Efficiency / throttle / ebikes.ca simulator

[El_Steak]

I have a 2806 in a 26inch wheel and a Methods 100V/100A controller.

I'm shopping for batteries and am considering a 24s Lipo setup (88volts 50amps).

When I look at the ebikes.ca simulator, this setup produces great torque all the way to high speeds, but a way too high top speed and poor efficiency of 70% at cruising speeds around 30mph.

Using a 2807 or "simulating" a 2809 or 2810 using a trick explained by Justin brings the top speed down a bit and produces better efficiency at lower speeds. It doesn't however improve torque noticeably. (The higher windings = greater torque myth was actually debunked by Justin).

Now this is all at 100% throttle. If I slide it down to 60%, then the top speed and efficiency become perfect for maintaining a 30-35mph speed. Is this accurate? If so, can I assume that simply throttling down to 60% will bring this 4.4kw beast in the sweet efficiency spot when cruising at 30mph on my commute? And yet allow me to burn rubber and reach crazy speeds when going full throttle?

 

[justin_le]

Now this is all at 100% throttle. If I slide it down to 60%, then the top speed and efficiency become perfect for maintaining a 30-35mph speed. Is this accurate? If so, can I assume that simply throttling down to 60% will bring this 4.4kw beast in the sweet efficiency spot when cruising at 30mph on my commute? And yet allow me to burn rubber and reach crazy speeds when going full throttle?

The answer is yes, as far as your motor is concerned. It will have pretty much the identical efficiency and characteristics when running 88V at 60% throttle as it would if you were 100% throttle at 53V.

However, your motor controller will get a lot hotter and your capacitors will wear out faster when using a higher voltage setup at partial throttle than an appropriate voltage setup at full throttle. At least this is the case with trapezoidal drive 6-step brushless controllers. At partial throttle, you have switching losses and freewheeling diode losses and a huge ripple current in and out of your main capacitors from the PWM activity, while at full throttle with no current limit, the mosfets aren't doing PWM but just stepping along in the sequence, so all the electronics are much happier.

With a sinusoidal drive controller, then it wouldn't really make a difference.

Justin

 

[Miles]

(The higher windings = greater torque myth was actually debunked by Justin).

Belief in it still persists on the forum, though............. and is probably at the root of a lot of the interest in delta/wye switching....

 

[El_Steak]

However, your motor controller will get a lot hotter and your capacitors will wear out faster when using a higher voltage setup at partial throttle than an appropriate voltage setup at full throttle. At least this is the case with trapezoidal drive 6-step brushless controllers. At partial throttle, you have switching losses and freewheeling diode losses and a huge ripple current in and out of your main capacitors from the PWM activity, while at full throttle with no current limit, the mosfets aren't doing PWM but just stepping along in the sequence, so all the electronics are much happier.

Thanks for the info Justin.

Would using the speed-limiter on the Cycle-Analyst have a similar effect on the controller? If not, using the POT mod to allow for on-the-road adjustments of the speed limit could be interesting.

 

[justin_le]

Would using the speed-limiter on the Cycle-Analyst have a similar effect on the controller?

Of course, because using the speed limiting function on the CA is exactly the same (as far as your controller is concerned) as backing off on the throttle.

Controllers _should_ be designed to handle doing PWM 100% of the time, but the units used in ebikes generally have undersized capacitors and do single quadrant rather than 2 quadrant PWM, both of which make them run hotter and with less efficiency in PWM mode. You can still use them this way, I'm just saying it's not quite as ideal as when the controllers are on 'full throttle'.

-Justin

 

[methods]

I agree with Justin.

When I put that line of controllers out there I was intending for people to run 24S lipo with a 5305. This allows you to enjoy those monster accelerations but quickly reach a reasonable freewheel speed (up and out of PWM in a hurry)

The controller has 4300uF of capacitance - but those are Chinese caps and I dont have any kind of long term data.

An example would be setting my controller for a 20A current limit -> try it. Ride around for a few miles and I bet the controller gets warm to the touch. Now roll the current limit back up to 100A and hammer it. Probably will stay cool.

Current limits are a funny thing.... People hook a high KV motor to one of my controllers and they feel it start to get warm - so they turn down the current limit and it gets warmer I try to convince them to crank the current limit and ride full throttle to bring the temps down and they just look at me cross-eyed

So - to answer your PM ... The best solution is to match the KV of your motor to the voltage you are running. Ideally you want to cruise at WOT. If you find yourself on the current limit all the time it is bad for everyone. A 9x7 wind is still kind of "hot" for 24S but you can easily reach the freewheel speed.

If you want to cut that 10x6 out of the rim and send it to me I will trade you for a brand new 9x7 from E-Bikekits.com (rear, black). I have been looking for a few more 10x6 for some 12S projects I have going.

It is a rear right???

-methods

 

[El_Steak]

Thats the dilema, lower the current limit and the motor stays cool but the controller gets hot. Raise it up to 100A and the controllers stays cools, but the motor melts down

Hummm, motor is cheaper...

Spring time is here and I want to ride my bike so I'll keep the motor and stick to an 18s config for now (still pretty fast).

I'll upgrade to something else next winter that will allow me to go 24s. Maybe a move to a 20" wheel or a switch to a 7x9 (at some point Jason from ebikekits mentionned that he had the intention of getting some of those). But who knows what will be available then. Things are evolving faster every year in the ebike world...

 

[methods]

I run a 9x7 in a 20" wheel with large cooling holes.
I run the current limit maxed out at something like 120A - I did overheat the motor once to over 200C but it recovered and for the most part it stays at a reasonable temperature with aggressive riding. So long as I dont don't lug it a lot - it can go all day long.

Sizing down to a 20" makes a big difference is you can do it... both for efficiency and performance. The acceleration around 25mph is staggering for a motor that is so inexpensive.

But 20" for a bike is not that reasonable... mine is on a recumbent trike.

-methods

 

[Mark_A_W]

(The higher windings = greater torque myth was actually debunked by Justin).

Belief in it still persists on the forum, though............. and is probably at the root of a lot of the interest in delta/wye switching....

Because in the real world, with current limits, it's true, at least as I understand.


If I have a motor with a no load speed of 40kmh, and the same motor wound for 80kmh, the torque will be the same if there are no current limits (same voltage).

But if I have a 40A limit, and the slower wind is drawing 39.999A under acceleration, then it will have double the torque of the faster wind, which needs 80A to match the torque.


If my understanding is wrong, please correct me, I want to understand this better.


Full-throttle's low speed Bafang BPM eats my fast MAC Shanghai during initial acceleration. The motors aren't identical, but are very similar sized geared hubs. The Bafang BPM was drawing about 30A and the MAC was drawing 35A (both due to controller limits). The big difference between the motors is the Kv of the MAC is way higher.

 

[Miles]

But if I have a 40A limit, and the slower wind is drawing 39.999A under acceleration, then it will have double the torque of the faster wind, which needs 80A to match the torque.

Yes, presuming the current limit is set by the battery output, or for the benefit of the controller.

The point is that, a given motor's maximum continuous torque output is limited by its ability to dissipate heat - this is independent of Kt.

This doesn't take into account the effect that the PWM duty cycle has on motor efficiency.

See also, this thread: http://www.endless-sphere.com/forums/viewtopic.php?f=30&t=16591

 

[liveforphysics]

But if I have a 40A limit, and the slower wind is drawing 39.999A under acceleration, then it will have double the torque of the faster wind, which needs 80A to match the torque.

Nope. This is exactly the misconception a lot of folks have.

With controllers both limiting battery current to 40A, and 1 motor with 2 turns, and one motor with 4 turns, the torque between them should be roughly identical. The only difference in torque would be due to efficiency differences rather than the motor wind differences.

This is because to limit the motor with half the turns at 40A, it has to run a phase duty cycle at ~50% of the duty cycle that the motor with twice the turns uses. This means the coil with half the resistance and inductance is seeing an average phase voltage of half of that seen by the 4 turn motor. Both motors are taking in the same amount of power (battery voltage x 40a current limit), and one motor is running at half the phase voltage, which means the phase current must be double, because energy doesn't just disapear.

This is how phase currents can get so high relative to battery currents.
Keep in mind, only your battery and your own instermentation see the battery current. Your motor only see's the phase current, and the phase currents get multiplied inversely with the change in voltage due to PWM duty cycle.

 

[harrisonh]

Controllers _should_ be designed to handle doing PWM 100% of the time, but the units used in ebikes generally have undersized capacitors and do single quadrant rather than 2 quadrant PWM, both of which make them run hotter and with less efficiency in PWM mode. You can still use them this way, I'm just saying it's not quite as ideal as when the controllers are on 'full throttle'.

I apologize for reviving an old thread, but I couldn't find the information after searching the forums and google.

What does single quadrant and 2 quadrant PWM mean? I think I understand there's 4 quadrant commutation (forward, regen, reverse, reverse regen), but what does it mean for PWM? (Are they separate, and can have Regen and be in single quadrant pwm?)