Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs

[DeathVirus]

Call me crazy but isn't motor speed affected by he KV rating? So a 3000kv motor will spin to 36,000 RPMs @12v whether it's a 2 pole or a 4 pole, correct? Why is there all this info about 2pole=rpm and 4pole=torque. I understand the 4pole portion of it already

From what I've read 4poles have more torque, have a smother startup, have a better drag brake (more magnets), and are more efficient and run cooler.

Other than cost, why use a 2pole?
Any benefit over a 4pole??

 

[E. Bryant]

The speed vs. torque thing comes from induction motors, where pole count does indeed provide a trade-off between speed and torque at a fixed supply voltage and frequency. That relationship does not apply to permanent-magnet motors that are driven with variable voltage and frequency.

Increasing the number of poles does typically lead to less torque ripple and cogging torque, and thus smoother motor performance - especially at low speeds. But increasing the number of poles also typically increases the number of slots (most hobby motors are wound with three slots per pole pair), and so we lose a bit of surface area in the stator with which to collect magnet flux, and so we might see a slight drop-off in performance (higher kV for the same number of turns per phase). 4-pole motors may also have more end-turn resistance per phase (we need to run more wire in the motor in order to connect those additional poles). Both of these things would tend to add up to a bit more resistance per phase, and so copper losses (heat) will be increased.

Perhaps more significantly for high-speed hobby motors, doubling the number of poles also doubles the electrical frequency (the rate at which current through each phase must be switched). This increases switching losses in the ESC as well as the iron losses in the motor.

It's not as simple as saying that one is "better". Generally speaking, you want more poles for larger motors (such as 1/8-scale stuff), and/or for lower-speed operation. But there are other ways to decrease the torque ripple and cogging torque of a small two-pole motor, and so I certainly wouldn't make a blanket statement claiming that more poles = more better.

 

[DeathVirus]

The speed vs. torque thing comes from induction motors, where pole count does indeed provide a trade-off between speed and torque at a fixed supply voltage and frequency. That relationship does not apply to permanent-magnet motors that are driven with variable voltage and frequency.

Increasing the number of poles does typically lead to less torque ripple and cogging torque, and thus smoother motor performance - especially at low speeds. But increasing the number of poles also typically increases the number of slots (most hobby motors are wound with three slots per pole pair), and so we lose a bit of surface area in the stator with which to collect magnet flux, and so we might see a slight drop-off in performance (higher kV for the same number of turns per phase). 4-pole motors may also have more end-turn resistance per phase (we need to run more wire in the motor in order to connect those additional poles). Both of these things would tend to add up to a bit more resistance per phase, and so copper losses (heat) will be increased.

Perhaps more significantly for high-speed hobby motors, doubling the number of poles also doubles the electrical frequency (the rate at which current through each phase must be switched). This increases switching losses in the ESC as well as the iron losses in the motor.

It's not as simple as saying that one is "better". Generally speaking, you want more poles for larger motors (such as 1/8-scale stuff), and/or for lower-speed operation. But there are other ways to decrease the torque ripple and cogging torque of a small two-pole motor, and so I certainly wouldn't make a blanket statement claiming that more poles = more better.

This is the best information I've ever come across. Well put.
So 2 pole motors have less amp draw and therefore a cooler ESC?

 

[E. Bryant]

So 2 pole motors have less amp draw and therefore a cooler ESC?

All else equal, yes - but all else is rarely equal, and so I wouldn't use this as the sole reason to pick a 2-pole over a 4-pole motor. For example, maybe I decide to play around with the magnet and/or stator geometry on a 2-pole motor in order to reduce cogging torque, and those changes might negate any advantage that the 2-pole motor had in terms of efficiency.

The bottom line is that a motor designer has a lot of choices to make and compromises to balance, and this is but one part of the overall puzzle. Sorry for the wishy-washy response, but this isn't the sort of thing that has a simple black-or-white answer.

 

[DeathVirus]

All else equal, yes - but all else is rarely equal, and so I wouldn't use this as the sole reason to pick a 2-pole over a 4-pole motor. For example, maybe I decide to play around with the magnet and/or stator geometry on a 2-pole motor in order to reduce cogging torque, and those changes might negate any advantage that the 2-pole motor had in terms of efficiency.

The bottom line is that a motor designer has a lot of choices to make and compromises to balance, and this is but one part of the overall puzzle. Sorry for the wishy-washy response, but this isn't the sort of thing that has a simple black-or-white answer.

So all things equal, a 2 pole is more efficient? As in longer run time?

 

[GLwagon]

Lower KV motors tend to have less overall draw at idle, they can be helped with more voltage.
Efficiency between 2 & 4 pole for equal KV the 4 pole will have more torque for the same KV hence better use of the available power.
More poles equal more control of the magnetic fields.
2pole = 120 degrees between fields. (potential longer distance/time to next field)
4pole = 60 degrees between fields.

 

[E. Bryant]

So all things equal, a 2 pole is more efficient? As in longer run time?

Yes, but...

1) All other things are never equal.
2) Even if all other things were equal, you'd be hard-pressed to ever notice a difference in an RC vehicle. We're talking fairly minor differences in this regard.

 

[E. Bryant]

Lower KV motors tend to have less overall draw at idle, they can be helped with more voltage.

Motors don't draw current at idle.

Lower kv with more supply voltage is always a good idea, assuming that the ESC is rated accordingly.

Efficiency between 2 & 4 pole for equal KV the 4 pole will have more torque for the same KV hence better use of the available power.

The average torque will not differ between two motors of the same kv. There is a parameter called kt (unit torque per unit current) that isn't used in the hobby industry, but is commonly used elsewhere, and if you do the math, you can easily see that kt is absolutely linked 100% to kv (it's an inverse relationship). So, two motors of the same kv rating also have the same kt rating; there is absolutely no other way.

What does vary is the ripple and detent torque of a given motor (what we call "torque perturbation"), and this torque is constantly added to/subtracted from the rotor as it spins. A motor with high torque perturbation can have significant torque peaks and dips, which can affect starting and low-speed operation. There are techniques to reduce these effect, but they tend to decrease flux linkage and thus it hurts the average torque of the motor. Like any other engineering exercise, it's all a matter of managing trade-offs.

More poles equal more control of the magnetic fields.
2pole = 120 degrees between fields. (potential longer distance/time to next field)
4pole = 60 degrees between fields.

Doesn't quite work that way. A "properly-designed" 3-phase motor (one with perfectly sinusoidal BEMF) will have constant torque around the entire rotation of the shaft, no matter the position of the rotor or the number of poles. I can design a 2-pole motor that has no torque pertubation and thus provides constant torque all the way around a full rotation. Now, it tends to be easier in practice to do this with a larger number of poles, particularly as the motor increases in size, and a 36mm diameter hobby motor falls into a bit of a grey zone where I could make arguments for either a 2-pole or 4-pole design. It's quite possible to design a smooth 2-pole motor, and it's quite possible to design a ripply 4-pole motor.

What's more important than the number of poles is that the designer understands the usage of the motor and has the skill to optimize the motor for that task. I know that's a lot tougher than saying "2 poles bad 4 poles good" or whatever it is that one wishes to believe, but such is life.

 

[JohnRobHolmes]

Since you have gotten the tech side out of the way, lol....

Used to be that two poles just didn't produce controllable torque in a crawler, and four poles were generally more controllable at low RPMs. Turns out it was lazyness of companies releasing "crawler" motors that had zero engineering done to them, simply race motors that may have a larger rotor thrown in. I spent some time refining a two pole and was very surprised to find that they can perform just as well as four pole at low speed. TrailMaster Pro is what came out of that work.

After working on the two pole, I focused on the four pole. With equal motor length and 36mm diameter, four pole can have a torque density advantage because of the increased flux gap area. Even though there is typically more end turn losses (depending on whether distributed or concentrated winding pattern is used), we also have a lower terminal resistance for equal KV of equivalent two pole. So four pole can have both power and torque advantage within the same motor size. ESC switching losses and steel losses do not offset this gain, in practice a four pole has both torque and power density advantage.

Working backwards, a four pole with equal power and torque of a two pole can be constructed smaller and lighter. This is where the Puller Pro 540 Stubby came from. I used the flux gap area of a my two pole to identify an equal FGA in four pole, landing at 15mm long stator stack. It is shorter and lighter than the TrailMaster Pro, but feels very similar on the rocks.


The performance or size factor of 4 pole comes at a price, and that is more expensive construction. It is basically the only downside to four pole, it costs more to make. Where theory and construction meet (in a crawler with the RPMs we need), the four pole does have an advantage of either smaller size for the same performance or higher performance in the same size.

 

[DeathVirus]

Since you have gotten the tech side out of the way, lol....

Used to be that two poles just didn't produce controllable torque in a crawler, and four poles were generally more controllable at low RPMs. Turns out it was lazyness of companies releasing "crawler" motors that had zero engineering done to them, simply race motors that may have a larger rotor thrown in. I spent some time refining a two pole and was very surprised to find that they can perform just as well as four pole at low speed. TrailMaster Pro is what came out of that work.

After working on the two pole, I focused on the four pole. With equal motor length and 36mm diameter, four pole can have a torque density advantage because of the increased flux gap area. Even though there is typically more end turn losses (depending on whether distributed or concentrated winding pattern is used), we also have a lower terminal resistance for equal KV of equivalent two pole. So four pole can have both power and torque advantage within the same motor size. ESC switching losses and steel losses do not offset this gain, in practice a four pole has both torque and power density advantage.

Working backwards, a four pole with equal power and torque of a two pole can be constructed smaller and lighter. This is where the Puller Pro 540 Stubby came from. I used the flux gap area of a my two pole to identify an equal FGA in four pole, landing at 15mm long stator stack. It is shorter and lighter than the TrailMaster Pro, but feels very similar on the rocks.


The performance or size factor of 4 pole comes at a price, and that is more expensive construction. It is basically the only downside to four pole, it costs more to make. Where theory and construction meet (in a crawler with the RPMs we need), the four pole does have an advantage of either smaller size for the same performance or higher performance in the same size.

Thank you sir. I was watching your videos on brushed vs brushless but there wasn't anything explaining 2 pole vs 4 pole. Great explanation.

I was wondering if you have any electronic upgrades for a Traxxas Summit. I have two trail-built stampede 4x4's for my twin boys. One of them has a 2 pole brushless and the other has a 4 pole brushless. I recently got an Axial Bomber and put a vanguard 3150kv motor on it. I've been wanting to upgrade the Summit to get more speed out of it but I read it has plastic gears and won't last long with brushless power.

 

[HahnsB2]

Having experience with both the 4 pole Puller Pro 540 and two pole Trailmaster Pros, I think the TM pro is really underrated, it performs quite well, I didn't find it lacking at all in my Wraith.

 

[DeathVirus]

Having experience with both the 4 pole Puller Pro 540 and two pole Trailmaster Pros, I think the TM pro is really underrated, it performs quite well, I didn't find it lacking at all in my Wraith.

That's probably the motor I should've gotten instead of the Vanguard 3150kv. Today I just ripped the left rear driveshaft apart and twisted the rear center driveshaf in the grass. I run Hyrax 2.2 which are quite grippy with stock foams but the problem was I decided to turn off punch control. I have punch control set to the highest setting on all my RC's because I hate wheelies (waste of acceleration) and it helps with that. Well I guess it was just too much power and messed it all up.

I replaced the rear center driveshaft with the plastic one it came with and popped the left rear driveshaft back in place. <---this is the fix, correct?
Anyway the driveshaft was just dangling in between the diff and whatever is stopping it on the inside. Th outside bearing closest to the hex was popped out.

I still have driveline drag. I checked both ends and it feels like the issue is inside the gear housing. I'll have to open that up tomorrow and see where the binding is. I run the 2 speed and this would be a good time to change the gearing from .93 to 1.08. It runs 27.5mph on 3s using GPS on hi and 13.5mph on lo. It's too much speed for my setup so I'll gear it down to run 24mphish on hi while I'm inspecting it.

 

[DeathVirus]

Here's my current setup but I'm gona change the front springs to 2.85lb/in springs and change front oil to 40wt.

RR10 Bomber
14/64 gearing
Castle Sidewinder 3 with Vanguard 3150 motor
XT60 connector
Passive cooling: (2) Bastens aluminum heatsink
Savox sw-1210sg (277.7oz/in, .15s)
Castle 10A BEC enclosed in TRX receiver case
Axial aluminum servo horn
Axial 2spd kit with optional high speed gearing
Traxxas 2075 shifting servo (125oz/in, .17s)
Stock suspension geometry
Stock King shocks/springs
Front/Rear springs: 1.43lb-in/3.27lb-in
Front/Rear oil: stock 30wt/45wt
Axial "soft" Rear sway bar
Removed rear spare tire and housing
Slipper clutch 1/2 turn out
Proline 2.2 Hyrax G8 compound tires
Proline stock open cell foams
Front: Proline 2.2 Denali wheels
Rear: Proline 2.2 Faultline wheels
1oz Beef Tubes Beef Patties per front wheel
Traxxas thick gray wheel hexes in rear wheels
1oz lead weight on top of steering servo
1oz wrapped around each end of front axle
1.25oz wrapped around each end of rear axle
Tactic TTX300 transmitter
Energizer Ultimate Lithium in transmitter
Steering dual rate: reduced by 2 notches
Sidewinder ESC Setting: 1.Forward/brake/Reverse, 2.Brakes 25%, 3.Reverse speed 75%, 4.Punch control level 1.High, 5.Drag brake 10%, 6.Dead Band Normal(.1ms), 7.Voltage cutoff Auto, 8.Motor timing Low(0 degrees), 9.Motor type Brushless
Gens Ace 3s 5000mah 50c "short" pack XT60
Front/Rear Ride Height: 1.73"/1.89" measured at pumpkin to ground
Front/Rear track width: 11.25"/11.4"

 

[E. Bryant]

After working on the two pole, I focused on the four pole. With equal motor length and 36mm diameter, four pole can have a torque density advantage because of the increased flux gap area. Even though there is typically more end turn losses (depending on whether distributed or concentrated winding pattern is used), we also have a lower terminal resistance for equal KV of equivalent two pole. So four pole can have both power and torque advantage within the same motor size. ESC switching losses and steel losses do not offset this gain, in practice a four pole has both torque and power density advantage.

Very interesting - thanks for your insight! While I'd love to know the details of your motor recipe (I have some guesses as to the differences between your two-pole and four-pole designs), I don't think that they are important; what is important is the understanding that the number of poles is but one of many variables that a motor designer can tweak to achieve the desired performance.

The performance or size factor of 4 pole comes at a price, and that is more expensive construction. It is basically the only downside to four pole, it costs more to make. Where theory and construction meet (in a crawler with the RPMs we need), the four pole does have an advantage of either smaller size for the same performance or higher performance in the same size.

FWIW, the small automotive motors that I helped designed typically used 4 or 6 poles (primarily to reduce torque ripple, vibration, and audible noise). While the automotive market is extremely sensitive to price, the performance benefits were deemed important enough to justify the additional cost.

 

[JohnRobHolmes]

Very interesting - thanks for your insight! While I'd love to know the details of your motor recipe (I have some guesses as to the differences between your two-pole and four-pole designs), I don't think that they are important; what is important is the understanding that the number of poles is but one of many variables that a motor designer can tweak to achieve the desired performance.



FWIW, the small automotive motors that I helped designed typically used 4 or 6 poles (primarily to reduce torque ripple, vibration, and audible noise). While the automotive market is extremely sensitive to price, the performance benefits were deemed important enough to justify the additional cost.

My motor recipe follows the basics. Start with high quality components within the target price range. Tune the "feel" of the motor for best control in all RPM ranges. Design the housing for maximal active motor within the frame.


As you know and have stated, design is always a trade off. Reducing torque ripple to improve low speed feel typically reduces flux linkage, and thus peak torque or power. Over the years we have found that a controllable motor wins over brute power, and the effort of chasing "max" numbers can be a waste of time where rubber meets the ground.

The current 4 pole offerings have about 10% higher Kv (10% higher terminal resistance) than what is "optimal" for power. On the u4 tracks they still run faster laps, are smoother in and out of corners, and keep nice and cool. We do have a special run of race motors in production that split the middle though, using higher grade magnets combined with a low ripple shape to get back the 10% while keeping that buttery smooth control. Trade off, higher costs and higher idle current. But in a race situation, a little more lamination loss is overshadowed by 10% lower copper losses. End effect is a very punchy AND controllable motor that runs super cool and doesn't fade as much during a race.

 

[Ditchrat]

My motor recipe follows the basics. Start with high quality components within the target price range. Tune the "feel" of the motor for best control in all RPM ranges. Design the housing for maximal active motor within the frame.


As you know and have stated, design is always a trade off. Reducing torque ripple to improve low speed feel typically reduces flux linkage, and thus peak torque or power. Over the years we have found that a controllable motor wins over brute power, and the effort of chasing "max" numbers can be a waste of time where rubber meets the ground.

The current 4 pole offerings have about 10% higher Kv (10% higher terminal resistance) than what is "optimal" for power. On the u4 tracks they still run faster laps, are smoother in and out of corners, and keep nice and cool. We do have a special run of race motors in production that split the middle though, using higher grade magnets combined with a low ripple shape to get back the 10% while keeping that buttery smooth control. Trade off, higher costs and higher idle current. But in a race situation, a little more lamination loss is overshadowed by 10% lower copper losses. End effect is a very punchy AND controllable motor that runs super cool and doesn't fade as much during a race.

\

I am very resistant to brushless for my own reason which I will not get into. However part of me wants to do a brushless rig to see the difference, but can't see the benefits to it against the initial costs.

That being said it is obvious you are knowledgeable, have you ever considered including some of this information on your website?

For example I run 35t motors on 2200 3s. I would love to know what brushless motor is comparable, then I would like to know how much more efficient is a brushless. Are we talking a couple minutes of run time, 10, 20, ect Obviously you can't answer that question for every variable, but there has to be some sort of observed percentage.

I deal with reviewing system designs in my line of work. As such I am very accustom to looking at pump curves to determine what size, frequency or style of motor is required to get the lift I need out of a pump.

Buying a motor for a rig feels like I need something visual to compare, and not just go off someone's recommendations.

You have any thoughts on that, including dude get over yourself, if that applies.:lmao:

 

[JohnRobHolmes]

Im not sure how to integrate that sort of information on my site at this time. I'm currently using Youtube to collect some ideas together. Tried to have a blog side, but that became a whole new project to manage and deal with constant fiddling/upgrading. Where would you expect or want to see it?

A brushed motor around 1500kv would be similar speed to a 35t. I would typically throw in a 2200kv and gear down to bring wheelspeed back in check. Almost any brushless motor you see as "540" will have as much or more power and torque as compared to your 35t.

Most people find that runtimes will be 1.5 to 2 times longer with brushless. This is only the case if you don't have and use extra wheelspeed .

You might find this recent video informative.

https://youtu.be/BcuhdkCucGA

 

[Ditchrat]

Im not sure how to integrate that sort of information on my site at this time. I'm currently using Youtube to collect some ideas together. Tried to have a blog side, but that became a whole new project to manage and deal with constant fiddling/upgrading. Where would you expect or want to see it?

A brushed motor around 1500kv would be similar speed to a 35t. I would typically throw in a 2200kv and gear down to bring wheelspeed back in check. Almost any brushless motor you see as "540" will have as much or more power and torque as compared to your 35t.

Most people find that runtimes will be 1.5 to 2 times longer with brushless. This is only the case if you don't have and use extra wheelspeed .

You might find this recent video informative.

https://youtu.be/BcuhdkCucGA

I have been following your videos on FB,(you are very well spoken and the videos are easy to follow"thumbsup") I assume that are the same as on youtube.

As to the information on your site, sorry I wasn't intending to have you change your site, just commenting on how information, such as the 1.5 to 2 times the run time, would help in the decision making process. To answer your question maybe in the description of the motor. I think you covered a comparison between brushed and brushless in a video I recently watch.

Just to be clear, I am currently getting 60 to 75 minutes of run time when I as long as I am not thrashing around between rocks, I could expect to get another 30 minutes to an hour out of a 2200mah battery switching to brushless? That is pretty significant, and honestly knowing that I am very inclined to give brushless a try in the next rig I build.

I appreciate the response.

 

[JohnRobHolmes]

Putting the info in one location in a coherent manner is my hurdle. Always been an issue. It could be quite the large amount of information to sift.

Yep, you could expect 1.5 to 2 hours runtime on the same 2200 3s pack. That's what I get on slower rigs with that size battery.

 

[Ditchrat]

Thanks again for the info and your time.