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09-27-2017, 01:20 PM | #1 |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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?? |
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09-27-2017, 03:05 PM | #2 |
Newbie Join Date: Dec 2016 Location: SW Michigan
Posts: 36
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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. |
09-27-2017, 03:21 PM | #3 | |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
So 2 pole motors have less amp draw and therefore a cooler ESC? | |
09-27-2017, 05:41 PM | #4 | |
Newbie Join Date: Dec 2016 Location: SW Michigan
Posts: 36
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
09-28-2017, 03:13 PM | #5 | |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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09-28-2017, 04:11 PM | #6 |
Quarry Creeper Join Date: Apr 2010 Location: Baraboo
Posts: 215
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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. |
09-29-2017, 05:22 AM | #7 | |
Newbie Join Date: Dec 2016 Location: SW Michigan
Posts: 36
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
09-29-2017, 05:38 AM | #8 | |||
Newbie Join Date: Dec 2016 Location: SW Michigan
Posts: 36
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
Lower kv with more supply voltage is always a good idea, assuming that the ESC is rated accordingly. Quote:
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:
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. | |||
09-29-2017, 10:46 AM | #9 |
owner, Holmes Hobbies LLC Join Date: Nov 2004 Location: Volt up! Gear down!
Posts: 20,290
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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. |
09-29-2017, 01:30 PM | #10 | |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
09-29-2017, 03:07 PM | #11 |
Quarry Creeper Join Date: May 2011 Location: Battle Ground WA
Posts: 429
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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.
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09-29-2017, 06:07 PM | #12 | |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
09-29-2017, 06:10 PM | #13 |
Pebble Pounder Join Date: Aug 2017 Location: Home
Posts: 105
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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" |
10-01-2017, 07:47 PM | #14 | ||
Newbie Join Date: Dec 2016 Location: SW Michigan
Posts: 36
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
Quote:
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10-04-2017, 07:51 AM | #15 | |
owner, Holmes Hobbies LLC Join Date: Nov 2004 Location: Volt up! Gear down!
Posts: 20,290
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
10-04-2017, 10:52 AM | #16 | |
RCC Addict Join Date: May 2015 Location: Price Twp.
Posts: 1,291
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
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. | |
10-04-2017, 01:02 PM | #17 |
owner, Holmes Hobbies LLC Join Date: Nov 2004 Location: Volt up! Gear down!
Posts: 20,290
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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 |
10-04-2017, 02:22 PM | #18 | |
RCC Addict Join Date: May 2015 Location: Price Twp.
Posts: 1,291
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs Quote:
I have been following your videos on FB,(you are very well spoken and the videos are easy to follow) 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. | |
10-04-2017, 02:43 PM | #19 |
owner, Holmes Hobbies LLC Join Date: Nov 2004 Location: Volt up! Gear down!
Posts: 20,290
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
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. |
10-04-2017, 03:02 PM | #20 |
RCC Addict Join Date: May 2015 Location: Price Twp.
Posts: 1,291
| Re: Advantage of 2 pole motor over a 4 pole? And don't tell me it's RPMs
Thanks again for the info and your time.
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