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  #1  
Old 11-01-2007, 06:32 AM
ev3k ev3k is offline
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Default DC / AC Efficiency?

I have been doing no small research lately re: the relative merits of dc vs ac drive systems, at least as related to a small vehicle, my intended project.

I see dc drives as significantly less expensive than ac. So with the power conversion efficiency of a series-motor PWM dc drive as a baseline, what are the performance advantages of ac that will offset the significantly higher cost, over a dc system?

So far, I have learned about a slight advantage in efficiency due to the capability for regenerative braking and some battery recharge by ac systems. However, could a dc series-wound motor be made to do the same thing with a slightly more complex controller installation?

I have learned that there are, or course, no motor brushes required for an ac induction motor, and I suppose I can assume a small amount of power is lost, and eventual brush wear must be accounted for, in the brushes of a conventional brushed dc motor installation.

I have learned that some ac inverters offered for electric vehicle service are water-cooled, so that tells me they are dissipating some amount of precious energy to their environment, energy that I would prefer be consumed/dissipated by the drive motor instead; I see that most dc controllers can be air-cooled, so I assume that less power is lost in thermal inefficiencies by dc controllers than by ac inverters.

I have assumed that most ac inverters are far more complex, and have the potential to generate far more EMI, than dc controllers. There is, of course, the increased failure potential for a more complex 3-phase ac system.

Is there some other power conversion efficiency measurement that I have failed to consider, that will give ac systems the upper hand?

So what have I left out? The way I see it, an ac drive system is more "elegant" than dc, but offers little (if any) real advantage in efficiency over a properly installed dc system. On the downside, the ac system is significantly more expensive (money I would perhaps rather invest in better batteries) and more complex, and therefore more prone to premature wearout/failure (motor brush replacement notwithstanding).

Please correct/expand on my elementary analysis.

Steve
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  #2  
Old 11-01-2007, 02:12 PM
Craig Craig is offline
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Default Re: DC / AC Efficiency?

Hi,
AC motors are more efficient than DC motors (due to inexistent brush friction I assume) as well as having regen. I can't remember the numbers though. Very useful I know . Regen is possible theoretically with DC motors, but the controllers that handle that are very rare, and a lot more expensive, so you're losing the advantage of DC.

Controller efficiency for AC systems is around 92% (siemens) to 97% (more expensive Brusa). I'm not sure how much is it for DC, but I'd imagine it's about the same thing

Here are the advantages of DC vs AC as I see it (fill in if there's anything wrong/missing)

AC:
- More efficient
- Regen braking
- Almost constant torque, so a single-speed gearbox can be used
- Electric reverse: no need for a reverse gear
- Unlike DC, the controller can't fail in a full "on" position

DC:
- Less expensive
- Maximum torque at 0rpm : lots of burnt tyres Google "white zombie", it's pretty fast =)

I think basically AC will be more expensive, even over several years (DC brushes don't cost much to replace), but you're getting more range, especially when driving in cities or mountains, as well as components made for electric cars. DC parts come mostly from forklifts, which are usually used indoors.
There are exception of course, and I probably left some things out, so you might want to wait for a few other opinions as well
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  #3  
Old 11-04-2007, 07:00 PM
CPLTECH CPLTECH is offline
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Default Re: DC / AC Efficiency?

Yes, AC is the way to go for slightly better efficiency. Water cooled also? There is someone in town that did convert a van to a 25HP, 3phase industrial motor. In my prior research, I figured that you could use a 240V industrial VFD [variable freq drive], since they take AC, rectify it to DC, then convert it to the voltage/freq it wants. Could someone bypass the AC input for a vehicle purpose???? Is 25HP enuff? My 26HP DC sure isn’t.

A big downside I see: Over 250V of batteries. I am very happy that I installed the 6V 72# US145’s that can handle some 254A before warping its plates. I get in that range & beyond many times. After 2K miles, I have yet to add water. But have never discharged below 100AH. Is that the reason?

Why not just get your feet wet with a less expensive 144V DC [not 120V] system {same cost as the 120V, except for the extra batteries]. Use it for local trips of 40-50 miles. I go 28-30 miles, 4 times a week at ~90AH. Then too, consider the coming cold weather when battery performance takes a nose dive, turning that vehicle into a late Spring-Summer-early Fall unit.

Since still in the learning curve, so these are merely my thoughts.
http://www.austinev.org/evalbum/1317
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Old 11-04-2007, 08:36 PM
Mr. Sharkey Mr. Sharkey is offline
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Default Re: DC / AC Efficiency?

Quote:
Originally Posted by CPLTECH View Post
After 2K miles, I have yet to add water. But have never discharged below 100AH. Is that the reason?
Nope. The reason you haven't had to add water is that you aren't charging hard enough, fully enough, nor are you giving the batteries a proper equalizing charge (apparently).

Fully charged batteries gas. Gas = hydrolyzed H2O = water usage. No usage = no gas = not fully charged. I expect to see about half the water between the "full" split rings at the top of each cell and the top of the plates need replacement about once each six months.

I'd suggest reviewing your battery charging regime. You should be taking your traction pack up to at least 2.43V per cell at ~every charge~, and holding it there for an hour or more. Once every couple of weeks, up the charge voltage to 2.5V per cell for a few hours to equalize the cells. Then you'll begin to see water usage, and your batteries will have a better chance to live out their full life expectancy.

Also, don't forget that temperature makes a significant difference in cell voltage, both during discharging and charging. You need to adjust the voltage up in the cold months to get a full charge.

Water usage is a normal -and necessary- part of good battery maintenance!
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Old 11-06-2007, 07:44 AM
CPLTECH CPLTECH is offline
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Default Re: Mr Sharkey

Mr Sharkey,
Thanks for your comments. Very helpful reminders. I have a K&W BC-20 charger that I always felt was less than adequate since, from what I had read, was that these US145 batts need a min 25A charge to knock the sulfation off the plates. However, I have tried to “make do” with the K&W. I was reading the manual after your post and noticed that it charges at a higher rate than the on-board amp meter displays(???). I normally charge at 10-12A ( then float of 2-3A after150V) because any higher causes the #10 gauge/30A circuit and the #12 charger cord to get warm, as well as the 30A DC fuse in the K&W. Perhaps the K&W wouldn’t be so bad if it just had a higher amp design. Any recommendations for a decent 240V/30-50A, value priced charger. Some use pulsed(?) DC for batt health, since I know straight DC is not best for longevity.

I was hit by a severe downsizing recently, just a few yrs before SS kicks in and applied at a fork lift/UPS equipment company that several before me had gone to. Since I was looking into EVs at the time, I thought this would be an ideal situation to use my current electrical/electronic skills plus see the latest industrial battery technology at work. Age bias outweigh skills.
http://www.austinev.org/evalbum/1317
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  #6  
Old 11-06-2007, 08:40 AM
Mr. Sharkey Mr. Sharkey is offline
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Default Re: DC / AC Efficiency?

The K&W chargers were a lightweight improvement over the Lester chargers, but they aren't very "industrial". Mine has the same heating problems you have experienced, as well as a partially melted ampere meter. I usually run 17-18 amps through it. It also has an awful power factor, which makes AC wiring and connectors heat up. I've melted a couple of AC inlet plugs on my car over the years, and my circuit breaker panel sounds like it has a hornet's nest inside when I'm charging.

If you are satisfied with the amount of time it takes to bulk charge your batteries, don't feel that you have to install a higher amperage charger to keep your batteries happy. I doesn't really make any difference how long it takes to recharge, as long as it's done completely. Slow chargers tend to make the EV owner take the car off the charger before the charge is complete, which is more damaging than charging slowly in the first place. A faster charger will allow the car to return to service faster, which means that there will be more time for absorbtion and float charging, which can't be hurried no matter how much current the charger is capable of putting out.

About the only inexpensive alternative to the K&W would be to find an old Lester and hope that you can get it running. I’ve also seen a couple of Todd Power Source 75 ampere chargers rewired to series the modules to provide high voltage output. They have the exciting possibility of advanced pyrotechnics if they fail. Much better chargers exist these days, but they aren't cheap.

Proper battery charge maintenance requires a multi-step charging regime, which none of these chargers can supply. Rather than retype it here, here's a link to an article I wrote for an RV magazine about battery charging. Obviously, it's written for 12 volt systems, so you'll need to do some math to apply the voltages referenced to a high voltage traction pack.

Sulfation is not a problem with properly charged batteries. The whole point of charging completely and equalizing frequently is to remove the sulfur from the plates before it crystallizes and hardens, effectively covering the active material on the plate and resulting in less area for the electrolyte to react with.

I don't know that anyone has actually scientifically proven that pulse charging is necessary/beneficial to the health of properly cared for batteries. There are many battery based power systems that use pure DC to charge and battery life is found to be normal in most cases. The pulse chargers are found to be useful for controlling sulfation in batteries that have been abused, either by neglect or system design constraints. The theory looks good, but in order for them to work as expected would require massive pulses, much more than can be delivered by the black box gadgets you buy online. I do own one of these pulsers, and throw it on my 12 volt batteries around the house from time to time, but I've never found it to be effective in "reviving" damaged batteries. Once I determine that a battery is beyond it's useful life, it goes to the city to be recycled. I have a good return relationship with my battery supplier, and take him many cells to be recycled each year. These days, it's actually profitable to return dead cells, lead prices are through the roof!
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Old 11-06-2007, 11:44 AM
CPLTECH CPLTECH is offline
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Default Re: Mr Sharkey

Thanks for the clarification. I feel relieved. The K&W thanks you, and will now stay plugged in a lot longer.
I am guilty of unplugging after a charge, but often after 5-10 AH (1-2 KWH)have passed, since it is not on a timer.
The explanation makes sense. I often wondered about the batteries on a slowly charged homepower system.
Thanks again. Will check out more of your pages as time permits.
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  #8  
Old 11-06-2007, 02:01 PM
Mr. Sharkey Mr. Sharkey is offline
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Default Re: DC / AC Efficiency?

No problem, glad to help.

BTW, if you need/want an electronic copy of the K&W manual, there's a PDF version here.

If you have some preverse reason to want to look at the Lester manual, it's available also. There's two files:

Operator's manual

Maintenance manual
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  #9  
Old 11-19-2007, 09:27 PM
pandaran pandaran is offline
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Default Re: DC / AC Efficiency?

How long does it generally take to get a complete charge? (I guess with a 144V SLA system, since that seems to be the most common one I'm seeing so far.)
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Old 11-19-2007, 09:48 PM
Mr. Sharkey Mr. Sharkey is offline
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Default Re: DC / AC Efficiency?

Depends on how deeply you've discharged the pack. On my EV (108 volt, flooded lead acid) I can count on charging about an hour for each five miles I drive. At 144 volts, the K&W charger is derated, and would not put out the full 17-18 amps of charge current, so the time would go up.

Here's some generalities about charging - trueisms that apply to my particular EV, but may have application over a wider spectrum of vheicles.

DC consumption: 3 ampere-hours per mile (stop-and-go traffic, slightly better on the freeway)

AC consumption: About 1 killowatt hour per each two miles

Battery efficiency: ~80% (must put 120% of Ah consumed back into battery to fully charge)

You can only pound the first 75% of the charge in at high current, then it must taper down to finish.

Finish charging takes longer than bulk charging. The last 25% of battery capacity goes back in at a reduced charge current to prevent the batteries from gassing heavily. For the most part, it doesn't matter how far the car was driven, finishing the last 10% can take 1-2 hours.
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