I see the above mentioned almost everytime someone has questions about installing a decoder in a locomotive. Is there a cut-off point where the test tells you it is time to replace the motor?
NP 2626 "Northern Pacific, really terrific"
Northern Pacific Railway Historical Association: http://www.nprha.org/
I got into this discussion when converting some Proto 2000 LifeLike diesel locos. I seem to remember that some early LifeLike PAs were notorious for higher amp motors and they could be an issue with at least some decoders. In my case, I luckily got some next generation PAs where the stall current measured 0.7 amps as I recall. That fit ok with the LokSound Select I was installing, with 1.1 amp rated motor output. And, as someone pointed out, it would be unlikely to achieve stall on the layout as the wheels would slip first.
Another aspect is how you run the layout and your booster(s) output. I like to run consisted diesels, so often have two trains with dual locos going. If all were going with heavy loads at once, it still would fit easily with my 5A NCE. But you can see that three or four such trains, on a lower amp starter system, could in some cases hit the booster limit. Of course added boosters with sub-districts would solve that.
Paul
Modeling HO with a transition era UP bent
NP2626I see the above mentioned almost everytime someone has questions about installing a decoder in a locomotive.
The stall current of a motor, when the motor is unable to turn with maximum voltage applied, is the maximum current the motor will draw. A decoder should have a current rating greater than the stall current in order to prevent damaging the decoder.
NP2626Is there a cut-off point where the test tells you it is time to replace the motor?
Replacing the magnets can make a big difference in motor current with the motor turning. The maximum current load is when the locomotive is blocked from moving while the wheel continue to turn. An advantage of open frame motors.
I don't understand why stronger magnets would reduce the stall current.
greg - Philadelphia & Reading / Reading
Should make no difference with stall current. Stall current is completely detemined by the resistence of the motor windings - which is determined by wire gauge used and how many turns were used.
Interesting to not in a 1946 MR ad where Pittman introduced their DC60 and DC70 motors, the larger DC70, while being listed at the same armature resistence as the smaller DC60, actually drew less current when running freely than the DC60 - .3 amps for the DC60 vs .25 amps for the DC70 - but the DC70 had a much larger magnet.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinker the larger DC70, while being listed at the same armature resistence as the smaller DC60, actually drew less current when running freely
the spining motor is a very interesting electrical device
when voltage is applied with the motor still, I agree with Randy that the current is limited by the armature resistance.
but the current creates a magnetic field opposing the field of the magnet, creating a torque and causing the motor to turn.
when the motor turns, the windings cutting through the magnetic field of the magnets induce a voltage in the windings (i.e. BEMF).
This induced voltage opposes the voltage applied externally to the windings, which reduces the total voltage across the windings and and the current (less armature voltage, same armature resistance).
with less current, there is less torque reducing the force causing the motor to spin. Ultimately, there is equilibrium point and current where there is sufficient torque to oppose the friction of the motor.
with a load applied, the motor slows, the BEMF reduces, the current increases until a new equilibrium is established where the torque equals the load on the motor plus the negligible friction.
A stronger magnet and field would create a greater BEMF for the same motor RPM resulting in a smaller current. I'm guessing the motor would operate at a lower RPM with less BEMF and greater current to create the needed torque.
I'm also guessing that while stronger magnets reduce current, they also result in slower speed but more efficient motors.
Wow, Greg, you remember lots more of physics (and how it applies) than I! My excuse is I was a chemical engineer. But chemistry was easy in my day as there were only 12 elements discovered by then.
gregc....The maximum current load is when the locomotive is blocked from moving while the wheel continue to turn....
I'm not a DCC guy, nor an electrical whiz, but I have to disagree with the definition of maximum current load shown in the link. The maximum occurs just before the wheels begin to slip, not while they're slipping. When the wheels are slipping, the only physical load is the weight of the locomotive, which is much less than that weight plus the load of a trailing train.
This locomotive weighs 32oz. and has a large can motor. The tender weighs another 12oz. It can pull a 100oz. train up a curving 2.5% grade, the equivalent of 4.85% grade on a straight track.
The loco is at it's limit, though, and is labouring, with some wheelslip. If I add just one more car, the locomotive will actually stall: the train stops, the loco stops, and the loco's wheels and motor stop turning, too. In decoder country, I believe this is where the smoke is let out.
The locomotive isn't mine, and I don't have it here to test. Next time the owner visits, though, perhaps I'll test it again and take a look at the ammeter before shutting things down. If it were mine, and running regularly, I'd remove an ounce or two of weight, which would let the drivers slip if a train were too heavy.
I've also managed to stall a Tyco C-430 - that was the one with a pancake motor on only the front truck. I added quite a bit of weight to this one, too, and it really improved the running qualities - smoother starts and stops, and no shimmying while running. However, it didn't take more than 4 or 5 trailing cars to stall it. I ended-up removing quite a bit of weight, but sold it, along with most of my other diesels.
Wayne
To get back to the original intent of this thread, as long as the maximum amperage rating of the decoder is not reached, it's best then, if the stall current of the motor is less than the max. amperage of the decoder?
Since locomotives rarely stall their motors in real life, wouldn't a more precise test be to take the amperage reading for the loco when it has stopped moving and is spinning it's wheels? To actually stall the motor, we need to press down on the locomotive with our hands, something that will never happen on the layout.
NP2626 wouldn't a more precise test be to take the amperage reading for the loco when it has stopped moving and is spinning it's wheels? To actually stall the motor, we need to press down on the locomotive with our hands, something that will never happen on the layout.
that's what the link I posted earlier describes as the "maximum load" (at least by Lenz) in the text I copy below. But what if the wheels get jammed up and stop turning? Wouldn't the decoder be subject to damage because the stall current is greater than the maximum load current and the decoder was selected based on the maximum load?
I guess, although I would consider it a real rarity, occasionally a loco could lock-up, due to rod failure (Steam) or gear failure (both steam & diesel) and cause a spike in amperage damaging the decoder, if stall current was higher than the decoder’s max allowable amperage. For this to happen, you would have to be running the loco at full throttle, which isn’t likely, at least for me.
rrinker Should make no difference with stall current. Stall current is completely detemined by the resistence of the motor windings - which is determined by wire gauge used and how many turns were used.
Rich
Alton Junction
richhotrainHow does the number of turns, or windings, of the copper wire on the armature affect the operation of the motor?
I believe that motor torque is proportional to current. Current is proportional to the voltage and inversely proportional (i.e. decreases) with resistance.
the more windings, the more resistance and less current. The larger gauge the wire, the less resistance.
I thought I read that some RC car motors have only a dozen or so windings of heavy gauge wire. This draws lots of current to give them more torque. (it's also dependent on armature diameter and magnet strength).
So there are design tradeoffs that balance the size of the motor (winding diameter) to provide sufficient torque with some limit on current and desired RPM.
I hope someone can confirm this. It's been a few decades since I took the motors course.
gregc rrinker the larger DC70, while being listed at the same armature resistence as the smaller DC60, actually drew less current when running freely the spining motor is a very interesting electrical device when voltage is applied with the motor still, I agree with Randy that the current is limited by the armature resistance. but the current creates a magnetic field opposing the field of the magnet, creating a torque and causing the motor to turn. when the motor turns, the windings cutting through the magnetic field of the magnets induce a voltage in the windings (i.e. BEMF). This induced voltage opposes the voltage applied externally to the windings, which reduces the total voltage across the windings and and the current (less armature voltage, same armature resistance). with less current, there is less torque reducing the force causing the motor to spin. Ultimately, there is equilibrium point and current where there is sufficient torque to oppose the friction of the motor. with a load applied, the motor slows, the BEMF reduces, the current increases until a new equilibrium is established where the torque equals the load on the motor plus the negligible friction. A stronger magnet and field would create a greater BEMF for the same motor RPM resulting in a smaller current. I'm guessing the motor would operate at a lower RPM with less BEMF and greater current to create the needed torque. I'm also guessing that while stronger magnets reduce current, they also result in slower speed but more efficient motors.
All this is interesting, but since the torque produced by the motor is a function of the current in the armature and the strength of the magnet, wouldn't the simple answer be that a stronger magnet requires less current in the armature to produce the same torque?
I have the right to remain silent. By posting here I have given up that right and accept that anything I say can and will be used as evidence to critique me.
richhotrain rrinker Should make no difference with stall current. Stall current is completely detemined by the resistence of the motor windings - which is determined by wire gauge used and how many turns were used. I wish that I understood more about these motors. So, let me ask a dumb question. How does the number of turns, or windings, of the copper wire on the armature affect the operation of the motor? Rich
I wish that I understood more about these motors. So, let me ask a dumb question. How does the number of turns, or windings, of the copper wire on the armature affect the operation of the motor?
It comes down to the same with any wire - a smaller diameter wire has more resistence than a larger diameter wire. A longer length of wire has more resistence than a shorter length of the same size wire. For a constant voltage, if the resistence goes up, the current has to go down: E=IR
Lower current means less power thoough, since power is dependent on current. SO as mentioned, you have those 'hot' slot car and RC car motors that have fewer turns of heavier wire. This is less resistence than many turns of thinner wire, so the current has to go up. More current = more power, but also quicker battery drain (in the case of an RC car).
The physics of it gets into the magnetic flux that is foormed around any conducting wire. More current = stronger magnetic field. Combined with a stronger permanent magnet field, and you get even more power. This is why replacing an old weak magnet in the motor results in it drawing less current - for the same amount of power, you need less armature field strength to work off the stronger fixed magnet field and since the resistence is fixed by the size and number of turns, and the voltage is fixed, the current goes down. In truth, since the power output of the motor goes up, you need to turn the throttle up a lot less to get the same work out of the motor, so what's really happening is the voltage is dropping, which means the current drops. Say with the old magnet, you need to turn the throttle up to 5 volts just to get the loco to move. With the new magnet, you only need 2 volts. Say the windings are 100 ohms. At 5 volts you have 5=Amps x 100, or Amps = 5/100 or .05. At 2 volts, you have 2= AMps x 100, or AMps = 2/100 or .02
NP2626 To actually stall the motor, we need to press down on the locomotive with our hands, something that will never happen on the layout.
I agree with you, except in the case of something that might have a traction tire. I'm wondering if the Tyco C430 mentioned above had one of them. I don't know how easily something like that would slip its wheels. Throw some weight in the engine and it may not slip at all, but just stall the motor.
No traction tire on the C-430, just too much weight added to the shell.
betamaxMore flux, more torque. Old electro-mechanical electric meters spun faster as the current flow increased the flux, increasing the torque on the disk.
Hense the "Flux Capacitor"?
There's no better flux than GERN brand flux - makes everything 3% better:
Just add GERN and your motor's current draw will drop 3%, better for DCC AND DC operation.
Reminds me, I need to pick up a can of GERN to add to my gas tank, I'm about to head off on a long drive and that extra 3% mileage could come in handy.