P2K GP30 speed?

[NW Modeling List]

Nigel:  I didn’t want to get into these details about DC motors because this
stuff usually makes the average modeler fall asleep.  The guy trained in
physics or engineering enjoys the discussion and information, but to those
not trained in the art it is information that they can’t readily understand.
Personally, I like it.  

 

Never the less, based on my engineering experience, and you will probably
agree with this, it is likely that the simple dimensional, fit and alignment
issues are causing the differences between the two locomotive models.  “Make
it like the print!”, is something you have probably said too.  90% of the
time it is a dimensional issue.  

 

Somewhere in one model is a binding situation that is likely causing most of
the trouble.  

 

The basic problem is the Chinese production lines have poor or no quality
control.  While the motor manufacturing line which uses some pretty definite
hard tooling to product a motor which tends to limit the range of
variability between motors to a rational Bell curve of performance ( Let’s
not get off into this assumption!), the hand assembly line for the model
itself has very limited control on quality.  They build ‘em.  And if they
run, they get shipped.  Customer service will sort out the bad ones by
shipping out a new one hot off the production line to replace the bad one.  

 

Gary 

 

 

  _____  

From: nw-modeling-list-bounces at nwhs.org
[mailto:nw-modeling-list-bounces at nwhs.org] On Behalf Of NW Modeling List
Sent: Monday, June 08, 2009 1:50 PM
To: NW Modeling List
Subject: Re: P2K GP30 speed?

 

Hi Gary;

 

Your note seems to indicate that you are assuming the motor is perfect.
There are also variations motor to motor such as:

 

*	the variability of the steel in the laminations 
*	the number of laminations - being off ±1 is not uncommon on low end
end motors due to thickness variation of the lamination material.
*	variability in the magnet material - high energy NdFeB magnets are
typically ±5%; cheaper ferrite magnets can be "tuned" resulting in tighter
tolerances, but less efficient motors.
*	geometric variability in the magnets.

For low end motors like in our model locomotives, three sigma limits of ±10%
within a particular model of motor should be expected.

 

I worked on spindle motors and voice coil motors in hard disk drives.

 

Nigel

http://www.linkedin.com/in/nigelmisso


----- Original Message -----
From: "NW Modeling List" <nw-modeling-list at nwhs.org>
To: "NW Modeling List" <nw-modeling-list at nwhs.org>
Sent: Monday, June 8, 2009 9:46:16 AM GMT -08:00 US/Canada Pacific
Subject: RE: P2K GP30 speed?

Remember that a DC motor has a straight line output curve on a horsepower
diagram.   For a given RPM there is a fixed torque output at a certain
voltage.  ( Horsepower equals torque times rpm.)  Basically at zero RPM you
have very high torque and at max rpm you have virtually no torque.  To
change this curve- really the slope of the straight line, the internal
windings have to be changed.  Gearing is a way to match the motor output to
the load, in other words, change the torque output at the gearbox output.
Thus, the slower locomotive with the higher gear ratio should be able to
pull more cars assuming that the wheel adhesion does not come into play.
Speed control via gearing change is not really the right way to look at the
issue.  Output torque is.

 

For two models of the same production batch to run differently, there are
some ‘quality’ issues within one of the models that increases its frictional
losses as compared to the other model.  It could be alignment of shafts,
binding somewhere in the mechanism, brush to commutator issues, friction or
binding within the trucks or wheel sets (is the wheel spacing correct?). 

 

The point of all of this is that when one observes a change between two
models one needs to compare the gear ratios and also see if the motor was
changed to determine what might be done to correct the issue.

 

Digital controls pulse the voltage to the motor (Pulse width modulation) via
the semiconductors to bring the dissimilar models into similar speeds, which
makes the motor not run on the straight line curve, but to one side of the
curve.

 

Gary Rolih

Cincinnati  

 

  _____  

From: nw-modeling-list-bounces at nwhs.org
[mailto:nw-modeling-list-bounces at nwhs.org] On Behalf Of NW Modeling List
Sent: Sunday, June 07, 2009 6:10 PM
To: NW Modeling List
Subject: Re: P2K GP30 speed?

 

It has been my experience that in many cases two identical locomotives from
the same run often operate at different speeds.  Not using DCC, my solution
has always been to put the faster locomotive behind the slower.  That way
the faster one is slowed down by doing most of the pulling of the train.

 

Dick Dunford

Blacksburg VA

----- Original Message ----- 

From: NW Modeling List <mailto:nw-modeling-list at nwhs.org>  

To: NW Modeling <mailto:nw-modeling-list at nwhs.org>  

Sent: Friday, June 05, 2009 7:55 PM

Subject: P2K GP30 speed?

 

Hi Listers,

 

            I was able to run some new engines on a friend’s layout this
afternoon.  For the first time I ran my new Athearn N&W SD45’s.  They ran
great!  I tried a new N&W P2K GP30 and the 45’s just dragged it along.  It
appears to run about half speed of the Athearn's.  I didn’t have any of my
older Athearn's or PPW or Atlas to compare it to.  So my question is that
the P2K’s will only run with other P2K’s?

 

Thanks in advance,

John Hecker 


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