Electric power primer

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Glenn Brooks
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Electric power primer

Post by Glenn Brooks » Tue Sep 15, 2020 10:19 pm

Hello all,

I could use some recommendations for how to power my old center cab yard goat.

NOTE: I just re read an old thread and got some good ideas from previous suggestions. However, one unknown area is what gear reduction are people using now days to drive the axles - and is a gear box necessary with say a 600 watt variable speed DC motor?

This loco was built as an overhead wire DC electric trolley in 1968, and has been laid up in storage for maybe well over 40 years. Trucks, but no chain drive, and whatever motor was since installed is long gone.

So thinking it can best come to life with a 24v or 36 v battery pack and some small elextric traction motors retrofitted to the trucks or axles. 12” gauge



Thanks
Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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BigDumbDinosaur
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Re: Electric power primer

Post by BigDumbDinosaur » Wed Sep 16, 2020 12:18 am

Any idea what that thing weighed when it still had its propulsion machinery installed?
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Glenn Brooks
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Re: Electric power primer

Post by Glenn Brooks » Wed Sep 16, 2020 1:31 am

BDD, hard to tell. The original builder was running a low voltage DC overhead wire system, so guessing the power plant wasn’t all the big. Right now the body and frame are fairly light - maybe 300-400#. So guessing loaded it might have not tipped 600# range...

Glenn.
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

jcbrock
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Re: Electric power primer

Post by jcbrock » Wed Sep 16, 2020 3:04 am

Just a data point here Glenn. We have a 7.5 " gauge electric-drive loco powered with 4 motors of 450 watts each, 24 volt. The dc motor controller is a Dimension Engineering Sabertooth 2x60 controller. It can move the 850 lb loco with 4 adults on 2 riding cars on 2% grades. I'm not sure if it would do twice that load, I need to add some instrumentation and circuit protection before trying to max it out. No gear reduction and I can't check it at the moment but I believe there is a chain drive reduction of maybe 2:1.
John Brock

Glenn Brooks
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Re: Electric power primer

Post by Glenn Brooks » Wed Sep 16, 2020 3:35 pm

Thanks John,

Good comp. This is the kind of info Iam looking for...

One question: should each motor be the same wattage or torque rating, or does it matter?

Are you under fire watch at the moment?? We’re inundated with heavy smoke but no fires nearby...

Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

jcbrock
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Re: Electric power primer

Post by jcbrock » Wed Sep 16, 2020 5:32 pm

I'd make the motors identical to try to make their speed profiles as close as possible. Another good source of information might be Nelson Riedel's Railbus project: https://nelsonslocomotive.com/Railbus/R ... erview.htm

No fire watch here, so where there's smoke there's not always fire. It seems to be least smokey in the morning and then thickens through the day. We're more a rural Willamette Valley farmland neighborhood so the danger is pretty low. I worked on a grass seed farm in college and spent some time burning field stubble back when that was a thing but it did not compare to this. I hope we get some rain tomorrow.
John Brock

kcameron
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Re: Electric power primer

Post by kcameron » Wed Sep 16, 2020 7:00 pm

One thing to consider about motors, they can fail. If you had 'such a deal' on some great but very rare motors, it might be very hard to get the motor repaired or find a replacement. I've known a few locos sidelined due to not being able to find a replacement motor of the same specs. Size, torque, speed, these things do matter. Yes you can adjust somewhat with fancy motor controllers to run the motors differently to make their outputs match, but it isn't easy and likely to need adjusting all the time.

I'd make sure I'm using some common motors so I felt I might find replacements later. Then again if I got a deal on a pallet load....
-ken cameron
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Pontiacguy1
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Re: Electric power primer

Post by Pontiacguy1 » Thu Sep 17, 2020 7:37 am

To figure out your gearing, you need to decide what your top speed is going to be, and not everyone has the same idea about what that should be. In 7.5" gauge, I usually use around 8 MPH as my wide-open full voltage top speed, or Wide-Open-Throttle top speed if it is gas powered. You then have to calculate the gear reduction based on the diameter of the wheels that you have, and the max RPM of the motors you are using.

To give you an idea of how that worked for me, I am building a small 4-wheel critter with my son, in 7 1/2" gauge. The wheel diameter is 4.25", and the motors are 350 watt, 2750 RPM, 24 volt. the motor has an 11 tooth sprocket on it already. I'm using a jack-shaft so mine will have double-reduction, but that isn't necessary most of the time. The circumfrence of the wheel is right about 13.4", so to go 8 MPH, the wheel has to spin at approximately 635 RPM. So now I divide: 2750/635 and come up with a gear reduction of 4.33:1 I'm going to be close to that with gears that are available, and will have a final speed of just under 8 MPH, and a gear reduction of 4.5:1 on the critter. With a total weight of around 200 lbs, and two 350 watt motors, it should be pretty well balanced as far as speed, power, etc...

So you really need to ask yourself how fast this thing needs to go. That and your motor speed will determine your gear ratio. The amount of power you need will be determined by what you intend to pull with it and the amount of traction available. Be sure to pick a motor that is large enough for the maximum weight/load/grade/curve resistance you will need to overcome. If in doubt, go with a larger motor, especially since this is grand scales. I would also consider using larger motors and gearing it for a higher top speed since this is grand scales. While a top speed of 8 MPH might be perfect for your small pike, if you ever take it somewhere else to run it, that might be way too slow. It's no fun if everyone else is running 10 or 12 MPH and you're the slowpoke running only 8 MPH.

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BigDumbDinosaur
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Re: Electric power primer

Post by BigDumbDinosaur » Thu Sep 17, 2020 11:24 am

Basic formula for power transmission calculations:

V = (u × D) ÷ (R × 336)

R — Gear ratio.

D — Wheel diameter in inches.

u — Traction motor revolutions per minute.

V — Speed in miles per hour.
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Glenn Brooks
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Re: Electric power primer

Post by Glenn Brooks » Thu Sep 17, 2020 1:30 pm

Good discussion on figuring gearing for top end. This helps a lot. I certainly agree with designing a proper top speed from the get to.

Which brings up the question of low end torque. I keep thinking about my old Farmal Cub- first gear, lots of torque and pulling power, shift to third gear to roll down the road...obviously hi and low gear makes a big difference in work output.

So, what do people do for gearing to start up and pull a heavy train? For example, an extreme example perhaps, pulling a 20,000# train with several loaded cars, or maybe a ballast car with two yards of gravel? Seems like one would need a very low gear ratio to deliver high tractive effort at slow speeds, then somehow, shift to a higher gear, once momentum is built up?

I guess the question is, in our miniature train gauges, is it simply a matter of adding a larger motor to a fixed chain and gear ratio? Or would I want a lower gear ratio AND a larger motor - to pull a heavy load. If so, how to achieve that, without breaking the bank...

(Note: I suspect I am overlooking the idea of how adhesion and tractive effort fits into the picture)


Thanks
Glenn
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Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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BigDumbDinosaur
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Re: Electric power primer

Post by BigDumbDinosaur » Thu Sep 17, 2020 3:06 pm

Glenn Brooks wrote:
Thu Sep 17, 2020 1:30 pm
So, what do people do for gearing to start up and pull a heavy train?
In full-sized practice, most traction motors are series-wound and powered by direct current. Assuming a power source whose voltage stays relatively constant under load, a DC series-wound motor is a "constant horsepower" device. In functional terms, what this means is torque is very high at or near stall and decreases as motor RPM increases. In most of the speed range, the product of torque and RPM remains relatively constant, resulting in constant horsepower output. Hence changing gear ratios is not necessary to accommodate the full speed range of the locomotive. The motors develop the very high torque at standstill needed to start the train, and the natural behavior of the series-wound configuration acts as the "gear shift."

Motors obviously have a limit on how much current flow they can tolerate without burning out. Again with full-sized practice, locomotives usually start out with series-parallel combinations of motors to limit current to a reasonable level. For example, an EMD GP-40, which has two-axle trucks, starts out with each truck's motors in series. The two series combinations are in parallel across the power source (traction alternator).

As the train accelerates, motor connections are changed to put all motors in parallel across the power source, maintaining the current flow in each motor to best use the available electrical power. At still higher speeds, something call "field weakening" is used to extract the maximum amount of power from the motors. In this case, resistance is used to divert some current around the field. The armature current is increased without increasing field strength, which makes for more motor torque at higher speeds. When the train decelerates the process will be reversed. The entire sequence I described is called transition, which is common in electric traction of all types.

Most electrically-propelled models use permanent magnet motors, whose characteristics are more like shunt-wound DC motors. They will not generate as much starting torque as an equivalent horsepower series-wound motor, but do not need the complicated transition system to work well at higher speeds. Modern DC motor controllers do much of the work that used to be done with relays, contactors and mag-amps, which makes use of PM motors more attractive, even though they are inferior to series-wound motors in traction applications.
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I'm a caterwauling curmudgeon. What's your excuse? ☻

Glenn Brooks
Posts: 2147
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Electric power primer

Post by Glenn Brooks » Thu Sep 17, 2020 3:17 pm

BDD, thanks much. This explains a great deal. I’ve just been reading about permanent magnet DC motors and the new small scale motor controllers that do something similar to the field weakening process you mentioned. So, gives me a much clearer picture about how this all works. Much appreciated.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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