Pulse switch

[jambo101]

I have an MRC Tech2 Railmaster 2400 DC controller and i've always wondered what that pulse switch was for as nothing seems to happen when i move the switch.

[hickstmj]

From NMRA:
The next step is to take a pack and add a switch for "Pulse." In a pulse throttle, half the a.c. wave cycle is used to produce a rapidly pulsating d.c.; a switch controls whether this is on or if the regular rectified current is used in the output. A good example of this throttle type is the MRC Throttlepack 501, which is a strong pack delivering 1.9 amps and has the pulse option. The reason for the pulse feature is better low- speed control, and it does work. In a sense, the pulses "push" the motor armature to help overcome inertia and friction. However, there is a fly in the ointment. The half-wave pulses cause extra heat to build up in the motor. Too much of this heat can cause the motor to burn out; some types of motors are more susceptible to this than others. While this isn't that common a problem, you should be aware of it. Pulse power should not be used for high speeds (it isn't needed there), and any loco that runs too hot may have other problems as well, (lubrication, tight gears).

[Russ Bellinis]

Pulse is generally not recommended for use with can motors. The sealed can tends to hold heat in so they will burn out more easily than open frame motors. If you have Athearn blue box locomotives that have been converted to can motors, or Atlas, Kato, P2k, Athearn Genesis or r-t-r, or other of the newer locomotives, they will have can motors installed and pulse is not needed. The main thing the pulses do is kind of give the armature a series of "kicks" to get the motor turning at very low speeds. That way your locomotive starts smoothly at low throttle settings when switching. If you have a stock train set engine from an old dept store train, you may have noticed that you need a lot of throttle to get it moving and then it seems to jump from a stop to high speed immediately. That is the sort of thing that pulse was designed to eliminate. Can motors start turning as soon as power is applied.

[pgandw]

Originally Posted by Russ Bellinis

Pulse is generally not recommended for use with can motors. The sealed can tends to hold heat in so they will burn out more easily than open frame motors. If you have Athearn blue box locomotives that have been converted to can motors, or Atlas, Kato, P2k, Athearn Genesis or r-t-r, or other of the newer locomotives, they will have can motors installed and pulse is not needed. The main thing the pulses do is kind of give the armature a series of "kicks" to get the motor turning at very low speeds. That way your locomotive starts smoothly at low throttle settings when switching. If you have a stock train set engine from an old dept store train, you may have noticed that you need a lot of throttle to get it moving and then it seems to jump from a stop to high speed immediately. That is the sort of thing that pulse was designed to eliminate. Can motors start turning as soon as power is applied.

Not so sure I agree 100% with you, Russ. Pulse power does create some extra heat, but certainly no more than operating a motor at lower speeds using the pulse width modulation which is standard with DCC, and probably quite a bit less.

Can motors do have less cogging than a conventional open frame motor because the magnets are curved in a cylinder around the armature. (As a side note, can motors with flat sides give back some of that even magnetic field to install in tighter spaces. Skew wound armatures, regardless of open frame or can, do a lot to reduce cogging, too.) And can motors do have less ability to dissipate heat. But can motors have higher resistance windings, and operate on lower current, so they produce less heat, too.

Coreless motors are the problem area with both pulse power and DCC. Because they do not have iron mass in their armatures, they do overheat very easily when operated with pulses and/or loads.

Both present-day can and open frame motors do not have overheating problems on the various types of pulse power, provided a reasonably free running mechanism. Some older, less efficient can motors with poorer magnets might get hot with pulses, but so do open frame motors where the magnets no longer have their intended strength.

Conventional pulse power will not produce the same top speed as full wave DC because the average voltage is less with every other pulse removed.

hope this helps

[Russ Bellinis]

Thanks Fred, I was repeating what was conventional wisdom regarding pulse back in the early 1980s from the various Model Railroad mags. I didn't realise that things had changed since then, but I should have.