Bafang BPM Motor

[Scottyf]

Afternoon All,
Quick question - The Bafang BPM Hill Climber motor (350w) does this naturally draw more current through its prm range compared to say the cheaper 250w motors using the same battery / controller combination.

For Example,
The Alien Aurora doesn't use a larger battery than any of the usual suspects on ebay to acheive it 20mph limit on the flat. It still uses the 36v 10ah battery which i would imagine has a continous discharge of 1c whith a max of 2c.
On the spec's it does say it uses an 18amp controller which is slightly up on most kits of 15amp.

So does the 350w motor naturally just have a higher draw of current through too the motor or is it something special in the controller and wind to help acheive the additional speed?

Your thoughts!

 

[flecc]

If the battery voltage is the same, it's rating as 350 watts indicates a 40% higher current than a 250 watt version.

The speed is due to a combination of the voltage fed to the motor, the motor's internal gearing and the wheel size, any one varying it.

For example, the legal assist speed eZee Quando and the Torq 1 both used the identical battery, 250 watt motor and controller, but the Torq 1 assisted to 22 mph. The difference was the 20" wheels on the Quando and the 28" wheels on the Torq, the same motor revs going further in a larger wheel.
.

 

[Scottyf]

Well for now forget the actual diameter of the wheel for differences in speed etc.

The actual motor itself must draw a higher constant current that the other 250w motor's to enable it to be faster without loosing torque etc (Even if the wind is a lower rpm)

Ie You input the same 36v, the same max current figure of say 18amp.
Then the level at which that motor will draw current would be greater along all rpm in order for it to be more powerful.

Unless the controller plays a big part in this in which case why not just get the 250w motor and use the same controller as say the 6fet Aruora bike.

 

[Deleted member 4366]

As Steve (Saneagle) found out, you can exchange a 250w Bafang for the 350 BPM motor and the bike goes faster and climbs better with the same controller (15 amp) and battery. It can't do it without drawing more current from the battery. How much? We can only guess, but as Flecc says, it should be about 40%, but the controller limits the current, so under maximum power conditions (up a hill), they're probably drawing about the same, or perhaps a little more for the BPM, which seems to be more efficient under these conditions. Under any other conditions (not max power), the BPM will always draw more current and will reach the controller's maximum more of the time.

If you get a controller that is suited to the BPM (25 or 30 amp), then you'll go faster and it'll draw much more current all the time unless you use less throttle or a power reducing switch on your pedal sensor.

Your battery can provide the current with either controller, but when you go over 1C (10amps for a 10aH battery), you shorten the battery life (number of charges). I don't know what battery the Aurora uses, but I've got a feeling that it won't last as long as it would in a 250w bike - by quite a bit!

 

[Scottyf]

Exactly, although the battery would still be within its max discharge rate.

But if i'm honest if a battery lasts roughly 500 cycles i'd be more than happy. Never really that fussed about something lasting 2000 cycles because its a push bike. In a different application ie a car it would be more important.

 

[Deleted member 4366]

Try it - It's only money!

Incidentally, I just ordered another BPM kit from BMSbattery. I got everything except the battery for about £150 including shipping. How cheap is that?!!

 

[Scottyf]

Yeah i think i may buy a bpm motor to try it out with the existing controller and battery as they are brand new anyway.

Off topic how,s your build coming along?

Although i have to admit the free wheeling of the bafang motors in my eyes are very good. Hardly notice its there. But my bike could do with mos substantial front suspension or a rigid frame. The bpm motor its self is cheap from bms. An it hardly cost anything more if they lace it into a wheel for you too. Bargin. I'm in talks with jack xie regarding it.

Least the bpm on endless sphere people are hitting it with 1kw no real issues. But that would mean a big battery upgrade and at the moment my commute 18mile each way the battery is on its last 20% on the indicator each time. But it is new and need's time to condition. Using 4.4ah total. Don't think the indicator is very accurate compared the the turningy watt meter.

 

[NRG]

Something dosent ring true, I don't think Aurora owners are reporting a ~40% drop in range compared to the GSII I own with the same battery....maybe a few miles in range down but not 40% and that due in all probability to the slightly higher rated controller.

To the best of my knowledge fitting a BPM in place of the 250W Bafang with the same controller wont result in more current draw given the BPM is wound for the same wheel size and RPM. As these motors come in different speed ratings the report above of increased speed with this swap is most likely down to a different RPM varient of the motor. Increased speed gives an illusion of increased hill climbing ability as the motor will produce its peak power at a slightly higher speed, I've experienced this first hand with increased voltages on my DIY Peugeot.

The current the motor actually 'sees' is very different to the current drawn from the battery. The controller uses PWM (acting like a buck converter) to vary the applied voltage to control motor speed and also to control phase current...the current limits and ratios to battery current are set parameters in the controller chip firmware and are not altered just by swapping the motor. Some controllers like the E-Crazyman allow for user adjustment of these parameters but they don't change by themselves.

 

[Deleted member 4366]

If you want to use your existing controller, it might be a good idea to make sure it gets plenty of air because it might get a bit warm. If you get a complete kit from BMS battery, you'll get a controller with it, so at least you'll have a spare, but the last one I got from them was quite large.

I've got too many projects on at the moment. They're all progressing slowly and steadily. On my mid-drive bike I've just finished the important difficult drive side fixing bracket. Now I need a bit of inspiration as to how to mount the motor. Then I'm virtually finished. Everything else is ready. I've a few ideas spinning around my head and soon I'll have a solution.

I think a 350w BPM motor is as far as I'd go on a normal bike. After that you might as well get a petrol moped. My next project is to build a bike for camping and towing a trailer. I already have the donor bike and I'm debating whether to have an additional 250w motor on the front and use the BPM only on the steeper hills. I'll be towing a generator in he trailer so that we can re-charge while under way because we want to do some big distances - starting with the show at Presteign (about 50 miles each way, and progressing from there. God luck with your project and let us know how you get on.

 

[Deleted member 4366]

Increased speed gives an illusion of increased hill climbing ability as the motor will produce its peak power at a slightly higher speed, I've experienced this first hand with increased voltages on my DIY Peugeot.

I don't believe it's an illusion. We did side by side comparisons and there's no doubt that the BPM climbs with more power at the same speed. If you had seen it towing the 40kg trailer up Ironbridge gorge with a 105kg rider, you'd know what I mean. With the old motor, the controller never got warm. It does now because it's providing more current to the motor - however the theory says it does it. I've no idea about the winding, but this motor still pulls at 25mph with a 25amp controller,

I remember someone else on the forum a while ago changed to this motor. I'm not sure whether he had a new bike, but he reported that he used less battery for the same journey. I think he was travelling at the same speed. Maybe he had a three position switch to cut the power, so I'd guess that the BPM can be more efficient under some circumstances like you said, but most people will want to use that power and you can't get that for nothing. It can only come from the battery.

 

[Scottyf]

As you have ordered one from bms battery have you found out what wind and wheel size they have put together for you at all?

I'm sure with the higher speed the peak torque and peak current would move to say a band between 7 to 9 mph on the 250w motor to maybe 10 to 14mph with the 350w. Given that they use the same wheel size and rpm.

It should cost more peak current across a wider band at the expense of battery. But even the aroura with 26 inch wheel and 18 amp controller is hitting the 20mph barrier.

 

[NRG]

I don't believe it's an illusion. We did side by side comparisons and there's no doubt that the BPM climbs with more power at the same speed. If you had seen it towing the 40kg trailer up Ironbridge gorge with a 105kg rider, you'd know what I mean. With the old motor, the controller never got warm. It does now because it's providing more current to the motor - however the theory says it does it. I've no idea about the winding, but this motor still pulls at 25mph with a 25amp controller,

I remember someone else on the forum a while ago changed to this motor. I'm not sure whether he had a new bike, but he reported that he used less battery for the same journey. I think he was travelling at the same speed. Maybe he had a three position switch to cut the power, so I'd guess that the BPM can be more efficient under some circumstances like you said, but most people will want to use that power and you can't get that for nothing. It can only come from the battery.

Hi d8veh,

All power drawn from the battery is governed by the controller, if that has not changed then just changing the motor does not automatically mean more current is drawn from the battery, in fact there is current limiting circuits in the controller to prevent this. I suspect the increase in speed is due to a motor with a different Kv rating or perhaps as Flecc mentioned the internal gearing is different. As these controllers operate like a buck converter and given the same PWM duty cycle for a given speed (controller programmed) and the limitation on current draw from the battery I can't see how the controller can deliver more phase current for more power....I may be missing something of course but I'ms scratching my head on this one especially the 40% more current drawn mentioned earlier in the thread, that makes no sense either.

 

[NRG]

Had a bit of a think about this over a cuppa....

If the motor Kv is the same or similar to the original motor then I think what I've said above is correct. Given that this motor increased the speed of the bike then the KV must be different IE: it spins faster than the original motor for a given voltage.

This makes sense now of the controller heating up comment, if climbing a hill at the same speed on the new motor a lower PWM duty cycle and therefore lower voltage would be used as the Kv rating is different....assuming 25% say this would mean that as these controllers only guess at the phase current (there's no shunt in each phase for measuring the actual current) the 25% drop in voltage would mean a corresponding increase in phase current to compensate.....this is the way buck converters work; trading volts for current and visa versa. Increased phase current will increase the power loss through the driving FETs hence the controller getting warm but there will be a corresponding increase in torque output of the motor due to this higher phase current.

The draw from the battery will remain more of less the same but more power will be lost in the controller...

It's a glorious day so I'm off to test my new 9T motor sprocket on the Kalkhoff to see what range it gives me.....

 

[Scottyf]

I sent a couple of emails of to Jim @ Alien bikes who has been very quick to respond and very helpful.
Although not as in depth as say the above points I just wanted some plain laymans answer.

The below is a conversation through email from myself and Jim.

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Hi Scott

Yes, a BPM motor fitted paired with a 36v 15ah controller will draw more current from the battery and provide more torque than a 250w version.

Good luck with your search for a BPM motor, I’ve had quite a few enquiries from people wanting to buy one but there seems to be a distinct lack of suppliers in the UK.

Regards

Jim



From: Scott Farnham
Sent: 22 March 2011 17:34
To: Jim @ AlienOcean
Subject: RE: Query

Afternoon Jim,

Thanks for the quick response.

What I am trying to get to the bottom of really is... "If I upgrade to a BPM Hill Climber motor, will this motor for with the same controller (36v 15amp max) provide me with more torque and top speed.

Ie the motor will just naturally draw more current due to its larger size than the original bafang motor.

Thanks for your help

Regards
Scott Farnham

 

[flecc]

Pending your receiving that reply, with the same controller, effectively no.

There might be a very small gain due to winding differences but I doubt that would be worth the effort of changing the motor.

You'd probably get a better result by keeping the 250 watt motor and changing the controller to a higher current limit one, assuming the battery was up to delivering the extra current.
.

 

[Scottyf]

To be honest I've took the plunge and brought the BPM motor laced in a 700c wheel with a 9fet controller to suit.
I've based the RPM on 36v cutting out at around 22mph, so no load speed should be around the 26mph limit.

Not say it will acheive this additional speed limit with the same battery I have (36v 9ah) but if all goes well I can always get a larger amp battery to offer up more current.

Even if I do loose the amount of cycles by half that still gives me a good 2 years (I'm a fair weather rider for the summer).

I'll keep you all updated on progress.
The whole electric bike thing is addictive though.

Love riding it to work, epscially the sunny mornings.

 

[Deleted member 4366]

NRG, I can understand what you're saying and I'm not disagreeing with you. As I said in my earlier thread, I don't believe that the controller is giving a 40% increase in peak current, but I'm sure the battery will go flat more quickly because the motor must be drawing peak current for more of the time. The only time I've seen maximum current on the 250w motor is when it's about to stall, when it's lost all efficiency, while as the 350w motor can be running nicely at that current. Maybe this accounts for the difference in power. In the end all the theory doesn't really matter. I think the main point is that with the upgrade to a BPM motor, you get more speed and climbing ability, but if you use these attributes, you'll flatten your battery more quickly and will probably wear it out a bit sooner - plus, there's a small risk that you might fry your controller if you can't keep it cool.

 

[NRG]

Sorry Jim, if you're reading this then no the draw from the battery won't increase as draw is limited by the controller, it may overshoot a touch before the controller cuts power back but essentially the draw will be the same.

Edit, Actually I'll take some of that back, peak won't be higher but the overall continuous draw may be.

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Posting from my iPhone using Tapatalk

 

[Scottyf]

Indeed the controller does limit / provide the draw from the battery.
However with the larger motor it would need more current at a higher nominal rate.

Thus increasing torque across more of the range, looking at the insides of the motor's im the BPM (Bafang Permanent Magnet) there appears to be a more dense amount of wire plus a larger magnet for any given wind.

I'll let you know once its fitted and I'll try it with my original controller (15amp) and then the 9fet controller. I'd prefer to keep the 15amp controller as it would make the battery have a longer life. But if its not much of an improvement I'll bang the 9fet controller straight in.

 

[jbond]

There's a lot of guess work in here but,

The Aurora comes with a 26" rim and a BPM motor marked as 36v26" 350w. The motor has a no-load speed of 24mph and a real world flat speed of 20mph. The controller is marked as 9A continuous, 18A max.

Now the guess work. BMSBattery sells these spec-ed by expected rim size. I guess this corresponds to what Bafang engrave on the outside. And I'd expect a 700c rim with a 700c motor to do the same speeds. And the same for a 20" motor in a 20" rim. There are some old actual specs out there but these talk about "code 10" or "code 11", not 26" or 20".

Now work this back to the typical 250w legal motor. These seem to have a no-load speed of ~20mph and a real world speed of 17-18mph which is then typically limited by the controller to 15mph and a bit.

Now let's say the controllers have a fairly low current limit. The motor-controller is going to be in current limiting most of the time except in the last 10% of speed when pedaling hard. It's possible that the BPM might spend less time in this region than a 250w motor as doing most of the work at 20mph is quite a bit harder than at 15mph. All the rest of the time, the two motors are in the same current limiting mode. So now we're into different PWM and hence different efficiency in the controller.

A guess at the bottom line. The net result is that I wouldn't expect a QWSX and a BPM motor with the exact same controller to be much different in overall battery usage. The BPM would top out going a bit faster. And it's possible that it generates bit more torque at low speeds.