Beginner Q about motor power

Dinama

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Jul 18, 2018
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I want to build Ebike with 3KW 9kg rear hub motor / 72V 15AH C8 / 60A controller with CA to control Amps
EBIKE weight should be around 35KG and my weight is 60KG max

I want to know how the motor work and drains the battery in different situations, it's a little hard for me to put it so I'll just write it and hope you understand me(sorry for the broken language)

Is there a minimum power that the motor must work on? or just a maximum?
let's say 250W motor can handle 800W more then that and it will be destroyed right? in the same way an 3KW motor will not handle 7KW,it will be destroyed..true?

But what will happen in the opposite scenario? 3KW motor will get only 1200W (72V x15A) what will happen then?
it will adopt him too much or it will make him work without effort?

Right now i have E BIKE with 250W small motor / 48V 15AH and 15A controller,i drive around 42-45kmh for 40km(i believed i drive on 800W +-).
I want to know what happen with a 3 Kw motor / 72V 15AH and controller that i can function to different A through CA.
Now i want to use 3 different Amps for drive,and want to know what happen each situation:
With 10A DC - i will drive up to 45kmh for max 60km (800W)
With 25A DC - i will drive up to 60kmh for max 40km (1800W)
With 45A DC - i will drive up to 77kmh for max 25km (3400W)

Are these assumptions correct?
Or you say that everything I've put up until now is a mistake, and the motor must have minimal power and oblige me to use let's say 40A min and drive on 3000W and 70 KMH for 30KM and there's nothing I can do?!

Thanks to all the responders!
 
Well, you are optimistic with the range to be expected, because you don’t take into account the power used in acceleration. Everytime you stop and start, every hill you climb, every windy day, are stealing some range.
 
Go over to ebikes.ca and play around with the MOTOR SIMULATOR

Then while your at it, you could go to TRIP SIMULATOR BETA and play around with your route and your requirements. You have to click on a pull down menu to get the GOOGLE MAP, then just zoom out from Vancouver BC Canada to your area and you are GOLDEN!
 
For a mid drive - internal gears and clutches are the failure point. For hub motors - heat is the enemy. Most hub motors can be run for a few seconds at 2x-5x rated power. Mid drives will usually break if you run more than the rated power through them.
 
It's not as simple as you put it.

A motor has an unloaded speed. If you limit that speed via current, rather than effective voltage, the motor becomes less efficient.

So if your motor is capable of 77km/h, but because of lack of current is only going 45km/h, it's going to be using that power inefficiently.
 
Sunder said:
A motor has an unloaded speed. If you limit that speed via current, rather than effective voltage, the motor becomes less efficient.

That's not really the case. Heavy current limiting reduces peak efficiency slightly, but it broadens and flattens the efficiency curve, which can often lead to greater overall efficiency. You can see the effects clearly using the Ebikes.ca simulator. Performance-wise there's a pretty big difference with a gradual smooth acceleration, and set up that way seems to make wind and small grades have a big impact on speed. Plus it will be harder to predict speed. These reasons are why speed limiting is so commonly done the other way.

A benefit to limiting speed by current limiting is that you're less likely to have heat problems on hills.
 
Weird. I did try it:

72v, 10A gave me an efficiency of 33.4% on 10% hill a Clyte
36v, 20A gave me an efficiency of 50.6% same settings. A gain of over 50%.

You might be thinking that 10% is a very steep hill. And it is, but that's not why I'm setting it so high. Cruising should take very little power whatsover - gains there are not what matters, and sure enough, the same figures on a 0% grade, gives you only 1% gain in efficiency.

Accelerating - either to slow down for traffic, stop for lights, or after a corner, is what burns your power. Since there's no acceleration field in the eBike calculator, I just used 10% to simulate a 0.1G acceleration. That's about 11 seconds to hit 40km/h, or a really gentle take off. Many hills would be 5-10% grade in many cities as well.

Edit: Just thought about it a bit more, and to cruise 40km/h takes about 400w at the rubber. So I'm thinking maybe there's so little difference between 72v and 36v, because neither of them are actually limiting current? How this would actually play out in real life, and whether it matters to OP really depends on hills and amount of start/stop he has.
 
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