2 pole motors faster than 4 pole motors?!

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Greywolf74

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So, someone (TurboChargedHonda) in a thread mentioned that 2 pole motors were faster than 4 pole motors. I was intrigued as I had never heard this. The little bit of back and forth got me curious so I started reading up on it and heres some excerpts of what I found for anyone geeky enough to be interested 🤣
I'm learning all kinds of stuff about brushless motors lately. Thanks to Bri26 and TurboChargedHonda.



"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."


Originally Posted by DeathVirus View Post

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."



Originally Posted by GLwagon View Post

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."


Quote:

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."


Quote:

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."


"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."




"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."
 
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Chubaka

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Interesting. I always saw the two as two stroke vs 4 stroke. In dirt bikes it doesn't make sense as a 2 stroke usually takes a 4 stroke of twice the displacement, but rather weed eaters. You have two stroke and four stroke. Owned and used each. Then you have the Stihl 4mix. 2 stroke rpm with the valve train and gobs of low end tourque of a four, on mixed gas.

Combine the two stroke and 4 mix have similar power but one hss nore torque, it can be geared for more speed.

I'm gonna have to look into this. Hard to find two pole motors these days. The above 4mix I've been thinking of a few rc projects with over the last few years.
 

olds97_lss

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Interesting. I always saw the two as two stroke vs 4 stroke. In dirt bikes it doesn't make sense as a 2 stroke usually takes a 4 stroke of twice the displacement, but rather weed eaters. You have two stroke and four stroke. Owned and used each. Then you have the Stihl 4mix. 2 stroke rpm with the valve train and gobs of low end tourque of a four, on mixed gas.

Combine the two stroke and 4 mix have similar power but one hss nore torque, it can be geared for more speed.

I'm gonna have to look into this. Hard to find two pole motors these days. The above 4mix I've been thinking of a few rc projects with over the last few years.
I just thought of it more like a 4cyl car vs a 6cyl or 8cyl. More poles to smooth out the pulses and to give you more linear torque.
EDIT: I'm not saying this is right, just saying this is how I interpreted it. But it seems there's far more to it than that. lol!

I think the only possibly 2 pole I have is a losi micro-t/sct motor. It had so little torque though that I went back to brushed for running around the house as I couldn't gear it down far enough to make it slow enough to drive in my house. lol!

I had seen some 6 pole motors and wondered how that affected what the esc saw. To run a 2 pole motor or 4 pole, the esc must be built to deal with it. Or so I thought. Then a guy on youtube was using a 6 pole motor on a 4 pole esc and said it worked fine. Even if it did, I thought it would at least spin at a lower kv because the pulses from the esc aren't coming out fast enough for a 6 pole...

Then again, I'm in the frame of mind with these that I need things more simply stated, such as "more is good, less is not good". But I wouldn't mind understanding or knowing if an esc made for a 4 pole motor can drive a 6 pole or 2 pole motor without issues.
 
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Greywolf74

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Then again, I'm in the frame of mind with these that I need things more simply stated, such as "more is good, less is not good".
Yeah sometimes its nice not to have to decipher all the jargon
But I wouldn't mind understanding or knowing if an esc made for a 4 pole motor can drive a 6 pole or 2 pole motor without issues.
I always thought that any brushless esc would run any brushless motors regardless of poles but in all honesty I've never really thought about it that much cuz 90% of everything on the market seems to be 4 pole
 

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From my understanding, 2 pole and 4 pole motors one 4 pole esc. 2 pole esc is only for 2 pole. A 4 pole motor on a 2 pole stutters or has low rpm and power from what I heard.

I would check, but I have 2 old 380 vxl, and my current 10th scale setup: first year 2006 castle mamba esc on a trinity 4450kv vxl replacement motor 2 pole circa 2012.

Never thought of different cylinders. Mostly in v8 gas and v8 diesels since early 2000s.
 

Greywolf74

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I know people were running the 6 pole tenshock motors back in the day on Castle ESCs and I dont think Castle makes any 6 pole motors so I'm guessing they are compatible with 4 pole ESCs? I have a never run Tenshock SC410 6 pole motor that I need to try out.
 
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