The battery game

[safe]

Recumpence (the guy with the PK Ripper bike that just sold for $5K) builds custom geardowns for RC motors that fit on ebikes. For whatever reason there is a shortage of high quality geardown units that make sense for ebikes. Most industrial geardowns are built with no thought about weight and are unusable. (many weigh 25 lbs)

The brushless motor has the advantage of being highly efficient at speeds up in the 6,000 to 10,000 rpm range. Brushed motors seem to fade out at around 5,000 rpm.

Without good geardowns you cannot achieve the performance potential of electric motors on ebikes very well. Hub motors need to be massive because in order to achieve any kind of efficiency at such low rpm (500 rpm or less) they require lot's of copper.

So if you want light weight, and high efficiency, then you need to go brushless and have a lightweight geardown to make the thing workable.

Recumpence has a strong niche in this potential emerging market. (I hope he can somehow profit by it)

Him selling this PK Ripper for $5K (only a few days after building it) is a good sign that profit is possible in this hobby.

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The brushless is the superior motor, but it more or less is a computer controlled device. It seems to me that in the loooooooong term that something like a Switched Reluctance motor is actually cheaper to build (it's just a solid block of iron at the core) and the computer control is no worse than on a brushless. Also, the Switched Reluctance has a powerband more like the traction motors with would make for a better motor across a wider powerband. The Switched Reluctance might be good enough to make for a one speed ebike that still performed as well as a geared ebike. (maybe)

http://en.wikipedia.org/wiki/Switched_Reluctance_Motor

That's all ramblings about future ideas... nothing I'm planning to pursue any time soon...

 

[duivendyk]

Driving the crank elecrically will take a substantial reduction. Motors typically run at a couple of thousand rpm while the crank speed is just a few hundred.

 

[safe]

That's why a good quality geardown that is lightweight is such a need for ebikes. Recumpence has the potential to capture an emerging market.

Back on that Switch Reluctance motor... since they use no permanent magnets they have no backEMF. This means that they can run at very high rpm or very low rpm and have high efficiency everywhere.

If there were a 1000 watt input ebike racing class this type of motor could deliver the exact power allowed across it's entire rpm range without much efficiency loss anywhere.

It's the ideal I think.

Have you heard of these motors before?

 

[duivendyk]

Yes I have,but I know little about them.There must be a snake alurking in the grass somewhere.Most motors like fixed voltages are these the exception?

 

[Fuzzo]

Motors either produce power or heat. It's the ratio of power verses heat that defines efficiency. So there is no such thing as "not caring" about efficiency and only caring about heat because they are directly related.

The essential problem with overvolting (with no ability to change your gear ratio of motor to rear wheel) is that your "coolest and most efficient" rpm gets increased by whatever amount you overvolt.

So if the "sweet spot" is 10 mph when at 24 volts then the "sweet spot" becomes:

10 * 36 / 24 = 15 mph

...when overvolted to 36 volts.

This means that you need to climb hills FASTER to keep the heat down, but that tends to require more power. When you run the calculation the heat scales proportionally with the slope of the hill.

So you can't win. More volts means more speed, which means more power, which requires more energy that creates more heat.

We were talking about different kinds of inefficiency, I think. You meant 'the proportion of power that goes to producing work'. I meant Wh/km. I'm not overly concerned if Wh/km drops on hills - I expect that, in fact. If I get more horsepower, then that saves one of the most inefficient factors in my equation - me. I get real tired real fast on hills. A bit more power means that on a hill I enter my efficient cadence, and that helps the motor too - together we can pull out of the inefficient low RPM.

Of course some hills are so steep that it's impossible to go up them fast, for any amount of power. As a driver, I need to watch that in any vehicle - it's easy to overheat a car driving up a really steep hill in first gear too. The solution is simple - stop for a while. I could even drip some water from my water bottle on it if I was really concerned

Obviously I'm very concerned if it overheats my motor, or burns out my controller, or snaps my chain. But no amount of calculating is going to tell me if that is going to happen. It just has to be tried.

 

[safe]

Yes I have,but I know little about them.There must be a snake alurking in the grass somewhere.Most motors like fixed voltages are these the exception?

The Switched Reluctance motors are nearly identical to the brushless motors except that they don't use permanent magnets.

The biggest negative I've read is that there is a significant (and somewhat unpredictable) torque ripple to these things. They make a lot of noise and if you are trying to use them for some kind of control device they can be jerky at slow speeds. However, given that on motorcycles you have "thumpers" like the Ducati bikes the riders are not going to complain if the motors make a lot of noise and vibrate.

Many of the "politically correct" types seem to make low noise a high priority, but for the more raw and animalistic types that want to race (my type of animals) the noise would just add to the excitement.

 

[safe]

We were talking about different kinds of inefficiency, I think. You meant 'the proportion of power that goes to producing work'. I meant Wh/km. I'm not overly concerned if Wh/km drops on hills - I expect that, in fact. If I get more horsepower, then that saves one of the most inefficient factors in my equation - me.

Efficiency and motor rpm are related.

...study the "death cross" which is the crossing of the power curve (black) and the heat curve (red). When the "death cross" fully sinks in you understand why gearing is everything. If you can get a low enough gear ratio then you can climb just about anything. If you can't then you overheat. If you overvolt it increases the power overall, but also increases the maximum rpm, so the heat doesn't get better because of overvolting and can even become worse.

Multispeed gears are the answer for ebikes... and since most bicycles already use gears the trick (in the future) is going to be to seemlessly integrate the motor and the pedals. This is difficult to do and so we wait as technology lags behind.

 

[Fuzzo]

Yup, gears are cool. In my case, I don't have any on the motor so I need other options. Higher RPM would be nice - my max no-pedaling speed is about 24 kmh, and if that got up to 30 kmh I'd be quite pleased. That would save me having to push through that last 6 kmh - maintaining it is nowhere near as hard as getting to it. I'm assuming that acceleration would also be faster, so less time would be spent at low RPM, to counterbalance the extra heat created there. Only on hills am I in danger.

Considering that the extra battery is not meant to be permanent, I already need a way of easily switching between 24 and 36 volts, basically disconnecting the extra battery. This could be my safety valve if throttle control was not sufficient to keep the heat down on a hill. We shall see if it is needed.

 

[Fuzzo]

I did already consider just changing the gear ratio on the motor, so that max RPM is at a higher speed. This would make the torque much less at the low end, and could end up sucking much worse on hills, but on the flat I'd get a higher top speed. Unfortunately Auckland is full of hills, so that plan was scrapped.

 

[safe]

This is where rewinding fits in...

You could rewind the motor with more turns thus making it have a lower top speed. You then compensate with a higher voltage. If you can do this and squeeze a thicker wire into the existing grooves in the motor you can essentially have your cake and eat it too.

If you can simultaneously increase the turns and decrease the resistance (with thicker wire) you just get more power with less heat at lower rpm.

You would have a truly better motor. (but rewinding is not easy)

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Otherwise... as you increase the voltage that just stretches the maximum speed up higher and that's not going to do any good on the hills. It all cancels each other out with overvolting and hills and heat. (you make no progress really)