Motor efficiency,volts,amps and 'watt' not.

[awol]

I'm more mechanical than electrical so am having to learn about the electrics side of things and have been tinkering on the hub motor ebike.ca simulator. For my 1st build I'm considering overspeccing it for reliability and longevity.
So hypothetically if I have a 500w motor running at 100% on throttle (throttle=watts?)at 16mph it uses battery-20A and runs at 75% efficiency, which I believe is optimal for the motor, therfore a 20ah battery will last aprox 1 hour at this constant?
Now if I decide to use 50% PAS level regularly instead I get 8mph at motor efficiency at 75% at 200w and 9A.
So does the controller adjust the amps and/or watts so when I pedal along 8mph the motor is still running at an optimal efficiency but the 20ah battery now lasts just over 2hrs at this constant?
So the controller can 'look after' the motor and battery depending on what speed I choose to regularly do?

 

[Deleted member 4366]

Very interesting question. The answer's not so easy.

First, you have to be mindful of the RPM of your particular motor at the voltage it's running at. The simulator doesn't take into consideration wind or pedalling.

Different controllers behave differently. They used to be all speed controllers, so your PAS level or throttle position makes the controller try and run at a particular speed regardless of conditions, so you could still get maximum power when going up a hill slowly. The Ananda controller in Oxygen bikes was an exception, where there was a ckear difference in power at each PAS level regardless of conditions. The new "torque simulation" controllers are supposed to be the same, but the one I have isn't noticeably different to the speed control ones, although it has smoother power profiles. The Simulator doesn't take into consideration all these new types of controller.

Assuming you have the old type of controller, the throttle or PAS position sets the maximum rpm of the motor, and as the max speed is reduced, the whole efficiency curve moves down with it, so with a 20 mph motto, maximum efficiency would be at about 15 mph, but at quarter-throttle, it becomes a 10 mph motor, so peak efficiency comes at 7.5mph' however, power is also less because of the lower rotation speed, so consumption would be much lower.

There's lots of relationships between speed, power, torque, current, throttle, software, wind, hills, etc, and it's very hard to pin down exactly what's happening at any point in time, so one can only generalise. The most important thing is that choose a motor. controller and battery, that give optimum performance under the conditions that you normally encounter on your rides, which means speed, weight, hills, wind etc. The simulator helps, but real-world experience is also needed to choose the right stuff.

The simulator doesn't work for any crank-drive motors.

 

[awol]

Thanks for a great reply, I have read it a couple times and probably will a couple more. Your clearly highly skilled in the subject.
So power=num of watts produced at motor to spin at required rpm to get my desired speed and amps=power drawn by the motor from the battery to produce this power and my battery and controller must be able to provide that amount of amps for the motor to spin at the rpm I require to get the speed I want.? (phew!)

I have been basing things on a bmsbattery 36v 500w cst rear wheel with a controller it recommends also you posted good comments about last week called a S12S. I am still checking out the differences in controllers other than amps. As for battery I was simulating the difference between a 36v20ah (pingbattery)and a 48v10ah (bms shelf battery), as I need it to last at least 20miles round trip or more in multiples of 10miles.
I take it a 48v10ah will not last charged for longer than a 36v10ah equiv but can give more amps to give faster speed? (but the controller must handle the more amps)

 

[Deleted member 4366]

A 22 amp controller at 48v will feel similar to a 30 amp 36v one, but the motor will spin faster at full throttle, but have less efficiency when hill-climbing, so could overheat.

A 15aH 48v battery will give you approximately the same distance as a 20aH 36v one. I wouldn't advise a Ping battery. They're really good batteries, but very heavy. Your bike will be much more enjoyable with alighter LiNiMnCo one (Li-ion).

For the 500w CST motor, the S12S controller will get you up virtually any hill if you weigh up to about 85kg. If you''re heavier, and you have steep hills to tackle, the S12P would be better. Bear in mind that most people have only 15 amps and a 250w motor, and they often find that adequate.

 

[awol]

I tend to have a habit of purchasing overprecced products for what ever it is I need with the assumption they will do what they do easier and last longer but you've got me thinking this is not the case with this project.
My choice of motor was because of that and I prefer the idea of rear wheel motor and the cst fitting also I am thinking the constant pulling on front forks with a front motor may wear the forks bearing out or damage them early or am I wrong on that?
As for battery I was lifting a 7kg weight around yesterday and your right, I think it may affect my balance at rear shelf height.
I'm small and weigh less than 60kg, so you've now got me considering 250/350w motor, but what could I do about the cst fitting for a rear wheel or are front drives ok? also I could then consider a smaller battery to help my balance but need to look at the ah ratings for my 20mile journey.

"A 15aH 48v battery will give you approximately the same distance as a 20aH 36v one" - this worked out from the voltsxamps, both give same watts at motor?

 

[Deleted member 4366]

At 60kg, you don't need a 500w motor. I just sold one of my MXUS kits to Jcubbin on this forum. Hopefull, he'll give a honest opinion of it once he's installed it if you can wait. I have a few motors left. They're a CST motor rated at 350W, and I run mine at 22 amps. I put details in the classified section if you're interested, and I did a review here:
http://www.pedelecs.co.uk/forum/electric-bicycles/15211-mxus-motor.html

 

[awol]

Yes I would like one of those kits, sound ok for someone my size but I have not decided on a bike yet so you would have to hold it for me until I decide and find out the wheel size. How do I contact you for payment etc ?

 

[Deleted member 4366]

I'll send my Paypal by personal message, then you can pay when you're ready and send your address in a PM or Paypal message with the payment.

 

[awol]

great, thanks d8veh
Will be in touch. I am off to look at bikes later, will the motor fit hybrid bikes or what gap will the wheel need to fit into.

 

[Deleted member 4366]

It'll fit any normal bike with 135mm wide rear. Some hybrid bikes have 700c rims. I only have 26" ones.

 

[awol]

Will the MXUS happily push me along rapidly if I go the 700c hybrid route or should I really be looking at an MTB instead for this motor?