60-66 vdc available, need 67-74 vdc! Help!

Thread Starter

DJ27

Joined Feb 12, 2019
2
Hi everyone, I’m new to the forum so thank you in advance for any help or guidance.


My end goal: Power a 74vdc (67v min input, 15 amp max) 120vac (potentially 7 amps) output inverter.


My problem: I will often be using the standing voltage of Two 32 volt batteries in series. Standing voltage is often 60-66.


I have been able to find many step up converters that were 10-60 Vin (60 wouldn’t be high enough for the 61-66v I will commonly have) and 12-90 Vout. Theoretically, they’re up to 25 A at the 60 volt with a conversion efficiency of 92-97%. Which should get me around enough wattage to use around the maximum output potential of the end inverter. For what it’s worth, I will probably rarely ever draw around or more than 500 watts on the 120v output of the inverter.


Does anyone have any suggestions (preferably under $100) on how to take my standing voltage of (lower than 60 would be great but not necessary) at least 60-66 and get it to 67-74 (MAYBE as high as 80v at the very high end? Manufacturer didn’t provide much for specs, so I’m going to email them to figure out things like accepted voltage range.)


I have no problem building a step up converter if I were guided in the right direction. However, at my experience level with this side of electricity, I’ve greatly struggled trying to figure out the specs/values needed on the components. I’ve done my homework and it would seem that the easiest way is through using an inductor, a switch (transistor, pwm, maybe more options I’m unaware of), a diode and a capacitor. But frankly, I’m not smart enough just yet to figure out exactly what parts I need. Even when I look at the formulas, I’m not able to quite put the pieces together.


I also wouldn’t be opposed to dropping the voltage from 61-66 to around 55-60 so I could used the aforementioned step up converter. I just couldn’t find (or maybe figure out) a rheostat, pwm, variable resistor (that wasn’t huge, I’m a little limited on space) or anything along those lines that would work for my given needs.



I do understand the inherent danger of working with these components/levels of voltage and amperage. I will proceed with caution on any advice/guidance given and at my own risk. Thank you for reading, I’m young and I know I can be scatterbrained when it comes to explaining my ideas/situation. If you have any questions, ask away. Thanks!
 
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Papabravo

Joined Feb 24, 2006
21,217
Every time you do a conversion there will be losses. Sometimes these losses can be large. You will be better off finding a solution that takes the DC input you have available and produces the AC output you are looking for. Then you only have to account for the efficiency of one conversion process rather than two.

e.g. if you have two conversion processes that are each 80% efficient, then your output power will be a maximum of 65% of your input power. 80% is usually fairly achievable, but going higher runs you into diminishing returns in terms of cost and time.
 

Thread Starter

DJ27

Joined Feb 12, 2019
2
Hi Papabravo, thank you for the feedback.

Unfortunately, at this voltage of DC, it appears they only make inverters for the charging voltage (typically 74v, 67 volts is the lowest cutoff for input I could find) of the 64 volt system. They’re making the inverters for the guys driving the locomotives, not the guys who work on them while they’re not running.

As far as efficiency goes I’m not too worried about that. The batteries we use are hellacious, each of 32v batteries is good for like 650 ah @ 77f to 1.75 VPC.

So it seems as though I’m left with 2 options. Make my own inverter from scratch or figure out how to raise my voltage 12%.

Anyone have recommendations on where to find oddball inverters/converters?
 
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