Bafang-BPM and MAC comparison for a mid-drive

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spinningmagnets   100 GW

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Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jun 27 2013 9:37pm

I have recently been evaluating these two similar motors in preparation for making a BB-drive. I've given each motor its own thread including a complete teardown with lots of pics. I will populate those threads with more data as the info and links become available. If you cut off the spoke flanges, both these motors are roughly 6-1/2 inches in diameter.

BPM: http://endless-sphere.com/forums/viewto ... 16&t=51237
MAC: http://endless-sphere.com/forums/viewto ... 50#p719603

The short story (this is just my opinion), the BPM is about $100 cheaper (approximately $150, vs $250 for bare un-spoked motors), but the MAC is capable of about 25% more power due to the magnets and stator-teeth being 25% wider.

I measured the space between the flats from one side to the other, and the BPM is about 4.5mm wider. Clearly...at a glance, the edges of the motor cases show that the BPM EDGE is narrower, but the BPMs planet-gear side "bulge" is also much more prominent. The MAC is wider at the spoke flanges, and the BPM is wider at the housing next to the axle For a mid-drive, the MAC is 4.5mm thinner at the axle area of the housing..

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BPM

PROs:
If you occasionally abuse the motor you're using, saving $100 each time you need a replacement can add up. Also, it's a cheap experiment if you are unsure whether you want a geared hub or not.

I initially thought that since the planet gears are a little wider than the BPMs, that this was a benefit (14mm wide gears on the BPM, vs 10mm on the MAC). But, they are thicker because they are still using a softer grade of plastic. This is not to save $2 per motor, the planet gears are a mechanical fuse that will strip when the motor is being overheated. The plastic planet-gears are cheap, so keep an extra set on the shelf if you have a BPM.

In spite of the affordable and common 0.50mm thick laminations, the low pole-count means this motor can still be run up to 2,000 motor RPMs without significant eddy-current losses. However, the low pole-count also means it may occasionally stutter from a dead stop if you are using a sensorless controller. Clearly it has hall sensors, so this shouldn't be an issue for most.

The magnet bowl is already skeleton-ized with 6 large holes to save weight.

CONs:
If you truly limit this motor to the advertised 500W, this is a great motor at a very affordable price. But...nobody here is interested in 500W (36V X 14A = 504W / 48V X 11A = 528W). The stock 16-Ga phase wires are very small (.060"/1.54mm copper diameter on the BPM for each wire, vs .068"/1.74mm on the MAC) and the small axle-bore limits how much you can upgrade the wires.

The stator teeth are 17mm wide, vs the 22mm wide stator teeth on the MAC. The stator-teeth become an electromagnet when energized, so a roughly 25% larger magnet, combined with more copper mass means the MAC starts out with more power, and has more hot-rodding potential.

When running higher amps, a frequent upgrade is to swap-in one steel gear so the motor can take more heat. This is noisier than having all the three planet-gears be made of the stock plastic.

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MAC

PROs
cell_man has started ordering all new MACs with the thinner 0.35mm laminations. So, in spite of the high pole-count, it can run to the higher motor RPMs typically found by using 72V. The older MAC was good up to 48V, but suffered eddy-current issues at the high-RPMs provided by 72V.

For only $15, cell_man will use thicker phase wires, and add a temperature sensor to the stator from the factory, so you don't have to disassemble the motor to add these features.

The MAC stator is a hair larger in diameter, but the difference is negligible. However, the wider stator and wider magnets are a significant benefit whether you will use the stock motor, or hot rod it with higher amps. This is the major difference between these two motors.

The high pole-count means the MAC works well with sensorless controllers at 36V/48V. At 72V the electrical frequency may be too fast for some controllers to work well in sensorless mode.

CONs
It costs $250 for the bare motor, instead of $150 like the BPM.

The back-iron on the MAC is only 2mm thick, so a small amount of un-shorted magnetic field is enough to hold a steel paper-clip onto its outside without falling. The BPM's 6mm back-iron is excessively thick and heavy. 4mm would have been better for both of them, but it should not be a performance issue.
MACpaperclip 001.JPG
Back-iron is 2mm thick on the MAC.
MACpaperclip 001.JPG (115.99 KiB) Viewed 8416 times
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When I was researching the BPM (the largest geared hub made by Bafang), I found the Q-BPM, which is the same motor, but made for the front wheel. If you are considering this motor for a non-hub build, the Q-BPM is an intriguing option. The axle is solid instead of hollow, and the phase wires exit the motor-case through the side-plate under a larger-than-average shaft bearing, and the axle is already shorter with threads up to the case already.
http://szbaf.com/uploadpic/201110171547322.pdf
http://endless-sphere.com/forums/viewto ... =2&t=41375

The Q-BPM does not have a freewheel on the right side, but it does have the 6-hole disc brake flange on the left. There are fixed sprockets (15T-22T) that bolt right up to the disc brake flange. Also, SBP has an adapter that attaches to the disc brake flange that has freewheel threads. You may have to run the Q-BPM to a jackshaft to get its benefits.
http://www.endless-sphere.com/forums/vi ... 25#p401162
http://www.endless-sphere.com/forums/vi ... 25#p746386
http://www.elifebike.com/peng/iview.asp ... QCN9.1A2VC


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Any input (pro or con) is welcome. Especially experience on how much these motors can take before the weak link in it snaps. By giving the motor some gears to downshift into, the motor RPMs can stay up higher on a slow uphill climb, which helps keeps the amps and heat down, so either one of these motors should perform better as a non-hub compared to how they are known to perform in the wheel.

Also, oil-cooling either one of these motors will definitely help it shed heat. Stock geared hubs are known to have a poor heat-shedding path. Without oil-cooling, but using these motors as non-hubs, I would be very comfortable using 25A on the BPM, and 30A on the MAC. If they both had temp sensors, you could slowly raise the amps even more.
Last edited by spinningmagnets on Sep 03 2013 7:04pm, edited 17 times in total.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by briangv99 » Jun 27 2013 11:04pm

Hi SM,
spinningmagnets wrote:cell_man has started ordering all new MACs with the thinner 0.35mm laminations.
has Paul confirmed the switch to 0.35mm stator laminations? I've heard it rumoured but haven't seen anything to confirm the upgrade.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jun 27 2013 11:09pm

All I have is the posting from the em3ev.com website about the new stator being able to run higher RPMs. My statement may be premature, but the upgraded stator was in response to the 0.35mm laminations in the BMC, and when ordering thinner laminations, the 0.35mm thickness is the next common size. I have PM'd Paul to get a verification as to the actual details...
Last edited by spinningmagnets on Jun 28 2013 1:55pm, edited 2 times in total.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by briangv99 » Jun 27 2013 11:41pm

Thanks SM, I hadn't seen the new motor listed. I'm sure you must be right, definitely sounds like a switch to thinner lams
spinningmagnets wrote:All I have is the posting from the em3ev.com website about the new stator being able toi run higher RPMs.
http://em3ev.com/store/index.php?route= ... uct_id=138
This is our new and improved version of the Mac motor, supplied as a kit. Our new version Mac now includes the same mods as before, both the upgraded phase wires and LM35 temp sensor, but now also includes an upgraded stator which offers improved efficiency and the ability to operate at higher speeds than were possible with the standard stator.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by friendly1uk » Jun 28 2013 3:28am

The BPM gears have turned blue I think. I wouldn't call it a downgrade though. I never considered the old one's to be mechanical protection. Just a weakness. They are cheap and readily available though.
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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by -dg » Jun 28 2013 5:29am

friendly1uk wrote:The BPM gears have turned blue I think. I wouldn't call it a downgrade though. I never considered the old one's to be mechanical protection. Just a weakness. They are cheap and readily available though.
That happens to be a brand new BPM that I loaned to spinningmagnets for this teardown thread. I ordered it from greenbikekit.com on April 20, 2013. If BPM gears have turned blue, it is a very very recently.
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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by full-throttle » Jun 30 2013 8:48pm

spinningmagnets wrote:BPM

CONs:
If you truly limit this motor to the advertised 500W, this is a great motor at a very affordable price.
Many of us successfully run these motors @ 3x their rating. So it can be done.
spinningmagnets wrote:MAC

CONs
The back-iron on the MAC is only 2mm thick, so a small amount of un-shorted magnetic field is enough to hold a steel paper-clip onto its outside without falling. The BPM's 6mm back-iron is excessively thick and heavy.
This is true. I have measured the flux at the flux ring for both motors. Don't have the numbers handy, but MAC exceeded 1mT (max rule of thumb) significantly. BPM is 0.5mT.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jun 30 2013 9:27pm

Thanks! That's just the type of info that is helpful.

edit: just measured these two motors' thicknesses twice, because I though the MAC was wider. I had thought I would initially try making side plates for a mounting bracket that attach to the disc-brake flange on one side, and on the other side a single hole that slips over the freewheel threaded stub (1.38 X 24) , with a common lockring holding that side on.

So, I measured the space between the flats from one side to the other, and the BPM is about 4.5mm wider.

Clearly...at a glance, the edges of the motor cases show that the BPM EDGE is narrower, but the BPMs planet-gear side "bulge" is also much more prominent. The MAC is wider at the spoke flanges, and the BPM is wider at the housing next to the axle.

waynebergmans bracket mounted to the spoke flange holes, which was a clever solution I had not considered. http://endless-sphere.com/forums/viewto ... 67#p752461
Last edited by spinningmagnets on Sep 03 2013 7:02pm, edited 2 times in total.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by neptronix » Jun 30 2013 9:59pm

What would be wonderful is a 2kW MAC motor stator in the same case as the hub. Half the poles.. what looks like more copper.. it would be better than the bafang, for sure.

If we have a 0.35mm lamination MAC though, then it could beat out the bafang for sure. I'm awaiting Paul's response as well, just outta curiosity.
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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 03 2013 10:52am

The MAC is still capable of more power than the BPM, but I was just now able to double the wires going to the Bafang-BPM

I was easily able to insert the paired stock 16-Ga wires, so phase wire copper is now doubled. With stock insulation stripped, two of the stock wires result in a copper diameter identical to the Hobby-King 12-Ga.

I re-did it a second time and included 3 of the hall wires to see if they would fit (they did), so they could be used for a temp probe. The five hall wires have been cut at the hall board, with just enough colored insulation left on the short stub for identification, so that they could be re-soldered easily if wanted in the future. The halls were removed because this motor is intended to be run in sensorless mode, and I wanted to see how much the phase wire thickness could be increased.

I tried several different schemes, but after doubling the phase wires, they are no longer the weakest link, and additional wire cross-section would not help, and it would just make insertion more difficult.

The result is a bundle that includes six stock phase wires instead of three (with stock insulation intact), three of the original hall wires, with the bundle covered by a single sheath of Hobby-King 6mm heat-shrink tube.

The stock cable-sheath was fairly thick-walled, and by switching to a section of heat-shrink and using some WD-40 as lube, the bundle easily inserted through the axle with double phase wires.

Hollow shaft ID: 0.300" / 7.60mm, MAC: 0.275" / 7.10mm

Axle flats are: 10mm on both

wall thickness on BPM: 1.2mm + 1.2mm + 7.6mm bore = 10mm

I did not drill out the axle bore at all, because the average quality of metal and the modest wall thickness indicates the axle is just adequate as it is. With this motor mounted in the rear wheel, I would not recommend taking jumps on a hardtail frame, but as a non-hub motor, the stock axle strength would not be an issue.
Last edited by spinningmagnets on Jul 04 2013 5:25pm, edited 1 time in total.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 03 2013 1:36pm

Notice the soft silicone insulation on the red Hobby-King 12-Ga wire is very thick. The stock BPM 16-Ga wire insulation is thin and fairly stiff.

http://endless-sphere.com/forums/viewto ... 37#p758156
Bafang6 020.JPG
two 16-Ga wires have the same cross-section of copper as one 12-Ga.
Bafang6 020.JPG (81.91 KiB) Viewed 8182 times
Bafang6 024.JPG
Six BPM phase wires, plus three of the hall-wires for a temp probe, with the entire bundle covered by one sleeve of 6mm heat-shrink.
Bafang6 024.JPG (115.26 KiB) Viewed 8182 times
Last edited by spinningmagnets on Jul 03 2013 5:38pm, edited 2 times in total.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by Tom L » Jul 03 2013 5:06pm

If the motor is being modified for use as a mid drive can't you have the phase and hall cables exiting through the cover? Thus allowing pretty much any gauge wire to be used?

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 03 2013 5:26pm

You are correct. I am only borrowing this motor to perform the tear-down, examination, and pic-tutorial. And in exchange for me avoiding the need to buy one ($150 +postage savings), I am upgrading the phase wires for the motors owner since I have plenty of parts to do that already (per the owners request).

That being said, Whiplash (among others) has shown that a BB-drive can be made with an unconverted motor (link below). I personally prefer the improved heat-shedding of the "shaft-drive" converted sideplate, and also how the converted motor is slightly thinner so it does not require a wider BB-crank.
http://endless-sphere.com/forums/viewto ... 25#p705659

Does anyone know the difference between the BPM, and the BPM-II? (other than the obvious screw-on side cover). The listed specs suggest that they have the exact same internal parts.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by crossbreak » Jul 11 2013 3:03am

CONs:
[...]this is a great motor at a very affordable price.
:D Is this really a con? Thx for all your pics! I will definitely go for the QBPM. Still more powerful and lighter motor than a GNG Gen1 ;) Looks like a big SWXB motor, the first one which was ever converted maling the axle rotate. Still i'm pleased with the 1350W i run through the SWXB and it's only 4kg drive weight.Still I think the QBPM has better power/weight ratio.
I have measured the flux at the flux ring for both motors. Don't have the numbers handy, but MAC exceeded 1mT (max rule of thumb) significantly. BPM is 0.5mT.
Again the point for well done engineering goes to the BPM. There must be a reasons making it more efficient than the MAC.

Do you know the gear module of the BPM? The SWXB has large 1.25 module, the smaller 1.0 BTW which makes it a lot less noisy.
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SWXB drive weight.jpg
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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by edoug » Jul 11 2013 11:49am

I just order to Bafang 48V500W BPM Front Driving Hub Motor 328 and plan to converted to a mid-drive. Does anyone know how to do this exactly. Is there a key holding the axial to the stator like the Mac motor?

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 11 2013 5:42pm

edoug, there are several ways to make a hubmotor into a non-hub mid drive. It all depends on what tools and skills you have. Whiplash made a mid-drive without any mods to the hubmotor, you might check his build for some ideas.

http://endless-sphere.com/forums/viewto ... 25#p705659

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by SolarRay » Jul 12 2013 2:08am

Please add xofo to comparison.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 12 2013 6:31am

Solar Ray, If you have an XOFO, and you send it to me...I will perform a tear-down and take pics of the internals. In exchange, I will add a temp probe/re-assemble, and I will pay the return postage. PM me for the address if you are interested.

It apprears the XOFO is the same diameter/width as the MAC/BPM, and it uses a cassette instead of a freewheel.
http://szxfmotor.en.alibaba.com/product ... sette.html

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by SolarRay » Jul 12 2013 10:26am

spinningmagnets wrote:send it to me...
Now, I choose which motor to buy for conversion.
Unfortunately, the shipment round-trip will cost more than the motor itself. :(

xofo has a copy qbpm: XFV (F).

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 12 2013 2:54pm

I see you are in Russia, you are correct my friend. Sorry. (Thanks, Google translate!)

Я вижу, вы в России, вы правы, мой друг. Извините.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by speedmd » Jul 13 2013 11:45am

Both motors look to be strong well made efficient hub motors, but it seems to me that you are paying a significant weight penalty using them in a light weight mid drive conversion unless you can swap out / or significantly modify some of the heavy steel parts. Does anyone have data on power output of these puppies running on 18s. :twisted:

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by waynebergman » Jul 13 2013 4:50pm

I think these motors are plenty light enough. If you are running 1800 to 2000 watts through them who cares if you have a 60 or 70lb bike. Running a mac as a mid drive is like night and day compared to a DD in the hub. The rear wheel of a mid drive bike is balanced and way way lighter than an HS3540 or something in the wheel. I feel the weight of these little motors would be a small thing to worry about for a usable and working conversion.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by -dg » Jul 13 2013 5:54pm

speedmd wrote:Both motors look to be strong well made efficient hub motors, but it seems to me that you are paying a significant weight penalty using them in a light weight mid drive conversion unless you can swap out / or significantly modify some of the heavy steel parts.
What heavy steel parts do you have in mind? I don't see much weight in steel parts that are not part of the magnetic structure.
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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by spinningmagnets » Jul 13 2013 7:05pm

The face of the magnet-bowl could have some holes drilled into it (on the MAC). I could easily remove about 1/4 of its weight. Of course the magnet backing steel could use an extra 2mm of thickness.

Either one of them in a mid-drive would be 3-times better than the same motor in the wheel. The shaft-drive conversion draws the stator heat away quite well, and the ability of the motor to use the bikes gears reduces the heat by at least 60% on steep hills, so...you're not making as much heat in the first place, and whatever heat you do make is sucked away by some very large aluminum mass.

Remember, the stator in the shaft-drive conversion (whether a MAC or a BPM) is connected directly to the aluminum housing, which a stock MAC and 9C does not have.

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Re: Bafang-BPM and MAC comparison for a mid-drive

Post by speedmd » Jul 13 2013 10:41pm

I could easily remove about 1/4 of its weight.
Agree, 1/3 is possible in most mass produced items without too much structural loss. That would bring it down to around 3- 3.5 KG? I don't have one to look over and only going by photos but could aluminum replace the axle possibly as it is used mainly as a stator spacer / bearing supports and also thinking the planet gear support can be made more swiss cheese style. Do like the built in gear reduction but also like the rc in runners like the ones from astro that I am comparing also. At 3kg it is most likely a wash when you add chain/belt and jack shaft reductions. I am shooting for a under 50 pound bike, full suspension w/10-12ah, 18s lipo as a design goal.

What kind of power can these put out. Is 3 or 4 kw out of the question? Agree, they will be way better in a mid drive, taking advantage of the bike gear reduction/ selection and better stator contact / cooling to the housing.

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