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DCC Decoder Ratings

The goal of all this information is to make sure the user does not overload the decoder with a to powerful motor.  More specifically make sure the motor never presents a current level that will exceed the current ratings of the decoder.

Historically in the early 90's with the first generation DCC decoders and old motors, there was a real concern about making sure the decoder could handle the motor's current draw.  The worse case motor current draw is stall current.   Decoders were very expensive and not as robust as today's decoders.  In that light, the best conservative advice became the recommended procedure to determine the stall current rating of the motor in order to select a suitable decoder.  

SUMMARY:  TODAY for On30 and smaller scales, the motor requirements have become stable and mature so that DCC decoder manufactures have standardized on current rating that will work with all of the motors such that one does not need to worry about stall current ratings anymore.
   People who worry about stall current ratings are really over specifying the decoder requirements to deal with a rare mechanical failure condition of a locomotive.  They are paying extra decoder money in the form of an "insurance policy" in case lighting strikes. 

Below is a discussion about technical issues that support the above summary.

1) CURRENT RATINGS: DECODER VS MOTOR
2) MOTOR STALL CURRENT: TWO VERY DIFFERENT OPERATING CONDITIONS
3) WHICH MOTOR AND DECODER RATINGS ARE IMPORTANT TODAY?
4) HOW DO I CHOSE A DECODER TODAY?
5) WHEN SHOULD I BE CONCERNED ABOUT STALL MOTOR CURRENT RATING?


1) CURRENT RATINGS: DECODER VS MOTOR

DCC Decoders have 2 motor current ratings:

1) Basic Current Rating:  This is the maximum continuous current the decoder can support without overheating itself. 

2) Stall Current Rating:  This is the maximum peak current the decoder can take without blowing one of its motor drive transistors up.

Note:  The DCC standards do NOT state how a decoder manufacture it to determine the decoder's current rating.  Most simply state a "Current Rating" without necessarily saying which current it is referring to.

Motors have 2 current ratings:

1) Slip Current draw: This is the maximum current the motor will draw when spinning its wheels while sitting on track with its own weight on the rails.  This is the normal operating condition that draws maximum power from the decoder.

2) Stall Current draw.  This is a motor's locker rotor (no spin) motor condition where a abnormally high current will will be drawn from the decoder.  Worse Case. 


2) MOTOR STALL CURRENT: TWO VERY DIFFERENT OPERATING CONDITIONS

Stall current s determined by the winding resistance of the motor.  Stall current is always higher than Slip current.  Typically the stall current is often twice the maximum slip current.

There are two situations where stall current is seen by the decoder.

1) Starting up a motor:  Motor is FREE to spin and drive the train.
It happens every time you start your engine (motor) and disappears as soon as the motor starts to spin.  It only last for a fraction of a second.  Decoder's can always handle this type of stall current.

2) Mechanical drive failure: Motor is LOCKED up so that it cannot spin.  Bind or Jammed.
This is the extremely rare event that requires a mechanical failure in the drive transmission.  This is the type of event that can overload and destroy a decoder causing it to burn up.

They key difference is the DURATION OF TIME that the motor does not spin.  That determines the DURATION OF TIME that the stall current is present as a load on the decoder.    Very Simplistically:  DECODER HEAT = CURRENT * CURRENT * TIME. 

What does this mean?  The decoder heat can quadruple when the motor current doubles.    But the TIME duration that the stall current is present directly regulates how long that heat exist.  Very little time = Very little heat.

Conclusion:  Decoders can takes the startup motor stall current all day long.  They cannot take a locked rotor stall current without being damaged.


3) WHICH MOTOR AND DECODER RATINGS ARE IMPORTANT TODAY?

Decoder Basic Current Rating > Motor Slip Current draw  (For scales On-30 or smaller)

Stall current information is not important anymore.

Why?  Stall current is much less important because the DCC manufacture has already taken into account the motor stall current issues by the choices of the transistors (MOSFET) they use.   DCC Decoder's motor peak current capacities have greatly improved over time due to progress in technology.   Modern MOSFETs used to drive the motors are much more rugged.  

NOTE: Although not a factor, it should be noted the DC motors have gotten a lot better (more efficient, lower stall current) at the same time.  However, some older O scale motors still in use are not so efficient and require a lot more current.


4) HOW DO I CHOSE A DECODER TODAY?

If you got a motor in a given engine, then you automatically know the motor is intended to be used with that scale.  This is what greatly simplifies the selection process for a decoder and allows one to not worry about the motor current rating.  Got a HO locomotive, then buy a HO decoder for it.  Got a O or large scale locomotive then get a large scale decoder. 

Why does this work?

The DCC decoder manufactures design the decoders for use with a certain scales already have, as part of the design process, done the motor current investigation for you.    They know all of the type of motor that you will encounter by testing and experience.   All you need to worry about is physical shape/size of the decoder relative to the space available in the engine you want to put it into. 


5) WHEN SHOULD I BE CONCERNED ABOUT STALL MOTOR CURRENT RATING?

1) When you are using a decoder outside of it intended scale.

2) When you are using a very very old decoder made back in the mid 90's or earlier.   1st generation.

3) When you are using questionable motors.  Very old, worn out, unknown manufacture, known problem history.  This really means you should replace the motor.

4) When you are using motors for full size O scale, not narrow gauge, or larger.

RECOMMENDATION

If possible replace any old inefficient or "hot running" poor performing motors such as

1) Open Frame/Brass Engine motor.  (Very old motor designs)

2) Any real old/Cheap toy motor of the 70's or earlier. 

Why?  This is not about current but about the high heat "under the shell" the decoder will have to work with.   Heat is heat regardless of where it comes from and to much heat will burn any decoder up.

1/31/14
2/9/16