dc motors

[Anonymous]

Thanks for you explanation. Makes sense. The idea of having prototypical response in the motor is interesting, although I would think it works better in larger scales where the engines are heavier and inertia comes into play more than in smaller scales such as H0.

/Ricard

[lionelsoni]

A permanent-magnet DC motor running on a constant voltage will slow down and pull harder as the load increases. The slowdown is accompanied by a great increase in the power dissipated as heat in the motor. A series motor (such as the universal motors traditionally used in toy trains) will slow down more, but will not overheat nearly as easily.

A permanent-magnet motor is a generator as well as a motor. The voltage that it generates is proportional to its speed. (It would also be proportional the field strength; but the permanent magnet produces a constant field strength.) The difference between the self-generated voltage and the voltage that you apply to the motor causes a current to flow through the armature, limited only by the low resistance of the armature winding. That current produces a torque that tries to change the motor speed until the two voltages are equal. If the load keeps that torque from making the voltages the same, the armature current heats up the armature.

A series motor has the same current flowing through the armature and the field, which therefore interact in a complicated way. At any speed, part of the applied voltage is across the armature and part across the field. With no load at all, the motor would accelerate up to an infinite speed. At any particular speed, the armature and field voltages sort themselves out, and the motor pulls the load without having to draw harmful currents. Since the torque is proportional to both the armature and field currents, it is porportional to the square of the motor current. This ability to adapt to a variety of speeds when run from a constant voltage is the reason for using series motors in prototype locomotives.

Permanent magnet motors run from constant voltage and not overloaded are handy for making a train run at a steady speed without operator attention. The modern control schemes do even more to control speed tightly. I personally dislike this effect and prefer the more realistic situation in which the train speed responds to the load.

[Anonymous]

When you say "it will adapt its speed to the load.", does that mean it will (try to) increase speed with increased load, or decrease speed with increased load? Or put in another way: A small permament-magnet DC motor will loose speed when the load increases, will a series motor do this to a lesser or larger degree?

/Ricard

[Anonymous]

Thanks, Bob.

Bob

[lionelsoni]

Yes, the motors are isolated (at least, all that I've ever seen). Because of the bridge rectifier and unlike the traditional universal motors, they must be isolated from the locomotive frame (if any). Dual-motored locomotives normally have the motors themselves wired in parallel; but, if you should happen to want especially slow operation, you can rewire them in series to cut the speed in half and reduce the current draw.

Actually, one end of the field winding of a universal motor is usually soldered to a lug on the motor to ground it. But you can easily unsolder it and completely isolate the motor. It won't behave like a permanent-magnet DC motor, however, which tends to run at a constant speed proportional to the applied voltage. Instead, it will adapt its speed to the load, just like the prototype motor which it is a miniature version of. You would also have to insert a bridge rectifier in the field winding or the armature, not both, to be able to reverse the motor with the polarity of the applied DC voltage.

[Anonymous]

No, I didn't. I was wanting to put dcc and soundtraxx in one. Guess I would take out the rectifier. My concern is makinfg certain the motor is isolated.

Thanks for the info.

Bob

[lionelsoni]

Bob, you do understand, don't you, that the modern Lionel and MTH locomotives with DC motors also include rectifiers and are almost always run on AC?

[Anonymous]

Thanks, Ben. I was thinking of buying an MTH switcher at the local hobby shop or ordering a K-Line docksider.

Bob

[ben10ben]

Yep, except in the "Traditional" engines from their Tinplate Traditions line.
As does K-line, Williams, Weaver, and Sunset.

Lionel is the only train company still using universal(Pullmor) motors, with the possible exception of companies making very accurate Prewar reproductions/

[Anonymous]

I know Lionel and Atlas do. Could some one tell me if MTH uses dc motors?

Thanks. Bob