How does a generator an a fan motor 'pulser' control a dc motor?

These paragraphs are from an article about a homebuilt hybrid car in Mother Earth News.

"Any project fresh off the drawing board has its share of problems, and

the Opel hybrid was no exception. When David pressed the accelerator for the first time, he got a 300-amp surge which melted his relays. So he searched his graduate texts for the answer ... and finally found it in-of all places-an old high school physics book: A pulser was necessary to "chop" the current flow and prevent a heavy initial draw to the drive motor.

As Dave explains it, "The motor will always have full voltage and full current, but the pulser makes it 'think' the voltage and amperage are cut down to about 1/4 of what's actually available. With this gadget-which is simply a combination of a reworked car generator and an

old fan motor-I can keep the draw within limits and effectively control

the car's acceleration . . . without sacrificing the maximum current or

voltage that's necessary for high-speed driving. I could have achieved the same results with a commercially available FCR control ... but one of those units would have cut my power slightly, and cost in the neighborhood of $800! I can build my own device for about $25, and I can fix it myself if it breaks!"

Boyntonstu

Reply to
stu
Loading thread data ...

It is hard to say for sure what he is talking about, however, maybe his car generator has brushes and contacts inside of it. Maybe what he did was hook the fan motor up to the car generator to make it spin. Then he somehow wired up some of the brushes and contacts such that they make and break the main power connection for his main motor as they spin around. In this way he made a crude mechanical method of reducing the average voltage applied to the main motor at startup. Seems like a primitive, heavy, and unreliable method to me. Unless I was stranded on a remote desert island I would go for the electronic PWM approach with some nice beefy MOSFETs.

Reply to
Fritz Schlunder

critical

evidently

electronics.

The inventor was out $800 and was not willing to get 'burned' again. For $25 he claims to get slightly more power that is possible with PWM. Is that factual?

For a non-elctonicer, replacing brushes is much simpler than cracking open a Curtis controller and trying to repair it.

BoyntonStu

Reply to
stu

Seems like a fair assessment. On the other hand I shouldn't be too critical of him since he did use his grey matter to fashion something that evidently does function, and in fairness his background probably isn't in electronics. Kudos to the developer even if I wouldn't do the same thing myself.

Reply to
Fritz Schlunder

If he had say a relay in parallel with his "pulser" to make hard contact after the motor was up to speed, and if the PWM controllers didn't allow for full 100% duty cycle, then it is conceiveable his method could yield more full power power. Alternatively if the PWM controller had a really large semiconductor voltage drop for some reason, maybe this could explain why he claims more power. But it is hard to say for sure without physically seeing what he did and how the electronic counterpart it was designed to replace functioned.

Indeed.

Reply to
Fritz Schlunder

It appears the complete article can be accessed here:

formatting link

I see it is dated summer 1979. I don't know what an FCR controller is, but power electronics have come a long way since 1979. This is probably why he claims to get more power from his method, whatever an FCR controller is probably wasn't very good by today's standards

As for the rest of the performance claims, I would say they are rather fanciful. Five horsepower just isn't enough juice to push a normal car on level terrain at 50mph. Four regular automotive batteries would not be able to supply too tremendously much more power than 5hp, and they wouldn't last very long at all. Additionally they would require replacement very quickly since automotive batteries aren't intended for that kind of use. Expecting a top speed of 90mph is also fanciful. The 1.0L Geo Metro has a rated maximum output of something around 55hp if I recall correctly. That car, which only weighs 1800 lbs and has very small tires for high efficiency has a top speed of around 80ish miles per hour on level terrain.

I suspect the car described in the article would be lucky to sustain 25mph on level terrain, with a total inability to go up even the smallest of hills at any speed.

Reply to
Fritz Schlunder

I read the article and it's full of mistakes. For one, it takes more than 5 HP to move that Opel car at 50 mph. For another, it states that the 5 HP lawnmower engine is efficient in it's use of gasoline. This is not true. The lawnmower engine does not use much fuel, but compared to the power output per gallon of gas by even the original engine in that Opel it looks pretty bad. You can figure this out your self by measuring how much gas your 3.5 HP rotary lawnmower uses when it is run till empty and scale it to your car. You can get the HP figure for your car at the library. Also, the wind drag goes up by the square, so the HP required to push the car goes up the same amount. So it takes

3.24 times as much energy to push the car at 90 than it does at 50. This comes out to 16.2 Hp required to drive at 90 if it only takes 5 HP to drive at 50. Even with the batteries it seems really unlikely the thing could be accelerated to 90 mph. ERS
Reply to
Eric R Snow

formatting link

At a guess someone said S and he heard F. The two sounds are very close and get confused all the time especially over a phone line.

Robert

Reply to
R Adsett

I read in sci.electronics.design that Fritz Schlunder wrote (in ) about 'How does a generator an a fan motor 'pulser' control a dc motor?', on Sun, 27 Mar 2005:

The fact that the inventor only found out about starting current when the contactor melted suggests that such a method would be all he could cope with.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
 Click to see the full signature
Reply to
John Woodgate

myself.

I think so too. And I'd say his design works - though I think the claims made are, if not factually/legally wrong, certainly blatantly misleading.

OK no, I stand corrected, "AND IT'S REALLY PRACTICAL!" is clearly not true.

"the little Opel' really has some get-up-and-go"

maybe be legally true claim, but plainly misleading. Ie only true for the most miniscule values of 'some'. A vehicle like that takes minutes to get upto a reasonable speed, is unable to accelerate away from problem situations, and is forever limited to little more than the slowest speed bits of a journey, ie corners.

again.

PWM.

cracking

Quite a good idea actually - this thread shows even tronic designers cant choose the cheaper more reliable design.

Its very old tech, but thats no reason to reject it. It was 25 instead of 800, an important consideration for any engineer, car alts are reliable, ditto fan motors, and using a carbon copper switch means near zero v drop - a good design really, and clearly a good choice in the

70s.

formatting link

rather

car on

Read again.

  1. The gas motor puts out 5hp, and the batteries will put out a lot as well.

  1. The car is 50lbs heavier than it was with a gas engine and clutch, so it has quite a bit of battery in it, not ordinary car batts.

  2. The article sells plans and may mislead the unwary, ie 90 may be only achieved on the steepest straight downhill stretch of road on the planet, and 50 may only be achieved for short times under favourable conditions.

"the engineer installed four 12-volt, heavy-duty automobile batteries-in series" not regular ones. I'm thinking very heavy duty, like 100-200 Ah. The car gained 50lbs.

yup, as is implied in the article.

yes, a backyard tinkers design. I wonder if one could improve that to some extent by refilling them with gel electrolyte. Still wont give adequate lifetime though, or anywhere near.

rated

car,

efficiency has

You dont need anything approaching 55hp to hit 90. A 30hp peak rating engine, run at just over half speed, so roughly around 15-20hp, will take a 1 tonne car to 85mph on the level, and more downhill. But... it will take minutes to get there, as the excess power for acceleration dwindles to zero at those speeds.

A smaller car doing 55 will need very much less.

25mph

of hills

One human can push a small car like that at 10mph, on the level. Its acceleration where more serious power becomes needed.

Having driven wildly underpowered vehicles, there are tricks. Hills: take a run up, and most of the power you need comes from the inertia of the vehicle.

This car shoudl meet its claims just about, but with many gotchas. The prime reason for 75mpg is simply the dangerously low power output. Hybrid tech helps too. But the driving experience would be dire. The designer consider a clutch unnecessary for example. IOW if you stop the car just once without going down thru the gears youve got your first issue. And you have to get the gearwheel speeds dead right at every single change. Fun.

Runnable, sure, but not workable on todays roads, ones with other cars on them. Might have been fit for very occasional daring use out in the middle of nowhere in the 70s, underpowered to the point of dangerous.

NT

Reply to
bigcat

Thanks for a well thought out and quite balanced response.

I wonder how such a device would allow the vehicle to stIn other words; with the battery 'ON' how does the commutator come to rest with the current 'OFF'?

Today, how does one build a really cheap/simple mosfet 'pulser' (not PWM)?

What I mean by my question is this. Assuming a 150 Amp/100V mosfet is available as a replacement for the carbon/copper switch, what is the simplest way to modulate the mosfet switch?

BoyntonStu

Reply to
stu

words;

My best guess is the fan motor + rewired alternator stay spinning as long as the ignition switch is on. At least thats how I'd probably do it. Taking foot off the gas pedal will mean no current to the main motor during stopping.

NT

Reply to
bigcat

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.