3 pole vs 5 pole motors

[don7]

I have been trying to research the specifications of a 3 pole motor vs a 5 pole motor.

A recent trhread on the Bachmann board indicated that Bachmann had replaced the 5 pole motors on their Spectrum 2-8-0's. The new 3 pole motors are supposedly rated the same.

 

http://www.bachmanntrains.com/home-usa/board/index.php/topic,4500.0.html

 

This is where I am confused, there apparently are a number of ways a 3 pole motor could outperform a 5 pole motor. The wire on the poles for example, if there are more windings and the wire was thinner this would make a substantial difference, if there was a flywheel attached makes a difference as well. In addition there is the aspect of the gearing. These

I am still trying to determine if this is a cost cutting measure or has Bachmann actually taken a step backword in quality in order to maximize profit?

 

 

[jeffrey-wimberly]

There's no way a three pole motor rates the same in performance (I've seen that thread also) as a five pole motor. I suspect Bachmann did this as a cost saving measure and is trying to blow smoke up everyone's caboose about it being the same. The three pole motor is going to be more jerky, this is why many locos that had them also had flywheels.

[don7]

There's no way a three pole motor rates the same in performance (I've seen that thread also) as a five pole motor. I suspect Bachmann did this as a cost saving measure and is trying to blow smoke up everyone's caboose about it being the same. The three pole motor is going to be more jerky, this is why many locos that had them also had flywheels.

If that is the case I will try to find a couple more of the older DCC ready Spectrum 2-8-0's rather than buy one of the new DCC on board engines with the 3 pole motor

I think the older 2-8-0's with the 5 pole are one of the best engines as far as running qualities go.  I  installed Digitrax decoders in the ones I have.

I would like a couple more but am hesitant to buy the newer ones with the three pole motors so will probably have to keep on eye on e-bay for them.

[Vail and Southwestern RR]

Over in Atlas land, a poster who enjoys tinkering, and experimenting did some actual investigation.  By the way, he started out assuming that 5 poles motors were always superior.  When he was done, his conclusion was a that good three pole motor performed as well as a 5 pole motor.  He was surprised, and disappointed, even.

I think the only fair thing to do is to run the actual loco, and judge for yourself.

[don7]

Over in Atlas land, a poster who enjoys tinkering, and experimenting did some actual investigation.  By the way, he started out assuming that 5 poles motors were always superior.  When he was done, his conclusion was a that good three pole motor performed as well as a 5 pole motor.  He was surprised, and disappointed, even.

I think the only fair thing to do is to run the actual loco, and judge for yourself.

I found that Atlas thread and there was certainly a lot of discussion between the two motors. The parties involved certainly made some points but not being well versed in the matter can only speculate. I was not convinced the 3 pole would be as good a runner as a 5 pole. 

I agree that as long as the engines have good running capabilites that is the most important aspect. However as none of the other manufactures such as LBI, Proto (Heriatage) or the newly re-released Mantua's have used a 3 pole I have my doubts.

Would be nice if someone with both the 3 and 5 pole Spectrum 2-8-0's could give us some advice on a comparison basis of the two motors. 

 

[Vail and Southwestern RR]

What I came away with (and if I remember right, the loco in question was a little one) was that a good three pole in a small space was better than a 5 pole compromised to fit into the space.  Also, the rest of the drive train, and the amount of reduction in the gears, could also make a big difference.  The bottom line, I think, is that you probably shouldn't try to make a blanket statement based on a single parameter.  The performance of the system is what counts.  A great five pole motor could be hooked up to a crummy drivetrain, and the performance will still be crap.

As they say, never say always.

[Vail and Southwestern RR]

Would be nice if someone with both the 3 and 5 pole Spectrum 2-8-0's could give us some advice on a comparison basis of the two motors. 

That would be the best way to tell.

[jeffrey-wimberly]

Going on what I know of my antique Bachmann diesel (I consider 30 years to definately be antique) which I remotored last year with a PPW (5-pole) can motor that there was a very definate and positive change in performance from the old open frame (3-pole) motor that it came with. There's no more jerky starts, the power consumption dropped way down (1.7 amps down to 0.4 amps) as well as a drop in the noise factor. I also have a couple of old Athearn diesels that were greatly improved after I pitched the old motors with red wire windings with the newer 5 pole motors with the copper colored windings. In every case there was a marked increase in performance and a marked decrease in power draw. To me, 5 poles beats 3 poles any day of the week.

[Vail and Southwestern RR]

Going on what I know of my antique Bachmann diesel (I consider 30 years to definately be antique) which I remotored last year with a PPW (5-pole) can motor that there was a very definate and positive change in performance from the old open frame (3-pole) motor that it came with. There's no more jerky starts, the power consumption dropped way down (1.7 amps down to 0.4 amps) as well as a drop in the noise factor. I also have a couple of old Athearn diesels that were greatly improved after I pitched the old motors with red wire windings with the newer 5 pole motors with the copper colored windings. In every case there was a marked increase in performance and a marked decrease in power draw. To me, 5 poles beats 3 poles any day of the week.

It seems to me, Jeffrey, that you are comparing fresh apples to rotten oranges.  Not surprising that the apples win!  I suspect that modern 3 pole motors would have given similar improvement!  They might still lose, but at least give them a fair fight!

[don7]

When I was checking to see who uses 3 pole motors I did find that NWSL uses 3 pole for their small to medium sized HO replacement motors.

NWSL certainly is noted for quality performance motors. Perhaps the new 3 pole motor with the latest technological changes is a match for most older 5 pole motors.  

[Vail and Southwestern RR]

When I was checking to see who uses 3 pole motors I did find that NWSL uses 3 pole for their small to medium sized HO replacement motors.

NWSL certainly is noted for quality performance motors. Perhaps the new 3 pole motor with the latest technological changes is a match for most older 5 pole motors.  

I would suspect that this is true.  As you might have noticed, I'm not a believer in "one solution for all problems".  There are so many factors, it comes down to how the system really performs, in an actual application.

[ICRR1964]

Jeffrey had a good point on this. I had an old Bachmann GP40, that had this type of 3 pole motor. They ran pretty good out of the box, But I put a A-Line motor in it, and you could not beleive the difference.

There are 3 pole, 5 pole, and 7 pole motors out there, the skew wound with offset feilds make a real good running motor. So it is possible to make a good 3 pole. One draw back though would be heat, the 3 pole would possibly run hotter.

But it is possible to get a 3 pole to run efficent. I guess it would depend on windings, and type of mags.

[Alan Robinson]

In engineering terms, what is the difference between a three pole motor and one with five poles? A DC permanent magnet motor has a magnetic field established by a permanent magnet (of course) and there are many variations in what that field looks like. Ideally, you want a strong field with uniform strength at all points between the pole pieces. This is never achieved in practice, but compare the configuration between an old-fashion open frame motor design such as Pittman made for many years and a new can motor design, or even better a coreless motor. The open frame used small pole pieces with not much steel in the cross-section between the magnet and where the armature sat. This limited the strength of the field. the pole pieces did not extend very far around the armature, sometimes being less than 90 degrees engaged with the armature. Sometimes the gap between the armature steel and the pole pieces was not very well controlled and was often quite wide. How did these factors degrade the performance of the motor?

One of the problems with the old style motors was "cogging", an irregular motion that was characterized by the rotation proceeding in jerky steps rather than in a smooth even motion. This limited the ability of the motor to operate at slow speeds with smoothness. A three pole motor would cog six times per revolution, because that was the number of times an armature tooth (the correct name for what we usually call an armature pole) lined up with one of the field pole pieces. A five pole motor would cog ten times per revolution. All else being equal, which would produce the smoothest slow speed operation? The five pole, of course. The cogging would be worse if the pole pieces were relatively narrow because the strength of the fixed field would have lumps in it. So what do we do about cogging?

The newer design can motors are characterized by permanent magnets molded into a shape that can fit all or nearly all the way around the armature, thereby creating a smoother, less lumpy field. This by itself would go a long way toward reducing cogging.

Another thing that can be done is skewing the armature windings so that the slots between the teeth do not align with the armature shaft. This allows each tooth to engage the permanent magnet field smoothly as the motor rotates, thus eliminating cogging even more.

A third thing is to increase the motor diameter, as the greater the diameter of the armature the longer the moment arm for the magnetic forces to work on. This usually causes the motor to operate at slower speeds with more torque, exactly what we need in our model applications.

Put these three elements together and you have a good motor with either three or five teeth.

Coreless motors take these factors to an extreme, with the permanent magnet field created by a central magnet surrounded by a metallic shell that completes the magnetic path. The narrow and very uniform slot between the central magnet and the surrounding shell create a very uniform and strong field.

The armature has no iron or steel (hence the term coreless) and is instead composed only of copper windings in a heavily skewed configuration. Because the armature contains no iron or steel, the commutation lossses of such motors are very low. There is no need to constantly magnetize and then remagnitize those armature teeth, with each magnetizing action creating wasted energy and heat. So these motors run very efficiently, often at efficiencies considerably better than 50%.

In addition, the armature diameter of such motors is maximized, since it is usually outside the central magnet and thus as large diameter as possible within the overall mechanical envelope.

One drawback of coreless motors is that if they stall due to a mechanical bind they can quickly burn out because the windings have a relatively low resistance. When the armature is turning, the back electromotive force generated by the armature moving through the fixed field creates a voltage that counteracts the applied voltage and this limits the current to a safe value. Stall the motor and that back-EMF goes away, causing the armature to draw high current nearly instantly. Since there is no heavy steel in the armature to act as a heat sink, the armature windings heat up quickly and will burn out if the heavy current stays on for more than a second or so.

Placing a resistor of the proper value in series with the motor can eliminate this problem without causing operational problems. These motors draw such low current during normal operation (sometimes as little as 25 milliamps under load) that the voltage drop across the resistor does not matter much.

An even better solution is to use an incandescent lamp as the current limiting resistor. The lamp should be chosen so that the lamp operating current is equal to the continuous current limit of the motor. During normal motor operation, when the lamp is not lit, the filament has a very low resistance, only about one tenth the resistance when the lamp is lit. This low resistance value has practically no effect on normal motor operation. When the motor stalls, the lamp lights, limiting the current to a safe value.

I like the coreless motors, even though they are a bit pricey. Just be sure you get one well matched to the load and made to run in both directions. Not all of them are. Can type motors are also a great choice and nearly all modern precision can motors give good performance in our applications. Pick the largest diameter motor you can stuff in the loco and have fun!

[larak]

Very comprehensively said Alan (and you saved me a lot of typing)