It's easiest to explain this with a diagram.
I'm simply thinking the controller is a switch that either completes the circuit or opens the circuit.
The controller still sees 24 volts...but the motor sees 48 volts.
Can someone explain why this will not work...I must be missing something.
What if we put in more batteries? The controller still only see 24 volts, but we can let the motor see 72 volts or even more.
It feels this should also work even for brushless motors like our favorite Xtreme bikes.
There are two problems.
When the powerstage on the controller is off, and the motor isnt turning, it sees full pack voltage, regardless of whether it is outside the string, or inside it.
so if the components in the 24v rated controller cant deal with 48v (or higher) then they will go bang.
the other problem is, brushless motors are actually 3-phase permanent magnet synchronous motors. so you would need an extra pack on each phase in each direction (6 extra packs, rather than one).
you could overcome the first problem (if you are using a permanent magnet motor) if you switch in the extra pack after the controller, only after the motor is up to speed.
This is because a permanent magnet motor as back emf (voltage) while rotating with no power applied.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
Nice idea - in theory should work on a permanent mag motor - I´d like to know if it works in practice!
That's not the way controllers are hooked up.
Whether a given controller can handle 48v depends on the components. Manufacturers always use components with higher ratings than the rating they put on the controller. That way they have some safety room in case of higher voltages or components that don't quite meet their specified ratings.
In any case ... the way controllers hook up is to interpose themselves between the battery pack & motor. Controllers are not a simple on-off switch but they do electrical things to make the power higher/lower depending on the throttle setting.
Normal wiring is for the +/- of the battery pack is connected to the B+/B- of the controller, and the M-/M+ of the controller connects to the motor.
If you put two controllers in series such that M- of one goes to Motor's -, then that M+ goes to the M- of the 2nd controller and the M+ of the 2nd controller goes to the Motor + ... that would mean the controllers would be seeing 48v and the controllers would have to support 48v through their components. I've also never heard of connecting controllers in series.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
Thanks for the replies.
If there's an engineer familiar with the power stage design can draw a circuit diagram of how the FETs are connected, then we can understand better.
Otherwise, this simple explanation of the FET indicates it acts like a simple switch:
http://en.wikipedia.org/wiki/Field_effect_transistor ...
Consider an n-channel "depletion-mode" device...If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.
I now see what I missed...
Basically, even if it is conceptually a switch, any physical switch will have a spec on the highest voltage that it can switch. So it's futile to try to mod a controller to do more by arranging the batteries in a particular way.
Among the links I have collected on http://evsearch.net is
http://tech.groups.yahoo.com/group/osmc/
That's the Open Source Motor Controller group. They have several open source controller designs in their files section. The discussion is pretty light there.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
Thanks David.
Also, I believe that in practice the FET is not the component that limits the battery pack voltage, rather it is the capacitor.
The reason is most designs use a capacitor to smooth out the spikes of turning on and off.
See http://www.4qdtec.com/pwm-01.html for a detailed explanation of the capacitor:
no, its the lowest of the rating between the drive FET and the capacitor.
The capacitor only does smoothing on the battery side. the inductance between the battery and capacitor is *very* small relative to the size of the capacitor.
the inherent inductance of the motor smooths the motor side.
in practice the freewheel mosfet is usuaully a diode.
the controller powerstage diagram shown is for a DC motor.
here is the powerstage for a 3-ph controller:
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km