Modified sine wave inverter used as DC-DC converter?

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A better plan would be to use a low voltage brushless motor/controller direct from your batteries.

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That sure would save a lot of expensive high-power electronics, improve reliability, efficiency, and reduce cost.

-- Cheers, James Arthur

I suspect that Paul is planning on using off the shelf induction motors and VF drives, which tend to come in the voltages that he mentioned.

For a proof-of-concept, using off the shelf parts isn't a terribly bad idea. Induction machines are going to be bulky and heavy for the power that they'll produce compared to PM brushless motors, but they'll be a lot less expensive, particularly in onesi-twosi quantities.

But, for even short-run production it may make more sense to come straight off the battery rail to power the motor drive, and either use PM brushless motors or have the induction machines rewound to work at higher voltage/lower current (the size, weight, and power capability won't change much -- you'll replace X amount of copper in the form of thin wire with X amount of copper in the form of shorter, thicker wire, all to get the same magnetic field at a different voltage).

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

If the inverter is pretty close to a sine wave, then that sine wave is going to be generated from DC pretty much the same way that a VF drive generates its sine waves. So if you can find a 240V inverter with sufficient power to match what you're doing with your motors, then you will probably find a DC rail inside the thing that matches the needs of your 240V VF drive.

You could also look for literature on DC-DC converters. You need a honkin' big one, and if you're designing from scratch, and to a point the higher the input voltage the better off you'll be -- the currents are lower, and the transformer turns ratio is less. But you're getting into some pretty serious power levels, which means that the design of the supply isn't trivial. If you want to focus on tractor-building and not power supply design, then finding some existing thing and hacking it is the way to go.

See the comments about just using battery voltage -- they're well meant and meaningful. In particular, keep in mind that if you rip all the wire out of a motor and rewind it with thicker wire, filling the space just as efficiently as it was filled before, you end up scaling the current up exactly as much as you scale the voltage down -- so all the efficiencies, maximum torques, etc., stay the same, just at a different voltage vs. current.

Of course, if you string 30 12V batteries in series then you have your

360V DC rail right there. Just don't lick your fingers and touch them to the end points to see if the thing is live.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

getting

I think off-the-shelf DC-DC converters at the power levels I'm looking = at=20 will be rare and hugely expensive. I'm sure I won't find any for as = little=20 as $100/kW as the Harbor Freight units are. And this is now a=20 proof-of-concept design, and targeted to people who have tractors and = want=20 to do a conversion. See my thread(s):

And I have an idea for a simple DC-DC converter. I can use a nominal =

500VA=20 toroid and rewind it with less turns of much heavier wire, and drive it = with=20 a 600 Hz square wave. It should be able to produce about 5 kVA, and such = a=20 toroid is about the size of your fist and costs only about $50 and is = much=20 more rugged than ferrite cores for higher frequency magnetics. I think = it=20 will work out about the same size and efficiency as a high-tech 5000W=20 switching supply, but it can be much simpler to build.

Actually, I did that very thing about 8 years ago. I pulled the original =

windings out of a single phase 120V 3/4 HP fan motor, and rewound it = with=20 much heavier wire (something like #18 or #16). And I converted it to a 6 =

pole three phase motor. It was designed to run on about 8 VAC so that a =

12V=20 battery could drive it directly, and I wanted to be able to overclock it = by=20 about 4x. I used a 2HP VF motor controller running on 240 VAC single = phase,=20 and I used two step-down transformers to match the motor voltage. It ran = OK=20 at 180 Hz with a fan load. And I made another motor that I drove using a =

three-phase bridge and modified sine wave (rectangular wave) directly = from a=20

12V battery, using a PIC controller.

Here is a picture of the stator:=20

It's probably not filled as tightly as the original windings. I wound by =

hand rather than making belts externally and then fitting them in the=20 lamination slots.

My ultimate goal is to make a low-voltage motor that can be overclocked = up=20 to 6x (360 Hz), and theoretically get 6 HP from a 1 HP frame. I have = some=20 motors that I got cheap with the intention of rewinding them, but it's=20 difficult to do so with the heavier wire, and I am hesitant to rip out = the=20 windings from these motors since they are new and unused. So, first I = want=20 to use them as-is, and maybe overclock the 1.5 HP 240/480 4 pole motor = to=20

2x, which is 3600 RPM, same as the gas motor in my tractor, and I was = going=20 to use a 480V controller so I can get as much as 3 HP. The gas engine is =

about 7 HP peak, while the induction motor should be able to produce 2x = to=20

3x normal as peak, so it should be about equivalent. This is the tractor = I=20 may use for this project: 30 batteries will be a problem. I'm looking at using 100 A-H SLA = batteries,=20 so two of them with 12V-120V inverters should give me the 360V I need, = and=20 2400 W-H which may provide up to one hour running time at 3-4 HP = average.=20 Then I can go with 4 batteries to get 2 hours or twice the power.

I'm really focused on using a three-phase induction motor rather than = the=20 more expensive and fragile BLDCs and PM or series wound brushed DC = motors=20 that are the usual technology.

Thanks,

Paul

When you get around to wanting to give that a try, consider that it's pretty common to wind a motor with multiple strands off finer (individually insulated) wire, lightly twisted. It appears to reduce skin effect at high frequencies (good if you're PWM-ing the thing), and it leaves you with a flexible bundle, instead of a rigid wire.

In theory you could unwind a motor, make your wire rope out of the existing wire, then put it all back in. In practice you'd probably be much smarter to just get some wire that fits what you need.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Essentially there is a 12VDC to 170VDC converter in there that turns on and off. This is followed by a full-bridge that switches output polarity at mid-point of the off-phases. That's pretty much it.

Mind that super-cheapo versions sometimes save a few pennies or "boost efficiency" by only generating 160VDC and roaching that on top of the incoming 12VDC. Then the input and output are not isolated. This has resulted in some "interesting" experiences for some folks. "Yo, dude, why's there smoke comin' out of your camper?"

Here ya go:

--
Regards, Joerg

http://www.analogconsultants.com/

But thou shalt not PWM the thing with at least some sort of LC filter. That takes some of the RF energy away from the windings.

I've done that with transformers and even a speaker when I was a kid. Never again. My fingers looked all calloussed and stuff back then, from pulling the windings taut.

--
Regards, Joerg

http://www.analogconsultants.com/

You can use a commercial 240 V variable frequency drive off 360 V DC. Many have DC terminals for power input. I guess you could use a 480 V VFD on a 720 V DC bus, although that scares me a bit.

Are you looking for a, say, 48 V to 360 V DC-DC converter? I would not use a bunch of sine wave inverters. What you want is a simple DC-DC boost converter, although the power devices are going to be fairly big.

Jon

You are planning on using 60 Hz transformers at 600 Hz? The core heating will be at least 10 X higher (depending on Bmax, though.)

A non-isolated boost converter would be much simpler. Basically a big inductor in series with the battery input and a power transistor to short it to ground. Duty cycle is controlled by monitoring the output voltage. Then, there's a big rectifier feeding the higher voltage to the VFD. This rectifier might even be the AC input rectifier already in the VFD.

OK, 6 HP is 4500 W. **TWO** 12 V batteries gives you 24 VDC. The current draw will be 4500 /24 = 187 A. SLA batteries have much lower current capacity than car batteries. ONE HOUR??? You will likely get about one minute before the batteries explode. Even BANKS of massive trolling motor batteries would have trouble handling this. I built an electric car some years ago, ran 48 V with 60+ Lb. 90 AH trolling motor batteries, and the terminals ran quite hot. Anyway, simple calculation shows that at 100% efficiency, and totally neglecting battery internal resistance, you would get 32 minutes with these batteries (100/187 * 60). Check the max continuous current rating of the batteries you are looking at.

Jon

I found a 150 VDC bus in the 175 watt unit I took apart. And the = switching=20 frequency is 43 kHz. The transformer is quite small, a little over 1" = cube.

I had thought it was isolated. So that makes it riskier to use two of = these=20 in series from separate batteries, as at least one of the batteries will = be=20 about 150 volts above the other. I measured the resistance from the = input to=20 the output and there is no direct connection. But from the schematic you =

provided, that's probably because the output is bridged by MOSFETs, = which=20 have high resistance until turned on.

That's very helpful. It looks like the 150V bus could be isolated by=20 removing some of the output components and the connection of the diode=20 bridge to battery +12V. But that defeats the purpose of using an=20 off-the-shelf component. I think I can make a simple DC-DC converter = that=20 can produce, say, 1200 watts at 180 VDC for each battery, with = isolation.=20 Then I can use two or four in series for 360 and 720 VDC bus voltages, = for 3=20 HP and 6 HP. I could make them as modular components designed for series = and=20 parallel connection, and have a way to produce a regenerative charging=20 current during dynamic braking (although that might not be very useful = for a=20 tractor unless it is used for frequent up/down trips rather than, say, = lawn=20 mowing where it needs power at all times. I could also add ultracaps for =

surge power and energy storage, so the the battery would see only = average=20 current within its more efficient range, but they are expensive.

Thanks,

Paul=20

I have a AC-Delco 2500W Msine inverter. No schematics, but it has 2 front end converters ~170VDC stacked for 340ish VDC. Which is then switched at 60 hz into AC. There is a common ground point, so there is +/- 170V to the output. Looks like this...

Must have been a school project, uses a TL494!

Cheers

That can be deceiving, you have to open it up and look. For example, the inverter in my link is not isolated and that is a fairly typical one.

Yes, but you have to rewind the little ferrite transformer and also make sure the caps can take it. Sometimes they are quite marginal in voltage.

Ultracaps? Make sure they can take the enormous current. And mind the battery, as Jon was pointing out. You can't pull cranking amps out of them for more than a minute, if that.

Sometimes it's better to design this kind of stuff from scratch.

--
Regards, Joerg

http://www.analogconsultants.com/

That particular model was sold out, but they had others from 200 watts = up to=20

6000 watts. The 900 watt model was the lowest price per watt, at $0.07. = Most=20 of the others were about $0.10/watt, which is about what the Harbor = Freight=20 units sell for. It does seem to be better to design this from scratch, = and I=20 think it should be possible to make it for less than $0.05/watt.

A 200 VA toroid kit is just $63:=20

kits.htm.=20 This is a toroid core prewound for 120 VAC 60 Hz, and has 0.23 V/turn. I =

would just remove the primary winding and replace it with something like =

10=20 turns of much heavier wire (probably actually four or more wires in=20 parallel), which would be about 23 VAC at 600 Hz. Maybe with a center = tap=20 for 12-0-12. Then I would wind a secondary with 140 turns, for 320 VAC. = A 12=20 volt square wave using a center tap design with just two power = transistors=20 in push-pull should produce a square wave output with 320 volts peak, = which=20 can be rectified to 320 VDC for the link voltage. This should provide up = to=20 2 kVA, more than enough for a 2 HP motor. The battery draw would be 167=20 amps, however, so I should really design for about 1000 watts average = for=20 each module.

In that case, I could even use the smallest (80 VA) toroid kit, which is =

just $52, and should provide at least 800 watts for 1 HP, and a more=20 realistic 87 amps draw. I'm sure I could get the unwound cores for much=20 less, and even have them custom wound, for under $100. Then a pair of = husky=20 MOSFETs like IRFB3077 at $3.50 each, a 5A 600V bridge and 470 uF 450 V=20 capacitor, controlled by a PIC or SG3526, and a few minor parts, and = I'll=20 have a 1 kVA module that can be connected in series or parallel with its = own=20 battery for the voltage and current needed. The whole thing should fit = in a=20

2" x 4" x 6" metal box which will be a heat sink for up to 100 watts if = it=20 is only 90% efficient.

Well, maybe that does come out as more than $0.10/watt. But if I made=20 thousands of them, the cost would go way down. And the biggest expense = will=20 be the batteries, at about $200 each:

I couldn't find actual technical specs, but this seems to be the = battery:

chure.pdf

It seems like a good battery, but actual A-H capacity seems to go down=20 sharply even at an 8 hr. rate compared to 20 hr. and 100 hr. rates. So I =

probably need to design for no higher than a 4 hour rate for continuous=20 duty, so a 100 A-H battery would be good for only 25 amps. Yeah, battery =

technology is really the kicker for practical EVs. Here is a pretty good =

reference:

Thanks for the reality check.

Paul=20

[Exide Road Force 100 AH battery]

I=20

continuous=20

battery=20

good=20

I did a little more investigation, and found some Lithium batteries = designed=20 for EVs. Here is a 100 A-H battery for $150:

item256c18331c

But it's only 3.2 volts so four of them are needed for the same W-H = capacity=20 as the Lead-Acid. So about 3x the cost. Here is the spec:

But the good news is that it holds 3 volts even at 100A draw (1C), and = can=20 even handle 5C surges. Here is more information from a US distributor:

Their home page

has some = interesting=20 information. They show about $10,000 for a complete system for an = ordinary=20 car, for 100 mile range with a 500 pound battery pack, and an expected=20 lifetime of 200,000 miles (about 2000 charge/discharge cycles). A = typical=20 small car is probably about 40 HP, so a 4 HP tractor might be only about =

$1000 (although $2000 is probably more like it, since it probably does = not=20 scale linearly). So 10 batteries will give 3000 W-H or 4 HP for one = hour,=20 and cost $1500. The other components could probably be purchased/built = for=20 $500.

Things may become even more interesting, and EVs more competitive, when=20 gasoline passes the magic $5 point (probably within a year), and if = battery=20 technology can cut the present costs in half (which I think is very=20 realistic).

And I'd also like to put a datalogger in my controller to determine how = much=20 power and energy are actually needed for various lawn tractor needs. I = have=20 a feeling that basic lawn mowing on a flat surface might take only 1 to =

2=20 HP. And hills may average out if some regeneration is possible. Of = course=20 there will be some tough jobs that require raw power, and for that a = diesel=20 engine may be the way to go. I just bought a diesel lawn tractor with a = 15=20 HP Kubota engine for $1000, and you can see it in action at=20 or go for a ride with me at=20

Paul=20

If you happen to own a factory in Guangdong, yes.

"just $63"? That's expensive, puts you at over $0.30/W which is six times your goal, and that doesn't yet contain anything else.

BIG warning flag: While these transformers may work at 600Hz the core could run quite a bit hotter there. But the big issue is wire diameter. You can't just say "Oh, with higher frequency the core will deliver 1-x the power". Because at some point the copper starts to glow red, the white, then ... phssssss

With many products, the minute something gets the word "Solar" added to its name the price seems to go up by a factor of two. Or more. It's like with food and the word "Bio". Not sure about this one but $200 sounds rather steep.

Sorry to rain on your parade regarding to the switcher, just wanted to prevent some major disappointment (and maybe a fire on your lab bench ...).

--
Regards, Joerg

http://www.analogconsultants.com/

That's 200 VA at 60 Hz. 2 kVA at 600 Hz.

Of course, the wire size must be increased to handle the current = involved.=20 And it may be better to use a bifilar winding at the higher frequencies, = but=20

600 Hz (or even 2000 Hz) is still mid-audio and there is not much skin=20 effect or other phenomena. Toroids are especially well suited to high=20 frequency use, and I would not expect much increase in core temperature = at=20 600 Hz. And these transformer kits come with an internal thermostat, so=20 they'll be protected.

I think it will be safe enough with some thermal and overload = protection. I=20 already have a transformer wound for this purpose from when I was = actively=20 pursuing a similar project a few years ago. But I never built it, and = now I=20 think it is time to do so. My main concern is the switching transients = at=20 the time of drive reversal. A center tapped push-pull design always has = a=20 point where both windings are open, and that can create a big inductive=20 kick. But a full bridge circuit can turn on both top or both bottom = drives=20 during the transition, allowing the inductive current to flow. This also =

enables duty cycle drive, although that may not be necessary, desirable, = or=20 effective for a DC-DC converter.

Thanks for the warnings, though. It's always best to look carefully at = all=20 aspects of a design, and I could have missed something.

Paul=20

It ain't quite that simple and you may find it difficult to cram enough turns through there with the respectively larger wire.

[...]

2.5kW is almost full-bridge turf. But even with a half-bridge you can provide fast diodes. They aren't expensive anymore these days.

If you build it from scratch anyhow, why not use a ferrite core and higher frequency?

--
Regards, Joerg

http://www.analogconsultants.com/

snip

ry

todays gasoline price here is ~13dkk/liter, that's ~9usd/gallon...

We "just" need a revolution in battery tech the rest is cheap, look at the stuff you can get for RC models.

6500W motor and driver for ~200$

-Lasse