# Q about volts, kV, efficiency, serial/parallel switching



## major (Apr 4, 2008)

ewert said:


> Okay, volts and kV equal RPM. I understand switching losses from controller drop efficiency in the controller:
> 
> About how much do some top of the line controllers lose efficiency at say 50% "volts"? (Soliton, Zilla etc.?) (just a side question really)
> 
> ...


Hi ew,

Careful about using kV. It usually stands for kiloVolts 

I think I understand what you are asking. And simply, no. Efficiency of the motor and controller is about the same at half speed as full speed, give or take a few points. In other words, there is little sense in switching to a 72 volt supply to run at 60 and then use a 144 V pack to run at 120. The controller, which you need regardless, can do this voltage reduction and get half speed on the motor very efficiently. And the motor will likely be happy and efficient running at either condition.

Different gears for 60 vs 120? Yeah, you might want to do that. Most do. And the difference in the 60 vs 120 gear is likely negligible, efficiency wise. You need a gear reduction between the motor and wheel regardless, so most guys keep the donor's OE tranny. This seems to work well for them, using 2nd for city travel and 3rd, maybe 4th, for highway. 

With your typical series motor, as long as you have it running at 2000 to 4000 RPM for your travel speed, no need to start doing anything fancy with battery voltage. The PWM controller will do the job very efficiently.

Regards,

major


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## ewert (Sep 5, 2009)

Oopsies about the kV =P. Didn't spot that heh.

Okay so messing around with parallel pack switching is no use as the controllers are good enough, gotcha.

I guess I'll google up some dc motor efficiency related texts to read up what matters. I'm personally most interested in motorcycles. Trying to figure out the motor efficiency stuff at the moment ...


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