Motor Poles - How Many Do They Have?

[KenS]

After seeing a couple of recent posts mentioning Kato's "five pole" motors, I realized we didn't actually have a thread about this, just a few mentions on threads about other topics.  So I thought it would be useful to start a thread where we could share our knowledge of which models have which kinds of motors (as well as some discussion about how much this actually matters).

 

I looked into this for my mostly Kato EMU fleet a few months ago, and was rather surprised by what I found.

 

After taking apart a number of my Kato EMUs, a DE10, and a couple of other locomotives, I came to the conclusion that Kato in the last six or eight years has been using two motors: a 5 pole version with straight windings that appears in all of the "not DCC Friendly" EMUs I own (without flywheels), and a 3 pole, skewed winding model found in newer designs (equipped with flywheels).  Both are presently being sold.

 

Although I only took apart two of Kato USA's products (a new GG1 and a slightly older SD80MAC) both used the five-pole motor, but equipped it with flywheels.

 

I took apart one Micro ACE EMU (the new "strengthened skirt" Sobu E231) and found it also had a three-pole, skewed motor (with flywheels).

 

I began to suspect that in N-scale, you just couldn't build a 5-pole, skewed motor. But it turned out I had a Walther's model (a GP38 I'd bought on a clearance table for layout testing) that had one.  What's interesting is that this is in one of their Proto N Life-Like engies often said to be Kato clones, and the hardware and circuit board do look identical, only the motor is different.

 

Which all begs the question: how much, if at all, does this matter to performance, and how much of it is pure specsmanship?  Both more windings and skewed windings can help low-speed performance, but more poles increase the space wasted between poles, which produces no power, and to some extent thus hurts low-speed performance a bit and probably reduces pulling power more. One thing both are good for is reducing the tendendcy of the motor to want to stop between poles (called "cogging torque"), but flywheels help counter that by storing rotational momentum to carry the motor past the gap. Flywheels are also helpful for maintaining constant speed on dirty track or through tournouts with unpowered frogs.

 

My guess, and it's only a guess, is that "5 pole" has marketing cachet in North America, which is why Kato and Walthers made the choices they did here.  But that both Kato and MA see the three-pole, skewed, flywheel-equipped design as superior.  There may also be a difference in pulling power, as an EMU or DE10 has a limited number of cars to pull, while a North American freight locomotive is expected to be able to pull long strings of cars. But I'd expect three poles to be superior to five for that (and I believe skewing also reduces power, so the Life-Like 5 pole, skewed, design would appear to be even worse in that sense).  One of these days I need to do some tractive-force measurements.

 

So, anyone taken apart any other Japanese (or other) trains and can comment on what was in them?  And thoughts on what matters and why?

 

Photo 1: Kato's older "not DCC Friendly" EMU motor: 5 poles, straight windings, no flywheel

Photo 2: Kato's "new" motor (this one from a DE10): 3 poles, skewed windings, flywheels

Photo 3: Kato USA motor: 5 pole, straight windings, flywheels

Photo 4: Micro Ace: 3 pole, skewed, flywheels

Photo 5: Walthers Life Like: 5 pole, skewed, flywheels

[The_Ghan]

Great post Ken,

 

Could this be a power v torque arguement?  Maybe it is simply a matter of keeping the cost of DCC friendly models down.  We have a limited number of speed steps with DCC anyway, so maybe the smooth speed transitions of a 5 pole motor are lost when you add a decoder.

 

One thing is for certain, it appears to demonstrate that there are horses for courses.

 

Cheers

 

The_Ghan

[Bernard]

I never thought (or realized) the "skewed" pole motor. What are the added benefits, if any, over a straight pole?

BTW - Ken, great thread and nice research! 

[Martijn Meerts]

The skewed pole motors have much less chance of stalling, because there's practically no dead zones. A 3 pole skewed is likely smoother than a 5 pole straight.

[grumbeast]

This is a fantastic post, thanks Ken!  I never realised the subtleties before, simply assuming that 5-pole is better, it now seems that it really is dependent on use.  I'm not sure i've ever seen a topic like this before

 

Graham