Electric Motors and Measuring Stall Currents

Often when I am writing about fitting DCC decoders into locomotives I recommend checking the stall current of the motor.  This should be done to make sure the DCC decoder is up to the job.  However I don’t think I have ever fully explained what this means or how to do it so this week’s post will be about exactly that.

Although DCC system supply a form of alternating current, AC, to the track, the electric motor still runs on direct current, DC.  The DCC decoder will convert the AC into DC using a bridge rectifier and supply the correct amount of voltage to make the motor run at the desired speed.  The higher the voltage, the faster the motor runs.  But what about power?  Simply adding more volts alone will not make the locomotive pull a heavier train.  The answer is current. Every electrical device will draw a current which is measured in amps.

Without going too deep into the mathematics behind all of this, current can be explained in a simple equation: the current ‘I’ in amps (A) is equal to the voltage ‘V’ in volts (V) divided by the resistance ‘R’ in ohms (Ω):

Ohm's Law

So, for example, a train running at slow speed, light engine, will have little resistance and may pull 0.2 amp at 6 volts.  Add a heavy train and the motor now has a lot of resistance so it might pull 0.5 amp but still at 6 volts.  As the resistance is increased, adding more freight cars for example, the current draw will also increase until one of two things will happen.  Firstly, and most commonly with N Scale, the locomotive will start to wheel spin as the resistance, in this case friction, between the wheels and the track is weaker than the motor.  The current draw will drop off but the train won’t be going anywhere.  Secondly, the motor will stall.  This means that the motor will draw as much current as it can but simply cannot spin anymore because the train is too heavy and the friction between the wheels and rail is too great.  This might happen if you have good traction tires on your wheels or something gets stuck in the gears.  When a motor stalls like this the current draw will peak sometimes up to and over 1 amp and it’s this that can damage a decoder.

The electrical components in a DCC decoder are only designed to take a certain amount of amps through all the tiny wires and connections.  This is because high amperage draws cause a variety of issues, one is heat. This is normally dealt with by using bigger wires and components.

All DCC decoder manufacturers state what their decoders are capable of handling.  For example, below is the instruction manual for a Digitrax SDN136PS sound decoder; I put these into my C-855 locomotives.

DC Stall Test 1

The manual says the chip has a 1.0 Amp /2.0 Amp peak capacity.  This means that the normal operating current draw that this chip can sustain is 1.0 amp and for short periods it can sustain a peak of 2.0 amps. Anything over this will damage the decoder or cause it to shut down.

So how do you measure the stall current to see if your chosen decoder will work with your motor? Well, you’ll need some wire, a DC controller and one of these…

DC Stall Test 2

It’s a multi-meter.  It doesn’t have to be an expensive one; it simply has to have the ability to measure current up to at least 2 amps.  This particular one will measure up to 10 amps, so it will do nicely.  The red wire is plunged into the hole marked 10A and the black into the common.  The dial is rotated to the red 10A marker and you can see below it’s reading 0.00 amps.  It’s now ready to use.

DC Stall Test 3

I should point out – DO NOT do this with a locomotive that has a DCC decoder already installed as you may do damage to the decoder.

Using a section of spare track which is not connected to anything else, connect one wire from your DC controller to one rail.  Connect the other DC controller wire to the black multi-meter wire.  Lastly, connect the red multi-meter wire to the other rail.  Now when you put a locomotive on the track and run it up and down, the multi-meter will display the current the motor is drawing.  Normally with DC locomotives this will also include any current draw from lights as well.  Remember the max current draw of the decoder will be for everything, not just the motor.

The main reason for doing all this was to measure the stall current of the motor and to do that you will need to find a way to stop it spinning when it’s under full power, i.e. full throttle on your DC controller.  With N Scale this can often simply be done by removing the locomotive shell and stopping the motor with your fingers, although I would not recommend doing this with larger HO and O scale engines as they have some big motors!

With the motor ‘frozen’ between your fingers and the power on, the multi-meter should be reading the max current draw from the motor.  If this value is higher than the manufacturer recommends for the decoder then it will not be safe to use it.

Normally with N Scale locos the stall current is about 0.6 to 0.7 amps and with a few LEDs it may go up to 0.9.  Add sound and it could be up to 1.5 amps but as long as that is below the manufacturers specification than it’s still safe.

This has been useful when I’ve wanted to run two motors from one decoder. For example, my Bachmann F7s, which you can read about here.  They have two decoders for four locos.

Bacmann F7 DCC Install 36

Next week I’m going to share with you some of my newly-weathered stock, I just hope my photos do them justice!