Locomotive current requirements / analysis

This section is to try to put some information down and some analysis of what the current (meaning electrical current) requirements for an interface in a LS locomotive is.

Here's the outline:

Definitions of current measurements needed:

Here are my suggestions: 

  • "average running"
  • full slip
  • locked rotor stall


Why do we need these? Because there are many different situations that occur during the operation of a locomotive. The current flowing through circuits and any standard interface has a great impact on the basic design, hardware components, etc.

Definitions (needs more detail)

Average Running Current.

The intent of this measurement/specification is to derive what the long term, steady state current can be in a locomotive. This is normally one of the specifications in any electronic component, like a 15 amp circuit breaker in your house. This is of course difficult to specify. What kind of load should be put on the loco? What kind of grade? What friction coefficient of the track? What ambient temperature?

The idea here is to understand what is the long term average. My suggestion is to go a little above the average train, so I'd say something like a 10 car freight train, with lighted caboose (more drag), ordinary wheel bearings, and a 1.5% grade.

 

Full Slip current.

This is a really important specification. Basically, this is the maximum current the locomotive will consume short of a mechanical failure (which includes derailment). This is how hard you could theoretically run the loco. All lights, smoke, accessories need to be included in the measurement. What is also important is the conditions, so some relative humidity, temperature, and friction between the wheels and track needs to be specified. The last is also a tough one.

The idea here, is that this is the worst case condition that the locomotive will ACTUALLY run under. This is the worst case current draw for the interface that could be maintained forever.

 

Locked Rotor Stall current.

Also known as full stall current. This is a condition where the motor in the loco is not turning. Applying power here produces a very high current. There are 2 cases where this occurs. One is mechanical interference, i.e. a gear breaks, a siderod comes loose and jams. The motor cannot turn. Under this condition, the locomotive cannot move, it is non-functional. Remember this for later.

The other condition is for a very short time, when a motor starts from a dead stop, you CAN draw the Locked Rotor Stall current. It is normally only for 1 millisecond or less. As soon as the motor starts to turn, the current rapidly goes back to your Average running current. This is important nontheless, since, for an instant, a very large amount of current is drawn. Unprotected electronic components could be damaged. 

 

At this point, before going further, it is very important to make the following observation, given to me by a very experienced person: You need to be very careful with these measurements. These measurements on a new locomotive can be very different from a locomotive with more hours on it.

He noted the aging of the drive train, aging of the motor itself, etc. One very important observation was the effect of the change in friction in drive wheels. Most locomotives are supplied with nice smooth, shiny drive wheels. They can slip much more easily than ones that have worn in a bit, or the plating has worn off. He indicates that the difference in pulling capacity of a loco might as much as TRIPLE from new to "broken in". This can have a great impact on the current draw, sometimes as much as 50% MORE at "full slip", which is a very important data point.

 

Now on to what all this means to a design:

What is the interface designed to?

I see two possibilities:

  1. Design the interface to continuously supply the full locked rotor stall current.
  2. Design the interface to continuously supply the full slip current.

(Note these currents have to be adjusted for locomotive "aging")

Idea number one sounds right to most people at first glance. Yeah, if this can happen, then build it bulletproof! The problem is that you are necessarily overbuilding something for a situation where the locomotive DOES NOT RUN. Furthermore, at some point, you would probably want to provide protection from a dead short. What is the benefit? I cannot see any, if you assume you also provide short circuit protection.

Idea number two sounds more promising. Supply the "worst case" current that can be drawn (based on measurements, adjust for aging, and put in maybe a 20% overhead). If you go over that current, shut down the power. Something as simple as "thermal switches" which are inexpensive can do this, properly specified and designed.

For example, you might have a locomotive with the following characteristics: average current about 1.7 amps, full slip 2.5 amps, and locked rotor 12 amps. (I have several such locos).

I would suggest the interface (in this case) would handle somewhere between 3.5 and 4 amps continuously, and disconnect power if it exceeded 5 amps for any length of time, like over a few seconds.

Determining the "right" limits for a standard interface means considering all locomotives on the market, and those likely to come on the market.

Consideration of what motors will "evolve" to is also important. One major decoder manufacturer has indicated that "high efficiency" motors are essential to the evolution of his products.

High efficiency motors have better low speed control, are more easily "read" by BEMF circuitry to increase very low speed running, use less electrical power for the same mechanical power output.

The downside is that they can have very high stall current ratings.

So, we have our work cut out for us.

The next step is to collect all the data possible on existing motors in use, especially high efficiency motors. We also need to work on some specification of how to measure full slip current.