Re: Grid-Battery "Hybrid" Tractors

On Fri, 25 Jul 2008 18:57:25 -0400, "Paul E. Schoen"


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Good point.
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wrote:

An AC induction motor exhibits a fairly flat torque curve from something like 10% of design RPMs at 50/60 Hz, and then is usually shown as decreasing, because, although motors can be driven by PWM to several times their rated speed, it is not usually recommended (or feasible) to increase the voltage accordingly (as a VF drive does). But there is nothing "magic" about 60 Hz as a limit for the magnetics, and it is possible to design motors that run up to at least 400 Hz. They are typically very high RPM, but with enough poles, it is possible to boost the HP of a motor by several times, using lower voltage windings and running at least up to 150 Hz. You can get 2 or three times the HP from the same size motor. This is very important for highway vehicles, where the weight and size of the motor contribute a lot to fuel economy and performance, but probably not as much for a tractor, where additional weight might be a good thing.
Since large induction motors are typically 92 to 95% efficient, a 75 kW 100 HP motor will produce something like 5000 watts of heat, which is removed by means of self-contained fans. A motor specially designed to be overdriven might be even more efficient, although there is a limit where magnetic losses take over. The good thing about electric motors is that they consume no power when they are idle, and their losses are at worst a percentage of the actual output power, and may even be less when lightly loaded. Losses are proportional to I^2, while torque is proportional to I. They can also be "pushed" to 2 or 3 times their nameplate ratings for short periods of time, so you can often get by with a smaller motor if your power needs are intermittent.
So the transmission requirements are mostly to provide the needed torque, and then the motor speed can be adjusted as needed. Large tractors probably have trannies with 10 or 15 speeds or more, while an electric motor might require only 3 or 4. This would be another saving. VF drives are so efficient and inexpensive now, that any other motor controller is just about unthinkable. And you can run a VF drive on 720 VDC directly, so it is ideally suited to a battery pack for use when transferring from one power source to another. This would require much less power than the tractor is actually rated for, so the battery pack could be quite small.
Paul
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....

several
100
short
power
probably
is
Paul,
On behalf of sci.energy readers, THANKS for a great overview of electric motor basics. Very seldom do we see postings of this quality, and I at least very much appreciate that.
Thanks again
Rob
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wrote:

It always impresses me that a lot of people participate in specialty newsgroups, and have opinions about technology and public policy, without making any effort to read a few books and understand the basics of what's actually going on, or the quantitative limits of what's possible.
My wife wanted me to put a windmill on our roof to make our own electricity.
John
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On Fri, 25 Jul 2008 20:37:17 -0400, "Paul E. Schoen"

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Very nice. :-)

Thanks!
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On Fri, 25 Jul 2008 18:57:25 -0400, Paul E. Schoen wrote:

I think we are really just arguing about the mythic lightweight super capacity battery pack and or the relative cost of the massive central pivot power supply, not to mentin the $AUSX00,000 is going to charge you to bring in an 11Kv feed.
<imagine the speed of the outside support of a central pivot supply as the tractor ploughs the inner rings {:-)>
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On Fri, 25 Jul 2008 16:17:47 -0500, John Fields

Unless the thing was 8 feet in diameter (which would have its own problems) you'd need kiloamps of coil current and megagauss field strengths to get the kind of torque a tractor wheel would need.
This guy is as ignorant of electrical and mechanical engineering as he is of farming.
John
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On Tue, 22 Jul 2008 16:27:09 -0700 (PDT), snipped-for-privacy@peoplepc.com wrote:

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Sorry, Charlie, nice try but the issue at the moment, brought up by
JL, is whether you can appreciate the economics of battery charging.
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On Wed, 23 Jul 2008 09:23:06 -0500, John Fields wrote:

Err, is this something to do with fast charging? Heard 110% for C/10 in dep-discharge lead acid batteries, but not that much.
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On Thu, 24 Jul 2008 23:38:48 +1000, terryc

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No, it's the typical coulometric charge-discharge characteristic of a
flooded lead-acid battery.
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On Thu, 24 Jul 2008 10:20:50 -0500, John Fields wrote:

Well, I guess I won't be ever buying their batteries then.
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On Fri, 25 Jul 2008 16:55:08 +1000, terryc

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I can't imagine why not.

What do find wrong with them?
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On Fri, 25 Jul 2008 09:13:42 -0500, John Fields wrote:

Requiring 140% of C put back in to get C back out.

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On Sat, 26 Jul 2008 00:48:24 +1000, terryc

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That's typical for _any_ flooded lead-acid battery, while much higher
efficiencies can be enjoyed by using SLAs. Including theirs, I
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On Fri, 25 Jul 2008 10:51:18 -0500, John Fields wrote:

Conflicts with other stuff I've read claiming 110%.
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On Wed, 23 Jul 2008 09:23:06 -0500, John Fields wrote:
notie this paragraph after I flicked the other answer....

So instead of 415V supply, you just tap the 11Kv lines instead.
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On Thu, 24 Jul 2008 23:40:20 +1000, terryc

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How would you do that?

JF

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wrote:

You throw a wire over it. Just make sure to wear rubber gloves :o)
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On Thu, 24 Jul 2008 16:40:52 -0700, Rob Dekker wrote:

ROFL.
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wrote:

--
;)

JF

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