experts weigh in: 72v at 10A or 36v at 20A, performance?

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voicecoils   10 MW

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experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Oct 30 2008 11:20pm

As some of you have read, I'm trying to run my Bafang geared brushless motor without shredding the gears, until a more robust (metal?) gear solution arrives. I'm on my last clean set of gears and have racked up ~50km without disaster, running at 36v nominal (lifepo4).

I'm getting 25-30km/hr on level ground with light peddling. Peddling and accelerating off a stop light draws 15-20A and gets me up to ~20km/hr reasonably quick. On hills, where 15-20A is sustained for more then a few seconds, I can hear the gears straining and have to pedal harder to lower the load on them. I'm obviously a bit gear striping paranoid.

My question is, by limiting current on the Cycle Analyst with my 36-72v 35A Crystalyte Analogue controller, how will the bike behave differently at:

* 36v 20A max limited

* 72v 10a max limited

With the latter, I know I should gain some top speed which I want, but will I lose acceleration off the line? The power is obviously the same but I regularly hear people say "voltage is speed, current is thrust".

Does anyone have an explanation or predictions?

Cheers!

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by recumbent » Oct 31 2008 12:06am

If you look at the "simulor on http://www.ebikes.ca" the difference huge.
Ebike in use:
LWB Recumbent bike, Sram i-9 internal gears.
Power: 58V, 16 amp/hr, lipo batteries.
max speed: 52 km/h (30 mph),
max range: 40 km's, incl stop&go few times, minimal pedaling.
New build: Delta recumbent trike

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by pwbset » Oct 31 2008 12:13am

I'd take 72v10a over 36v20a any day. Voltage rocks! To quote Jean-Luc Picard, "Energize!" :mrgreen:

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Ben   100 W

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by Ben » Oct 31 2008 12:17am

I think you can expect around the same acceleration.

While the voltage = speed, current = acceleration thing is loosely correct, there are other factors which will come into play.

The way I see it is that at 72V you have a higher EMF and therefore you use the current more efficiently.

I had a play around with the ebikes.ca simulator and it shows that the torque you'll get at 72V 10A will be slightly larger than what you get at 36V 20A.

So I reckon run it at 72V (like there was any real doubt in the first place).

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voicecoils   10 MW

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Oct 31 2008 12:51am

Well damn. I just looked at the simulator too. Thanks for the opinions guys. I tried two options with the eZee motor, which perhaps most closely approximates my bafang. I used 40V and 80V as they are closer to actual battery voltages through most of the discharge.

40 volts 20A:
40v20a-eZee.jpg
40v20a-eZee.jpg (67.3 KiB) Viewed 1315 times
Rather looks like the performance I'm experiencing.

80 volts 10A:
80v10a-eZee.jpg
80v10a-eZee.jpg (73.42 KiB) Viewed 1316 times
Last edited by voicecoils on Oct 31 2008 1:02am, edited 1 time in total.

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voicecoils   10 MW

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Oct 31 2008 1:01am

80V does look MUCH better, even with the restricted current. I suspect with wind resistance, I wont be hitting 70km/hr, but if the same power bumps me from 25-30km/hr to 35-40km/hr I'd be stoked.

At around 22kg without batteries (with 2 racks, controller, wiring, and lock) it's quite porky. At 4.7kg per pack, I was starting to like just carrying one :-) I need to get the suckers in the main triangle for one thing, once exams are over.

The other issue of course is gear stripping. No one knows why the failures occur, though there has been discussion. Gear speed, torque, heat, mechanical shock...? If it is primarily gear speed killing gears, then 80v is a bad idea. Nogwin is running 66v happily for nearly ~2000 km now with current up to 30A, but does peddle first before winding up the motor.

I will soon have one more spare set of nylon gears, but I cannot keep buying more!!! US$90 invested in gears so far, not including the set the motor came with.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by cerewa » Nov 05 2008 11:50pm

Besides high amperage and high voltage, another factor that could influence whether or not your gears will strip is rider+vehicle weight. If you put a lot of power into the motor and there isn't much resistance then why would the gears get damaged... but if there is a lot of weight to keep the wheel from spinning up to speed right away then that could cause problems.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Nov 06 2008 5:23pm

cerewa wrote:Besides high amperage and high voltage, another factor that could influence whether or not your gears will strip is rider+vehicle weight. If you put a lot of power into the motor and there isn't much resistance then why would the gears get damaged... but if there is a lot of weight to keep the wheel from spinning up to speed right away then that could cause problems.
Are you suggesting that because the motor is on the front and there is less weight on it that it is more or less likely to damage the gears? I didn't quite understand your response.

In my case:
~75kg rider
~22kg bike
~4.7kg battery pack (each, 1 for 36v, 2 for 72v)

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by vanilla ice » Nov 06 2008 5:32pm

He just means the load the motor has to pull, not the weight on that particular wheel. The resistance to acceleration. When your car's clutch starts to go out usually you're ok in first gear. Its top gear you'll first experience the slippage. For the same reason. Theres less resistance to the engine rpm increasing in first gear than the higher gears... Ok bad example.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by Ben » Nov 06 2008 6:33pm

Terrible example, just terrible :)

He's just saying that the heavier your bike is, the harder the motor has to work to propel you and therefore the gears would be more prone to stripping.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Nov 06 2008 6:50pm

Ben wrote:Terrible example, just terrible :)

He's just saying that the heavier your bike is, the harder the motor has to work to propel you and therefore the gears would be more prone to stripping.
Got it. I'm going to check my gears again today. I'm also, if I get the chance, going to try the 72v45A infineon controller which just came in with 10a current limiting with the direct plug in CA.

Perhaps it will run the bafang a bit more smoothly. At 80V10A it still seems a bit loud at times to me and keeps me worrying about peanut butter.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by Ypedal » Nov 06 2008 7:01pm

If we were discussing direct drive hub motors i'd say go high voltage..

but this is a motor not designed for high poer, and it's geared.. this makes a big difference in how they behave..

At 36v the no load rpm is much lower.. vs 72+v ..

At low speed, if you gently apply throttle and allow the gears to hook up, then ramp up the power slowly, you should be able to tell at what point more throttle does not = more pulling force.. if you are carefull and play nice.. there is a good chance that motor will hold up pretty good..

However.. if you like to crank the throttle WFO from a dead stop and don't pedal.. or worse yet pedal in pulses.. the higher the voltage you put into the motor the more potential for higher rpm you are playing with.. the more heat you make the softer the gears.. the more they wear down.. etc.. recipe for short gear life..

For making the gears last. 36 or maybe 48v.. but if you run 72v and the motor never manages to get up to full speed, sucking back amps and making heat.. expect problems.

If you want to run high power and play with the big boys.. get a bigger motor !
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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Nov 06 2008 8:26pm

Ypedal wrote:If we were discussing direct drive hub motors i'd say go high voltage..

but this is a motor not designed for high poer, and it's geared.. this makes a big difference in how they behave..

At 36v the no load rpm is much lower.. vs 72+v ..

At low speed, if you gently apply throttle and allow the gears to hook up, then ramp up the power slowly, you should be able to tell at what point more throttle does not = more pulling force.. if you are carefull and play nice.. there is a good chance that motor will hold up pretty good..

However.. if you like to crank the throttle WFO from a dead stop and don't pedal.. or worse yet pedal in pulses.. the higher the voltage you put into the motor the more potential for higher rpm you are playing with.. the more heat you make the softer the gears.. the more they wear down.. etc.. recipe for short gear life..

For making the gears last. 36 or maybe 48v.. but if you run 72v and the motor never manages to get up to full speed, sucking back amps and making heat.. expect problems.

If you want to run high power and play with the big boys.. get a bigger motor !
That's what my X5 will be for :twisted:

I'm just trying to get this bike going as a reliable commuter for my ~14km (each way) ride to work. The top speed at 36v is quite underwhelming, though the hill climbing assistance is quite nice. I need to clock more km's first but at 36v I'm peddling past the motors stop speed pretty quickly and beyond that there's no assistance. Perhaps a ~24v booster pack might add some assistance at speed without much weight. Something I could switch on when I get up to a certain speed. I think Mark_A_W is doing this with an RC lipo pack.

I do try to roll the throttle on smoothly, but I don't know that there is a good correlation between the mechanical motion of 0% - 100% throttle and the electrical response.

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Re: experts weigh in: 72v at 10A or 36v at 20A, performance?

Post by voicecoils » Nov 07 2008 6:27am

More no load testing at 40v & 80v with and without the wheel attached. May be of some interest to bafang users:
See direct post here: http://www.endless-sphere.com/forums/vi ... 90#p110590

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