Bafang SWXK-5

[trex]

for my little project, where can I find the Power out versus RPM of the Bafang SWXK-5?
thank you for your help.

 

[banbury frank]

Hi All brush less motors power is calculated in watts 750 watts = 1 HP power most motors are 80 % + good at converting energy so battery voltage X the output off the controller in amps

Equal watts 36 volt lithium battery is really 40 volts most controllers are 7 Amps +


So 40 X 7 = 280 Watts _ 20% = 224 watts

So it is the battery voltage and the controller Amps and volts NOT the motor

The only thing with the motor is how many amps and volts it will Handel without burning out


Frank

 

[trex]

Hello Frank, I know the maximum output of this motor is 282W at 180 RPM from the bafang tech sheet but I don't know what the power is at 100 RPM (climbing 5% hills) and at 50 RPM (climbing 10% hills).

 

[vhfman]

This link may be of use to you Trex. It has a table of spec and some curves of the motor under test.

Chris

 

[banbury frank]

Hi What I was trying to say was if you change the battery voltage or the power off the controller you can improve the performance off the motor


Frank

 

[trex]

vhfman, thank you for that link. Yes, it does answer my question but just to be sure that I understand the chart, the yellow line is marked N (rotational speed RPM) only goes from 223 rpm to about 180rpm.
How can I work out the power output at 100 rpm? is it straightforward lookup at the two Y scales (N and P) 100rpm -> 150W and 50 rpm -> 78W?

 

[flecc]

How can I work out the power output at 100 rpm? is it straightforward lookup at the two Y scales (N and P) 100rpm -> 150W and 50 rpm -> 78W?

Yes....................

 

[trex]

has anyone tried to get the Bafang SWXK-5 to cough up more Watts at low revs?

 

[jbond]

I think there's two things going on here. The controller current limit and the curves for the motor if it's allowed to draw as much current as it wants. In practice, the controller limit dominates at everything below about 95% of the no-load speed. The guys on ES have some experience of running these motors at 48volts and higher and anything up to a 25A on the controller. There's a couple of limits if you push them too far. First too much heat being generated and secondly the effect on the internal nylon gears which will strip their teeth if you try to push too much power though it.

I've read here that sensored versions can handle more power safely as the startup "judder" can break gear teeth if you allow too much power with a sensorless setup.

None of that actually answers your question of course! Pushing a 250w motor to 500w appears to be possible. 1000w, probably not.

 

[Scottyf]

Quite a few standard kits hit Peaks of around 600watt on the 250watt hub motors.
You could take the peak further but the motor will get warm to hot. There's not so much on the nylon gear stripping any more when you read on other forums.

But pushing 1000watt out of it is pushing it. A safer short peak would be 750 watt. As a total that is.

 

[NRG]

1000W peak is possible with the bafang motor but you need to treat it with respect at low speeds and when setting off. I suspect if abused at low revs it would strip the gears.

 

[Scottyf]

Ive thought it was stressed more once the motor becomes warm and hence the nylon gears become warm and thus not as strong. But nrg is correct making sure your able to Pedal before applying power is the best way to make sure it doesn't harm the motor.

 

[flecc]

The SB motor and eZee controller in the Torq 1 and Quando has at least twice been measured at 1000 watts peak consumption. Assuming 75% efficiency means about 750 watts output, and gradient climb capability calculations on the Quando bear that out.

 

[trex]

This link may be of use to you Trex. It has a table of spec and some curves of the motor under test.

Chris

The data from that link seems to suggest that the controller is not intelligent, P (Watts) = 1.54 * N (rpm) all the way down to zero. I feel so stupid but I have to ask, how do I add an image from my PC to this post?
Can anyone also help me find a chart for POWER versus VOLTAGE for the same motor?
The goal of the project is to improve the power output of the motor at below 100 rpm, not breaking the 250W legal limit.

 

[banbury frank]

Hi trex

If you buy an infinion controller and a cycle analyst and a 48 volt battery you can have all the power at your control

Remember 250 watts continues output so peak can be 500 watts

The Grin Cyclery


The Cycle Analyst Homepage - Ebike Amp-Hour and Watt Meter


Frank

 

[trex]

Hello Frank, I saw on google a picture of a SB motor burnt at 60V, the nylon gears could not take the power and melted. 60V may be the upper limit in input voltage for the SB motor, but then, the motor was pushed so hard that the gears may have sufferred from heat rather than just from excessive torque. I am thinking of making a stepup converter with a chip from Linear Technology. If the gears melted because of heat, the scope for boosting the SB at low revs is good, if they were damaged because of torque, then the possible improvement will be pretty limited.

 

[NRG]

Surely it will be excessive torque that strips the gears, the motor would have to get pretty darn hot to melt the Nylon composite gears...

Good thread on ES (many others too)

http://endless-sphere.com/forums/viewtopic.php?f=3&t=7031&p=105941&hilit=+knuckles+nylon+gears#p105941

 

[trex]

I don't know NRG.
If you climb hills at 100 rpm on 700C tyres, your speed is 13kph. The SB motor offers only 154W of assistance. At 50 rpm, your speed is 6.5kph, the motor helps with 77Watts. It's useful but not good.
If I can improve the power output in this band, there won't be so much appeal for the expensive Panasonic and Bosch drive systems. A mechanical solution (ES) is good but an electronic solution is better.

 

[NRG]

I've scanned the thread l linked to above and it seems excessive voltage and 'power off / power on' shock at speed was causing the breakages so localised heat my be a contributing factor but it seems the voltage and shock are the main culprits....

The SB motor will be putting out far more than 154W at half the no load speed when climbing a hill! This is typically where peak amps are hit and the motor is at its best for hill climbing. Going slower than half speed creates more losses in the form of heat in the controller, wiring and motor.

As the motor slows down the torque will increase and due to the way the controller works using PWM the current supplied to each motor phase will be greater than battery current by some margin (The keywin controllers for instance are set for 2.5x the battery current) so a 15amp controller can be putting around 38amps into each phase in these conditions.

The controller does this by trading volts for amps, they are very efficient voltage to current converters just like a 'buck' converter so what you are trying to achieve is already happening automatically.

IE: the motor is already providing more power at lower RPM than at full speed, its the nature of the beast...taking some rough figures at peak battery draw, lets say 36v and 15amps under load = 540W out of the battery, take away 50W for controller and wiring losses (gross estimation) = 490W and the motor is approx 65% efficient at half the no load speed gives ~320W electrical power at the motor.

 

[trex]

NRG:
I am sure your are right, that's why I was looking for power outputs at various RPM.
I thought the main feature that the Panasonic drive has over front wheel SB drive is more power at low revs.
Vfman pointed me into the right direction but with hindsight, the dataset I got from that web site is not good.

So, what is the output of the SB at 100 rpm and at 50 rpm in your view?
and which contoller is best at climbing hills?