Just to satisfy my curiosity…
I’ve seen this term used a lot, but what exactly is it (with respect to locos)? What does it mean, and how do you measure it?
Thanks.
“Stall current” is a term to describe the amount of electrical draw the motor pulls when the motor is locked into a “stall” condition…that is, the motor won’t turn, like if you gripped the armature shaft with pliers to prevent it from turning, and apply current. The “stall current” can be measured with a amp meter inline with the power supply to motor circuit.
I hope that helps!
Rotor
The correct way to do it is to turn your controller/power pack up to max so that when you place your loco over the rails and allow the wheels to make contact with them, the wheels will begin to spin. You then quickly press down on the loco just enough…just enough…to cause the wheels to bind to the rail and stall the motor. Look quickly at your ammeter, and there is your stall current. Lift your loco off the rails right away, or you could cause the motor to heat up and seize.
Stall current was promoted by the early proponents of DCC as a way to determine the current rating requirement of a decoder, so the decoder manufacturer couldn’t be held liable and be expected to replace a burned out decoder.
But how likely is it that you will ever deliberately push down on your locomotive until the wheels stop spinning, and thus possibly overload the DCC decoder?
what about when you hook on 137 cars to engine and the wheels wont turn?
You anticipate, with the laws on warranties behind you, that those who installed the decoder were wise enough to choose one that was able to handle the stall current. It if was yourself, then you provide the warranty. Simple, no?
Stall current also relates to “starting current”. If you apply full voltage to a stopped motor the inital inrush current will equal stall current. As the motor starts to turn back EMF is generated and the current deduces to “run current”. With large motors stall current is used to size motor starters. Since stall current (most often called starting current) can be 6 to 10 times run current the motor starter will not trip when starting but can still provide protection if the motor stalls while running. Not much of a problem in model railroading.
Keep in mind that with the PWM control by DCC decoders, you rarely if ever would see the stall current load on the decoder unless it was running wide open and it truly did stall. Since the voltage is not applied in a continuous fashion, the startup current is not going to be the same as the stall current, with DCC.
And if you put 137 cars on your train and the wheels don’t turn - get rid of the traction tires. The loco should always be able to spin its wheels if restrained by drawbar load.
–Randy
Thanks for the help, everyone. From this info, it sounds like it’s something that’s used mainly to pick a correctly-sized DCC decoder. Unfortunately, I don’t use DCC (at the moment), so I guess it’s not something I really need to be concerned about right now… unless maybe I had a really wimpy power pack without overcurrent protection.
So stokesda got his answer, but my question is how can you find the “stall current” without damaging the motor? I’m fairly confident that a DH143 or DH163 will work in my IHC 4-6-4 Hudson, but how do I know? The single sheet of documentation that came with my Hudson lists the motor this way:
part # 49…motor (complete)…136-930
which tells me I have a complete motor, but nothing else. Can someone give a brief overview on how to determine what the voltage draw is, how to find the stall current, and how to convert that information into something that I can use when searching the decoder pages that say the decoder handles 1.5 amps…What does that mean?
IHC locomotives draw very little current because they have can motors in them. Even the stall current of that IHC Hudson would probably only be around 1.5 Amp. The TCS T-1 decoder is rated at 1.5 Amps continuous, 2 Amps peak, so it would be more than sufficient for practically anything you wanted to put one into. If the current rating of the Digitrax decoders you have is 1.5 Amps continuous, use one of them.
The motor does not draw voltage. It draws current based upon the voltage placed across it, the rotational speed of the motor, and the load on it.
Stall current is as has been stated, it’s the maximum current you can force through the motor, which in this case occurs at maximum voltage with the motor stalled. You’re safe to do this test for a few seconds. It’s not like the motor will immediately burn up, but you should let it go within a few seconds. Don’t repeat the test any time soon either, as the windings in motor will heat up pretty good. Give it some time to cool down.
FYI, the MAXIMUM possible current going through a motor is when you force a change of direction. Maximum speed forward switched to maximum speed reversed under load without first stopping the motor. Not good in any situation, but kids will do it if you let them.
Momentum settings will help to minimize these potential disasters.
Mark in Utah
The “weigh down till it stalls” method works, but it’s hard on the shell. I’d recommend one start by removing the shell and/or weight so that you can get at the motor or at least the flywheels.
Next connect the motor to a power pack, with an ammeter in series between them. Fix the motor so that it cannot turn (I usually just hold the flywheel with my hands) and gradually turn up the powerpack to maximum voltage. Quickly note the current showing on the ammeter and then turn off the power supply.
This is the maximum current the motor can draw running on DCC (DCC won’t generally do “snap” reverses as described above). Select a decoder with a peak current rating equal to or greater than this value.