Field wound K&D open frame motors

[SWrailroader]

I would be interested in hearing about people’s experience with field wound open frame motors often found on the older steam from Lobaugh, Scale Craft, Hines, etc. Usually I swap them out for a Pittman, but occasionally I like to keep it stock. Yes, I know they are current hungry like 2 amps and they produce ozone. Most that I have encountered seem pretty robust except one recently in Scalecraft SP 4-8-2 was pretty weak despite looking in good shape.

[bob turner]

Check the rectifier. Wire for forward only and note the performance. No magnet to get weak, so these things go until the bearings or brushes die.

[up148]

No help from my end. All my old models have been converted to can motors.

[bob turner]

We had a 4-4-0 at the museum - scratchbuilt. An interesting model,but the gears gave out. Back then, NWSL had only the small Mod 0.5 box, so I put that in (long story short, once I put ball bearings on the worm shaft the thing did literally years of nonstop daily operation).

But the story here is while the gearbox lasted seemingly forever, we went through DC-91 Pittman about once every six months.

I finally went to a K&D (we simply ran out of the DC motors). That went for years, until the bearings got so much slop the armature started hitting the stator. Of course we had truckloads of the series-wound motors laying around.

[SWrailroader]

Bob,
Thanks for the great info. While the old stuff is less efficient, it may be more robust. This is very useful info!
Rob

[R.K. Maroon]

So here is my two-cents worth:

It is my experience that wound-field "universal" motors do not start as smoothly has permanent magnet motors. This may be an inherent property of the electromagnetics, or it may be because PM motors, especially PM can motors, were designed later and benefited from advances in bearings. As such, I would be inclined to replace a wound-field with a can in any engine used for switching. However, I find wound-field motors to be perfectly fine for operations at speed, so I never am tempted to replace a perfectly good wound-field motor with a Pittman or other can in a mainline locomotive. Note, in support of my argument, that one generally cannot tell what motor is in a model by observation of operation at speed.

In truth, I have never understood why so many people yank out the K&D and install a Pittman. It often seems reflexive -- that is, done without any sound justification. It's not even a trivial task, as the replacement motor almost always has a different bolt pattern than the original. Further, it is actually asking for trouble, in that modern can motors produce more torque. More torque sounds great, as long as the gears in the model can handle it. They may, but you can bet they were sized for the old motor, not the new one.

It should be noted that wound-field motors often get lumped together in the modeler's mind with open-frame PM motors. They are similar in construction but are definitely not the same. Wound-field motors were mature and reliable when the early open-frame PMs showed up. Many of these PM motors were junk, and I would bet that their well-deserved reputation tarred their wound-field cousins. This is speculation, but I would bet a lot of money that more than a few of the modelers at any random train show couldn't tell you the difference between the two.

One caveat to that last paragraph: Those concerned with sound systems are rightfully concerned about noise. It is undeniably true that a high-quality can motor runs noticeably quiet when compared to a K&D wound-field motor. However, the problem is the open-frame construction -- not the wound-field. Anybody with a vintage All Nation, CLW, or KTM locomotive with an original open-frame PM motor can verify this.

As to maintenance, if a model arrives with a lot of miles clearly on it, I remove the motor (usually but not always a K&D) and disassemble it for cleaning. I have collected several boxes of spares which I occasionally mine for new brushes, bearings, or other parts. It is a small detail, but the absence of a permanent magnet makes disassembly and reassembly easy.

Finally, I somehow came to think that wound-field motors were more efficient running on AC than DC. I mentioned this in conversation with a fellow at a train show once and he was skeptical. It turns out he was a PhD in electrical engineering and sent me a well-constructed proof that showed that the motor was equally efficient at DC as AC. I was glad to know it, but the damage to the ego persists. We mechanical engineers don't like to be proven wrong, especially by an EE.

Jim

[bob turner]

Efficient? We really don't care about efficiency. The voltage drop across the rectifier means we need a higher voltage to attain rated speed. SW may have a bad rectifier, which would further slow the motor.

The old rectifiers were Selenium. Replace those with a good Radio Shack bridge rectifier.

Anybody who runs enough trains to care about how fast the household meter runs doesn't have time to eat or sleep.

That said, I have a lot better luck with the 8x34 motors, and that's what is in the MM and Suburban.

[Rufus T. Firefly]

....Radio Shack....

Does the current incarnation of Radio Shack even sell such stuff?

[SWrailroader]

Jim and Bob,
Thanks for the very helpful discussion. My K&D did not come with a bridge. I almost wire them to run in the forward direction. The brushes look fine. I surmise that it needs to be disassembled, cleaned up and relubed. This is all info that I will never forget.

Rob