# DC powered AB units



## Lefty70 (Jan 10, 2016)

Hello, just curious how DC powered A and B units work as far as electrically? I am concerned that the B unit could cause the A unit to derail if it had more power as it is new versus running with a used A unit. Am I worried about nothing here? I have ran two separate DC units on the same track before and the last train ran down the front one....


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## JerryH (Nov 18, 2012)

With DC it is hard to get the locos speed matched. You can try running them together and see what happens. If there is too much of a difference, you could have problems. With DCC you can speed match them via programming the decoders.


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## DonR (Oct 18, 2012)

Usually you try to put the speedier loco first, not really a
choice for and A & B unit.

You could experiment with some resistors in the motor
circuit of the faster one and try to get a speed match.
However, if they are close they'll probably work out OK.

I have DCC but my Bachmann EZ controller does not have
tCV setting speed match capability, even so I have 3 consists. The
locos run fairly close in speed tho not exact. I notice only
a little bumping or nudging.

Don


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## Old_Hobo (Feb 20, 2014)

DonR said:


> Usually you try to put the speedier loco first, not really a
> choice for and A & B unit


It could be a choice, if the engines are the type (Athearn?) that the A or B unit body will fit the same chassis....you could put the A body on the faster chassis that way.....


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## Lefty70 (Jan 10, 2016)

Both are Walthers TrainLine DC. Question: if they sell them new as a pair, are you supposed to operate them together? Again one of mine is used, the other I am buying is new.


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## thedoc (Oct 15, 2015)

Lefty70 said:


> Both are Walthers TrainLine DC. Question: if they sell them new as a pair, are you supposed to operate them together? Again one of mine is used, the other I am buying is new.


 Usually an A unit chassis is used to power the B unit, so just swapping the shells is not a problem, test them to see if one is faster than the other, and the faster one will be the A unit chassis. If there is too much difference that results in wheel spin, you might want to consider different unit combinations all together.


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## traction fan (Oct 5, 2014)

*A&B units*



Lefty70 said:


> Both are Walthers TrainLine DC. Question: if they sell them new as a pair, are you supposed to operate them together? Again one of mine is used, the other I am buying is new.


Lefty70;

On the prototype, it was very peculiar for a B-unit to be operated without an A-unit controlling it.(though there were oddball examples like a B-unit with a window and control position in one end, used as a switcher.) The "B" stood for booster, and that's just what they were; more horsepower controlled by the crew in the A-unit's cab. Normally B-units did not have any controls or even a place for a crew; hence they were normally connected to one or more A-units.
If you are referring only to the models you have, you can run the A-unit separately, and run the B with a different A-unit that matches the B-unit's speed better. On whichever unit was purchased used; have you checked and cleaned the mechanism inside and out?
The used unit may have acquired enough dirt, or other mechanical problems to cause it to run slower.

Traction Fan


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## thedoc (Oct 15, 2015)

traction fan said:


> Lefty70;
> 
> On the prototype, it was very peculiar for a B-unit to be operated without an A-unit controlling it.(though there were oddball examples like a B-unit with a window and control position in one end, used as a switcher.) The "B" stood for booster, and that's just what they were; more horsepower controlled by the crew in the A-unit's cab. Normally B-units did not have any controls or even a place for a crew; hence they were normally connected to one or more A-units.
> If you are referring only to the models you have, you can run the A-unit separately, and run the B with a different A-unit that matches the B-unit's speed better. On whichever unit was purchased used; have you checked and cleaned the mechanism inside and out?
> ...



On one of the western RR's there is a case of a 'B' unit equipped with controls and a window for the engineer to bring a section of a passenger train to a junction where it was joined to the rest of the train to complete the trip. The 'B' unit and the cars were cut into the rest of the train and the whole train proceeded as one train.


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## Lefty70 (Jan 10, 2016)

I guess I am not asking my question correctly. With a DC power supply, do you normally run A and B powered units at the same time? Because mine did not come as a package will the power be different as opposed to a set? I have run other engines at the same time and the rear engine has run down the other. Not being DCC, how does the manufacturer get them to operate correctly? This is only a question as A unit hasn't arrived yet. B unit is in perfect working order, but MAY have some previous use.


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## Cycleops (Dec 6, 2014)

You just don't know, the only way is to suck it and see. With DC you have few, sorry read 'no' options.


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## tkruger (Jan 18, 2009)

Lefty70 said:


> I guess I am not asking my question correctly. With a DC power supply, do you normally run A and B powered units at the same time? Because mine did not come as a package will the power be different as opposed to a set? I have run other engines at the same time and the rear engine has run down the other. Not being DCC, how does the manufacturer get them to operate correctly? This is only a question as A unit hasn't arrived yet. B unit is in perfect working order, but MAY have some previous use.


Yes you can run them together. This is what I do for running a DC constant. Place each on the track equal distance apart. Then run them at 25%. See what catches what. Note that one may go through curves etc. better too thus effecting the speed. Then again at 50%. And finally at what you top operating speed will be. If you notice that they catch each other very slowly or not at all then you are in good shape. If it is the tortoise and the hare you have an issue.

General rule is that the fastest goes first to help stop the bumping between the locomotives. If they are real closely matched then it is not an issue. 

If they are to far different there are a few options. Clean and lube everything on both locomotives. Give them both a full tune. If from the same manufacturer and generation and model this should get them close.

If the B is always slightly faster than the A then swap the motors, they are the same model this should not be an issue.

I have done this mostly with Athearn Blue Box locomotives. They span many generations and their performance can be effected by the generation motor they have, how clean the comm is, are the brushes worn, dirty wheels, poor lubrication, planet alignment etc. Basically with used locomotives anything can be a culprit. Your best bet is to tune everything and then test.


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## ggnlars (Aug 6, 2013)

With DC, if the units are not wired together, the are independent motors. There are electronic controls that could couple them, but this is not usually the case here. You want to mix and match your units easily as desired.

Both motors will have functions of current draw, torque and rpm that are dependent on the voltage potential. In this case, rpm is speed and Torque is drawbar force. Think of current a fuel. All motors, new and used will have variations in these functions. The strength of the permenent magnets is the biggest variable followed by drag due to dirt or corrosion. 

Powered engines of any age and manufacturer can be coupled or consisted like your AB units. The reason the faster goes first is because it likes to pull. But does not like to push. Faster will pull the slower. The train speed will be set by the slower function at the train load. 

Larry
www.llxlocomotives.com


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## traction fan (Oct 5, 2014)

*Speed differences in models*



Lefty70 said:


> I guess I am not asking my question correctly. With a DC power supply, do you normally run A and B powered units at the same time? Because mine did not come as a package will the power be different as opposed to a set? I have run other engines at the same time and the rear engine has run down the other. Not being DCC, how does the manufacturer get them to operate correctly? This is only a question as A unit hasn't arrived yet. B unit is in perfect working order, but MAY have some previous use.


Lefty70;

OK, we are talking about models not the real thing. Two powered DC locomotives will run together, but as you have found out, they do not always run at exactly the same speed.This can happen weather the two are sold together, as a set, or sold separately. Other than putting the faster one in front, there's not much you can do about it. Both locos are running on the same DC voltage, which is fine, and the speeds should be at least close, but seldom identical at every throttle setting. There is the option of using a "dummy"(un-powered) B-unit which will simply trail along at the A-units speed like any other car being pulled.

Hope that answers your question;

Traction Fan


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## thedoc (Oct 15, 2015)

ggnlars said:


> With DC, if the units are not wired together, the are independent motors. There are electronic controls that could couple them, but this is not usually the case here. You want to mix and match your units easily as desired.
> 
> Both motors will have functions of current draw, torque and rpm that are dependent on the voltage potential. In this case, rpm is speed and Torque is drawbar force. Think of current a fuel. All motors, new and used will have variations in these functions. The strength of the permenent magnets is the biggest variable followed by drag due to dirt or corrosion.
> 
> ...


Even if the 2 units are wired together, both motors will try to run at their own speeds.  The only real solution is to mechanically link the units together via drive shafts between the units. Then they will run at the same speed, because both motors will be locked together.


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## Old_Hobo (Feb 20, 2014)

Or, like has been said, let the fast one pull the slower one....simple and effective resolution....:thumbsup:


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## Chet (Aug 15, 2014)

I operate DC only and mu locomotives together if I have a heavy train to pull up my 2% grade. The bulk of my locomotive fleet are Atlas locomotives. They are old units, bought and custom painted when they first came out some time in the 80's. 

I am lucky that they all run at just about the same speed. Here's a quick video. 

https://www.youtube.com/watch?v=wFIIWvaTiIk

If your locomotives are made by the same manufacturer, they could run fairly close to the same speed. If you have a fairly long and heavy train even if the B unit is slightly faster than the A unit, the B unit will be doing the bulk of the pulling with the A unit assisting and you shouldn't have a problem.


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## DaveCo (Nov 9, 2015)

In the past I've had to disconnect the drive shaft and basically have a dummy unit with a motor in it. I've also heard of people gearing their wheels different but that's way over my head.


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## rrgrassi (May 3, 2012)

Before I used DCC, I would run the slower locomotive first, but I would also speed test them. Usually the faster locomotive was slowed down by the load of the cars on it. When they ran equal, I knew I had the right number of cars coupled to the faster locomotive. Broke too many plastic couplers putting the faster locomotive on point.


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## thedoc (Oct 15, 2015)

rrgrassi said:


> Before I used DCC, I would run the slower locomotive first, but I would also speed test them. Usually the faster locomotive was slowed down by the load of the cars on it. When they ran equal, I knew I had the right number of cars coupled to the faster locomotive. * Broke too many plastic couplers putting the faster locomotive on point.*


I once had 3 heavy engines on the head end of 140 loaded hoppers using metal KaDee's and didn't have a problem. The hoppers were about 8 oz each and there was a PRR Q-2 with 2 motors, a B&O EM-1 with 2 motors, and a rebuilt B&O S-1a. The layout was an over and under 3 times around and at times the lead engine was running beside the caboose.


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## Old_Hobo (Feb 20, 2014)

rrgrassi said:


> Broke too many plastic couplers putting the faster locomotive on point


Yet another reason to ditch the plastic couplers......


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