of thumb for 240V inverter systems...
and I approximated the current draw and power consumed, and made allowance for the inefficiency. The induction cooktop rarely runs at full power. It is so much faster than the gas stove, and is typically used for little cooks during the day (when we have excess solar input).

Quote:
Originally Posted by deckofficer View Post
I do know of induction cook tops being used on a number of high end boats running off the inverter but all have had a LiFePO4 house bank.
When we had Lead/Acid cells in the dark old days, you could turn on the induction and watch the voltage drop and drop. No sag now with LiFePO4.

Chris
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