36V vs 48v efficiency.

[benjy_a]

In my thirst for knowledge I have another question...

I'm building a folding electric bike using the new Crystalyte G209 direct drive motor.

I will be using a 20A Infineon controller with a direct plug in Cycle Analyst which gives me full control over top speed and current drawn by the motor.

I am considering going up to 48v on this project and then limiting the current to say 8-10A. I will also limit the speed to stay UK legal.

Will this be more or less efficient than 36v at 15-20A? Is this even possible to answer without the full specifications of the motor?

Thanks,
Ben

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[cwah]

You can check different setup directly on the simulator:
ebikes.ca Hub Motor and Ebike Simulator

The closest motor to the G209 is the crystalyte 406.


If you plan to stay road legal, you better have the right voltage for your speed. So that the motor would be at his max efficiency.

If you increase the voltage but reduce the max speed, the motor won't be at his max efficiency band and will be less efficient.

So if you want to ride at 15 mph, the 36V setup is the best for you.


That's what's happening to me with my little DD motor, the top speed is at 30+ mph, but most of the time I'm around 17-23 mph. So now I have an highly inefficient bike using a lot of watt. I'm even thinking about lowering the voltage in order to remain on the high efficiency band.

 

[amigafan2003]

It's worth noting that higher voltage batters suffer less voltage drop - so they maintain the motors top speed and power up inclines/into headwinds more effectively*.

Take note of cwah's comments on motor efficiency - if you up the voltage you want a lower rpm motor (unless like me you're comfortable with exceeding the legal speed limit for ebike assist).

 

[NRG]

Exactly... although 48v is more efficient in terms of electrical power loss in the circuit as you can run a lower current, however, as cwah and amigafan point out get the voltage wrong for your speed and the motor inefficiency swamps any electrical circuit loss.

 

[benjy_a]

As I understand it, the cycle Analyst speed limit actually changes the voltage seen by the controller.

Would this not allow more torque and power up to the set speed limit, without running the motor inefficiently?

The following is from the cycle Analyst website:

Speed Limit: This has the utility of providing legal compliance to the speed cap imposed on ebikes in most jurisdictions. When used with a full throttle, it serves as a cruise control on the electric bicycle. It can also be beneficial to riders who want to increase their torque and power by using a higher voltage battery, without simultaneously increasing their top-end speed.

Current Limit: An adjustableampslimitisuseful to prevent damage to the batteries from excessive current draw, to increase the range that you'll get on a charge,and to protect themotor controller and motor in setups that draws too many amps.

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[NRG]

The motor inefficiency is built in, its how the motors are wound IE: wound for a certain RPM at a certain voltage. The CA as far as I understand scales back the throttle input voltage as would the rider in use, except the CA can monitor the road speed, and current draw from the battery and scale back the throttle voltage depending on what parameters have been programmed or set by the user. The controller still sees the full battery voltage but scaling back the throttle alters the PWM signal applied to the motor and thus caps the speed.

Edit: I see the throttle can also be configured as a variable battery current limiter which some my find useful to extend range...

 

[benjy_a]

The motor inefficiency is built in, its how the motors are wound IE: wound for a certain RPM at a certain voltage. The CA as far as I understand scales back the throttle input voltage as would the rider in use, except the CA can monitor the road speed, and current draw from the battery and scale back the throttle voltage depending on what parameters have been programmed or set by the user. The controller still sees the full battery voltage but scaling back the throttle alters the PWM signal applied to the motor and thus caps the speed.

Edit: I see the throttle can also be configured as a variable battery current limiter which some my find useful to extend range...

Ok so basically if you had 48v and just used the throttle carefully it would have the same effect? Surely then the motor inefficiency would only happen at wide open throttle but going less than optimum speed?

As far as I see it you only have the inefficiency of the actual controller and it's ability to step down the voltage to the motor, is that correct?

The motor is wound to go 15mph at 36v, so surely at about 70% throttle on 48v it still runs efficiently or am I completely wrong? There seem to be some posts on endless sphere supporting this view.

I will probably stick with 36v in any case unless the motor rpm is really slow. The problem is that it's an unknown quantity until I get it!

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[NRG]

As I understand it, yes, the CA controls the throttle input as would the rider except the CA does it automatically.

The controller uses PWM to control the effective voltage the motor see's, under gentle load the efficiencies at full throttle vs 70% or whatever are essentially the same. When under load though the motor slows down and to maintain forward progress the controller has to increase the phase amps to the motor in order to maintain the same speed, so phase amps rise to increase torque and the motor efficiency drops causing the windings and controller to waste power in the form of heat. The controller trades volts for amps, it does this whatever the throttle voltage is.

 

[benjy_a]

As I understand it, yes, the CA controls the throttle input as would the rider except the CA does it automatically.

The controller uses PWM to control the effective voltage the motor see's, under gentle load the efficiencies at full throttle vs 70% or whatever are essentially the same. When under load though the motor slows down and to maintain forward progress the controller has to increase the phase amps to the motor in order to maintain the same speed, so phase amps rise to increase torque and the motor efficiency drops causing the windings and controller to waste power in the form of heat. The controller trades volts for amps, it does this whatever the throttle voltage is.

Ok thanks I think I get it. I will wait and see what the real speed is with my old 36v battery...if top speed at fresh charge is acceptable then I will stick with that voltage but perhaps go for Lifepo4 and reasonably high current on the controller with higher capacity battery. I look forward to finding out!