high amperage dc-dc step down converter

[Sinatrablue]

Hey all,

 I have a project ive been pondering and im looking for some guidance. Ive been working on an all-off-grid setup using a battery bank as the primary energy storage for everything for my log cabin retreat.

My main idea is i want to use a bandsaw powered by a 72v motenergy IPM motor which is powered directly from my battery bank which is charged by solar/wind.

Ideally i would have no external batteries to my bandsaw build, and to my chainsaws. My main issue is the step down conversion, i dont know of any high amperage (200+ amps continuous @ 72v) with a high voltage input (120-240v input DC). Ideally i would be stepping down from my 200v main battery bank to some lightweight cables to power my dc motors.

Perhaps its more efficient to just convert everything over to AC and run standard induction motors? I would think that keeping everything high voltage DC would reduce some of the losses during the conversion process.

[Berni]

72V at 200A is 14.4kW so that must be quite the hefty motor. Is this a lumberyard bandsaw for cutting entire trees lengthwise or something?

Running something that huge will certainly need a correspondingly huge power supply. Tho you could cheat a bit and use the motor itself as part of your power supply. You could PWM control the motor and limit the duty cycle to the point where it would not go above applying more than 72V of average voltage. The motors own inductance will smooth that out into a constant DC current. This simplifies the power supply down to just one pair of huge power transistors and some filtering around it so that it doesn't turn into a radio transmitter.

But if you don't have experience with power electronics, then it might indeed be easier to go pick up a big 3 phase motor from a yard sale and put a VFD on it.

[T3sl4co1l]

One of the bigger contraindications for DC might well be plain and simple fusing. That's a lot of power to deal with, even more under fault conditions from a battery that size.  Need special fuses for that, and moreso for the higher ratings.

And yeah, I don't know where you'd be using 10kW+ for, that's light-industrial processes like a machining center, induction heater, etc.  Which, would be pretty badass applications, but that's also quite a big solar panel to keep it supplied on a day-to-day basis?  Would be interesting I suppose.  Needless to say, much care is required to operate such equipment, so too its wiring.

An inverter has a small downside (a little less capacity; so what, tack on another panel or two?) and many strong upsides, especially that it's a standard setup that is widely supported and well understood by many manufacturers and installers.  If you want to deep dive into these things, at whatever level, wiring, equipment, electronic design, that's fine, but if you just want a system, don't underestimate the value of a standard approach.

Tim

[Sinatrablue]

One of the bigger contraindications for DC might well be plain and simple fusing. That's a lot of power to deal with, even more under fault conditions from a battery that size.  Need special fuses for that, and moreso for the higher ratings.

And yeah, I don't know where you'd be using 10kW+ for, that's light-industrial processes like a machining center, induction heater, etc.  Which, would be pretty badass applications, but that's also quite a big solar panel to keep it supplied on a day-to-day basis?  Would be interesting I suppose.  Needless to say, much care is required to operate such equipment, so too its wiring.

An inverter has a small downside (a little less capacity; so what, tack on another panel or two?) and many strong upsides, especially that it's a standard setup that is widely supported and well understood by many manufacturers and installers.  If you want to deep dive into these things, at whatever level, wiring, equipment, electronic design, that's fine, but if you just want a system, don't underestimate the value of a standard approach.

Tim

Im leaning towards this, as the higher voltage and lower fire danger of using AC vs DC due to arcing i think your right to just rectify everything into AC using a large inverter to bear the load.

72V at 200A is 14.4kW so that must be quite the hefty motor. Is this a lumberyard bandsaw for cutting entire trees lengthwise or something?

Running something that huge will certainly need a correspondingly huge power supply. Tho you could cheat a bit and use the motor itself as part of your power supply. You could PWM control the motor and limit the duty cycle to the point where it would not go above applying more than 72V of average voltage. The motors own inductance will smooth that out into a constant DC current. This simplifies the power supply down to just one pair of huge power transistors and some filtering around it so that it doesn't turn into a radio transmitter.

But if you don't have experience with power electronics, then it might indeed be easier to go pick up a big 3 phase motor from a yard sale and put a VFD on it.

I was mainly attempting to use a more direct approach, using power directly from the 200v battery bank to run my bandsaw mill. Yeah it is a large bandsaw similar to what you can see online for sale to cut logs into boards. The idea is that i would not use this continuously, and i would run a generator if i knew i was going to run it continuously for many hours. But generally sawmills are used intermittently as logs come in.

I would like to have a 10kw PV array and another 1-2kw from wind generation. If i had 20kwh of battery storage then i should theoretically be able to run this sawmill daily and not tap into my reserves. This is an off-grid venture to be rid of gasoline and run only on electrical power.

[schmitt trigger]

I also believe that Berni’s suggestion is a solid one. You could employ one of those rugged block IGBTs available from IXYS or Infineon.
If you follow that approach, please use one of the driver ICs specifically designed to drive these hefty IGBTs.
Avoid the temptation to use less-capable drivers.

Lastly as others have mentioned, it will require a substantial filter to prevent radiating RFI all over the place.

[jbb]

Not sure if you’d be able to get something off-the-shelf for a straight-up DC-DC converter.

But it sounds like what you need is really a big DC motor driver.  Is that right? If so, maybe an electric forklift controller block (or a small EV or whatever) could do it.