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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

variable speed DC motor drive


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  #1  
Old 09-13-2005, 03:20:49 PM
John Newman Jr.'s Avatar
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Default variable speed DC motor drive

Maybe a little off topic, but here goes:
A friend has (for lack of a better description) a "Kiddy Train". A scaled down railroad on 18" track. He's got a circle of track about 1/4 mile total, plus a siding to park it on with a shed (converted dog house) for the engine - all flat & level. Locomotive currently in use has a tiny little Continental 4 cyl flathead with a GM Powerglide rigged to it. He is thinking about building a new engine and I suggested that he copy actual engine configuration a little closer and use electric traction motors in the axles and have a generator on board for power. He is interested enough to consider it if I can help with the plan and show him that the equipment is available and affordable. The goal is to stay low voltage, so looking at what I have laying around, I've got:
- an ex- military generator that gives 28VDC @ 55A.
- 3 (24V) golf cart (actually more like 'wheel chair') type motor/transaxle assy's that say they draw 20A @ full load.
With a couple of 12V deep cycle batteries in the mix, I think this will provide enough pulling power to move the load. He own 5 cars with 4 seats each but currently only can run 3 cars with that engine. So that only leaves 2 cars that this new engine would pull.
The big question is how to get variable speed on these motors while still getting full amperage for the torque required to start out (smoothly & gently) from a dead stop. Is such a device available off the shelf? It could be a foot pedal or a hand lever. Reversing would be great, but not required.
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Old 09-13-2005, 04:13:14 PM
B.Sparks B.Sparks is offline
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Default Re: variable speed DC motor drive

There are a number of electronic DC motor controllers available. Curtis Instruments makes a variety of them: http://www.curtisinst.com/index.cfm?...dustryID=10#10

Or you could use a more modern system. Generate at 240 vAC (60 hZ, single phase) and feed it to a standard 3 phase induction motor for traction power via a variable frequency drive. This will give incredibly smooth control. There are many AC drive locomotives in service.

Before you get too carried away with the repower project, compare the costs, weight and complexity of the proposed system compared to what you have now. The ONLY reason railroads use the engine/generator/traction motor system is that there is no mechanical transmission system that can handle the sort of horsepower and operating conditions encountered in RR service. Many people think of the engine/generator/traction motor system as some sort of perpetual motion scheme. It isn't. The energy lost in those multiple power coversions is substantial.

There was a car built in the 1920's that featured a DC generator, driven by the engine, which provided power to an electric motor that drove the rear wheels. It's "claim to fame" was a pushbutton automatic transmission - a full 2 decades ahead of the "Hydromatic." The transmission control was mounted on the steering column, not unlike the Edsel of 30+ years later. No clutch, no gearshift and regenerative braking. It featured smooth, stepless accelleration that's only now being matched by the CVT transmissions coming on line. Unfortunatly the car was costlier, heavier, slower and thirstier than it's contemporaries.
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Old 09-13-2005, 05:16:20 PM
Raymond Raymond is offline
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Default Re: variable speed DC motor drive

While you are at it, take a look at hydrostatic. The variable speed with 100% torque is easy as well as reversing and braking. It is heavy and dependable and short of leaks its virtually indestructable. There are also many environmentally friendly and fire safe fluids for those who fear environmental destruction from oil leaks.
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Old 09-13-2005, 05:26:30 PM
B.Sparks B.Sparks is offline
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Default Re: variable speed DC motor drive

Raymond - hydrostatic.. Great idea! Here's a possibility:
http://www.surpluscenter.com/item.as...atname=engines
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Old 09-13-2005, 07:30:53 PM
Home Brew Home Brew is offline
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Default Re: variable speed DC motor drive

The guys are right, engine to generator to motor to traction wheels isn't very efficient. But since you mentioned you wanted to do this at an affordable price. . . . .

Using what you have without going out and spending big bucks there are some things to check first.

One thing I would check is the duty cycle on the motors before going too far. The military generator is more than likely 100 % duty cycle. Wheel chair motors would not necessairly be 100 % duty cycle and you might end up spending $$$ on the tail end of your project to keep the traction motors in good health.

Do the dc motors have sleeve or ball bearings ? One thing I've seen happen to shielded ball bearings is after some use the carbon dust collects in them and drys out the grease. If ball bearings are used in the motors be sure to get sealed bearings. Also generator brushes will have to be maintained. So maintence with DC generator / DC motors is an issue.

As an experiment I would see if the military generator could have a throttle control put on it and idle the engine down to see what the generator output will go down to. If there is still a healthy amount of voltage you could break in to the generator regulator circuit. I"m assuming the generator has a regulator on it. If the generator is wired like the old Delco Remy automotive generators you could break into the regulator circuit or do away with the regulator all together and use a variable resistor between the field lead and ground. You would want to use one of the larger variable power resistors and not a little volume control type pot.

As you decrease the resistance between the field lead and ground the voltage out of the generator would come up, as the engine RPM is brought up more voltage would be produced and eventually you will have enough volts and amps to start the dc motors to move.

The idea is by controlling the generator, it becomes your transmission / clutch. In this design / idea I would not put batteries in the mix. If the dc motors / transaxles are still attached to the golf carts you could put the military generator on a little trailer and hook things up so you are powering the golf cart from the generator and see what range of speed / power you can get out of this type of setup before going and building another locomotive and installing everything to find out there is a weakness some where in the design. If it does work then you could add a load such as a small fold down camper or a trailer load of dirt / gravel to see what 1 transaxle setup is capable of.

This is just a suggestion based on using what you mentioned is available in your post as well as some things to check. Good Luck ! Sounds like a fun project !
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Old 09-14-2005, 12:12:51 AM
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Default Re: variable speed DC motor drive

I need to clarify - this is not any kind of commercial duty issue! It is a private toy of an individual. We are not talking about an attraction that is open to the public. While maintenance is certainly a point to consider, it is not going to see daily use. The efficiency of the hydrostatic transaxles you've mentioned is understood, but the whole project is just for fun and seeing what we can come up with that would be unique.

B.Sparks:
your link to the Curtis controllers looks promising.
Doing it with a 240VAC gen, variable frequency drive and a 3ph motor does not. That would require buying everything we already have over again. Even if we could find all of it through surplus outlets (as we have with the stuff we have so far) it would still be a lot of $$$. Plus, it all has to fit in an enclosure about the size of a kids' pedal car (well, actually about twice as long, but about the same width).

Homebrew:
The gen is a surplus unit like those sold by Colemans Surplus with the 2 cylinder Mil Std engine (16 cu.in., I think). The control box has a variable pot on it that will adjust voltage from about 22v- 32v. I put a load bank that I made on it consisting of 6 pairs of automotive driving lights with each pair wired in series to take 24v. These are 55w @ 12v bulbs X 12. The meter on the gen shows I am pulling about 28A with all of them on and the voltage turned up to about 26v. Forcing the throttle down against the gov dims them, but when rpm got below a certain threshhold, I lost all output. Forcing the throttle up against the gov they maintained pretty much the same level, but turning up the pot with the revs up got the lights so bright I was fearful of blowing them (turning the pot up to max at governed speed made them brighter, but didn't frighten me - maybe my gov isn't set high enough....)

The 'traction motors' have shielded / sealed ball bearings on the axles, not sure about the motor. I don't have them here, but I have included a picture from the catalog we got them from (actually we got them from a 'close-out sale' catalog from the same company for $140 ea.) Here is a link to the company that made them
Stature Electric
Look at their 5136 Series Transaxle for details.
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Last edited by John Newman Jr.; 09-14-2005 at 12:49:38 AM.
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Old 09-14-2005, 10:37:40 AM
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Default Re: variable speed DC motor drive

Give this site a try:

http://www.evparts.com/firstpage.php...21006b4e369682

If the URL doesn't end up on one line, you will have to cut and paste it to make it fit.

Take care - Elden
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Old 09-14-2005, 07:31:21 PM
Home Brew Home Brew is offline
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Default Re: variable speed DC motor drive

One caution about permanent magnet motors, if you need to work on the armature, change bearings, true up the commutator or whatever, be sure to put another armature in the field case while you work on the original armature. The reason is that the armature is part of the magnetic circuit. If you remove the armature to work on it and leave the case empty, the field magnets will demagnetize themselves. We've had customers pull their armature to turn the commuatator, undercut the mica and do a final polish of the commutator and put things back together to find their motor now has no power. That is because while the case sat with no armature in it, it demagged itself. You can have the magnets remagnetized by a motor shop, and not all motor shops are set up for this procedure. The "dummy" armature can be slightly small in size and even longer, just as long as the case doesn't sit empty.

With 20 to 1 gear ratio, it looks like what you have might work, since things are flat and you aren't going to be pulling up any hills. I see the transaxles are 22" wide and your track is 18". I assume you have something figured out for this, that is why I mentioned the caution about the motor, in case you take things apart to modify the transaxle for the width of your rail road.

It sounds like your generator might have electronic control, and you apparently have enough range in the engine to slow things down enough where the generator quits generating. If you want to experiment, one thing to check is if the pot resistance increases to lower the output voltage, and of course the pot resistance decreases to increase voltage. You could put a larger value pot in place of the original voltage pot. Say if the original pot is a 10K you could put a 50K pot in place of it to see what happens. You may not, however, be able to lower the voltage enough for a smooth soft start.

Going with one of the off the shelf controllers the guys have mentioned might be the more plug and play way to go.

If you want to see someone elses backyard railroad go to
www.monorails.org/tMspages/NMT01.html
This is a monorail and it runs on DC from on board batteries. There is also a link for the dc controller this guy used !
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Old 09-14-2005, 10:31:29 PM
Ed Stoller Ed Stoller is offline
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Default Re: variable speed DC motor drive

This looks like a real fun project. The key here maybe to come up with the right junk to start with. I currently am running a Tecumseh HH120 engine simulator for testing ignition systems. It is driven be a 90 Volt DC motor from a road side rescue tread mill. I come complete with the controller which was importent to me.

I also salvaged some bigger 30 volt motors off a floor scrubbing machine. The two motors that drive the brushes had a gear reduction on them. The motor that propelled it is on my WheelHorse as a generator putting out 120 VDC. The motors ran off batterys. One of them is in use on my buddies big honey tank, like 100 galons. He uses it the stir the honey. You might contact a company that sells / services floor scrubbing machines to see if they have any good junk or coustomers wanting to get rid of one. I would think you would want to use a chain drive between the axel and the motor / reduction gear.
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