My Carrera kraken E 250w Bafang bbs01b

[wheeliepete]

I was trying to understand the back emf thing d8veh, so when you increase the voltage the motor will have a higher max. rpm and more torque, but will still reach the point where the back emf will reduce the current, just at a higher rpm.

 

[anotherkiwi]

I was trying to understand the back emf thing d8veh, so when you increase the voltage the motor will have a higher max. rpm and more torque, but will still reach the point where the back emf will reduce the current, just at a higher rpm.

Back EMF kicks in once you pass the maximum rpm just like on a hub motor. The thing is depending on gearing you can spin a mid drive motor faster than max rpm before the cut off speed. Back EMF is at about 90 rpm on the GSM so when I am climbing in a lower gear than necessary I have to remember not to spin at my natural rhythm.

The rpm for all the Bafang drives is on the spec sheets for both unloaded and loaded speed: http://www.bafang-e.com/en/components/motor.html

 

[Benjahmin]

Put another way, any DC motor is also a dynamo - they are one and the same thing. So, when you apply a voltage to the motor a current flows through the stator windings creating a magnetic field which intereacts with the permanant magnets in the rotor - so causing the rotor to spin. As soon as it spins the magnetic field of the rotor magnets intereacts with the windings (copper coil) in the stator, so generating a voltage in the windings BUT this has a reversed polarity to the applied voltage. Hence back EMF (electro motive force). As the rate of spin increases so does the back emf, when the back emf equates to the supply voltage there can be no further increase in rpm. So rpm limit of any electric motor is a function of the way that motors work and, ultimately, not the controller or firmware.
So it seems to me that the relationship between cadence and motor rpm is one of the internal gearbox between motor and final drive.
Sorry if I'm stating the bleedin' obvious, but writing it out helps me understand it.

 

[Ebyke]

Thanks people
Nice bit of homework there..lol as the old saying goes you can't polish a turd.But you can cover it in glitter..yes I do understand where your coming from with the back Emf. So maybe we should put together a good list of what works the best for the bbs01 250w uk legal system.ie programm settings. The best chain ring size for what type of riding. And what type of rear gearing works well for the type of bike being used there's plenty of us on here. I don't know how many ride uk legal bbs systems. But there must be a few. There seems to be a shortage of info for this motor. Plenty for non uk legal specs out there. But when your new to it it's all a bit confusing.
Jim

 

[wheeliepete]

Put another way, any DC motor is also a dynamo - they are one and the same thing. So, when you apply a voltage to the motor a current flows through the stator windings creating a magnetic field which intereacts with the permanant magnets in the rotor - so causing the rotor to spin. As soon as it spins the magnetic field of the rotor magnets intereacts with the windings (copper coil) in the stator, so generating a voltage in the windings BUT this has a reversed polarity to the applied voltage. Hence back EMF (electro motive force). As the rate of spin increases so does the back emf, when the back emf equates to the supply voltage there can be no further increase in rpm. So rpm limit of any electric motor is a function of the way that motors work and, ultimately, not the controller or firmware.
So it seems to me that the relationship between cadence and motor rpm is one of the internal gearbox between motor and final drive.
Sorry if I'm stating the bleedin' obvious, but writing it out helps me understand it.

Nice one, makes more sense now. I knew that a motor could be used as a dynamo, but hadn't considered it would be producing voltage when used as a motor. As you say this is down to the physical characteristics of any given motor and can only really be changed by external gearing, hence why I asked earlier how this could be improved by firmware or controller improvements.

 

[peter.c]

great idea I suspect you are right most people go power mad and fit the most power they can bbshd 1700watt and 52 volt battery but I think it may work if we all collaborate the answer may be found

 

[Deleted member 4366]

Nice one, makes more sense now. I knew that a motor could be used as a dynamo, but hadn't considered it would be producing voltage when used as a motor. As you say this is down to the physical characteristics of any given motor and can only really be changed by external gearing, hence why I asked earlier how this could be improved by firmware or controller improvements.

There is one thing you can do with the controller. You still see it on some controllers, and that's then three speed switch. You get an extra connector with three wires. One is ground and the other two are basic digital inputs to the controller, so the switch only acts as a command to the controller when to operate high-speed mode. High-speed mode changes the timing slightly - a bit like you can with an internal combustion engine to get more torque or speed. I'm not sure about exactly how the changed timing deals with the back emf, but the end result is a 10% boost in maximum speed.

Not all three-speed switches do the timing change. some are just programmed for different speed limits. The KU63 controller that we used to buy from BMSBattery used to have it.

 

[anotherkiwi]

Changing the gearing won't help your natural cadence which sounds to be similar to mine at about 100-110?

If you put a bigger chainwheel on to slow your cadence you will be running into other issues. The GSM is a BBS01 clone and is supplied with a 46 tooth chainwheel, it is on a 28" wheel bike at the mo'. I managed to stall the motor with that and an 11-32 cassette so went to a 42-32 double chainring on the front. The 42 is a little too small, a 44 is probably the ideal for a 28" wheel but with the 32 I can climb anything in bottom gear, seated...

The motor will be moved to the trike which is 26" rear wheel so the big chainwheel will be upgraded to a 48 tooth. At the moment the cassette is an 11-40 8 speed but I think I will upgrade to 11-46 10 speed. I have also ran the ratios for the stock single 46 tooth chainwheel (http://www.bikecalc.com/speed_at_cadence) which is also an option as 1x10. I hardly ever use the 11 tooth top gear feeling more comfortable in 48:13 which still gives about 55 km/h before spinning out. 46:13 is still a healthy 50 km/h and the 11 tooth is always there for those moments you must be pedalling at 60-65 km/h...

The advantage of the GSM external controller over the BBS0x is... the external controller! I have tried 12S - 44.4v nominative battery and hot off the charger seen about 680 Watts on the LCD. That is too much for the 36v controller I have at the moment which is using the thermal cut off to save it's skin. Although all that power is lovely I expect to run a 48v controller at 12 Amps which gives the same power as a 36v at 15 A give or take a Watt or two. The cadence of the motor is 110 rpm with a 44.4v battery so no back EMF in the tests I have done so far.

 

[wheeliepete]

Maybe then it may be worth trying the BBS01 with an external controller at 48v and 12A to improve the back EMF effect. Not sure if the Bafang PAS will work, but I think it would be possible to fit an external one on the non drive side.

 

[peter.c]

Is that what a gsm sold by Woosh £199 for the 36v can it be over volted ?

 

[wheeliepete]

Yes, same unit